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New Possible Applications of Heavy Main-Group Elements in Organic Synthesis.

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the Symposium further examples of the importance of steric
effects (E. Vismara) and polar effects (P. Boldt) were given.
The author hopes that those not specifically mentioned above
will accept this general acknowledgment. He must however
acknowledge his own colleagues and in particular J. C. Walton.
Received: December 2, 1981 [A 413 IE[
German version: Angew. Chem. 94 (1982) 433
[ I ] Unless otherwise stated the kinetic data are taken from: a) A. F. Trotman-Dickenson, G. T. Milne: Tables of Bimolecular Gas Reactions.
NSRDS-NB 99, Washington D. C. 1967, and the Supplementary Tables;
1st Supplement: E. Ratajczak, A. F. Trotman-Dickenson, Uwist, OSTI,
Department of Education and Science, 1970; 2nd Supplement: J. A.
Kerr, E. Ratajczak, University of Birmingham, 1972; 3rd Supplement: J.
A. Kerr, E. Ratajczak, University of Birmingham, 1977; b) J. A. Kerr, M.
J. Parsonage: Eualuated Kinetic Data on Gas Phase Addition Reactions.
Butterworths, London 1972; c) P. A. Grey, A. A. Herod, A. Jones, Chem.
Rev. 71 (1971) 257.
121 Unless otherwise stated, bond dissociation energies are taken from: J. A.
Kerr, Chem. Rev. 66 (1966) 465: S . W. Benson: Thermochemical Kinetics,
2nd ed., Wiley, New York 1976.
[3] J. M. Tedder, Tetrahedron 36 (1980) 701: 38 (1982) 313.
[4] B. Giese, Angew. Chem. 89 (1977) 162: Angew. Chem. Int. Ed. Engl. 16
(1977) 125.
[5] C. Ruchardt, Angew. Chem. 82 (1970) 845: Angew. Chem. Int. Ed. Engl.
9 (1970) 830.
161 C . Ruchardt, H.-D. Beckhaus, Angew. Chem. 92 (1980) 417; Angew.
Chem. lnt. Ed. Engl. 19 (1980) 429.
[7] J. M. Tedder, J. C. Walton, A h . Free Radical Chem. 6 (1980) 155.
[S] D. S . Ashton, J. M. Tedder, J. Chem. SOC.Perkin Trans. I 1 1972, 965.
[9] A. L. J. Beckwith, C. J. Easton, J. Am. Chem. Soc. 103 (1981) 615.
[lo] G. Russell in J. K. Kochi: Free Radicals. Wiley, New York 1973, Vol. I,
p. 275.
[ I I ] D. A. Coates, J. M. Tedder, J. Chem. Soc. Perkin Trans. I1 1978. 725.
[I21 W. H. Davis, W. A. Pryor, J. Am. Chem. SOC.99 (1977) 6365 (and earlier
papers); R. W. Henderson, ibid. 97 (1975) 213 (however, see D. D. Tanner, P. W. Samal, T. C . 4 . Rua, R. Henriquez, ibid. 101 (1979) 1168).
New Possible Applications of Heavy Main-Group Elements in
Organic Synthesis**
methods (36)
By Thomas Kauffmann*
Dedicated to Professor Leopold Horner on the occasion of his 70th birthday
Aside from elements of the 2nd row, and one element of the 3rd row of the periodic system-%, P, S, and Se, respectively, whose organoelement groups such as Me3Si and Ph3Po
have proven useful in numerous organic syntheses-other elements of the 3rd as well as 4th
and 5th row (Ge, As, Sn, Sb, Te, Pb, Bi) can also be used as components of synthetically
useful organoelement groups, the elements As, Sn, and Pb, in particular, offering certain
advantages over the others. Some of these organoelement groups are suitable equivalents
for Li- or halogen-substituents attached to carbon; they stabilize carbanionic centers (minimum of this effect at the 3rd-row elements), and owing to their suitability as leaving groups
in (3-eliminations, also open up interesting synthetic possibilities. The thermally induced
syn- and silica-gel induced anti-elimination of Ph3Sn, Ph,Sb, Ph3Pb, together with p-OH,
are novel. With the newly synthesized compounds Ph,EI-CHz-Li
(El = Sn, Pb, As, Sb, Bi)
and other a- and p-lithiated R,EI- and Ph,As(O)-reagents such organoelement groups can
be introduced into organic compounds and exploited in organic and organoelement syntheses.
1. Organoelement Functional Groups
Organic chemistry largely concerns the chemistry of
functional groups. More and more “organoelement functional
containing elements previously not considered by the organic chemist are presently acquiring importance in organic synthesis. There are some groups of
this type whose central atom is a transition metal (examples: 3rd row in Scheme 1); however, groups with a main
group element are presently of much more importance: for
instance, regarding hydroboration, Wittig reaction, organoselenium reagents. Unlike classical functional groups
such as -OH, -NH2, -CO--,
-COOH, the organoelement functional groups are cleaved off again during the
course of the synthesis. The designation “mobile functional groups”[’’, however, would raise objections in that
such a designation frequently also holds for the classical
functional groups CI--, Br--, I-.
HO-
RzB[*] Prof. Dr. Th. Kauffmann
Organisch-chemisches lnstitut der Universitat
Orleans-Ring 23, D-4400 Miinster (Germany)
[**I New Reagents, Part 24.-Part 23: [83]: 7th Summary Progress Report on
Organic Anionochemistry.- Earlier Progress Reports: [I].
410
0 Verlag Chemie GmbH, 6940 Weinheim. 1982
0
RCu-
HZNMe$&
-co-
Hal-
Ph3P-
Me2S-
(CO)3Cr-
(C5H5)2ClZr-
e
0
RSe-
Scheme I . Examples of classical functional groups and organoelement functional groups.
0270-0833/82/0606-04IO $02.50/0
Angew. Chem. I n t . Ed. Engl. 21 (1982) 410-429
Main group elements that have proven particularly useful as constituents of novel functional groups are to b e
found in the region of the periodic system shown in
Scheme 2. Hitherto, their use has almost been restricted to
the light elements B, Si, P, S, and Se; only Sn (see Section
2.1) constitutes an exception. This prompted the question
as to what extent the heavier elements Ge, As, Sb, Te, Pb,
and Bi are suitable[41as constituents of new functional
groups.
In general, we examined compounds with singly-bonded
groups of the type Ph,EI-, which are less volatile and SO
presumably less poisonous than their Alk,El-analogues.
Such groups can be replaced by Li-, halogen-. o r H-atoms;
they are electron-withdrawing, and some can be readily
eliminated together with a p-0- or fl-H-atom. The attractiveness of these groups largely lies in the fact that-with
limitations in the case of P, Bi, Te-the C-El bond is just
as stable as a C-N, C-0 or C-Hal bond towards H 2 0
and 02.Hence, when using them the organic chemist can
keep to conventional methods and apparatus.
i
Ge
As
@
The possibilities offered by seleno/Li exchange have
been exploited, in particular, by Seebach et al.[loland by
Krief et al.[”].
The following features are common to both halogen/Li
and organoelement/Li exchange: a) The reactions usually
proceed rapidly, even below -50°C. b) Since equilibria
are established, good yields are achieved only when the negative charge in the organolithium compound that is
formed is stabilized better than in the Li-reagent (exceptions: shift in equilibrium, e. g. precipitation of a sparingly
soluble product). c) With the exception of boron (sextet
-octet on formation of the ate-complex), the exchange
takes place only in the case of elements having valence
electron shells that can be expanded to a decet; the exchange takesplace markedly easier with 4th- and 5th-row elements than with 2nd- and 3rd-row elements.
Organoelement/Li exchange has the advantage that at
least nine elements (Scheme 4) can be conveniently used,
as opposed to only two elements in the case of halogen/Li
exchange (C1 is very seldom suitable).
1
Hg
T1
Pb
Bi
Scheme 4. Elements suitable for organoelement/Li (A) and halogen/Li-exchange (B).
Scheme 2. The main group elements so far frequently used in organoelement
groups are circled.
Our interest in organoelement functional groups was
awakened when it was shown that the scope of application
of 1,3-anionic cycloadditions[’] can be considerably extended by cleavable electron-withdrawing organoelement
groups (see Section 4.2.2).
2. Organoelement Groups as Li-Equivalent
2.1. Organoelement/Li Exchange
The activation of C-atoms by Li-substituents enables the
coupling of a large variety of electrophilic moieties. Aside
from H/Li exchange, the most important method for introduction of Li into organic compounds is by halogen/Li exchange- which permits carbanionic centers to be generated at sites where this is impossible by H/Li exchange
owing to relatively low CH-acidity.
The third possibility, namely “organoelement/Li exchange’’16], is much less known[’]. Of these reactions, in
which the intermediary occurrence of ate-complexes is
rarely questionable[*], Seyfrth et ~ 1 . ‘ have
~’
mainly used
stannyl/Li exchange on a preparative scale, e . g . for the
very convenient synthesis of allyllithium compounds
(Scheme 3)[91.
Scheme 3.
Angew. Chem. Inl. Ed. Engl. 21 (1982) 410-429
A further advantage is that a reactive, and, in the case of
stannyl- and plumbyl-groups, coordinatively saturated
compound e. g. tetraphenylstannane (Scheme 3) is usually
formed as by-product. Therefore, other than in the case of
halogen/Li exchange, which leads to alkyl or aryl halides,
undesirable secondary reactions of the lithiated product
(see Section 2.2.2) are rarely encountered.
All the elements amenable to organoelement/Li exchange are less electronegative than the halogens CI, Br,
and I. Scheme 5 shows how this can be favorably exploited: Whereas H/Li exchange with the Li-equivalent
Br- (1st reaction step) takes place at the 2-position of the
thiophene, the Li-equivalent n-Bu,Sn directs the Li into
the 5-position, resulting in the isomer l b being formed instead of la”*]. This reaction could have wider synthetic
utility.
Br
Li
Sn(trBu),
I ) LDAiEther
2) CI-LMe,
Scheme 5. All reactions at -60°C [IZ]; LDA=lithium diisopropylamide,
THF = tetrahydrofuran.
41 1
A further reason for organoelement/Li exchange being
preferred over halogen/Li exchange could lie in the better
accessibility of organoelement educts. This holds true, e. 9..
for some of the compounds (Ph,E1)zCHz 3 dealt with in
Section 2.2.
As expected, the suitability of an organoelement group
for organoelement/Li exchange strongly depends on the
electronegativity of the site of attachment: for example, a
PhzAs group attached to an sp3 or spz C-atom can be replaced by Li only with difficulty, whereas arsino/Li exchange takes place rapidly, even at O'C, in the case of the
arsinoethynes 2 (Scheme 6)'13];this is also of preparative
interest because of the thermal instability of stannyl- or
plumbylethynes and the much higher thermal stability of
arsinoethynes.
We checked whether the methyllithium derivatives 4
listed in Table 1 are accessible by organoelement/Li exchange (Scheme 8) from the symmetrically disubstituted
methanes 3, which, with exception of the Bi-compound
3f['8~24n1,
were all well known"].
a
d
e
r
Scheme 6. G=AsPhz, SiMel [13].
Me
I
+
MeLi
- AsMes
Me
I
Me3Si-N-Li
Ph
- &Me4
PhAs(Li)GeMe3 [14b]
Ph
Scheme 7. Organoelement/Li exchange at heteroatoms.
OrganoelemenULi exchange offers still further inexhaustible possibilities. Some of these are outlined in the
following Sections.
G
RLi
4, Yield [%I
PhGe
PhSn
Ph,Pb
Ph2As
Ph2Sb
Ph2Bi
nBuLi or PhLi
PhLi
Ph Li
nBuLi
PhLi
PhLi
0 la, cl 123fj
36 la], 52 Ib] 12411, el
89 [a], 100 [b] [24m, n]
72 [c] [23d, 25al
100 [a] [25b]
70 [a] [24n]
The synthesis of 4b and 4c appeared particularly problematical, since it was not expected that the electropositive,
heavy elements Sn and Pb would stabilize a carbanionic
center. Moreover, secondary reactions were expected to
take place. Some stannyl- and plumbylmethyllithium derivatives had indeed already been described in the literature,
but they were of type 5, with carbanion-stabilizing substituents Z and Z'.
Z
Z
R3E1-C-Li
= Z' = H a l [191
Z = 2' = SR' [ZOa, bl
5, E l = Sn, P b
2
I
z = z'= B(oR'),
= SMe,
[ail
2'= SiMe3 [ ~ O C ]
More interesting in this connection was the synthesis of
PhTe-CH,-Li
4h by phenyltelluro/Li exchange (yield
2 100%) described by Seebach et ~ 1 . ~ "
Te~ ;is, of course,
markedly more electronegative than Sn and Pb.
With exception of 4a the desired methyllithium derivatives 4 are formed in preparatively satisfactory yields (Table 1)12.4.221. Whereas PhLi could generally be used-in
which formation of a mixture cannot occur by ligand exchange-the more reactive nBuLi was required in excess
for the synthesis of 4d. Small amounts of 6 are formed by
ligand exchange via an ate-complex (Scheme 9).
2.2. I . Synthesis of C C H - L i by OrganoelemedLi and,
for Comparison, by Halogen/Li Exchange
To the reagents which have proven useful in organic
synthesis belong anions of the type G-CH?,
where G (as
throughout this review) symbolizes an organoelement
group. Some of these reagents have been synthesized in the
form of compounds G-CHz-Li
4 by organoelement/Li
exchange from disubstituted methanes 3 (G = PhSe,
PhTeIlO. 171), or G-CHZ-G'
10 (e.g. G=PhS--,
G'= PhSe--["bl 1.
-[
PhZAs-CH,-Li . 4 d , 72%
2.2. Organoelement/Li Exchange at GzCHl
412
4
[14a]
+Meh
PhAs(GeMe3),
3
Molarratios:[a] 3 : P h L i = l : l ; [ b ] 3 : P h L i = l : 1 . 5 ; [c] 3:nBuLi=1:4.
Finally, it should be mentioned that organoelement/Li
exchange is not just restricted to C-bonded organoelement
groups: whereas groups such as R3Si--, R,Ge--,
and
RZP-, which attach to carbon are not as a rule amenable
to this exchange, they can be exchanged if they are attached to a relatively electrophilic heteroatom (examples:
Scheme 7).
Regarding the stereochemistry it was established that
stannyl/Li exchange at vinyl compounds takes place with
complete retention of configuration[Is1, which could recently be demonstrated also for the exchange at an sp' Catom in the synthesis of an a-lithiated
Me3%-N-AsMe,
G-CH,-Li
4 from 3 by organoelement/Li exchange
Table 1. Synthesis of G-CHz-Li
in THF according to Scheme 8.
C
=loo%
RLI
G,CH,
Scheme 8. M = Li, Na, K.
b
2
- 2MG
HalzCHz
(Ph,As),CH,
4 nBuLt
THF
3d
t
PhzAs-CHz-AsPhz
I
nBu
nBu
I
PhzAs-CHZ-AsPh
1
Li'
6
Scheme 9. [23d, 25al.
[*I In compounds 3, 4, 7, 12, 13, 34, 35, 57 with G=Ph,EI, a means:
Ph,,EI=Ph,Ge, b : Ph,Sn, c : PhlPb, d : PhZAs, e : PhzSb, I:Ph2Bi, g: PhSe,
h : PhTe.
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
The reaction 3b-4b was examined more closely: Demonstration of the back reaction shown in Scheme 10 reveals that, as expected, an equilibrium is set
Accordingly, the yield of 4b can be increased by increasing
the concentration of PhLi (Table 1). The reaction analogous to 4b-3b (Scheme 10) using Pb instead of Sn could
likewise be demonstrated (yield 19%)124”1.
PhLi/Ether
Ph3Sn-CHz-I
Ph,Sn-CH,-Li
- 60°C
7b
KSn-suspension
(Ph8n)zCHz
THF/Ether (5 : 1)
3 h, 2OoC
3b, 2 1 %
Scheme 10. [24e].
For comparison, the methyllithium derivatives 4 were
also synthesized from halogen compounds 7 by halogen/
Li e ~ c h a n g e l ~ . ~(Table
. ’ ~ ’ 2). With exception of 4alz7I,none
of the compounds 4 in Tables 1 and 2 had hitherto been
described in the literature.
G-CH,-Hal
RLi
--+
G-CH,-Li
7
4
Table 2. Synthesis of G-CH,-Li
a
C
d
i
i
4 from 7 by Hal/Li exchange.
4, Yield [%I
PhlGe
PhSn
Ph,Pb
PhzAs
MelSn
(nBu),Sn
t:
100 [a, bj [23fl
98 [a] [24k], 86 [b] I24eI
70 [a] [24kl, = 100 [bl [24ml
= 100 [a, b] [24i]
92 [a] [24k]
= 100 [a] [24k]
t:
[a] In ether, molar ratio 7 :nBuLi= 1 : 1; [b] in THF, 7 : PhLi= 1 : 1.
Educts 7a and 7d were synthesized by using the reagent
8, which is stable for only a short time at -100°C
(Scheme 11). The trickIz4j1for achieving good yields is to
add the organoelement halide at the same time as the
CH212, and not later[281.-The rest of the halides 7 (Table
2) are accessible by means of the Simmons-Smith reagent12Y1,
whose nucleophilicity is inadequate for the synthesis of 7a and 7c.
G-Hal
+ CH,I,
PhLi
-l20T
G-CH,-I
THF, - 50°C
7b, c
[Li-CH,-II
-
8
Scheme 12. Yield of norcarane in the case of 7b, G = Ph&, 28%. in the case
of 7c, G= PhlPb, 49% [24e].
In contrast to most of the organoelement/Li exchange
reactions listed in Table
no sparingly soluble product
separates out in the reactions given in Table 2, i. e. establishment of equilibrium is not disturbed. From these reactions, therefore, it can be concluded that the relatively
electropositive, heavy main-group elements Sn, Pb, As, Sb,
Bi readily stabilize a neighboring carbanionic center; this
was confirmed by H/Li exchange reactions (see Section
4.1). Experiments with the triakylstannyl compounds 7i
and 7j (Table 2) show that phenyl ligands on the element
are not decisive for this stabilization.
The reagents 4 synthesized can be used for indirect nucleophilic lithio- (El=Sn, Pb, Sb; Section 2.2.3) and
halomethylation (El=As, Sb, Bi; Section 3.3.1) and for
carbonyl olefination (El =Ge, Sn, Pb, S b ; Section 5.1.1).
2.2.2. Stability and Reactivity of G C H - L i
G
b
-
A
nBuL1
[Li-CH,-II
-
8
G-CH2-I
7a, G
7d, G
were surprised at the stability of the
Seebach and
phenyltelluromethyllithium 4h which they synthesized (no
decomposition after 24 h in T H F at 0°C under inert gas).
Triphenylstannyl- 4b, triphenylplumbyl- 4c, diphenylarsino- 4d, and diphenylstibino-methyllithium4e are likewise
astonishingly stable: 4b (in ether) or 4c-e (in THF) could
still be detected to the extent of 20, 17, 100 and 96%, respectively, after 24 h at 20°C124k.h1.
Free electron pairs on
the heteroatom would thus appear to effect additional stabilization (cf. e. g. 9). However, the Bi-compound decomposes relatively rapidly in T H F at = 2 0 ° C ; only ~ 2 0 %
was still detectable after 30 min[24’1.
4a--f12.4,26.31,24e,h-k.m.nl
and 4h(’7,24a1
react with aldehydes, ketones (see Table lo), organoelement halides,
and-as far as tested-with oxiranes in moderate to good
yields according to Scheme 131321.
= Ph3Ge, 7 2 %
= PhzAS, 6 6 %
I ) RR’CO
Scheme I I. CHLBr, can also be used instead of CH211 [24j].
4
Since the lithium compounds 4 are distinctly more reactive in T H F than in ether (see Section 2.2.2), they were also
prepared by halogen/Li exchange with PhLi in T H F (Table 2). By way of contrast, nBuLi in T H F proved to be unsuitable in the cases investigated (Scheme 12), since competing organoelement/Li exchange takes place: After addition of cyclohexene, norcarane could be detected, thus
indicating that the reaction follows the course formulated
in Scheme 12124e1.
Angew. Chem. Int. Ed. Engl. 21 11982) 410-429
OH
T’
I
G-CH,-LiG-CH,-G’
G-CH,-&RRI
CI4
Hal-Alk
lo
G-C HZ-Al k
G-(CHZ),-OH
Scheme 13 (see also Tables 3 and 10).
Reactions with alkyl halides proved more critical: As a
rule, only 4a undergoes reaction in ether; in THF, on the
other hand, use of 4a-d leads to CC-coupling in prepara413
Table 3. Synthesis of G-CH,-Alk
to f 2 0 " C . Yields based on 4.
from 4 and alkyl halides in THF at -30
G
Hal-Alk
Yield [Yo]
[a1
[bl
4a
Ph3Ge [24j]
4b
PhiSn [24e]
lodomethane
lodoethane
I -1odopropane
I -lodobutane
1 -1odopropane
1-lodobutane
1 -1odopentane
lodomethane
Bromoethane
1 -1odobutane
Benzyl bromide
I-lodopropane
I -Brombutane
I -Bromoctane
I -1odobutane
I-Iodopropane
I-Brompropane
92
85
86
86
75
Ph3Pb 1241111
4c
Ph,As [24i, 25a, 311
4d
Ph2Sb [26,31]
4e
,
and the analogous Br-compound (extensive decomposition even after a few minutes at - IOOOC) (Scheme 15) is
very limited, despite the variant involving generation of the
halomethyllithium immediately in the presence of the electrophile (see Scheme 1 I ) indicating a potential improvement in this connection. Moreover, possible indirect lithiomethylation with the Simmons-Smith reagent in the
case of strong electrophiles (e.g. Ph3SnCI[291)(Scheme 15)
also fails, even with compounds like Ph3SiC1 o r
Ph,GeBr[24J1.
(34)
46 (35)
72
78
68
69
89
19
93
85
57
67
[I-CH,-Li]
8
30
2
0
48
CHZIZ
I-CH,-ZnI
;
Ph3Ge-CHz-PbPh3
~
10a
b-CHz-Sn(nBu)p
+
65%
I
,-
h-CH2-SePh
Li-CH,-E
4
Ph&e-CHz-Sn(nBu)3,
3 PhLr
nBuL1
89%
10b
1
RL,
+
A favorable extension to the methodology is provided
by organoelement/Li exchange: Like PhSe-CH2-Li
4g1'O1,the new reagents 4b, 4c, 4j, and-to a lesser de-
[a] 4 prepared by organoelement/Li exchange (in the case of 4a see Section
2.2.3). [b] 4 generated by bromine/Li (values in brackets) or iodine/Li exchange.
Ph3Ge-Br
I-CH,-E
Scheme 15. EX = electrophile.
is
12
yT3
2i
4g
72%
*
nBuLi
Ph,Ge-CH,-SePh
* Ph3Ge-CH2-Li
t
4a
93%
10c
Scheme 16 124jl.
tively satisfactory yields (Table 3). 4e, 4f, and 4h
(G = Ph2Sb, Ph2Bi and PhTe, respectively), however, also
react only slowly or not at all (4e) in THF. This would suggest that these compounds form stable, less reactive aggregates of type 9 with participation of the free electron
pairs.
As follows from Table 3, the method adopted for the
synthesis of the reagent 4 also influences the yields of the
products obtained in its reactions with alkyl halides: The
halobenzene formed as byproduct during synthesis of 4 by
halogen/Li exchange with PhLi deactivates the reagent 4
by complexation, while the by-product n-butyl halide can
undergo CC-coupling with 4.
The very moderate yields obtained on reaction of 4e
to give alkyl(dipheny1)antimony could be noticeably increased by transmetalation to the copper(1) compound
(Scheme 14)[24.3'1.
4e
-
I-(CHd&Hj
CUCl
THF
gree-4e can also be used for nucleophilic lithiomethylation (Schemes 16 and 17). Of importance is that these compounds are thermally considerably more stable than bromomethyllithium or 8 (see Section 2.2.2). Compared to
10a and 1Oc the intermediate 10b has the disadvantage of
not being crystalline. Disadvantageous in the case of 10a
is that 2-3 moles of PhLi are required per mole of educt
for the orgmoeIement/Li exchange.
Free electron pairs on the heteroatoms of reagent and
educt car, have an unfavorable effect in such reactions, as
shown in Scheme 17: Although PhLi reacts smoothly with
methylenebis(dipheny1stibane) 3e with organoelement/Li
exchange (Table I), the analogous compound 10e can be
quantitatively recovered after reaction with PhLi or nBuLi
and hydrolysis with water[24h1.
PhzSb-CH,-Cu
THF
L-CH2-PbPh)
PhZSb-(CHz),CH3
m = 3,6,9 :80,45,61%
Scheme 14. [24h, 311
2.2.3. Indirect Nucteophilic Lithiomethylation with
4C
j
/
*
10d
PhzAs-Cl
LICHz-SbPh2
23%
I
4e
7
1
2 PhLi
Ph2As4H2-PbPh3
Ph,As-CHz-SbPh,
10 e
++Ph,As-CH,-Li
4d
CCH-Li
The possibility of carrying out indirect lithiomethylations with the extremely thermolabile iodomethyllithium f l
414
Scheme 17 [24b].
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
Either LiR is deactivated by complexation in accordance
with l l a , or an aggregated energy-deficient ate-complex
is formed, e.g. l l b , which yields the educt again during
the hydrolysis.
r
1
Li
one plumbyl group, also replaceable by Li (examples:
Scheme 20[24.351).Isolation of the pure product is, as a rule,
easy, since plumbyl compounds very readily crystallize. A
disadvantage is that plumbyl/Li exchange usually requires
2 moles of PhLi per mole of 14, which in turn necessitates
the use of 2 moles of electrophile in subsequent reactions.
On reaction with PhLi, the trirnethylstannyl compound
14c exchanges exclusively the trimethylstannyl group for
Li. In the corresponding triphenyl compound, on the other
hand, exchange of all three groups takes place to the same
2.3. OrganoelemedLi Exchange at GJCH
2.3.1. Bis(triphenylp1umbyl)methyllithium
8
Numerous organoselenium compounds can be synthesized from 3g by H/Li exchange to give 13g, reaction with
an electrophile, seleno/Li exchange, and renewed reaction
with an electrophile (Scheme 18)['0."1.
(PhSe),CH2
-
El
(PhSe),CH-Li
+
-+
+
PhSe-C?
E2
13g
3g
9
CI-SMe3
j
-
88%
**..
I L C
( Ph3Pb),CH-GePh3
(PhsPb)&H-SiMe,
14a
Ph-CHO
OH
(Ph,Pb)zCH--&H-Ph
[
8
1
%
*
*-I
H3 C a
, .-(Ph,Pb)&H-SnMe,
14c
14 b
W%pl
15 a
The compounds (Ph,E1)&H2 (El = Sn, Pb, As, Sb) 3b-e
can also be lithiated by H/Li exchange to 13b-e, but only
under conditions which lead to strongly deactivated Licompounds (see Section 4.1.1). In the case of 3c, El = Pb,
this difficulty could be overcome by using organoelement/
Li exchange on a compound of the type G3CH 12: The
well-known tris(triphenylplumby1)methane 12c, which can
be conveniently prepared from chloroform and
Ph3Pb-Li[341, and whose yield could be increased from 66
to 91%'23hl,reacts quantitatively with PhLi to give
which proved to be surprisingly stable (45-50% still existing after 5 h in boiling THF) and to be very versatile, owing to its exceptional reactivity and selectivity and its exchangeable plumbyl groups (see Schemes 19.20): On reaction with non-enolizable aldehydes and ketones[361it forms
b-hydroxybis(plumby1)compounds (57- 81!/0)'~~"~,which
can be decomposed to plumbyl- and lithio-alkenes (see
Section 5.1.3). Primary alkyl halides (secondary and tertiary d o not react) are converted into alkylidenebis(p1umbanes) (25-96%)[24"1, from which a-plumbylalkyllithium
compounds can be prepared (see Section 5.1.4).
(Ph3P;)fH-Li
87% BrCePh3
'"%I
looj-lPhL1
PhSPb-CHLi -SiMe,
Scheme 18.
(Ph3Pb)3CH
-
15b
I
75% CI-SIMC~
78% CI-BMe3
SiMe?
Ph,Pb-CH(SiMe,),
Ph3Pb-CH:
GePh,
Scheme 20. Use of 159: Scheme 61 [24].
(PH3Pb),C
Ph)[25'1.
is not attacked by LiR ( R = M e , nBu,
2.3.2. Bis(tripheny1stannyl)methyllithium
Other than quoted in the literature[371,the heteroanalogue tris(triphenylstanny1)methane 12b of 12c is currently
only available by an involved synthesis (see Section 4.1.1).
The Li-compound 13b, which is obtainable in almost
quantitative yield from 12b, is a promising educt, but very
few reactions have been tried out (Scheme 21)[23'1.
/
(Ph3Sn)&H-CH3
\
';i
7H3
(Ph3Pb)zCH-CH,-CH-OH
(Ph,Sn),CH-(-Ph
011
E l = Ge, Sn, As: 7 3 , 81, 6 7 %
MI
96%
ph L1
13c
Ph3Pb-CHLi-Ge Ph,
R = H, Ph: 82, 6 1 %
Scheme 21 [23e].
(Ph3Pb),CH-CH3
Scheme 19 [24m].
2.4. Compounds of Type G-CH2--G'--CH2--G
13c reacts with organoelement halides to give derivatives 14, which contain a further organoelement group besides two plumbyl groups and which can be decomposed
to a daughter generation of Li-reagents 15 containing only
Compounds of type 16 would be convenient educts for
the synthesis of more complex organoelement molecules, if
they would allow the peripheral groups to be exchanged
for Li without the central group being attacked. However,
Angew. Chem. Inr. Ed. Engl. 21 (1982)410-429
41 5
preliminary investigations (Scheme 22) have shown that
limitations are soon encountered. Though 16a could be relatively easily lithiated, further reaction in preparatively
satisfactory yield was possible only with Ph2AsCI, and the
product 16b did not undergo stannyl/Li exchange (almost
quantitative recovery after addition of water). Presumably
the resulting ate-complex is stabilized to such an extent by
the diphenylarsino group that SnPh4 is not eliminated[24e1.
68%1“ P h h
C12SiMe2
Me\
/ ‘p
PhLi
/Me
Me\
<Si>
f Si>
Li
Scheme 22
/Me
SnPh,
’, /
3.1. Motivation and Previous Uses
Halogen substituents are important functional groups,
since they can be readily exchanged, often stereospecifically, for other functional groups and C- and metal-atoms.
However, there are limits to their use, in that a negative
charge in the a- or b-position to the halogen has a strong
destabilizing effect. Thus, the groups and compounds depicted in Scheme 23 can be used as nucleophiles only at
very low temperatures, if at all[381.17e is explosive.
I
\
/
Hal
-c-c\
Li’
H,
/Hal
/C =c\
H
Li
17a
H
C1
HCEC-Li
Li-CX-Cl
Li\
/C
H
/Hal
=c\
Prerequisites for the use of halogen equivalents in organic synthesis are easy introduction of the equivalent and
favorable exchange of halogen. Should organoelement
groups function as halogen substitutes in carbanionic reagents, high thermostability and nucleophilicity of these anions is also desirable.
As a rule, the coupling of organoelement groups with Catoms is unproblematical, since both nucleophilic as well
as electrophilic introduction is possible. Moreover, on using suitable organoelement groups (see Sections 2.2.2;
2.3.1 ; 3.3.1-3.3.3) carbanions exhibiting satisfactory stability and nucleophilicity can be generated. However, difficulties are encountered on exchange of the organoelement
group for halogen.
H
17b
Hal
X = CH, N
It is meaningful, therefore, to search for equivalents for
C-bonded halogens. These equivalents should, first of all,
tolerate a neighboring negative charge or partial charge,
and, moreover, be strongly nucleophilic.
were the first to use methylmetal reagents
Corey et
(Li, Cu) containing an organoelement group ( = PhS) as halogen-equivalents for nucleophilic halomethylations. Disadvantages of their method (Scheme 24) are long reaction
times, limitation to introduction of iodine, and, above all,
that practically only alkyl halides can be used as substrates. If instead phenylthiomethyllithium is allowed to
react with aldehydes, ketones or oxiranes, then the hydroxy group of the product in the “onium cleavage” with
MeI/NaI is, according to our observations[25a1,partially
methylated.
416
Krief ef
carried out analogous but more rapid reactions with the Se compound 4g and also described indirect
nucleophilic a-bromoalkylations with Se reagents (see Section 3.3.2). Brown el al.I4I1likewise used an organoelement
group as halogen-equivalent in the indirect anti-Markownikoff addition of HHal to olefins via borane addition and
halogenolysis. The same can be achieved by hydrozirconation and subsequent halogenolysis at 0 0C[421.
3.2. Organoelement/Halogen Exchange at C-Atoms
Li
3. Organoelement Groups as Halogen Equivalents
I
Scheme 24 [39]. “Onium Cleavage”.
P h 2 A s ,SnPh4
124eI.
Li-C-Hal
PhS-CHZ-Cu
3.2. I . Alkyl-Bound Organoelement Groups
Since halogens attached to sp’ C-atoms are generally far
more versatile than those attached to sp’ or s p C-atoms
there is mainly a demand for equivalents for alkyl-bound
halogen. In the case of the organoelement groups which
represent potential candidates for this purpose, exchange
for halogen is most difficult if the group is attached to a
primary sp3 C-atom.
The disadvantages of the “onium cleavage” used by Core~~~~~
and Krieft401were pointed out in section 3.1. “Halogenolysis” with halogen or SO2CI2might as a rule be more
favorable (example: Scheme 26). As in the “onium cleavage”, an element with a free pair of electrons is also required. The electrophilic attack of the halogen leads in
these cases to “dihalides” such as 18a (Scheme 25A),
A)
Br\
&Alk-AsPh,
Alk-AsPh
I
XBr
110-130°C
Br-Alk
18a
Scheme 25
Angew. Chem. Inr. Ed. Engl. 21 (1982) 410-429
which eliminate alkyl halides on heating. The remaining
valences of the element (as e. g . in the diphenylarsino
group) are more appropriately blocked with phenyl
ligands, which d o not participate in the exchange reactions.
If the central atom of the organoelement group does not
bear a free electron pair, all the alkyl moieties participate
in the exchange reactions, or the halogen initially attacks a
phenyl ligand (if present), which leads to undesirable
cleavage of this moiety (Scheme 25B). Such groups are
thus unsuitable as halogen-equivalents on sp’ C-atoms.
The formation of alkyl halides by halogenolysis of organo-arsenic, -antimony, and -selenium compounds is already well knownC4’’.But this approach has only recently
been used for the directed synthesis of primary alkyl halides, while Krief et al.’401had already previously synthesized secondary and tertiary alkyl halides by productive
halogenolyses of organoselenium compounds.
Alkyldiphenylbismuthanes such as 19, and thus also
their halogenolysis, were previously unknown. We synthe19
A
PhsBi-R
*
+ Br-Ph
Br-R
86 70
%C1-R
Ph2Bi-C1
7%
+
58%
(84%)
C1-Ph
2 %,
(14%)
Scheme 26 [18,24nj; R=n-C5HII.A : Br2,ether -70- +2O“C; B: analogous
to A with S02C12or with CIdTHF, 0°C.
sized them according to Scheme 26-the stratagem here
being the use of alkyllithium compounds at low temperatures instead of Grignard compounds at elevated temperat ~ r e s [ ~ ~ ~ ] - found
a n d that bromolysis and chlorolysis already take place below 0 ° C (Scheme 26).
Table 4 enables a comparison of the methods and
groups for organoelement/Br and organoelement/I exchange at a primary sp3 C-atom. O n bromolysis, PhaAs
and Ph2Bi give the highest yields; the air-sensitive group
Ph2Bi is only used at low exchange temperatures. In the
“onium cleavage” the yields with PhS, PhSe, PhTe are
more favorable than with Ph2As and Ph2Sb; surprisingly,
however, the lowest exchange temperature was with
Ph2Sb.
In the bromolysis of allyldiphenylarsane 20 with bromine in the molar ratio 1 : I the CC bond remains intact
(Scheme 27), while in the case of cyclopropyldiphenylarsane the exchange is accompanied by ring ~ p e n i n g [ ~ . ~ ~ g ] .
Br, ,Br
Ph2As-CH2//
B’L, P h 2 A s - C H 2 d 2B r - C H 2 d
20oc
20
PhCl
18b
82%
Scheme 27 14, 23gj.
Organoelement/halogen exchange by halogenolysis at
secondary and tertiary sp3 C-atoms is unproblematical.
When attached to such C-atoms the otherwise unfavorable
PhSe and PhTe groups can be exchanged for bromine already at room temperature in moderate to good yields; on
the other hand, “onium cleavage” fails in this case1401.
On
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
using Ph2As, halogenolysis with Br2 or I2 is practically
quantitative but requires a temperature of 110-130°C.
Table 4. Synthesis of n-CaHlzBrby bromolysis of n-ChH13Gwith Br2 and of
n-C6HIzIby “onium cleavage” of n-C6H13Gwith Mel/Nal. Cleavage temperatures and yields.
G
Ph2As
Ph2Sb
Ph2Bi
PhS
PhSe
PhTe
n-C6H ,,I
T [“Cl
Yield [%]
n-ChHI3Br
T [“C]
Yield [%]
130
220
- 10
80
80
86[46,471
65 [24h, 25bJ
84 [24n, 181
44 [23a, 461
27 [24al
20
20
< 3 [?5a]
41 [25b] [a]
80
80
66
93 [391
80 1401
77 [24a]
[a] Since methylation with Mel was not possible, ethylation was carried out
with [Et30]BF4.
To our knowledge the only comment made so far in the
literature about the stereochemistry of organo/halogen exchange at sp3 C-atoms is that PhSe/Br exchange largely
proceeds with inversion[481.In the diphenylarsino/halogen
exchange studied by us there is little prospect of an optically active arsane being converted igto an optically active
halide, since secondary or tertiary halides rapidly racemize
at 110-130”C[491.
Organo-transition metal groups can be cleaved just as
readily as Ph2Bi from alkyl moieties by halogens (examples: Scheme 28). The possibilities of using such groups as
halogen equivalents are however very limited, since all attempts to synthesize reactive carbanions containing organo-transition metal groups have so far failed. Thus, the
synthesis of 21 could not be achievedL24J!
c1
B I ~0°C
.
cp2zrw
Br
96%[42]
H
D
PhLl
Cp(CO),Fe-CH2-SnPhs
+ Cp(CO)2Fe-CHz-Li
[Wl
Scheme 28.
21
3.2.2. Vinyl-Bound Organoelement Groups
Groups such as Me3%, ( r ~ B u ) ~ S(central
n
atom without
free electron pair, alkyl ligands), which are attached to an
alkyl moiety are unsuitable as halogen equivalents. On the
other hand, when bound to a vinyl group or a nucleophilic
arene they are very useful: In the case of I-alkenylsilanes,
Me3Si/Br exchange already takes place specifically at
room temperature with rupture of the sp2 C-Si bond and
with retention, whereupon intermediary bromonium ions,
e . g . 22, are formed (Scheme 29)[5’1.Stannyl groups are exchanged for bromine even more readily than are silyl
groups (Scheme 40)“3.521.
We have found that PhSe and Ph2As groups attached to
vinyl are exchanged for Br in high yields without the double bond being attacked. In contrast to Ph2As/Br exchange
(Schemes 29,41, 42)[13,23b1,
PhSe/Br exchange (Scheme 38)
already takes place at 20°C[4.47b1.
417
The main advantage of 24a is its strongly acidifying diphenylarsinoyl group, which enables coupling of a total of
three residues in the a-position (see Section 3.3.2). The disadvantage that a LiAl H,-reduction is necessary after the
I ) MeI/NaI
22
+AsPhZ
nPr
1) 2 PhMgBr
I,AsCH,
(+ small amount of
E -i s o m e r )
BIZ, F'hCl [I31
v
I130-C
n Pr
II
PhzAs-CH3
2) HzOz
23a 70%
I
60%
Scheme 29.
Scheme 3 I .
3.3. Nucleophilic Reagents with Halogen-Equivalent
Groups
K*
0
1) R-CO-R
2 ) HzO
3.3. I . Indirect Nucleophilic Halomethylation
Diphenylarsinomethyllithium 4d : Nucleophilic halomethylations of alkyl halides previously required the fourstep process Alk-Hal
+
Alk-CN
+ AIkC02H +
Alk-CH20H
+ Alk-CH,Hal.
With organoelement
groups as halogen equivalents the process nowadays involves
rather
Alk-Hal
+
AIk-CH2G
+
Alk-CH2-Hal.
Besides the well-known reagents
PhSCH2Li and PhSCH,Cu (Scheme 24) and 4g, whose
disadvantages have been mentioned in Section 3.1, the Asreagent 4d is particular suitable for this purpose. The Bianalogue 4f is less suitable, since on reaction with electrophiles it yields difficultly separable mixtures (so far a pure
product has been obtained only from benzophenone),
while on using the analogous Sb reagent 4e (or the analogous Cu(i) compound) organoelement/halogen exchange
is relatively unfavorable (see Table 4).
Since reaction of 4d with aldehydes and ketones or oxiranes yields p- or y-hydroxyalkylarsanes (yields: 53-94%),
it should be pointed out that a y-hydroxy group causes
little interference in diphenylarsino/bromine exchange,
whereas a 0-hydroxy group may markedly reduce the yield
of halide, or, in certain cases, lead to cleavage of water
(Scheme 30). In cases where cleavage of water interferes
with halogenolysis, it can be profitable to esterify the hydroxy group with acetyl chloride beforehand[24g1.
Phz As-C Hz-C H( OH) P h
Ph,As-CHz-C(OH)Ph,
BIZ
63%
BIZ
75%
Br-CHz-C H( 0 H ) P h
Br-CH=CPh2
Scheme 30 [44, 45, 47aI.
For reactions with aldehydes, ketones, and oxiranes, 4d
can be prepared by iodine/Li exchange (Table 2). For
reactions with alkyl halides, on the other hand, the uneconomical synthesis (larger excess of nBuLi necessary) by
organoelement/Li exchange according to Scheme 9, which
leads to reactive 4d (Table I), is necessary.
Diphenylursinoylmethyllifhium 24a : An alternative to 4d
is the more reactive and somewhat more conveniently ac(Scheme 3 1 :
cessible arsane oxide derivative 24a[44,45,47c1
avoidance of the volatile poisonous intermediate Ph2AsC1
and the low temperature synthesis formulated in Scheme
1 l), which reacts with electrophiles according to Scheme
32.
418
24a
P h ' zII< H ~ - C H ( O H ) R R '
PhzAs-CH2-R
25
26
Scheme 32
Table 5. Reaction of 24a with electrophiles in THF at -40molar ratio 1 : 1 according to Scheme 32.
Electrophile
Butyraldehyde
Benzaldehyde
Cyclohexanone
Benzophenone
Ethyl bromide
n-Propyl bromide
n-Butyl bromide
Ally1 bromide
Benzyl bromide
Oxirane
Methyloxirane
Ethyloxirane
Yield [%]
Product
25a
25 b
25c
25d
26a
26b
26c
26d
26e
27a
27 b
27c
R
R
H
H
nPr
Ph
+CHI),,Ph
Et
n Pr
n Bu
CH?-CH=CHz
CH:-Ph
H
Me
Et
+2O"C in the
Ph
60 [47a]
82 (47al
53 (47al
81 147al
56 [24h]
69 [24h]
72 [47a]
70 [47a]
72 [24h]
74 " 3 1
68 "4
70 ~ 4 g i
CC-coupling, is not so serious, since the reduction proceeds quantitatively and can be combined with the CCcoupling in a one-pot process.
In reactions with carbonyl compounds and oxiranes the
arsinoyl reagent 24a does not lead to better yields (Table
5) than the arsino reagent 4d. On the other hand, the yields
in reactions with alkyl halides are essentially better (Table
5) than when using 4d (Table 3), in that the more convenient route via Ph2As-CH2--I
7d was chosen for the preparation of 4d.
Reaction of 24a with oxiranes and then with water does
not lead to y-hydroxyalkylarsane oxides but to 2-hydroxy1,2h5-oxarsolanes 27[24g1,
which, however, are smoothly reduced by LiAIH, to y-hydroxyalkylarsanes, i. e. the readily
accessible compounds of halogenolysis (cf. Scheme 36).
3.3.2. Indirect Nucleophilic a-Haloalkylation
For the introduction of a-bromoalkyl groups into electrophiles (Scheme 33) Krief et ul.'40.531
used the reagents
28, which are obtainable by PhSe/Li exchange (cf. e.g.
Scheme 18).
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
E
Brd-Alk
I
R
I) EX
Alk
2) BrzlNEtl
I
R
Scheme 33. EX = electrophile
In our method (example: Scheme 34), on the other hand,
a-lithioalkylarsane oxides, e. g. 30, are used, which are also
readily accessible by H,O,-oxidation and lithiation after
reaction of PhAsCl with carbanions or of Ph2AsLi with alkyl halides. The halogenolysis can be combined with a nucleophilic substitution in a one-pot process, whereupon
hydroxy compounds or thioethers are immediately
f ~ r r n e d ~ ' ~ ~ The
. ' ~ ' . a-lithioalkylarsane oxides thus offer a
variety of possibilities for the synthesis of functionalized
hydrocarbons. -Also secondary alkylarsane oxides, e. g.
31, can be lithiated with LDA; further reaction with electrophiles, however, is more difficult than with compounds
of type 30.
Owing to their high thermostability (in contrast to corresponding selenium oxides, which already decompose below 20 0C[551)
and tendency to crystallize, the alkylarsane
oxides, e. g. 29, necessary as precursors o r intermediates
0
Li
II
I
Ph,As-CH-R
-
7 yH3
Me1
72%
Ph,As-CH-R
-
3) KOH
::
7H3
HO-CH-R
64%
PhzAs-C Hz-R
29
I)LIAIH~
II
Li
I
I
C H2-R
A
- LiR
60%
1 ) Allgl-Br
2) HzOz
Ph,As-Li
C"CIZ
I
HO-CPh,
Scheme 37 [241].
2) CllSOZ
3.3.3. Indirect Nucleophilic a- and fl-Halovinylation
7H3
Cl-CH-R
77%
can easily be isolated. Also their a-lithiation products are
usually so thermally stable that they can be heated during
reaction with electrophiles. Exceptions are the compounds
listed in Scheme 35, and similar compounds: They fragment by LiR e l i m i n a t i ~ n [ * ~ ~ . ~ ~ ~ .
0
T
7
H3
Br-CH-R
93%
Scheme 34 [24h, 541: R=CzH,.
Ph2As-CH
Since they react with electrophiles-in analogy to allyllithium compounds with sulfinyl, sulfonyl o r phosphonate
$r~ups~~~I-exclusively
in the a-position (Scheme 37), and
the C C double bond remains intact during diphenylarsino/bromine exchange at allyldiphenylarsane 20 (Scheme
27), 32 is in all probability suitable for the indirect nucleophilic introduction of a-haloallyl groups into electrophiles.
Also worthy of mention is the oxidative coupling of 32 in
good yields to give 33, a diene containing two halogenequivalent substituents. In the synthesis of 26f according
to Scheme 37 it is advisable to oxidize the very unstable allyldiphenylarsane 20 with 30% H,02 immediately after its
::
PhzAs-CH
The thermal instability of 17c and 17d[381
(Scheme 23)
considerably lowers the value of these reagents for syntheses. A substitute for 17c is I-phenylselenovinyllithium
35g : According to independent observations by Krief et
ul.[snland by US[^.^^^^ the electron-withdrawing phenylseleno group of 34g enables H/Li exchange, to give the sufficiently thermostable and nucleophilic reagent 35g. Moreover, according to our finding^'^.^^^^ on the products obtained by reaction of 35g with electrophiles (examples:
Scheme 38), phenylseleno/Br exchange in ether o r chlorobenzene proceeds quite smoothly already at room temperature, even if a hydroxy group is in the b-position to the
phenylseleno group. 35g is therefore a suitable reagent for
indirect nucleophilic a-halouinylation.
CHZ
Br
Scheme 35 [24h, 561. Fragmentation of the educts with R=Ph, Ph2As,
PhzAs(0) at > - 55, < 20, and <20 " C , respectively.
BrzlElOH
Like 24a, a-lithioalkylarsane oxides also react with oxiranes to give 2-hydroxy- 1,2h5-oxarsolanes, which can be
readily decomposed to open-chain b r o m i d e ~ [ ~(Scheme
~gl
36).
0
ll
PhzAs-CHLi-C,Hl
A74%
Ph,As,
HO
Ph-CHo
43%
LDA/THF
HO
H
34g
+seph
Li
-qSeBr
nC8H'rBr
69%
nC8Hll
3%
Ph
qePh
1) LiAlH4 (- 100%)
2) Br2 (80%)
-
qSePh
YHOH
83%[47b]
00% [58]
t
1
/;=jSePh
Li
36
Scheme 36 [24g].
Scheme 38 [4,47b]
Of the specific a-lithioalkylphenylarsane oxides prepared[41,the allyl compounds 32 deserve special mention.
Angew. Chem. hi.Ed. Engl. 21 (19821410-429
When using 35g, account should be taken of its tendency to undergo elimination of PhSeLi-particularly in
419
the
presence of
hexamethylphosphoric
triamide
(HMPA)[581.A small HMPA content is, however, advantageous for reaction with alkyl halide~[~~I.-The
tellurium
analogue 35h (see Section 4.1.3) is no alternative on account of its poorer accessibility and lower nucleophilicitYThe thermolability is greater when halogen and lithium,
as in 17a and 17b, are attached vicinal to a C C double
bond than when attached at the geminal position (Scheme
23). Whereas a substitute organoelement reagent for ciscompounds of the type 17b is to our knowledge unknown,
there are equivalents for the trans-compound 17a: Unlike
the extremely unstable Se-compound 36'''"], the Li-compounds 38a and 38b synthesized for another purpose by
Corey et
and Cunico et ~ 1 . " ~ 'respectively,
,
are suitable in this respect, as is demonstrated by synthesis of the
halides 39 and 40 (Schemes 39 and 40). The Sn-reagent
38a is, of course, hardly recommendable, since it is extremely difficult to separate the desired product from the
tetrabutyltin and other products that are always formed.
=--G
HSn(nBu)3
37a, G
37b, G
( ~ B U ) ~ S ~ 3 7 ~ G,
G
/=/
(Scheme 6). That the diphenylarsino- and not the trimethylsilyl-group of the intermediate is exchanged for bromine is surprising.
-
CISiMe3ITHF
PhzAs-S-Li
45
= 100%
Br2
PhzAs---SiMe3
Br-=-S1Me3
49%
3.3.5. Strongly Nucleophilic Equivalents for Hap
When halide ions with extremely high nucleophilicity
are desirable in synthetic operations, reaction with Ph2Ase
(Ph2As-CI
or PhZAs-AsPh2
Li, Na, K) or PhSeO
(PhSe-SePh
NaBH4) with subsequent organoelement/
halogen exchange is recommended. Here, the diphenylarsenide ion generally has the higher nucleophilicity. If in
such a reaction the organoelement group is coupled with a
primary C-atom, Ph2AsQ might, as a rule, also be more
suitable than PhSeo, since the yield on organoelement/halogen exchange is better (see Table 4). Scheme 42 shows
some applications.
+
+
= s n ( n B ~ ) 88%[59]
~,
= SiMe3, 98%[60]
= AsPhz, 80%[13]
Scheme 39.
R = H: 85%
R = Ph: 68%[23d]
R = H: 7 2 %
R = P h : 55%[23d]
H-
I)L3AsPhz/HNEt2
-Ph
2) H z 0
- /=/"
iMe,
37b
""' Li
ph
Asph,
>LJ
Br,
PhCI, 130°C
Scheme
2) B12(20~C),86% 12%;
38b
Br
71 %[4 7a J
67%[63]
I)nBuBr,81%[60]
+
Ph
~
42.
nBu
40
Scheme 40.
3.4. Electrophilic Reagents with Halogen-Equivalent
Groups
In the case of the As-reagent 41 synthesized by us, the
Z-configuration is presumably stabilized by the complexation formulated in Scheme 41. Unfortunately, the Ph2As/
Br exchange requires such severe conditions that inversion
of configuration occurs[621.-The educts for 41 are accessible by hydrostannation according to Scheme 39, where u p
to = 10% of the Z-isomer 43 is formed as well as 37c.
In attempts to halogenate carbanions with halogens,
CC-linkage by oxidative coupling is not rarely the main
readion. As a rule, better results are obtained with CCI, o r
CBr>641.If the Li atom attaches to an sp3 C-atom a favorable alternative is arsanation with chloro(dipheny1)arsane
followed by diphenylarsino/halogen exchange. In the case
of alkenyl- and aryllithium compounds, silylation with trimethylchlorosilane followed by trimethylsilyl/halogen exchange is more favorable (examples: Scheme 43).
nBuLi
CI-AsPhz
95%
nBu-AsPhz
SOZCIZ.
or Iz
Br2
nBu-Hal
Hal = C1, B r , I: 73, 89, 91%
[4,47al
100%
43
D
AsPh,
LJ
nP'r
44
BIZ.20°C
Scheme 41 "31
3.3.4. Indirect Nucleophilic Haloethynylution
Scheme 43
Such conversions have rarely been investigated. An Asreagent, namely 45, is also suitable for this purpose: it is
conveniently accessible by H/Li or Ph2As/Li exchange
Organoelement equivalents for vinyl halides are mentioned in Section 4.2.1.
420
Angew. Chem. I n t . Ed. Engl. 21 (1982) 410-429
4. Electron-Withdrawing Effect of Organoelement
Groups and Synthetic Uses
4.1. Acidification by Heavy Main-Group Elements
4.1.1. H/Li-Exchange at G2CH2
According to the findings in Section 2.2.1, Ph,El-groups
with E=Sn, Pb, As, Sb, Bi stabilize a lithiated C-atom directly attached to them. Moreover, the reactions with the
trialkylstannyl compounds 4i and 4j (Table 2) showed, in
the case of tin, that phenyl ligands are not decisive for this
stabilization. The question now arose, whether heavy
main-group elements in general could detectably acidify
an attaching CH-group.
We initially investigated the effect of bases on bis(triphenylstanny1)methane 3b, whose tendency to undergo
stannyl/Li exchange precluded the use of organolithium
compounds as base. After a number of fruitless atlithiation was finally accomplished with the aid
of the bulky lithium dicyclohexylamide (LDCA) in presence of u p to 92% HMPA123e'.Analogous compounds were
also lithiated under the same conditions, at room temperature according to Scheme 44[671(Table 6 ) ; in the case of the
Bi-compound 3f, decomposition reactions prevent detection of lithiation.
That the acidifying action of heavy main-group elements
has only just been discovered might be due to the high
electrophilicity of the corresponding organoelement
groups. This promotes direct attack at these groups by
bases such as nBuLi or PhLi, which effect organoelement/
Li exchange (cf. Scheme 51).
G-CH,-G'
THF,2OoC
3, G
10; G
-
LM!A/HMPA ( I : 1)
* G'
N(iPr),
I
47
Ph,Sn-CH,-SnPh3
I
Li
In compounds 3, two organoelement groups acidify the
methylene group. It could be shown with the aid of the
reactions outlined in Scheme 45, that even one diphenylarsino group makes the a-methylene group capable of lithiation if a second group of the same kind supports it by
~helation~~~'~.
Scheme 45 [23c, 24b, 24cJ.
Conditions
48,n= 1
2
0
63
36
49
LDA/THF
LDCA/HMPT
G-CHLP-G'
13, G
= G'
Section 2.3.2). As experiments with 13b in the presence of
dicyclohexylamine or HMPA have shown, deactivation is
effected by the amine and not by HMPA[23e1.Probably the
1 : 1 complex 46 is formed, whose nucleophilic center is
strongly shielded by bulky residues.
Since LDA has a greater tendency to undergo nucleophilic addition than the bulky LDCA, it was surprising
that 3b could be recovered largely unchanged after reaction with LDA (practically no lithiation). Presumably the
ate-complex 47 is formed, which decomposes on addition
of water, with generation of 3b.
Yield/[%]
3
36
60
4
5
6
32
54
21
0
= G'
15, G 9 G'
4. I . 2. Influence of Element and Ligand on the Acidification
Scheme 44.
Table 6. Lithiation of 3 and 10 to 13 and 15, respectively, according to
Scheme 44 [67].
_________
G
Yield [Oh]
131
131
13b
13c
Ph,Si
Ph1Ge
Ph,Sn
Ph,Pb
0 [24n]
0 (23fl
92[23e]
67 (25al
15c
1Sd
Ph,Sn-CHLi--AsPh,,
PhqPb-CHLi-AsPh2,
-1
1%
13e
13f
G
Yield [%]
PhZAs
PhlSb
Ph,Bi
63 [24e]
68 [24h]
decomp. [24n]
We have compared the acidifying effect of heavy and of
light main-group elements by H/Li exchange on the compounds 3 under competing conditions, the experimental
conditions having to be varied for practical reasons[66b1.
Scheme 46 A shows a typical competition experiment. The
results are collected in Scheme 47.
yield 67% [24b]
yield 32% [24b]
The Li-compounds 13 obtained by H/Li exchange disappointingly showed little reactivity towards carbonyl
compounds and other electrophiles: For example, the most
investigated of such compounds, namely bis(tripheny1stannyl)rnethyllithium 13b, reacted in preparatively useful
yields, if D 2 0 or H 2 0 is ignored, only with Ph,SnCI,
whereby tris(triphenylstanny1)methane 12b123e.671
was obtained for the first time[37J.
If, o n the other hand, 13b is synthesized by organoelement/Li instead of H/Li exchange, it is very reactive (see
Angew. Chem. I n l . Ed. Engl. 21 (1982) 410-429
Ph3Si-CH,-SPh
log
Ph3Ge-CH,-SPh
10h
I-
Ph,Si-CHLi-SPh
77
+
*.
Ph,Ge-CHLi-SPh
23
I
55%
Scheme 46. Conditions: A) LDCA/HMPA (1 : I ) , THF, 20 "C, 0.5 h: B) LDA/
THF, -50- - IO"C, 2.5 h.
Since a comparison of the groups Ph3EI in which El = Si,
Ge, Sn, Pb was either impossible, or possible to only a lim42 I
ited extent, owing to the very low acidity of compounds 3,
El = Si and G e [3a], these groups were combined with the
distinctly acidifying phenylthio group to give, e. g., compounds log and 10h, respectively. The results of competition experiments with these c o m p o ~ n d s [ * ~(example:
j~
Scheme 4 6 9 ) are given alongside the dotted brackets in
Scheme 47. The degrees of lithiation were determined by
deuterolysis with D 2 0 and 'H-NMR analysis of the products.
From the molar ratios of the lithiation products it is concluded that the elements of the 4th row (Sn, Sb, Te) are
more strongly acidifying than the homologous elements of
the 3rd row (Ge, As, Se), and equally as strongly acidifying
as the homologous elements of the 2nd row. Furthermore,
a marked increase in acidifying action was observed on going from the 4th row element Sn to the 5th row element Pb,
and-as
expected-on
going from the 4th-5th-6th
group.-Since the molar ratios of the lithiation products
did not change in experiments carried out over prolonged
periods, thermodynamic acidity must be involved.
Accordingly, the failure of attempted lithiation of 31 can
only be explained thus: Owing to the small C-Si bond
length of = 1.93 (corresponding bond lengths in the case
of Ge, Sn, Pb: = 1.98, 2.18, ~ 2 . 2 9
the methylene
group of 31 is so strongly shielded that, other than in the
case of the corresponding Sn- or Pb-compound (see Section 4.1.1) or 3k, it cannot be attacked by lithiating reagents.
a
4. I . 3. H/Li Exchange at Phenyltelluroethene
Since the PhTe group is superior rather than inferior to
the PhSe group in the acidifying
(Scheme 47),
and since phenylselenoethene 34g is amenable to H/Li exchange (see Section 3.3.3), it was of interest to us whether
this also holds for phenyltelluroethene 34h. The question
at hand was also whether organoelement/Li exchange can
be suppressed in favor of H/M exchange.
Whereas reaction of nBuLi with the recently prepared
compound 34h[24"1(Scheme 48) leads to formation of only
= 10% a-lithio(pheny1telluro)ethene-as expected the
main reaction is organoelement/Li exchange-the yield
increases to ~ 5 0 %
on using the bulky bases lithium dicyclohexylamide (LDCA) or lithium-2,2,6,6-tetramethylpiperidide in T H F at - 70 "C. A further increase in yield was
not p~ssible[*~''~.
-4TePh
-
lTePh
-
LDCA/THF
CI-SiMe,
Li
51%
34 h
17 : 83
SiMe,
35 h
MgBr
Scheme 47. For explanation see text. Lithiation conditions: A as in Scheme
46A 124j. 24111: B as in Scheme 46B [24a, 24jl: C nBuLi+ZTMEDA, THF,
20"C, 0.5 h [24g].
+Teph
80%
=/
dTePh
dSiMe3
SiMe,
YHOH
Ph
49
Scheme 48 [24a, 47al.
Since bis(trimethylsily1)methane 3k is lithiated largely at
the methylene group by nBuLi HMPA[68"1,whereas all attempts at lithiation on the analogous hexaphenyl compound 31[24"1failed, it would appear that phenyl ligands in
comparison to methyl ligands reduce the acidifying effect
of the element[41.Individual and competition experiments,
mainly performed on compounds 10, in which the group
under examination is combined with a phenylthio- or phenylseleno-group, later showed a reverse pattern in the ratios (examples: Table 7).
+
3k (Me3SihCH2
31 (Ph3Si)*CHZ
In view of the very good yields obtained for organotelluro-compounds on reaction of 35h with benzaldehyde or
trimethylchlorosilane (Scheme 48), the failure of the reaction with ally1 bromide or I-iodopropane was surprising.
The products to be expected, and actually synthesized
from 35h and electrophiles, contain the phenyltelluro
group, which is readily exchangeable for Li; hence, the
vinyl group can be coupled with a second electrophile
to give, e . g . . 49[68h1.35h may therefore be regarded as
equivalent for I , I-dilithioethene 50, similarly to 38a[591
(Scheme 40) as equivalent for E- 1,2-dilithioethene 51.-
Table 7. Competing lithiation of G-CH,-G'
10 for determining the influence of ligands on the acidifying action of main group elements.
Yield G-CHLi--G'
rel.
abs. [Oh]
Substrates
Me,Si-CH,-SPh/Ph,Si-CH2-SPh
MeiSi-CH,-SePh/Ph,Si-CH2-SePh
Me,Ge-CH2-SPh/PhIGe-CH2-SPh
Me,Ge-CH:-SePh/Ph,Ge-CH2-SePh
[a]
[b]
[a]
[b]
6 : 94
23 : 77
12 : 88
0 : 100
80 [47a]
53 [24j]
ii: 100 [24j]
56 I24jl
~
la] LDA, THF, - 5 0 + - 1 0 ° C 2.5 h: Ib] LDCAIHMPA (1 .l), ether/THF
(3.5 I), 20"C, 2 h
422
SnMe3
R-CH=(
Li
50
Li
Li
51
52
Equivalent reagents for 2-substituted I,l-dilithioethenes
are the recently described compounds 52, which are accessible by stannyl/Li exchange[701.
Angew. Chem. I n t . Ed. Engl. 21 (1982) 410-429
4.1.4. What Causes the Stabilization of Carbanions by Heavy
Main-Group Elements?
Table 8. Synthesis of 55 and 56 from 34 and alkyllithium compounds (molar
ratio 1 : 1) according to Scheme 49. Yields based on Alk-Li.
This stabilization of carbanions is proven by the exchange reactions (organoelement/Li, halogen/Li, H/Li exchange) outlined. The question as to the cause, however,
cannot as yet be answered unequivocally: According to ab
initio calculations on REI-CH:
anions (El=S, Se) the
stabilization of carbanions by thio- or seleno-groups is due
to the polarizability of S and Se, and to a stereoelectronic
effect[”]. If polarizability does in fact play an essential
role, then stabilization by the “soft” elements of the 4th
and 5th row is readily understandable.
The relatively high acidity of C-H bonds alpha to 2ndrow elements Si, P, S is still usually explained, even today,
in terms of a (p-d)n orbital overlapping in the respective
anions. A corresponding overlapping is unlikely in the
case of a 4th- or 5th-row element, owing to the distinctly
longer C-El bond. That long pd hybrid orbitals of 4thand 5th-row elements participate, as has been discussed
for explanation of the strong ring current effect of arsaand ~tiba-benzene~’~],
is, however, also worth considering
here. It might also be possible that alkyllithium compounds containing a heavy main-group element in the aposition form particularly stable aggregates and are thus
thermodynamically favorable. Here, the formation of atecomplexes, e. 9-53,is the first likelihood, provided that the
heteroatom has no free electron pair.
Alk
Ph3Pb-CH,-bb(Ph),-CH2-Li
53
--+
4c
4.2.1. Chain Expansion of Carbanions
It was known from the literature that R,El-substituted
ethenes 34, El = Sip, S, Ge, add to the double bond of alkyllithium compounds[731because of the electron-withdrawing action of these elements. Since As and Se are likewise electron-withdrawing (cf. Section 4.1.2): and since alkyl-bound Ph2As- and PhSe-groups are favorable halogen
equivalents (Section 3.2. l), we checked whether the chainlengthening of carbanions with diphenylarsino- 34d and
phenylseleno-ethene 34g formulated in Scheme 49 could
be realized, and found our expectations confirmed.
G
+ ==/
34
-
Li
I
Alk-CH,-CH-G
54
Hi0
Alk-(CH,),-G
55
1
BIZ
d, G
= Ph,As
g, G
=
PhSe
A1k-(CH,),-B
r
56
Scheme 49. Yields: Table 8.
In each case, diphenylarsinoethene 34d gave far better
yields than phenylselenoethene 34g (Table 8) in the addition and the halogenolysis (see Section 3.2.1). The drawAngew. Chem. Int. Ed. Engl. 21 (1982) 410-429
C2HS
PhlAs
n-C4H9
s-C4H9
[240]
K4Ha
Yield [%]
56
37 [a]
95 [a]
71 [b]
57 [bl
Yield
PhSe
[23al
82
84
[%I
55
56
72 [b]
25 [b]
25 [bl
32
28
back of using 34g as substrate for the addition may be due
to the fact that a-lithiation to 35g (see Section 3.3.3) and
phenyl/alkyl exchange to give alkylselenoethene compete
with the additi~n[~’].-Thereactions formulated in Scheme
49 reveal 34d as equivalent for vinyl halides, which cannot
add carbanionic compounds because of polymerization.
a-Lithiated alkyldiphenylarsanes 54d have been made
accessible for the first time via the reaction of 34d formulated in Scheme 49. With these, as with diphenylarsinomethyllithium 4d (see Section 3.3.1), a-arsino-, and thus ahaloalkyl-groups (see Section 3.3.2) can be incorporated
into electrophiles (example: Schemet 50).-Electrophiles
have also been reacted with Se derivatives 54g1581.
Li
I
Alk-CH-As
54 d
HO-C H-Ph
Ph,
1) Ph-CHO(THF)
2) Hi0
=
I
Alk-CH-AsPh,
68%
According to our observations, R,El-substituted ethenes
34 d o not react with 4th- and 5th-row elements according
to Scheme 49, most likely due to organoelement/Li exchange. Thus, there is a clear line of demarcation between
suitable elements (area circled in Scheme 51) and unsuitable elements for such reactions. The same applies for cycloadditions (Section 4.2.2).
4.2. Nucleophilic Addition and Cycloaddition to
Organoelement-Substituted Ethenes
Alk-Li
55
Scheme 50 [240]. Alk=n-C5H,,.
Li
2 Ph3Pb-CH,-Li
G
Scheme
51. The group of elements circled can react according to Scheme 49.
4.2.2. Organoelement Groups as Removable Auxiliary
Groups in I , 3-Anionic Cycloadditions
The scope of the “ 1,3-anionic cycloaddition” type of
reaction[’] is considerably limited by the fact that, for stabilization of the heteroallyl anions and activation of the
substrates, aromatic moieties are required which can be removed after reaction.-This problem was remedied by the
observation that organoelement groups like PhS, PhSe,
Ph2P and Ph2As, which are readily removable after the cycloaddition, so activate olefinic double bonds that cycloaddition of 2-azaallyllithium compounds according to
Scheme 52 is possible‘741(yields: Table 9). The cleavage of
these auxiliary groups can be accomplished by hydrogen423
ation with Raney-nickel (PhS, PhSe, Ph2As) or by bromolysis (Ph2As, PhSe) (examples: Scheme 53). The Ph2P
group may be removable after oxidation by Horner reaction.
Me3Si, Ph3Si1751
and Ph,Ge groups are likewise activating (Table 9), but they are difficultly removable, while
corresponding groups containing the 4th- and 5th-row elements Sn, Sb, Te, Pb, Bi are unsuitable because of organoelement/Li exchange (see Scheme 51).
Thus, as expected, the tendency to undergo cycloaddition
depends on the degree of stabilization of a negative partial
charge in the a-position to the organoelement group. The
comparison phenylthioethene : phenylethene (styrene)about four times more rapid reaction and easily removable
activating groups of the sulfur compound-clearly demonstrates the improvement achieved.
5. Organo Heavy-Metal Groups as Leaving Groups
in B-Eliminations
5.1. Novel Reagents for Carbonyl Olefination
1
1) "g!..CH,
Ph 'N
5.1.1. Synthesis of Terminal Olefns
N
H
2) H 2 0
58
Ph
Ph
The reagents 4 listed in Table 10 (synthesis: Sections
2.2.1 and 2.2.3) react with aldehydes and ketones to give 8hydroxy compounds of the type 59[2.4.26.3'1.
Scheme 52. G and yields: Table 9.
Table 9. Synthesis of 57 and 58 from 34 and 1.3- and I,l-diphenyl-2-azaallyllithium, respectively, in THF according to Scheme 52 [74].
A
G
-
Yield [Yo]
57
58
O=CRR
G-CH,-Li
a
Ph,Si
Me,Si
Ph,Ge
PhzP
PhzAs
PhS
PhSe
d
g
86 [75]
57 [75]
29 [2Sb]
>42 [23i]
65 [240]
68 [25a]
77 [23a]
43 [25a]
31 [25a]
59
Table 10. Synthesis of 59 from 4 and carbonyl compounds (molar ratio 1 : 1)
according to Scheme 55. Yield of isolated product.
,Br
G
c H3
+
P h G e 1230
BrAsPhz 73%
P h S n [b] [24k]
Ph,Pb [b] [24k]
PhzAs [25a, 24il
SPh
.QFh
A
P h aN - P h
Yield [%I on reaction with
PhzCO
nPrCHO
C,H,,O
[cl
MeCOPh
I
I
c H3
Ph
t
H I
Scheme 55. Methods A. B. C see text.
PhCHO
c H3
I
CH,=CRR'
B, C
31 [25b]
> 65 [23i]
0 [240]
64 [25a]
42 [23a]
AsPhz
G-CH,-CRR'
4
-
1
A
57%
PhlSb [a] [25b]
PhzBi [a] [24nl
I
H
Scheme 53 12401. A: Raney-nickel/EtOH, 20°C; B: 1) Br,/CCI, 0°C. 2)
120°C.
In competition experiments, in which two olefins were
allowed to react with 1,3-diphenyl-2-azaallyllithiumin the
molar ratio 1 : 1 :1, we determined the relative rate constants quoted in Scheme 54. The gradation in readiness to
74 [a]
58 @I
65
71
69 [a]
92 Ibl
39
65 la1
0
54
94 [b]
45
48
59
62 [a]
75 [b]
37
46
70 [a]
73 [b]
61
41
9
[a] Prepared by organoelement/Li exchange. [b] Prepared by halogen/Li exchange. [c] Cyclohexanone.
Table 1 I . Carbonyl olefinations according to Scheme 55, for example, of
benzaldehyde. Method A : dry heating to 180°C. B: HCI0dCH3OH, 20°C;
C: Si02/THF, 20°C.
G
SePh
A
Styrene yield [%I
B
C
34g ;=/
\
H
Ph
2 . 3 : 1.0
~
~
[a] Yield based on 59. [b] Yield based on 4, since 59 not isolated. [c] Method
A . Heating to I1O"C.
k,,, = 6.4
2.3
1.5
1.0
0.46
0.09
Scheme 54.
undergo cycloaddition along the series PhS >
PhSe > Ph2As directly corresponds to the relative
acidifying effect of these groups revealed in Scheme 47.
424
With exception of the Ge-derivatives and the Bi-compound obtained by reaction with benzophenone, the products 59 undergo p-elimination on heating to give terminal
olefins (Scheme 55, Method A)[4.26.761.The decomposition
temperature in the case of the Sn-compounds (100120°C) is lower than in the case of the Pb-, Sb-, and AsAngew. Chem. In[. Ed. Engl. 21 (1982)410-429
compounds (160 - 180" C). Accordingly, the yield of olefin
is usually somewhat better in the former case; the As-compounds give the lowest yields of olefin. Apart from by
heating (in toluene at llO"C, in chlorobenzene at 13OoC,
or without solvent at 180°C; Method A ) the formation of
olefin can also be achieved by reaction with HC104
(Method B)1761.
A much simpler and more gentle method is
to pass the THF-solution of 59 through a short silica-gel
column (Method C')176.771.
Method C fails in the case of Geand As-compounds and is thus apparently limited to compounds 59 containing organoelement groups of 4th- and
5th-row elements. Table 1 1 shows the yields obtained in
carbonyl olefinations for the example of benzaldehyde
(yields on the use of other carbonyl compounds: [4.26,3'.7611.
The Bi-compound obtained from benzophenone gave 0,
61, and 91% yields of I,]-diphenylethene on using Methods A , B, and C,r e s p e ~ t i v e l y ~ ~ ~ " ~ .
Prior to our investigations P-hydroxy compounds of type
59, G=Ph3Sn, Ph3Pb, were synthesized by Davis and
Grayr7'l by nucleophilic ring-opening of oxiranes according to Scheme 56, followed by decomposition with acid to
the olefin. Since the overall reaction proceeds with retention, and the C-0 bond is undoubtedly opened by attack
from the rear, the olefin is formed by anti-elimination. This
is of interest in connection with the findings described in
Section 5.1.2.
0
Scheme 56. El =Sn, Pb 1781.
Aside from 4a, the first Ge-reagent used for carbonyl
ole fin at ion^'^^',^^^,^^^ (Tables 1 1 and 12), the analogous anionoid trimethylgermyl compounds 4n and 60 are also
suitable for this purpose (Scheme 57)Iz4j1.They have the
advantage over 4a in that the P-hydroxyalkylgermanes 61
(cf. 5 9 ) obtained by reaction with aldehydes and ketones,
like the analogous S i - c o m p o ~ n d s ' ~are
~ ~also
~ ~ ~decom,
posed to olefins by K H (presumably syn-elimination) as
well as by acids (presumably anti-elimination), the yields
being very good in each case (Table 12). These findings
can possibly be exploited for stereospecific carbonyl olefinations (cf. Section 5.1.2). In comparison to the analogous, less-expensive Si-reagents, it would appear to be advantageous in the case of the Ge-reagents 4a, 4m, 60-as
-
nBuLdTHF
Br2lCIx;hu
Me4Ge
Me3Ge-CHp-Br
60%[81]
I
-
50+O0C,Smin
*
well as 63a and 63b (Table 13)-that the nucleophilicity is
somewhat higher as a result of the particularly low stabilization of carbanions by G e (see Scheme 47).
In the synthesis of organic compounds the organoelement functional groups must be removed again. Carbonyl
olefination with Ge-reagents (see also Section 5.1.4) thus
also deserves attention, since it is the mildest method for
the removal of organogermanium groups from Catomsls21.
Table 12. Carbonyl olefinations according to Scheme 57 with
4m and Ph,Ge-CH2-Li
4a
as well as
MelGe-CHz-Li
60 to give terminal olefins. Decomposition of the 8Me3Ge-CH2-MgBr
hydroxy compounds 61 obtained by hydrolysis with H 2 0 : Method A : with
HCIOa at ~ 2 0 ° C B:
; with KH at ~ 2 0 ° C
[24J].
4m la1
4a Icl
Carbonyl compound
Olefin yield I%]
A [bl
B
Heptanal
Phenylacetaldehyde
Benzaldehyde
4-(/erf-Butyl)-cyclohexanone
Acetophenone
Heptanal
Phenylacetaldeh yde
Benzaldehyde
Acetophenone
90 (91)
95 (88)
94 (87)
9 I (79)
91 (63)
61, G
+
cH,=cRR'
= Me3Ge,
Ph3Ge
I
Ph-CHO
'
Ph-CO-CH, ]
I
C H2-C H-R
62a, El
62b, E l
Scheme 57 [24j]. A. 5 see Table 12
Angew. Chem. Int. Ed. Engl. 21 (1982)410-429
= Ge
= Si
Ph-CH=CH,
%
P
h H3C
+ %CH,
3570
22%
Scheme 58 (24jl.
Table 13. Yields [Yo] of terminal olefins on using aldehyde-selective carbonyl
olefinating reagents 63 (molar ratio 63 :carbony1 compound = 3 : I ) [83].
[a] Heptanal
[b] Nonanal
[cl Benzaldehyde
Me3Ge-CH2-Li
OTiC13
96
4a, 4n, 60 and the analogous Si-reagents are unsuitable
for aldehyde-selective carbonyl olefinations, since they
also react rapidly with ketones (example: Scheme 58). O n
the other hand, we were able to carry out ''highly aldehydeselective" carbonyl olefinations for the first time with the
transitionmetal reagents quoted in Table 13. These were
generated in situ from 60 and the analogous Si-compounds
prepared by reaction with Tic], in ether or with CrCI, in
T H F (examples: Table 13)1831.
4m,~ 7 5 %
Me,E1
no olefin
formation
[a] Molar ratio 4m :carbony1 compound = 1.5 : I . [b] Values in brackets:
Yields on analogous reaction with 60. [c] Molar ratio 4s :carbony1 compound = 1 . 3 : l .
Me3Ge-CH2--TiC13
63a [24j]
C.C HZ-CRR'
OH
88
a7
81
90
95
MelGe-CH2-CrCIz
63b [24Jl
Me,Si--CH,-TiCI,
63c [47a, 841
Me3Si-CH2--CrCl2
63d [47a, 841
[a1 88
[bl 76
Icl 76
la1 67
Ibl 58
Ial 65
Ibl 61
[cl 59
la1 47
[bl 45
[a] 5-Nonanone
[b] Acetophenone
[c] 4-(ten-Butyl)cyclohexanone
Icl 0
[cl
la1
[bl
Icl
la1
lbl
[cl
0
0
3
8
0
0
@
425
The mechanism of carbonyl olefination with the reaGray[781(see Section 5 . I.]), an anti-elimination; hence, syngents 63 varies considerably. When Ti-reagents 63a, c are
elimination takes place on heating. The erythro-isomer 64b
employed, olefination already occurs before h y d r o l y ~ i s ~ ~ ~ l , was decomposable in the same way. Accordingly, benzalwhereas when the Cr-reagents 63b, d are used it occurs
dehyde can be converted at choice with phenylthio(tripheonly after the neutral hydrolyzate has been treated with
nylstannyl)methyllithium 15e into the E-olefin 65a (83%) or
mineral acid. The intermediates 62 obviously decompose
the Z-olefin 65b (84%). Under analogous conditions the
rapidly with formation of olefin.
Pb-analogue of 15e gave either pure 65a or a 1 :2 mixture
of 65a and 65b. Thus, Sn-reagents would seem to be more
5. I . 2. Stereospecific Carbonyl Olefination to the
suitable than the Pb-analogues for stereospecific syntheses
Z- or E-Olefin
of Z-olefins from carbonyl compounds.
In highly stereoselective olefinations a distinction is
made between methods “without isolation of intermediates” ( e . g . influencing the steric course of the Wittig
reaction according to Schlosser and Christmanda6I) and
methods “with isolation of intermediates”. Of the latter
[ ~ ~in~ principle, espethe Peterson c a r b ~ n y l - o l e f i n a t i o nis,
cially advantageous: Since the intermediary fi-hydroxalkyltrimethylsilanes can be decomposed at choice by syn- or by
anti-elimination (action of K H or acid)[801,a n aldehyde or
an unsymmetric ketone can be converted selectively and in
high yield into an E- or Z-olefin, provided the threo- and
erythro-form of the fi-hydroxyalkylsilane can be separated.
In contrast, in the case of other carbonyl olefinations
either only syn- (Wittig-like reactions) or only anti-elimination (use of S e - r e a g e n t ~ ~is~ possible.
~])
In principle, however, the scope of the Peterson carbonyl-olefination is considerably limited. In the case of the
readily accessible Si-reagents with an electron-withdrawing
moiety on the carbanionic center, the alcoholic intermediates are isolable in only very low yields o r not at all, owing to the instability of their anion (see Scheme 63).
According to our findings, stereospecific carbonyl olefinations to give either the E- or the Z-olefin are also
possible with stannyl- and plumbyl-stabilized carbanions[4~24k.25a.881.
Since the alcoholic intermediates can almost always be isolated when using these reagents (exception: Scheme 62) it would appear that they have broader
scope than those of the trimethylsilyl derivatives (Peterson
carbonyl olefination).
The synthesis of the fi-phenylthiostyrenes 65a and 65b
from benzaldehyde according to Scheme 5914.88’has been
investigated in most detai1[4,881:The action of perchloric
acid in methanol on the threo-intermediate 64a, separated
quantitatively by HPLC from the isomer 64b, led exclusively to the Z-olefin, while heating gave exclusively the Eolefin. The former reaction is, according to Davis and
5.1.3. Other Syntheses of Organoelement-SubstitutedOlefins
by Carbonyl Olefination
Organoelement-substituted ethenes, e. g. of type 34,
have a variety of uses in organic synthesis. Some of the
more useful reactions (examples: PhS group: indirect nucleophilic acylation with 1 - p h e n y l t h i o ~ i n y l l i t h i u m ~ ~ ~ ~ ;
PhSe, PhTe, Ph2As groups: Schemes 38,48,49,52) may be
extended to correspondingly substituted higher olefins.
The synthesis of such compounds-also using the stereospecific method presented in Section 5.1.2-is one convenient use of carbonyl olefination reagents containing
stannyl- or plumbyl-leaving groups. Some such syntheses
are outlined in Schemes 60 and 61.
PhwsePh
1) PhsSn-CHh-SePh
2) H ~ O / H @
+
Ph
I
66a, 9 3 %
Ph,C=O
I
1)F’hsF’b-CHL-AsPh~
15d
PhFAsPh2
~
2) HzOIH’
Ph
66b, 100%
Scheme 60 [24m]. The yields of 66a and 66b on elimination by heating are 74
and 75%. respectively.
Ph-CHO
(PhsPb),CH-Li
OLi
(Ph,Pb),CH-&H-Ph
81%
13c
/
P h3 Pb-C H Li-Si Me3
15a
PhLl
7
48%
67
Ph
=i
Ph
Scheme 61 [24m].
I ) F’h-CHO
I
SPh
93%
PhS
64a/64b
m i x t u r e (91%)
15e Ph3Sn-CHLi-SPh
-
I
100% LDA
64b 45%
65b
Scheme 59. For simplicity only one enantiomer of each of the alcoholic intermediates 64 is formulated.
426
The yields obtained on synthesizing the plumbyl olefin
67 via two routes (Scheme 61) show that Ph3Pb can be
more favorable than Me3Si as leaving group. In each case
only the E-olefin is formed.
In analogous experiments, the organo heavy-metal
groups behaved surprisingly: the Li-compounds 69,
G = Ph3Sn, Ph3Pb, Ph,Sb, PhTe, did not react with benzaldehyde in the sense of a Horner reaction but according to
Scheme 62, even at -7O”C, with immediate cleavage of
the organo heavy-metal group to give 0-(diphenylphosphinoy1)styrene 70[240.901.
Anqew. Chem. h i . Ed. Enqi. 21 (1982)410-429
-
O G
II
I
LDA
A) Ph,P-CH,
-
f: 7
PhXHO
Ph,P-CH-Li
THF
70 [240]
69
68
SO,Ph
1)PhXHO
(THF, O°C + + 20'C)
B) Ph3Sn-kH-Li
71
*
corresponding transition metal compounds of type 63 (cf.
Table 13)-show enhanced nucleophilicity. The only carbony1 olefination carried out so far (Scheme 66) is promising with regard to yield and stereoselectivity.
/dPh
2) HzO
PhSOz
GePh3
A)
61% [ 2 4 e ]
Scheme 62. Yield of 70 from 68, G = Ph,Sn, Ph,Pb, Ph>Sb,PhTe=92,71,25,
4fyo [90].
Thus, in this case even the phenyltelluro-group which is
otherwise unsuitable for this purpose is relatively suitable
as leaving group. The observed promotion of f3-elimination
by a strong electron-withdrawing group is reminiscent of
the analogous circumstances in the Peterson carbonyl olefination, where a rearrangement of the silyl group (Scheme
63) is assumed for explaining the observed effect["I.-The
triphenylstannyl group was also eliminated even at low
temperature in the reaction of 71 with ben~aldehyde[~~'].
=/
34a
RLi
+
Ether
R-CH,-CHLi-GePh,
75
R = Me, nBu, sBu, fBu: 2, 87, 90, 8 8 % [ 2 4 j ]
R = Ph: 60%[73c]
,SePh
B) CH3-C\H
76 G e p h ,
nBuLi, THF
C H3-C H Li-G e P h 3
63%[2411
77
Scheme 65.
I ) PhCHO, THF, ZO°C
dGePh3
2 ) H C I O ~ / M ~ OZOT*
H,
&CH=CH-Ph
8470, E ; Z = 5 : 1
Scheme 66 [24j].
Me3Si Oo
- cI 4IxA I
OSiMe3
4
0 1
-c-c-
A
X
I
+ x
X
Scheme 63.
5.1.4. Synthesis of Non-Terminal Olefins
Extension of the carbonyl olefination with Sn- and Pbreagents mentioned in Section 5.1.1 to the synthesis of
non-terminal olefins appears less worthwhile, since compounds 72, El = Sn, Pb, are relatively difficultly accessible
(see Schemes 19 and 21) and the equilibria set u p in the organoelement/Li exchange (Scheme 64) lie distinctly more
unfavorably than in the corresponding methane-derivatives as a result of poorer charge-stabilization in 73 (see
Table I).
7H3
(Ph3El),CH + P h L i
5 3 3
Ph3E1-CH-Li
+ Ph4E1
73
/
PhzCO
72
7H3
Ph,Pb-CH-C(OH)Ph,
74
E l = Sn: 2 6 % [ 2 4 e ]
E l = Pb: 3 6 % [ 2 4 m ]
Ph
H3cq..
Ph
Scheme 64. A : I 1O"C, toluene, 22 h, 15% [24m]: B: HC104, =90% [24m]
Moreover, the 0-hydroxy compounds, e.g. 74, formed
from carbonyl compounds and 73, are thermally more stable than the corresponding compounds 59 (Scheme 55).
a-Lithiated triphenylgermylalkanes such as 75 and 77
are far more easily accessible than the analogous Sn- and
Pb-compounds, namely by addition of organolithium compounds to triphenylgermylethene 34a (Scheme 65 A) or via
Se-compounds (e.g . according or analogous t o Scheme
65 B; 76 is synthesized according to Scheme 18). Whether
they offer advantages over the corresponding more
cheaply available a-lithiated triphenyl- and trimethylsilylalkanes is still undecided. Presumably they-as well as the
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
The synthetic routes formulated in Scheme 65 cannot be
extended to the Sn- and P b - a n a l o g u e ~ [ ~ ~ ~ .
5.2. Elimination of Organoarsenic- and
Organoantimony Groups together with fl-H Atoms
The importance of organoselenium reagents in organic
synthesis is largely based on the fact that selenium oxide
groups are rapidly eliminated together with a p-H atom
with formation of olefin, even at room temperature[551.
When the cleavage of seleno groups via the oxide (primary
seleno groups) is rendered difficult, the phase-transfer
reaction with chloroamine-T via selenium tosylimine has
proven u s e f ~ I [ ~ ~ ] .
We extended both methods to primary diphenylarsinoand diphenylstibinoalkanes (Scheme 67) and found that
the route via the oxide leads to olefin formation (ca. 2 1 23%) only in the case of the Sb-compounds, whereas the
expected olefins are formed in both cases via the tosylimine. However, since the yields were unsatisfactory[24h1
(El = As, 41-45%; El = Sb, 53-56%) and the required decomposition temperature (90 "C) was relatively high, this
method cannot be fully recommended. Surprisingly, in
analogous reactions with a secondary arsane and stibane
(2-diphenylarsinoo~tane[~~~~,
2-diphenylstibinopentane[12')
only minimal amounts of olefin could be detected.
PhZEl,
/H
7c-q
Scheme 67, EI=As, Sb [24h].
427
6. Conclusion and Outlook
Landes Nordrhein- Westfalen (Project ‘‘ Unreaktivierung
durch Ummetallierung ‘7 and the Fonds der Chemischen Industrie for very helpful financial support.
On examining heavy main-group elements for possible
uses in organic synthesis we found, in the case of the elements marked with a circle in Scheme 68, that there are
four possible modes of reaction. As shown in Scheme 68,
none of the elements investigated is completely unsuitable
for synthetic application. Moreover, each element-even
the inconvenient bi~muth[’*~-revealedspecial characteristics and useful features arising from a combination of various properties. The element Te, e.g., is exceptionally good
in that it has a strong acidifying effect in the functional
group PhTe, it is the most amenable to organoelement/Li
exchange, and it forms essentially more thermally stable ahetero-substituted carbanions (cf. 4h and 35h) than the
comparable elements Br and I (cf. 8).
Received: December 28, 1981 [A 41 I IE]
German version: Angew. Chem. 94 (1982) 401
[I] Th. Kauffmann, Angew. Chem. 76 (1964) 206; 77 (1965) 557; 83 (1971)
21;86(1974)321;91(1979) 1;Angew. Chem. Int. Ed. Engl.3(1964)342;
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121 Th.Kauffmann, R. Kriegesmann, B. Altepeter, F. Steinseifer, Chem. Ber.
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I31 A. H. Davidson, P. K. G. Hodgson, D. Howells, S. Warren, Chem. Ind.
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[4] First review: Th. Kauffmann, Top. Curr. Chem. 92 (1980) 109.
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[6] The designation “Element-Lithium Exchange” 1221 originally proposed
by us has thus been modified.
[7] Literature survey: B. J. Wakefield: The Chemistry of Organolithium Compounds. Pergamon Press, Oxford 1974, p. 66f.
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[9] D. Seyferth, M. A. Weiner, Org. Synth. 41 (1961) 30. - With this method
one avoids the Wurtz-coupling that always occours in reactions of ally1
halides with Mg or Li.
( P b
1101 D. Seebach, N. Peleties, Angew. Chem. 81 (1969) 465; Angew. Chem. lnt.
I
4
I
J
Ed. Engl. 8 (1969) 450; Chem. Ber. 105 (1972) 51 I ; D. Seebach, A. K.
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[ I I] a) D. van Ende, W. Dumont, A. Krief, Angew. Chem. 87 (1975) 709; Angew. Chem. Int. Ed. Engl. 14 (1975) 700; b) A. Anciaux, A. Eman, W.
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@
,
Krief, Angew. Chem. 88 (1976) 184; Angew. Chem. Int. Ed. Engl. I5
Elimination together
/C4E1Phn
(1976) 161 ; d ) M. Sevrin, A. Krief, Tetrahedron Lett. 1980, 585.
with P-OH
[I21 A. Tannert, unpublished results 1981.
1131 H. Stockelmann, unpublished results 1978/1979.
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1
I
I
[IS] F. Steinseifer, Th. Kauffmann, Angew. Chem. 92 (1980) 746; Angew.
Chem. Int. Ed. Engl. 19 (1980) 723.
Scheme 68. Four possible reactions of organoelement compounds
[I91 D. Seyferth, F. M. Armbrecht, Jr., E. M. Hanson, J . Organomet. Chem.
>C-EIPh,.
I0 (1967) P25; C. M. Warner, 1. G. Noltes, ibid. 24 (1970) C 4 ; D. Seyferth, F. M. Armbrecht, Jr., R. L. Lambert, Jr., W. Tronich, ibid. 44
The author is convinced that all the elements investi(1972) 299.
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stinghaus, Chem. Ber. I10 (1977) 841; c) B.-T. Grobel, D. Seebach, ibid.
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110 (1977) 852.
the practising synthetic chemist. The element As, which,
[21] D. S. Matteson, Synthesis 1975, 147.
[22] Th. Kauffmann, K.-J. Echsler, A. Hamsen, R. Kriegesmann, F. Steinseiunlike the neighboring elements Si, P, S , Se in the Periodic
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bility as air-stable donor atom in transition-metal ligands it
1977; j) F.-J. Wilgen, 1978.
still plays a major role in new investigations being carried
[24] Dissertation, Universitat Miinster: a) H. Ahlers, 1981; b) B. Altepeter,
1981; c) C. Beirich, planned for 1983; d) A. Busch, 1974; e) K.-J.
out by our group[951.
Echsler, planned for 1982; f) J. Ennen, planned for 1982; g) F.-J. Joskowski, planned for 1982; h) R. Joussen, 1979; i) N. Klas, planned for
For the results presented here I a m grateful to all those en1982; j) R. Konig, planned for 1982; k) R. Kriegesmann, 1980; I) H.
Lhotak, planned for 1982; m) A. Rensing, planned for 1982; n) F. Steinthusiastic and capable colleagues who have been engaged in
seifer, 1981; 0)H.-J. Tilhard, 1980.
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ess at handling toxic substances. l sincerely thank all colUniversitat Miinster, experiments 1978.
leagues cited in the references and hope that this report
(261 Th. Kauffmann, R. Kriegesmann, A. Woltermann, Angew. Chem. 89
might bring back memories of their having joined in the com(1977) 900; Angew. Chem. Int. Ed. Engl. 16 (1977) 862.
1271 A. G. Brook, J. M. Duff, D. G. Anderson, Can. J. Chem. 48 (1970)
bined effort as members of our research team studying
561.
organoelements. I also wish to thank Herrn H . Niewind,
[28] G. Kobrich, R. von Nagel, Chem.-Ztg. 94 (1970) 984.
Organisch-chemisches Institut der Universitat Miinster, for
[29] D. Seyferth, S. B. Andrews, J. Organomet. Chem. 30 (1971) 151; R. D.
Taylor, J. L. Wardell, ibid. 77 (1974) 3 I I .
the synthesis of numerous starting substances, as well as the
1301 The reaction 3 c - 4 ~ (Scheme 8, R = Ph), was practically quantitative, irDeutsche Forschungsgemeinschaft (Ka 144/34 and Ka 144/
respective of whether 1 mmol of 3c (Ph,Pb does not precipitate) o r 8
mmol of 3c (Ph,Pb precipitates) per 100 mL of T H F were used 1241111.
3.5-I), the Ministerium fur Wissenschaft und Forschung des
1
@
I
\
428
Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
[31] Th. Kauffmann, A. Hamsen, R. Kriegesmann, A. Vahrenhorst, Tefrahedron Lett. 1978. 4395.
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aldehydes and organoelement halides, although 4f was completely consumed.
[33] 1Of could b e obtained (66%) from PhSeNa and 7d. Reaction with nBuLi leads to 4d (45%) [24i]. - While organolithium compounds are activated by chelate ligands with hard donor centers, e . g . tetramethylenediamine (TMEDA), from our experience the reverse is the case with chelate ligands with soft donor centers.
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cases.
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Angew. Chem. Int. Ed. Engl. 21 (1982) 410-429
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be lithiated to only 5 and lo%, respectively [23e]. - In attempts to lithiate
bis(trimethylstannyl)methane under various conditions, substantial decomposition occurred in each case (H. Rohkrahmer, Dissertation, Universitat Miinster, planned for 1983). b) (Ph2P):CH2 already decomposes
at <O"C with LDCA+HMPA [24g].
[67] Th. Kauffmann, B. Altepeter, K.-J. Echsler, J. Ennen, A. Hamsen, R.
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[72] P. Jutzi, Angew. Chem. 87 (1975) 269; Angew. Chem. I n / . Ed. Engl. 14
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1771 This method was also discovered independently by D. Seebach (private
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j781 D. D. Davis, C. E. Gray, J. Organomet. Chem. 18 (1969) P 1 ; J. Org.
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1811 J. L. Speier, J. Am. Chem. SOC.73 (1951) 826; C. R. Hauser, C. R. Hance,
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I841 C. Pahde, Diplomarbeit, Universitat Miinster 1982.
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I891 E. J. Corey, B. W. Erickson, unpublished experiments 1967, mentioned
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[911 I. A. Carey, A. S. Court, J. Org. Chem. 37 (1972) 939.
1921 Exclusively stannyl- and plumbyl/Li exchange, respectively, was observed o n reaction of PhLi with Ph3El-CH2-SePh
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[951 Cf. Th. Kauffmann, J. Ennen, H. Lhotak, A. Rensing, F. Steinseifer, A.
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Tetrahedron L e f f .1981. 5035.
429
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