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Intermediates of -Eliminations.

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ANGEWANDTE CHEMIE
VOLUME 4 . N U M B E R 1
J A N UARY 1 9 6 5
P A G E S 1-106
Intermediates of a-Eliminations [*]
BY PROF. DR. W. KIRMSE
CHEMISCHES INSTITUT DER UNIVERSITAT MARBURG (GERMANY)
a-Elimination denotes the removal of two substitiients from the same carbon atom. Presentday knowledge of these reactions, their intermediates, and end-products is reviewed, with
particular emphasis on the question of the occurrence of carbenes and the role of organometallic (carbanion) intermediates.
1. Introduction
Elimination rextions cover a wide field in preparative
and theoretical organic chemistry. However, a- or 1 , l elimination has long been overshadowed by the more
familiar 3- or 1,2-elimination [l] which has been more
extensively investigated. This is the reason for the
cursory treatment of a-elimination even in modern
textbooks. Nevertheless, interest in these reactions can
be traced back a long way. Attempts were made to
prepare methylene from methanol [2], methyl chloride
[3], and methylene iodide [4] before the tetravalency of
carbon had yet been established. Influenced by his work
on isonitriles and fulminic acid derivatives, Nef [5]
published his “general methylene theory” in 1897, in
which he tried to explain all substitutions as resulting
from a-elimination and addition. This concept fell into
disfavor because of its excessive generalization, and
only recently has it been reinstated in certain special
cases. Isolated references to a-elimination may be found
in the works of Lab [6], Staudinger [7], and Zngold [8].
[*] Extended version of lectures delivered by the author in GieBen, Munchen, Freiburg, Darmstadt, Heidelberg, and Braunschweig (Germany).
[I] More recent summaries: J . F. Bunnett, Angew. Chem. 74,
731 (1962); Angew. Chem. internat. Edit. I, 225 (1962); R . F.
Hudson, Chimia 16, 173 (1962); D. V . Banthorpe: Elimination
Reactions. Elsevier, Amsterdam 1963.
[2] J. 5. Brimas, Ann. Chim. Physique 121 58, 28 (1835); H . V .
Regnault, ibid. [2] 71, 427 (1839).
131 A . Perrof, Liebigs Ann. Chem. 101, 374 (1857).
141 A . M . Eutlerov, Liebigs Ann. Chem. 120, 356 (1861).
[5] J. U. Nef, Liebigs Ann. Chem. 298, 202 (1897).
[6] W. Lob, Z . Elektrochem. angew. physik. Chem. 7 , 903
(1901); Ber. dtsch. chem. Ges. 36, 3059 (1903).
[7] H . Staudinger and 0 . Kupfer, Ber. dtsch. chem. Ges. 44,
2197 (191 1); 45, 501 (1912).
[8] C. K . Ingold and J. A. Jessop, J. chem. SOC.(London) 1930,
709.
Angew. Chem. internat. Edit.1 Vol. 4 (1965) / No. 1
The development of carbene chemistry 191 has given a
new impetus to the work, since a-elimination was of
interest as a possible route to “energy-deficient’’ carbenes. The results obtained were first interpreted simply
as carbene reactions. It was soon shown, however, that
organometallic (carbanion) intermediates could also
play an important part, and in certain cases the appearance of carbenes was denied [lo]. It is hoped that a
critical survey of the field, including own results,
some of which have not been published, will contribute
to the elucidation of these problems.
2. Solvolysis of Haloforms
As a result of the excellent work of Hine and his school,
the reaction of haloforms (trihalogenomethanes) with
bases is one of the most thoroughly investigated cases
of a-elimination. The most important results will now
be summarized, since they present several problems of
a-elimination in a particularly clear light.
In fluorine-free haloforms the base-catalysed H-D
interchange is 1 0 3 to lo5 times faster than any other
reaction [I 1-15]. Solvolysis is therefore introduced by
[9] J. Hine: Divalent Carbon. Ronald Press, New York 1964;
W. Kirmse: Carbene Chemistry. Academic Press, New York 1964.
[lo] H . Hoberg, Liebigs Ann. Chem. 656, 1 (1962).
[ l l ] J. Hine, R . C. Peek j r . , and E . D. Oakes, J. Amer. chern.
SOC.76, 827 (1954).
[I21 J . Hine and N . W. Burske, J. Amer. chem. SOC. 78, 3337
(1956).
[I31 J. Hine, N . W. Burske, M . Hine, and P . B. Langford, J.
Amer. chem. SOC.79, 1406 (1957).
[I41 J. Hine and P . B. Langford, J. Amer. chem. SOC.79, 5497
(1957).
[I51 J . Hine, R . Eutterworth, and P. B. Langford, J. Amer. chem.
SOC.80, 819 (1958).
1
the rapid and reversible formation of a trihalogenocarbanion [Equation (a)].
HCX3
0
+ B? C X3C: + BH
(a)
The rate-determining step of the solvolysis is to be
found among the subsequent reactions of the trihalogenocarbanion. There are two possible interpretations
which fit the kinetic data (the reaction is first order with
respect to the haloform and the base [16,17]):
a) A slow SN1 reaction of the trihalogenocarbanion to
give a dihalogenocarbene, followed by rapid conversion
of the carbene into carbon monoxide and formate (the
course of which does not concern us here) [see Equation
@)I.
0
xsc:
slow
xzc:
+
xQ
8 ) A slow SN2 reaction of the trihalogenocarbanion,
according to Equation (c).
Q
X3C:
+ HzO
0 @
XzC-OHz
slow
fast
H 2 0 , HO@
Q
63
X,C-OH2
@
--L
XAX
Haloforms with two fluorine substituents occupy a
special position. The soIvolysis of these compounds is
unexpectedly fast, and takes place without H-D exchange [14,21,22]. The behavior of haloforms containing one fluorine atom [13,23] shows that fluorine
makes carbanion formation more difficult, and stabilizes
the carbene. H i m has therefore suggested a concerted
a-elimination mechanism for haloforms containing two
fluorine atoms. The energetically unfavorable trihalogenocarbanion stage is by-passed, and the relatively
stable difluorocarbene is formed directly [Equation (e)].
+ XQ
CO, HCOF
In the carbene mechanism (b), the nucleophilic attack
takes place after the rate-determining step, whereas in
mechanism (c) the nucleophilic attack is identical with
the rate-determining step. It follows that the effect of
added anions X @on the kinetics provides a means of
distinguishing between the two mechanisms (b) and (c).
In (c), the starting materials and the end-products of the
substitution are identical:
x@+x3c:
that oIefins react with the dihalogenocarbene and not
with the trihalogeno carbanion. However, this view has
been shaken by the latest observations according to
which trichloromethyllithium, which is stable at low
temperatures, attacks olefins as an electrophile [20].
On the other hand, difluoromethyl phenyl sulfone,
C~HS-SOZ-CHF~,
follows the two-step mechanism
(stabilization of the carbanion) [24].
3. Scheme of the a-Elimination
On the basis of the special case of the solvolysis of
haloforms, a scheme for the a-elimination will now be
formulated and its general applicability tested [see
Scheme I].
xQ+x,c: Q
Such a reaction cannot affect the kinetics. In (b), on the
other hand, the step
xo t- :cx2 + x,c: Q
constitutes a reversal of the rate-determining step, and
hence leads to a decrease in the overall rate of the
reaction. Such an effect has in fact been observed on
addition of halide ions [17]. The action of nucleophilic
anions in the reaction of haloforms with bases therefore
consists, at least partly, in addition to the intermediate
carbene. The addition of olefins, preferably in aprotic
media, yields 1,I-dihalogenocyclopropanes[see Reaction (d)] [9,18,19].Competitive experiments have shown
that the olefin behaves as a nucleophile in this reaction
(the rate of the reaction increases with increasing electron density of the C=C bond). By analogy with the
behavior of nucleophilic anions, it could be assumed
Two substituents (R, R ) on the reacting carbon atom do no1
participate directly in the or-elimination. One group (X) can be
removed as a cation by means of a base (B:@), while a second
group ( Y )may be lost as an anion. If these steps occur in suc-
1161 J. Hine, J. Amer. chem. SOC. 72, 2438 (1950).
[17] J. Hine and A . M. Dowelljr., J. Amer. chem. SOC. 76, 2688
(1954).
[18] W . Y. E. Doering and A . K . Hoffmann,J. Amer. chem. SOC. 76,
6162 (1954).
I191 P. S . SkeN and A . Y. Garner, J. Amer. chem. SOC. 78, 3409
(1956).
1201 W. T. Miller jr. and D . M . Whalen, J. Amer. chem. SOC. 86,
2089 (1964).
[21] J. Hine and J. J . Porter, J. Amer. chem. SOC. 79,5493 (1957).
[22] J . Hine and A . D . Ketley, J. org. Chemistry 25, 606 (1960).
[23] J. Hine and S. J. Ehrenson, J. Amer. chem. SOC. 80, 824
(1958).
[24] J. Hine and J . J. Porter, J. Amer. chern. SOC. 82, 6178 (1960).
2
t
Scheme 1. The general principle of a-elimination.
Angew. Chem. internat. Edit. VoI. 4 (1965) I No. 1
cession, an organometallic (or carbanion) intermediate (2) is
formed. Direct formation of carbene (3) from ( I ) is also possible, as seen in the case of haloforms with two fluorine atoms.
These reactions finally lead to substitution products (5) and/
or olefin addition products (6). In certain cases, “dimers” of
the type RR’C=CRR’ are formed. All the end-products can
be derived from the intermediates (2) or (3).
A nucleophile Z O can yield a new organometallic (carbanion)
intermediate ( 4 ) , either by addition t o the carbene (3), or by
substitution in (2). This intermediate then adds on X t o form
the substitution product ( 5 ) . Direct substitution in the starting material ( I ) can also lead t o (5). It is generally comparatively easy to rule out this alternative. It is much more difficult t o decide whether the reaction proceeds via (2) or via (3).
The solvolysis of haloforms provides a n example of the addition of nucleophiles t o carbenes. On the other hand, halogen
in the halogenomethyl derivatives of zinc 125,261, aluminum
[ 10,261, and mercury [27] very readily undergoes nucleophilic
substitution. A n analogous reaction must therefore be expected in the case of unstable intermediates of type (2). The
origin of the olefin addition products (6) is also ambiguous.
Photolysis and thermolysis of diazo compounds show beyond
doubt that carbenes can add onto olefins. However, the
halogenomethyl derivatives of zinc [ICH2ZnI, (CICH2)2Zn]
[28,29], aluminum [lo] (RzAICHzX), and other metals I291
also convert olefins into cyclopropane derivatives. A preliminary decompositon into metal halide and carbene can be
ruled out with reasonable certainty.
The course of these organometallic syntheses of cyclopropanes is subject to some controversy. Hoberg [lo]
has suggested a two-stage mechanism (f) which he has
supported by isolating hydrolysis products (8) of the
intermediate (7). The strict stereospecificity of the cyclopropane formation requires that the Hoberg mechanism
This mechanism readily explains the stereospecificity
and the electrophilic behavior of MCH2X. A convincing argument in favor of mechanism (g) is provided
by the reaction of 1,6-dihalogeno-3-hexeneswith halogenomethyl derivatives of zinc [31], which yields only
the cyclopropane derivative (9) expected from mechanism (g), without any of the isomer ( 9 4 required by
mechanism (f) [see Equations (h)].
Furthermore, the cupric chloride-catalysed reaction of
diazomethane with olefins [32] does not proceed via
intermediates of type (7). Cis- and trans- 1,4-dichlorobut-2-enes are converted stereospecifically into cis- and
frans - 1,2 - bis(chloromethy1)cyclopropanes [33] in accordance with mechanism (g), whereas mechanism (f)
should lead via a common intermediate (10) to an identical mixture of the isomers [see Scheme 21.
ClHZC,
,CHzCl
c=c\
H’
H
CIHzC,
c=c:H
H’
(8)
I
I
-C-CI I
H CHzX
I
I
-c-c-
CH2N2/CuCI
*
ClCH
G H & 1
CHzC1
Scheme 2. The cupric chloride-catalysed reaction of diazomethane with
olefins.
\ I
CHZ
should involve not only stereospecific addition and
elimination, but also configurational stability of the
intermediate (7). (The addition products (7) obtained
from geometrically isomeric olefins are diastereoisomers.)
Wittig [29], Simmons [28], and Closs [30] have postulated a single-step mechanism (g), in which the organometallic reagent is assigned the role of a “carbenecomplex”.
.~
[25] G. Witfigand F. Wingler, Liebigs Ann. Chem. 656, 18 (1962).
1261 H . Hoberg, Liebigs Ann. Chem. 656, 15 (1962).
[27] A . Ledwifh and L. Phillipps, J. chem. Soc. (London) 1962,
3796.
1281 H . E. Simmons and R. D . Smith, J. Amer. chem. SOC.80,
5323 (1958); H. E. Simmons and E. P. Blanchard, ibid. 86, 1337,
1347 (1964).
[29] G. Witfig and K. Schwarzenbach, Liebigs Ann. Chem. 650,
1 (1961).
[30] G. L. CIoss and L. E. Closs, Angew. Chem. 74, 431 (1962);
Angew. Chem. internat. Edit. I, 334 (1962).
Angew. Chem. internat. Edit. / VoI. 4 (1965)
CHzN2/CuC1
/ No. I
4. Methyl and Methylene Derivatives
Attempts to prepare methylene by a-elimination from
methyl alkyl ethers and butyl-lithium [34], as well as
from methyl chloride and phenylsodium [35], or pentylsodium [36] led to homologous hydrocarbons. These
are substitution products in accordance with Scheme 1.
The organometallic compound functions as a base
(B:@)and, at the same time, as a nucleophile (ZO) [see
Scheme 31.
[31] G. Witfig and F. Wingler, Chem. Ber. 97, 2146 (1964); F.
Wingler, Ph. D. Thesis, Universitat Heidelberg, 1964.
1321 W . V .E. Doering and W . Rofh, Tetrahedron 19, 715 (1963);
E. Vogel, W . Wiedemann, H. Kiefer and W . F. Harrison, Tetrahedron Letters 1963, 673. Cf. also the corresponding reactions
with aromatic compounds: E. Miiller et al., Liebigs Ann. Chem.
661, 38 (1963); 662, 38 (1963); Tetrahedron Letters 1963, 1047,
1501.
1331 W. Kirmse and R. B. Hager, unpublished work.
[34] K . Ziegler and H . G. GeNerf, Liebigs Ann. Chem. 567, 185
(1950).
[35] L. Friedman and H. G. Berger, J. Amer. chem. SOC. 82, 5758
(1 960).
1361 W . Kirmse and H . J . Schladetsch, unpublished work; cf.
Angew. Chem. 73, 540 (1961).
3
+
CH3X
R-M
a-Elmin.
RH
+
MCHzX
lSN2
/+R;,<4;-hI
*
R-CH,
CH,X
R-CH,M
a-Elimin.
CH,X
-
/MCH,X/:CH~
X = C1, 0 - A l k y l
R = n-C4HS, n-C5HlI,
R-CH2-CH2M
i
C6H5
sterically more hindered product is favored in these
reactions [40,42]. The olefin again behaves as a nucleophile [43]. Consequently, benzene is attacked with diffiR - C H ~ - C H ~ culty [41,44], while phenoxides 1451, pyrrole, and indole
[46] react much more readily. Methylene chloride and
triphenylphosphine form an ylide [47,48]. The reaction
R-CH,-CH,-CH~ of methylene chloride with excess alkyl-lithium leads to
alkylcarbenes and their secondary products [49].
--+ :CH,
Scheme 3. The preparation of homologous hydrocarbons from organometallic compounds and methyl ethers or methyl halides.
Only the higher homologs (R-CH2-CH3r etc.) in Scheme
can definitely be regarded as products of cr-elimination, since
the R-CH3 h'drocarbons
mainly from
Wurtz reactions (SN2)' It has been
to show that *he
analogous reaction of methylene bromide or chlorobromomethane with methyl-lithium [371 probably involves pure aeIimination, induced by halogen-metal interchange (see
Scheme 4). (Earlier experiments with phenyl-lithium [38] did
not lead t o any definite conclusion regarding the mechanism.)
BrCH,Y
+
CH3Li
-j
CH3Br
+
Y-CHz-Li ---
-
Y ,/<kH,Li
+cH3L11
CH,-CH,-L~
Y = B r , C1
:CHz
BrCHzY
CH3-cHZBr
Closs has recently succeeded in preparing chlorodiazomethane in solution and decomposing it [501. Chlorocarbene Prepared in this way was substantially more reactive than that
obtained by a-elimination. This observation lead CIoss t o
believe,". . .that in them-elimination a truly free carbene might
be bypassed" [SO],The situation cannot be more accurately
described at present, since the different energy contents of
intermediates produced in different ways must also be considered. The recent successful preparation of dichloromethyllithium in solution at low temperature [51] (detection by reaction with carbonyl compounds) promises
elucidation
of this problem.
The a-elimination of hydrogen chloride from a-chloroethers in accordance with Equation (k) requires the use
of alkyl-lithium compounds (t-butyl-lithium on occasions, to suppress SN2 reactions) [52,53], while potassium t-butoxide is sufficient in the case of the corresponding sulfur [54] and selenium compounds [55]. The
/+ LiCH,Y/:CH,
R-A-CH&l
CH3-CHz-CHz-Li + CH3-CH2-CH2Br
+
0
B:
+
BH
+
0
R-A-CHC1 --+ R-A-CH:
A = 0, S, S e
(k)
;
Scheme 4. The preparation of homologous alkyl halides from methylene
halides and organometallic compounds.
Olefins can be converted into cyclopropane derivatives
in moderate yields by any of the a-elimination reactions
discussed above [10,35,39]. This fact can be no longer
regarded as evidence of the existence of free methylene
(cf. Section 3). The characteristic insertion into C-H
bonds, such as is observed with methylene from diazomethane or ketene, does not occur in a-eliminations.
The existence of free methylene is very improbable in
these reactions.
Similar problems arise in the chemistry of chlorocarbene. As expected, the abstraction of a proton from
methylene chloride is easier than from methyl chloride
[see Equation (i)]. Alkyl-lithium compounds are suitable
bases [40], poor yields being obtained with potassium
t-butoxide [41].
CHzCli
+ R-Li
-+
R-H
+ LiCHC12
----)
:CHC1
+ LiCl
(i)
Closs has successfully carried out stereospecific additions
to olefins, using a methylene chloride/base system.
Whenever isomerism is possible, the formation of the
.
[37] W. Kirmse and B.v. Wedel, Liebigs Ann. Chem. 666, 1
(1963).
[38] G. Wiftigand H. Witt, Ber. dtsch. chem. Ges. 74, 1474 (1941).
[39] W. T. Miller j r . and C. S . Y. Kim, J. Amer. chem. SOC.81,
5008 (1959).
[40] G . L . Cioss and L. E. Closs, J. Amer. chem. SOC. 81, 4996
(1959); 82, 5723 (1960).
[41] M . E. Volpin, D . N . Kursuvov and V. G. Dulovu,Tetrahedron
8, 33 (1960).
4
smooth addition of the intermediates to olefins was
interpreted as a carbene reaction. The objections to this
view have been discussed already.
In the reaction of trithio-orthoformates (IZ) with
potassium amide in liquid ammonia, the presence of the
carbanion intermediate (12) is indicated by a green color.
The intermediate can be trapped by alkylation with
methyl iodide [56]. The main product, tetraethylthio[42] G . L. Closs, R. A . Moss, and J. J. Coyle, J. Amer. chem.
SOC.84, 4985 (1962).
[43] G. L. Closs and G. M . Schwurtz, J. Amer. chem. SOC.82,
5729 (1960).
1441 G. L. Closs and L . E. Closs, Tetrahedron Letters 1960, No.
10, 38.
[45] G. L. CIoss and L. E . Closs, J. Amer. chem. SOC.83, 599
(,1961).
[46] G. L . CIoss and G. M . Schwurtz, J. org. Chemistry 26, 2609
(1961).
1471 G. Wittig and M . Schlosser, Angew. Chem. 72, 324 (1960).
[48] D. Seyferth, S. 0. Grim, and T. 0. Read, J. Amer. chem.
SOC.82, 1510 (1960); 83, 1617 (1961).
1491 G. L . Closs, J. Amer. chem. SOC.84, 809 (1962).
[SO] G . L . Closs and J. J . Coyle, J. Amer. chem. SOC.84, 4350
(1962).
1511 G. Kobrich, K . Flory, and W . Drischel, Angew. Chem. 76,
536 (1964); Angew. Chem. internat. Edit. 3, 513 (1964).
1521 U. Schollkopf and A . Lerch, Angew. Chem. 73, 27 (1961).
[53] U. Schollkopf, A. Lerch, and W . Pitteroff, Tetrahedron
Letters 1962, 241 ; Chem. Ber. 97, 636 (1964).
[541 U. Schollkopf and G. J. Lehmunn, Tetrahedron Letters
1962, 165; U.Schollkopf, G . J . Le.'zmunn,J. Puust, and H . D. Hartl,
Chem. Ber. 97, 1527 (1964).
[55] U. Schoiikopf and H . Kuppers, Tetrahedron Letters 1963,
105.
[56] J . Hine, R. P. Buyer, and G. G. Hummer, J. Amer. chem.
SOC.84, 1751 (1962).
Angew. Chem. internut. Edit. 1 Vol. 4 (1965)
1 NO. I
ethylene (13) is thought to result from the combination
of diethylthiocarbene with (12). However, there is no
independent evidence for the occurrence of the carbene.
5. Benzyl and Vinyl Derivatives
The reaction of benzyl halides or benzyl ethers with
bases has only occasionally led to intermediates which
can add onto olefins.
Diphenylmethyl bromide reacts with potassium amide in the
presence of 1-octene t o give 1.1-diphenyl-2-hexylcyclopropane in 7 ”/, yield (571. A 14 % yield of 7-phenylnorcarane
+ C,H,Li
C,H,-CH,X
+
,
Li
C6H5-Cy
X
(CsHsCH:)
X = C1, OC,H5
(14) has been obtained f r o m benzyl chloride, n-butyl-lithium,
a n d cyclohexane [SS], while the corresponding reaction with
benzyl phenyl ether gave only 2 % of 7-phenylnorcarane [59].
The action of bases on benzyl halides generally yields
stilbene derivatives [*]. Good preparative results are
obtained with alkali metal amides in liquid ammonia
[63]. The first plausible mechanism was suggested by
Hahn [64] and Kleucker [65]. According to this the
0
:C-#H + B:
X 116)
-+
:C=C:
+
BH
+
X0
~~
[57] A . Y. Garner, Ph. D. Thesis Abstracts 17, 224 (1957).
[58] G. L. Closs and L. E. Closs, Tetrahedron Letters 1960, No.
24, 26.
[59] U . Schollkopf and M . Eisert, Liebigs Ann. Chem. 664, 76
(1963).
[*I Early examples: [60-621.
[60] C. A. Bischof, Ber. dtsch. chem. Ges. 21, 2071 (1888).
[61] P. Walden and A. Kernbaum, Ber. dtsch. chem. Ges. 23,
1958 (1890).
[62] J. Thiele and A . Wanscheidl, Liebigs Ann. Chem. 376, 278
(1 9 10).
[63] Ch. R. Hauser et al., J . Amer. chern. SOC. 78, 1635. 4942
(1956); 79, 3142 (1957).
[641 G. Hahn, Ber. dtsch. chem. Ges. 62, 2485 (1929).
[65] E. Kleucker, Ber. dtsch. chem. Ges. 62, 2587 (1929).
Angew. Chem. internat. Edit.
1 Vol. 4 (1965)
No. I
carbanion intermediate (IS) is thought to be alkylated
by more benzyl halide, followed by elimination of
hydrogen halide from the “dimeric halide” (16) [see
Equation (l)].
Halides of type (16) have been isolated in good yields
[63], and recently even from the reaction of benzyl
chloride with butyl-lithium [65a]. The substitution
(1.5) + (16) in the case of a-phenylethyl chloride
proceeds with inversion of configuration [66]. H-D
exchange in the case of 9-bromofluorene is fast compared to the elimination of hydrogen bromide; the rate
of formation of difluorenylidene is proportional to the
square of the 9-bromofluorene concentration [67].
These results confirm the postulated course of Reaction (l), and largely rule out the possibility of a
carbene intermediate.
However, there are also observations which are less
compatible with Scheme 1 . Benzyl chloride, 4-nitrobenzyl chloride [68], and 4-nitrobenzyldimethylsulfonium salts [69] obey a first-order rate-law with respect
to the concentration of )CHX and of base. Only in the
last case has preliminary H -D exchange definitely been
observed [70]. No satisfactory interpretation has yet been
found for these reactions,especially with regard to the isotope effect [68,69], and the substituent effect [69,70].
l-Halogeno-2,2-diarylethylenesare converted into diarylacetylenes by alkali metal alkoxides [71-731, hydroxides [74], or amides [75], as well as by organolithium compounds [76]. The corresponding reaction of
1, I-dihalogeno-2,2-diarylethylenes
proceeds with lithium
[77], alkyl-lithium compounds [76], phenyl-lithium or
triphenylmethylpotassium 1781.
On the basis of careful investigations on the H-D exchange [79], the stereospecificity of the rearrangement
[80,81], and the effect of substituents [82], it seems
[65a] D. F. Hoegand D.I. Lusk, J. Arner.chem. SOC.86,928 (1964).
1661 W . R. Brasen, S. W . Kantor, P. S. Skell, and Ch. R. Hauser,
J. Amer. chem. SOC.79, 397 (1957).
[67] D. Bethe//, J. chern. SOC.(London) 1963, 666.
[68] S. B. Hanna, Y . Iskander, and Y . Rind, J. chem. SOC.(London) 1961, 217.
[69] C. G. Swain and E. R.Thornton, J. Amer. chem. SOC.83,
4033 (1961); J. org. Chemistry 26, 4808 (1961).
[70] I. Rothberg and E. R.Thornfon, J. Amer. chern. SOC.85.
1704 (1963); 86, 3296, 3302 (1964).
[71] P. Fritsche, Liebigs Ann. Chem. 279, 319 (1894).
[72] W . P. Buttenberg, Liebigs Ann. Chem. 279, 324 (1894).
[73] H . WiecheN, Liebigs Ann. Chem. 279, 337 (1894).
[74J M . M.Tifeneau, C . R. hebd. Seances Acad. Sci. 135, 1374
(1902).
[75] G. H. Coleman and R. D. Maxwell, J. Amer. chem. SOC.56,
132 (1934); 58, 2310 (1936).
[76] D. Y . Curtin and E. W. Flynn, J. Amer. chem. SOC.81, 4714
(1959); D.Y. Curtin and W. H. Richardson, J . Amer. chem. SOC.
81, 4719 (1959).
1771 W . Srhlenk and E. Bergmann, Liebigs Ann. Chem. 463,
71 (1928).
[78] F. B. Kirby, W. G. Kofron, and Ch. R. Houser, J . org.
Chemistry 28, 2 I76 ( 1 963).
[79] J. G. Pritchord and A. A. Bothner-By, J . physic. Chem. 64,
1271 (1960).
[SO] A . A. Bothner-By, 1. Amer. chern. SOC.77, 3293 (1955).
[81] D. Y. Curtin, E. W. Flynn, and R. F. Nystrom, Chem. and
Ind. 1957, 1453; J. Amer. chem. SOC.80, 4599 (1958).
[82] W. M . Jones and R . Damico, J. Amer. chem. SOC.85, 2273
(1963).
5
probable that the rearrangement takes place at a vinyl
anion stage (17), and that alkylidenecarbenes do not
take part in it [83].
X = H, Halogen
Y = Halogen
More recent work has shown [89,90]that cyclopropanes
are also formed from alkyl chlorides with hydrogen in
the (3-position. Whereas Whitmore had postulated a yelimination, experiments with D-labelled compounds
have shown that a-elimination occurs [89-921. Cyclopropane derivatives obtained from alkyl chlorides
doubly deuterated in the a-position contained only one
deuterium atom, e.g. (21) [89,90].
H3C\
l-Halogeno-2,2-dialkylethylenes(18) do not rearrange
to give acetylenes. On treatment with potassium t-butoxide [84], or with phenyl-lithium [85], substitution
and olefin addition have been observed. 1,l-Dibromo2-methylprop-1-ene behaves similarly in the halogenmetal exchange with methyl-lithium [85a]. These reactions do not permit any distinction between carbene
and carbanion intermediates.
(18)
6. Halogeno- and Dihalogeno-alkanes
The reactions discussed in Sections 4 and 5 were
ambiguous in the sense indicated in Scheme 1. On the
other hand, a-eliminations on halogeno- and dihalogenoalkanes permit a clear differentiation between the intermediates. Alkyl- and dialkylcarbenes are stabilized by
intramolecular insertion to form olefins (19) and cyclopropanes (20) [9]. Since such reactions are unknown
with carbanions, they provide a good indication of the
appearance of carbenes, particularly in the quantitative
comparison of elimination reactions with the decomposition of diazo compounds.
CH-CD2C1
H3C'
H3C,
H
(21J, 35-37%
(22). 44-52%
T h e olefins (22) a n d (23) produced i n t h e s a m e reaction, o n
t h e other hand, consisted of a mixture of singly and doubly
deuterated compounds, i. e. p-elimination competes with aelimination t o a varying extent. This competition depends on
t h e strength of t h e base. Hardly a n y a-elimination is observed
with alkyl-lithium compounds [891 or with sodamide [go] provided t h a t p-hydrogen is present. In the case of neopentyl
chloride, however, cr-elimination occurs even with sodamide
t911.
The activation energy of the a-elimination must therefore decrease much more rapidly with increasing bazestrength than does that of (3-elimination. This seems
plausible from a consideration of the transition states.
In the case of cc-elimination, the energy required to
remove hydrogen is compensated only by the energy
of the new bond to the attacking base. This bond will
be relatively strong, even in the transition state (24).
This is further substantiated by the small isotope effect
in the a-elimination [89].In the transition state of the
(3-elimination (25), on the other hand, not only is a
new B-H bond formed, but also a new C-C bond. The
B-H bond is therefore weaker (the reaction shows a
greater isotope effect [93]) and makes a smaller contribution to the energy balance.
(24)
a) or-Elimination of Hydrogen Halide
Wititmore has obtained cyclopropane derivatives by
reaction of neopentyl chloride [86,87] or neophyl chloride [*I [88] with sodium or alkylsodium compounds.
[83] G . Kobrich, Angew. Chem., in the press.
[84] M.Tanabe and R . A. Walsh, J. Amer. chem. SOC.85, 3522
(1963).
[ 8 5 ] H. Giinther and A. A. Bothner-By, Chem. Ber. 96, 3112
(1963).
[85a] H . D . Hartzler, J. Amer. chem. SOC.86, 526 (1964).
1861 F. C. Whitmore, A. H. Popkin, H. I. Bernstein, and J . P. Wilkins, J. Amer. chem. SOC.63, 124 (1941).
[87] F. C. Whitmore and H . D . Zook, J. Amer. chem. SOC.64,
1783 (1942).
[*I "Neophyl" is a contraction denoting the 2,2-dimethyl-2phenylethyl radical.
1881 F. C . Whitmore, C. A. Weisgerber, and A . C. Shabica j r . , J.
Amer. chem. SOC.65, 1469 (1943).
6
031, 11-21%
(25)
Easier cleavage of the C-X bond also favors p-elimination. The incidence of a-elimination decreases on passing from alkyl chlorides to alkyI bromides, and thence
to alkyl iodides (using the same base) [90]. Only pelimination occurs in the case of secondary alkyl chlorides, even with the strongest bases [89,90].
Experiments with deuterium-labelled compounds have
shown that the cyclopropane derivatives prepared from
primary alkyl halides result from an intramolecular
~-
[89] W . Kirrnse and W. Y. E. Doering, Tetrahedron 11,266 (1960).
[90]L. Friedman and J. G. Berger, J. Amer. chem. SOC.83, 492
(1961).
[91] L . Friedman and J . G. Berger, J. Amer. chem. SOC.83, 500
(1961).
[92] P . S. SkeN and A. P. Krapcho, J. Amer. chem. SOC.83, 754
(1961).
[93] W . H . Saunders j r . , and D . H . Edison, J. Amer. chem. SOC.
83, 754 (1961).
Angew. Chem. internat. Edit.
/
Vol. 4 (1965) No. I
carbene reaction (cf. [94,95]). In agreement with this,
the nature of the halogen atom has no effect on the
composition of the a-elimination products [90,91]. The
formation of the carbene stage is further corroborated
by good agreement with the results of the decomposition
of the diazo compounds.
Whether the carbenes are formed via an organometallic
(carbanion) intermediate or in a concerted mechanism
is less certain. The reaction of 1,3-dichloropropane with
phenylsodium [96] points to a two-step mechanism. A
mixture of 4 0 % cyclopropyl chloride and 6 0 % allyl
chloride is obtained in moderate yield. The photochemical or catalytic decomposition of 3-chlorodiazopropane, on the other hand, yields more than 95 % of
allyl chloride (cyclopropyl chloride occurs at most in
traces) [96]. It seems likely that the high yield of cyclopropyl chloride is due to a 1,3-elimination from the
organometallic intermediate (26) (cf. Section 6b). The
question as to whether allyl chloride results from cr- or
from ?-elimination must remain unanswered.
I 6-Ehmin.
C1CH2-CH=CH2
As long as corresponding rearrangements are unknown
in the case of carbanions, hydrogen shift to form the 1chloroalkene must be regarded as an alkylchlorocarbene
reaction [97].
b) a-Elimination of Halogen
Competition between a- and p-elimination must be
ruled out in the case of the removal of halogen from
gem-dihalides, since halogen-free products can be formed only by a-elimination. The cr-elimination of halogen
can be effected with alkyl-lithium compounds [see
Equation (m)] or, less smoothly, with metals [see
Equation (n)] [99]. In Reaction (m), at least one of the
halogen atoms must be bromine or iodine; the first step,
is halogen-metal exchange. In Reaction (n), both halogen
atoms are removed as anions, the necessary electrons
being supplied by the metal
,!I,'
1
.i ,.','
+& C1CH2-CH,-CHNz
C1-CH2-CH2-CH:
Carbene formation can again be detected by typical
The a-hydrogen atom in 131-dich10roa1kanes is more
intramolecular insertion reactions [see Equations (o)
readily removed than in alkyl chlorides, even with alkyland (p)l. The ratios of the product yields largely
lithium compounds [97]. Whereas the reaction of the
intermediate with more alkyl-lithium predominates in
c H3
+ (CH,),C=CH-CH,
the case of 1,l-dichloroethane [98], higher 1,l-chloro- (CH3)3C-CH12 + CH&i
alkanes yield considerable quantities of 1-chloroalkenes.
(28/, 9 5 - 9 7 1
1291, 5-9'10
These 1-chloroalkenes may be regarded as a-elimination
products for the following reasons : 1-chloroalkenes are
(CHB)BC-CHN~
128), 9 2 % +
( 2 9 ) , 7%
pcH
(CH3)&-CBr2-CH3
obtained in the same cis/truns ratio (approximately 9 : 1)
from alkyl-lithium compounds and chloroform as from
1,l-dichloroalkanes. A common intermediate (27), in
accordance with Scheme 5 , is the best explanation of
this fact. P-Elimination from 1,I-dichloroalkanes takes
place with alkoxides and gives an entirely different
cisltruns ratio (approximately 1 :1).
R-CH2Li
+
R-CH2-CHCl2
CHC13
1
LiCC13
i
:cc1,
I
Y
-CH,Li
+
R'Li
I
J
R-CH2-CC1,Li
4
R-CH2-C-CI
Y
Scheme 5. Syntheses of I-chloroalkenes.
R-CH=CHCI
[94] W. v. E. Doering and W . Kirmse, Tetrahedron 11, 272 (1960).
[95] H . C. Richey j r . and E. A. Hill, J. org. Chemistry 29: 421
(1964).
1961 W. Kirmse and H . J. Schladetsch, unpublished work.
[97] W . Kirrnse and B. G . v . Bulow, Chem. Ber. 96, 3316 (1963).
[98] W . Kirmse and B. G . v . Bulow, Chem. Ber. 96, 3323 (1963).
Angew. Chem. internat. Edit. / Vol. 4 (1965) 1 No. I
+ CH3Li
+
H:FVCH3
+ (CH3)3C-CH=CH,
correspond to those obtained in the decomposition of
diazo compounds [99].
2,2-Dibromobutane and 2-bromo-2-chlorobutane gave
identical butene mixtures with different alkyl-lithium
compounds. The eliminating agent and the departing
halogen atom cannot therefore participate in the
product-determining step. These results can hardly be
explained otherwise than by a carbene intermediate.
Organometallic intermediates were also detected in Reaction
(m) by intramolecular capture [loo]. When 1,1,3-trihalogenoalkanes are reacted with methyl-lithium, bromine, or iodine in
the I-position is exchanged for lithium whereas unactivated
halogen in the 3-position fails to react. The organometallic
intermediate (32) can undergo 1,3-eIimination of lithium
halide to yield a cyclopropyl halide. If the reaction proceeds
to the carbene stage (or if the carbene is formed in a concerted
mechanism), allyl halide (by @-hydrogen shift) and cyclopropyl halide (by insertion into the y-C-H or into the CI-C
bond), are to be expected as end-products. The source of the
[99] W. Kirmse and B.v. Wedel, Liebigs Ann. Chem. 666, 1 (1963).
7
cyclopropyl halides can be ascertainedby using different halogen atoms. On ring closure of (32), halogen (Y)in the l-position is retained, while cyclopropyl halide (33) formed from
the carbene contains halogen (X) in the 3-position.
L
Lil
Skattebiil has reacted 1,I-dibromo-2-(buten-3-yl)-cyclopropane (37) with methyl-lithium, and isolated the
spirane (39) in addition to the allene (38) [loll. The
ratio of the quantities of the compounds (38) and (39)
was found to depend on temperature. Unfortunately,
the mechanism of the conversion of 1,l-dihalogenocycIopropanes into allenes is not yet clear [102-1051. Tt
cannot therefore be stated which intermediate of this
interesting reaction is intercepted by the olefin addition.
J
The first of these alternatives is unambiguously supported by
experiment. Ally1 halides (or their secondary products) have
not been found in any case, whereas cyclopropyl halides have
been obtained in good yields. 1,l-Dibromo-3-chloropropane
gave only cyclopropyl bromide, and 1,3-dibromo-l-chloropropane gave only cyclopropyl chloride. It is thus shown that
ring closure proceeds from the intermediate (32) with retention of the halogen atom in the 1-position.
Substitution of iodine for Y in (32) is observed in the presence
of Iithium iodide (the starting materiat and the end-product
d o notreact with lithium iodide) [IOO]. Thus the high reactivity
of halogen in the a-halogenoalkyl derivatives of zinc, aluminum and mercury (Section 3) is also found in the corresponding lithium compounds.
The competition between hydrogen shift and olefin
addition has been investigated with 4-pentenylcarbene
(40) [106]. The reaction products are 1,s-hexadiene
(41) and bicyclo[l.3.0]hexane (42). The decomposition
of 6-diazohex- 1-ene proceeded only when catalysed by
copper, and gave compounds (41) and (42) in yields of
55 % and 45 %, respectively. The reaction of 4-pentenyllithium with methylene chloride leads, via the organometallic intermediate (43), to the same products (41)
and (42) in yields of 87 % and 13 %, respectively.
Q
CHzClZ
c) Addition of Intermediate Products of the
Elimination to Olefins
0/+
CH-Li
I
As has been seen in the foregoing sections, both carbenes and organometallic (carbanion) intermediates can
be detected in the cr-elimination reactions of halogenoand dihalogeno-alkanes. The question now arises: which
one of these intermediates is responsible for the addition
to olefins? This problem is best examined once more by
means of competitive intramolecular reactions.
The 1,3-elirnination of lithium halide involves organometallic intermediates of type (32). Addition to olefins,
which competes with this 1,3-elimination, would have
to be a direct addition of the organometallic intermediate. However, no such competition is observed
in the reaction of 5,7,7-tribromohept-l-ene
(34) with
methyl-lithium [loo]. Reductive dehalogenation of the
products yields n-butylcyclopropane (35), but no norcarane (36).
[IOO] W . Kirmse and B. Y . Wedel, Angew. Chem. 75, 672 (1963);
Liebigs Ann. Chem., in the press.
8
c1
(43)
The experiments described above, although as yet incomplete, give n o indication of a direct reaction between
ol-halogenoalkyl-lithium and olefins. The experiments
relate only to compounds of type R-CHX-Li and
give no information concerning organometallic intermediates with two or three a-halogen atoms.
[loll L. Skattebol, Chem. and Ind. 1962, 2146.
[lo21 W . V . E. Doering and P . LaFlamme, Tetrahedron 2, 7 5
(1958).
11031 W. R. Moore and H . R. Ward, J . org. Chemistry 25, 2073
(1960); 27, 4179 (1962).
11041 L . Skattebol, Tetrahedron Letters 1961, 167.
[lo51 T. J. Logan, Tetrahedron Letters 1961, 173.
[I061 W. Kirmse and D . Grassmann, unpublished work.
Angew. Chem. internat. Edit. I Vol. 4 (1965)
No. I
7. Quaternary Ammonium Salts and Tertiary
Sulfonium Salts
The Hofmann degradation of quaternary ammonium hydroxides can generally be described as an
E2 elimination with simultaneous removal of P-hydrogen
and tertiary amine [107]. This requires that the participating groups should have a planar tvans-configuration, as shcwn in (44). The possibility of a-elimination
ammonium salts (47) with organolithium compounds
generally follows this route [seeEquation(s)] [l 11,114 to
1 161. Remarks made in Section 6 naturally apply to the
transition from the p- to the “a,~”-elimination with
increasing base-strength.
+ DCHzN(CH3)z
(44J
would be expected to arise whenever this trans-configuration cannot be achieved. Nevertheless, careful investigations on trans - (2 - pheny1cyclohexyl)trimethylammonium hydroxide [IOS-1111 have shown only pure
?-elimination. The reaction probably proceeds by an
ElcB mechanism [see Equation (q)] promoted by the
phenyl group.
An ylide (a’$)-mechanism has been found only with
quaternary ammonium hydroxides in which the phydrogen atom is not activated, but is strongly sterically
hindered [112,113]. In such cases, an a-hydrogen atom
is first removed, and the resulting ylide (45) gives rise
to the Hofmann degradation [see Equation (r)] by a
cyclic proton shift.
cc’,p-Elimination can also be induced by halogen-metal
exchange on a-halogenated quaternary ammonium salts,
e.g. (48) [Equation (t)] [114].
Owing to the smooth intramolecular elimination according to Equation (r), no @-hydrogenatom need be
present in order to obtain carbenes from quaternary
ammonium salts. Treatment of tetramethylammonium
bromide with phenylsodium/phenyl-lithium in the
presence of cyclohexene gives norcarane in 10 % yield,
in accordance with Reaction (u) [117]. As in the experiments discussed in Section 4, the formation of
methylene in this reaction is questionable [*I.
+ NaBr
(451
Trimethyl-(2-t-butyl- 3,3 - dimethyl- [2-D] - buty1)ammonium hydroxide (46) decomposes to the extent of
75 % in accordance with Equation (r) [113].
The situation is clearer in the case of the tertiary sulfonium salts; p- and a‘,p-eliminations can again be detected according to the strength of the base [120,121].
However, alkyldiphenylsulfonium salts, e.g. (49), open
up a route to alkylcarbenes, which can undergo intramolecular insertion, e.g. according to Equation (v)
[I 221.
/CH3
S-CHz-CH,
C6d
CH3
C6H5s@
(49)
Whereas the ylide mechanism (r) is the exception with
ammonium hydroxides, the reaction of quaternary ammonium salts, e. g . [2-D]-cyclohexylmethyltrimethyl[I071 A . C. Cope and E. R. Trurnbull in: Organic Reactions.
Wiley, New York 1960, Vol. XI, p. 317.
[I081 R. T. Arnold and P. N . Richardson, J. Amer. chem. SOC.
76, 3649 (1954).
[I091 J. Weinstock and F. G. Bordwell, J. Amer. chem. SOC. 77,
6706 (1955).
[I101 A. C. Cope, G. A. Berchtold, and D. L . Ross, J. Amer.
chem. SOC.83, 3859 (1961).
[ I l l ] G. Ayrey, E. Buncel, and R. N. Bourns, Proc. chem. SOC.
(London) 1961, 458.
[112] A. C . Cope and D . L. Ross, J. Amer. chem. SOC.83, 3854
(1961).
[I 131 A. C. Cope and A. S. Mehta, J. Amer. chem. SOC.85, 1949
(1 963).
Angew. Chem. internat. Edit. / Vol. 4 (1965)
1 No. 1
(C&)Pa
( V)
H3C,
,C=CH, + (C&,),S
H3C
\ICH3
+
..
- ._
[ I 141 G. Wittigand R. Polster, Liebigs Ann. Chem. 599, 13(1956).
[ I 151 A. C. Cope, N . A. LeBel, P. T. Moore, and W. R . Moore,
J. Amer. chem. SOC. 83, 3861 (1961).
[116] F. Weygand, H. Daniel, and H. Simon, Liebigs Ann. Chem.
654, 11 1 (1962).
[I171 V. Franzen and G. Wirrig, Angew. Chem. 72, 417 (1960).
[*I See [118,119] for the formation of polymethylene from trimethylammonium methylide.
[I181 G. Wittigand R. Polster, Liebigs Ann. Chem. 599, 1 (1956).
[ I 191 F. Weygand, H. Daniel, and A. Schroll, Chem. Ber. 97, 1217
(1964).
[I201 V . Franzen and C. Merz, Chem. Ber. 93, 2819 (1960).
11211 V. Franzen and H . J . Schmidt, Chem. Ber. 94,2937 (1961).
11221 V. Franzen, H . J . Schmidt, and C. Merz, Chem. Ber. 94.
2942 (1961).
9
On the other hand, the addition of methylene to poiar
double bonds 1123,1241, which can formally be achieved
with trimethylsulfonium salts, as well as the formation
of ethylene from these compounds [124], must be regarded as a sulfonium ylide reaction. This is probably
also true of the similar reactions (w) of diphenylbenzylsulfonium salts (50) [ 1251.
8. Future Prospects
The results which have been referred to illustrate the
importance of cr-elimination in numerous substitution
and addition reactions. Investigation of these reactions
[1231 E. J. Corey and M . Chaykovsky, J. Amer. chem. SOC.84,
3782 (1962); Tetrahedron Letters 1963, 169.
11241 V. Franzen and H . E. Driessen, Tetrahedron Letters 1962,
661 ; Chem. Ber. 96, 1881 (1963).
“251 W. A . Johnson, V. J. Hruby, and J. L. Willimns, J. Amer.
chem. SOC.86, 918 (1964).
is made interesting, yet at the same time difficult, by
the various intermediates with similar chemical properties. According to present-day knowledge, both of
substituted alkylmetal compounds (or carbanions) and
carbenes behave as electrophiles. The conversion aolefins into cyclopropane derivatives, which originally
did much for the development of the carbene concept,
is becoming increasingly recognized as a reaction of
organometallic (carbanion) intermediates. Olefin addition has become completely useless as a means of
detecting carbenes. At present the only reliable criterion
for the occurrence of carbenes is the (intramolecular)
insertion into C-H bonds, which has permitted detection of alkyl- and dialkylcarbenes. It is not surprising
that these are the very compounds in which a-elimination proceeds to the formation of carbene intermediates.
With the transition from primary to secondary and
tertiary carbon, the tendency towards SN1 reactions
steadily increases, while the SN2 mechanism is suppressed. Alkyl groups should therefore also favor the
transition into the carbene, which is anatogous to the
SN1 mechanism, in the case of a-substituted alkylmetal
compounds (carbanions).
Received, J u l y 13th, 1964
[A 41 11190 IE]
G e r m a n version: Angew. Chem. 77, 1 (1965)
Translated by Express Translation Service, London
Cyclopropenylium Compounds and Cyclopropenones
BY DR. A. W. KREBS
ORGANISCH-CHEMISCHES INSTITUT DER UNIVERSITAT HEIDELBERG (GERMANY)
The predictions of Hiickel’s rule gave the incentive to numerous investigations which led
ultimately to a new definition of aromatic character, and which have added greatly to our
knowledge o j the properties and reactivity of aromatic compounds. lncfeasing use was
made of physical measurements as criteria for the aromaticity of a compound. The first
member ( n = 0 )in the series of Hiickel’s (4n f 2) x systems is the cyclopropenylium cation.
The predictions regarding the stability of’ this system have been confirmed at least qualitatively by the synthesis of cyclopropenylium salts. Properties and reactions of cyclopropenylium compounds and cyclopropenones were investigated.
I. Introduction
Ever since the discovery of benzene by Faraday in 1825,
the theory of the cyclic unsaturated systems has been
of interest to organic chemists [I, 21. The formulation
of the (4n + 2)x-rule by Hiickel[3] in 1931 was a
milestone in this development. The (4n + 2)x-rule can
be expressed as follows:
[ I ] Concerning the historical development of the theory of
aromatic compounds, which is not discussed in detail in the
present paper, see e.g. C . K . Ingold: Structure and Mechanism
in Organic Chemistry. Cornell University Press, Ithaca 1953,
pp. 156-196 and [Z].
[2] W. Y . E. Doering and H . Krauch, Angew. Chem. 68,661 (1956).
[3j E. Hiickel, Z . Physik 70,204 (1931); 76, 628 (1932).
10
Pfanar monocyclic systems with trigonally hybridized
atoms containing (4n + 2) x-electrons possess a characteristic electronic stability.
Not only did this rule provide an explanation on the
basis of quantum theory of the special position of benzene, which was already known, but it also allowed predictions to be made regarding the stability of systems
which had not yet been synthesized.
Ths validity of Huckel’s rule for n = 1 was verified by the
preparation of the tropylium cation [2,4-5 b] and by
the agreement between the observed properties and
[4] P. L . Puuson, Chem. Reviews 55, 9 (1955).
[5a] T . Nozoe in D . Ginsburg: Non-Benzenoid Aromatic Compounds. Interscience, New York 1959, p. 339.
[5b] T. Nozoe: Progress in Organic Chemistry. Butterworths,
London 1961, Voi. 5, p. 132.
Angew. Chen?. internut. Edit.
Vol. 4 (196.5) Nu. 1
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