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a-Silyl Diorganozinc CompoundsЧA New Class of Useful Zinc Reagents.

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35 mol% LiBr were allowed to react under 60 bar CO at 120 "C for 12h. The volatile
components were removed under high vacuum, and the residue was taken up in a
saturated aqueous solution of NaHCO, and then washed with chloroform and ethyl
acetate. The aqueous phase was adjusted to pH 2 with phosphoric acid and extracted with ethyl acetate The organic phases were then combined and dried over
magnesium sulphate, and the solvent was removed under vacuum. The product was
recrystallized from a suitable solvent mixture. All isolated compounds were characterized by means of NMR and IR spectroscopy and mass spectrometry. The purity
of the products, as measured by HPLC, was >98%
Received: November 2, 1996 (Z9727IEl
German version' Angew. Chem. 1997, 109, 1534-1536
Keywords: amino acids * carbonylations . homogeneous catalysis . multicomponent reactions * palladium
[l] a) M. Baviere, B. Durif-Varambon (Institute Fragais du Petrole), DE3514329, 1985 [Chem. Abstr. 1985, 103, 8767~1;b) C. Theis, W Latz (Hiils
AG), DE-3643204,1986 [Chem Abstr. 1988,109,170928vj;c) J. J. Lin (Texaco
Inc.), US-4720573, 1988 [Chem Abstr. 1988, 109, 3824Okl.
121 a ) H. Kessler, Angew. Chem. 1993,105,572; Angeiv. Chem. Int. Ed. Engl. 1993,
32, 543; b) J. Gante, ibid. 1994, 106, 1780-1802 and 1994, 33, 1699-1721;
c) A. Giannis, T. Kolter, ihid. 1993, 103, 1303-1326 and 1993, 32, 1 2 4 4
[3] a) U Kazmaier, J Urg. Chem. 1996, 61, 3694; b) U. Kazmaier, A. Krebs,
Angeu. Chem. 1995, 107, 2213; Angeu. Chem. Int. Ed. Engl. 1995, 34,
2012; c) R. M. Williams, Synfhesis ofOptically Active =-Amino Acids, Vol. 7 in
Organic Chemistry Series (Eds.: J. E. Baldwin, P. D. Magnus), Pergamon,
Oxford, 1989; d) R. M. Williams, M.-N. Im, J Am. Chem. Soc. 1991, 113,
9276.
[4] a) M. Beller, B. Cornils, C. D. Frohning, C. W. Kohlpaintner, J Mol. Catal.
1995, 104, 17-85; b) I. Ugi, Proc. Estonian Acad. Sci. Chem. 1995,44,237273; c) B. Cornils, W. A. Herrmann, Quo vadis?in AppliedHomogeneous C a r d ysis with Metal Comple-xes (Eds.: B. Cornils, W. A. Herrmann), VCH, Weinheim, 1996.
[5] a) H. Wakamatsu, J. Uda, N. Yamakami, J Chem. Soc. Chem. Commun. 1971,
1540; b) I. Ojima, Chem. Rev. 1988,88, 1011; c) J. F. Knifton, Amidocarbonylution in Applied Homogeneous Catalysis with Mefal Complexes (Eds. : B.
Cornils, W. A. Herrmann), VCH, Weinheim, 1996.
[6] J. Ojima, Z. Zhang, Urganometa/lm 1990, 9, 3122-3127
[7] We would like to thank E. Jagers (Hoechst AG) for carrying out the first
amidocarbonylation trials with non-cobalt catalysts; E. Jagers, H.-P. Koll
(Hoechst AG), EP-338330 1989 [Chem. Abstr. 1990, 112, 7795121.
[8] K. Izawa, J. Synth. Urg. Chem. 1988, 46, 218-231.
[9] a) D. L. Boger, R. M. Borzilleri, S. Nukui, J. Urg. Chem. 1996,61, 3561; b) K.
Burgess, D. Lim, C. I. Martinez, Angew. Chem. 1996,108,1162; Angew. Chem.
Int. Ed. Engl. 1996, 35, 1077
[lo] a) M. Beller, H. Fischer, P. Gross, T. Gerdau, H. Geissler, S. Bogdanovic
(Hoechst AG), DE-4415712, 1995 [Chem. Abstr. 1996, 124, 149264aJ; b) E.
Drent, E. Kragtwijk (Shell), GB-2252770, 1991 [Chem. Absrr. 1993, 118,
3941Op]
[I 11 a) J.-J. Parnaud, G. Campan, P. Pino, L Mu/. Carol. 1979,6,341; b) P. Magnus,
M. Slater, Tetrahedron Lett. 1987, 28, 2829
p-Silyl Diorganozinc CompoundsA New Class of Useful Zinc Reagents**
Stefan Berger, Falk Langer, Christian Lutz,
Paul Knochel,* T. Andrew Mobley, and
C . Kishan Reddy
Organozinc halides (RZnX) are useful organometallic intermediates for organic synthesis."] They are readily prepared by
the direct insertion of zinc into organic halides,l2] display an
exceptional functional group tolerance, and show a high reactivity toward a range of electrophiles in the presence of an appropriate transition metal catalyst."] Diorganozinc reagents
(R,Zn) have a significantly higher reactivity, but more importantly, they undergo highly enantioselective asymmetric additions to aldehydes to afford polyfunctional secondary alcohol~.[~]
The drawback for some of these
is that only one
R group of R,Zn is transferred. This disadvantage is especially
disturbing in the case of the asymmetric addition to aldehydes
when a highly functionalized, valuable R group has to be transferred, since a large excess of diorganozinc ( 2 - 3 equiv) is usually required to obtain high conversion and high enantioselectivity. Herein, we report a general solution to this problem with
novel mixed p-silyl diorganozinc reagents.
Recently, Bertz et al. have shown that mixed cuprates
RRCuLi-LiX (R' is a nontransferable Me,SiCH, (TMSM)
group) are thermally stable but highly reactive.[51This new class
of copper reagents proves to be synthetically very useful in allowing the performance of l ,4-additions and acylations in high
yields.[51Stimulated by this work, we found that it is possible to
prepare mixed diorganozinc compounds of the type
R(TMSM)Zn (1) and to use them for Michael additions and
enantioselective additions to aldehydes. The new zinc reagents 1
can be prepared by three methods: 1) by the reaction of
(TMSM)Li (2) with an organozinc iodide, 2) by the reaction of
(TMSM)ZnI (3) with an organolithium or -magnesium reagent,
or 3 ) by the reaction of (TMSM),Zn (4)[61with a dialkylzinc
compound (Scheme 1).
This last preparation is especially useful, because it allows the
preparation of salt-free diorganozinc compounds 1. This is of
RZnl
+
(TMSM)Li
2
RLilRMgX
+
(TMSM)Znl
3
RaZn
+
THF
-80 "C
THF
-80 "C
R(TMSM)Zn
+ Lil
1
R(TMSM)Zn
+
LiI/Mg(I)X
1
THF
(TMSM)2Zn T 2 R(TMSM)Zn
4
1
Scheme 1. Preparation of the fl-silyl organozinc compounds 1
[*I Prof. Dr. P. Knochei, Prof. Dr. S. Berger, Dr. F. Langer, Dip1.-Chem. C. Lutz,
Dr. T. A. Mobley, Dr. C. K. Reddy
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-35032 Marburg (Germany)
Fax: Int. code +(6421)28-2189
e-mail: Knochel(2psl51S.chemie.uni-marhurg.de
[**I We thank the Chemischen Industrie for generous support. C. L. thanks
Chemetall GmbH Frankfurt and SlPSY S.A. (Avrille, France) for a fellowship,
C. K. R. thanks the Alexander-von-Humboldt Foundation for a fellowship.
We are grateful to Witco (Bergkamen) for generous gift of chemicals.
1496
0 VCH Erlagsgesellschaft mbH, 0-69451 Weinheim, 1997
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Angel*. Chem. I n t . Ed. Engl. 1997, 36, No. 13/14
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prime importance for achieving high enantioselectivities in the
asymmetric addition of diorganozinc compounds to aldehydes.
The equilibrium between R,Zn and R(TMSM)Zn as well as the
evidence supporting the existence of the mixed diorganozinc
compound 1 was studied in more detail with modern NMR
techniques. The mixing of nearly equimolar amounts of iPr,Zn
and (TMSM),Zn (4) (6(CH,) = - 0.93 and 6(CH,) = - 3.0) in
a ratio of 47: 53 in [DJTHF at - 20 "C leads to the appearance
of a new set of signals attributed to the mixed species
iPr(TMSM)Zn ( l a ) (6(CH,) = 0.99 and 6(CH,) = - 3.7).
New resonances for the iPr and TMS groups were also observed.
The ratio between remaining 4, iPr,Zn, and the mixed species
l a , determined by integration of the methylene groups of the
TMSM of 4 and 1 a, was 20: 15:65, from which an equilibrium
constant of K,, = 14 can be calculated. To confirm the structure
of 1 a, we have acquired a gradient-selected, heteronuclear, multiple bond-coherence (HMBC) spectrum.['] In this experiment
we observed a cross-peak between the methylene protons of 1a
and the cc-carbon ( C H ) of the iPr groups, which suggests that
both substituents are covalently bound to the same zinc center.['] By mixing a primary diorganozinc compound (Et,Zn)
with 4, a similar result was obtained. However, the exchange
between Et(TMSM)Zn (1 b) and (TMSM),Zn (4) was fast on
the NMR time scale at room temperature in contrast to that
between iPr(TMSM)Zn (1 a) and 4.
The thermal behavior of 1a and 1 b were studied by variable
temperature NMR spectroscopy. In addition to a dependence of
the chemical shift on temperature, an exchange process between
the three species, 1 b, Et,Zn, and 4, in solution was observed.
The spectra for the methylene region of the TMSM groups at
several temperatures are shown in Figure 1. For the methylene
resonances of 1 b and 4 (Av = 42.3 Hz), the coalescence temper~ 1 bin [DJTHF)
ature r, is 305 K for the sample shown ( 1 . 4 of
with the observed rate constant at the coalescence temperature
for exchange k = 94 s-' and AG* = 63.2 kJmol-'.
In contrast, a 1.OM solution of iPr(TMSM)Zn (1 a) has a T, of
333 K (AC = 46.5 Hz).['] A dependence of the exchange rate on
~
concentration was observed upon dilution of the concentrated
sample of l a by a factor of 4. This resulted in a T' which was
greater than the boiling point of the solvent for the NMR spectroscopy (THF). Although the rate of exchange for the case of
a primary alkylzinc derivative [EtZn(TMSM) 1 b] is faster than
the rate of a secondary alkylzinc compound [iPr(TMSM)Zn
1 a], the rates of exchange for both 1 a and 1 b are substantially
faster than the observed rate of reaction with organic substrates.
Recently, we have shown that diorganozinc compounds
R,Zn add to various Michael acceptors in THFIN-methylpyrrolidone (NMP) ( 2 :l).[41As only one group R is transferred
in these reactions, one R group wasted. We found that the mixed
zinc reagents 1 react readily with a typical enone like cyclohexenone to afford the 1,4-addition product 5 and that this
1,4-addition requires only about 1.2 equivalents of the mixed
zinc reagent 1 (Scheme 2, Experimental Section, and Table 1).
The use of TMSBr["] to promote the Michael addition instead
of TMSCl gives higher yields in all cases (Table 1).
R(TMSM)Zn
A
+
J
295 K
290 K
280 K
4.7
4.8
4 . 9 -1.0
*
25 "C, 12 h
X = CI.Br
5
Scheme 2. Michael additions of 1 to enones.
Table 1. Michael addition of R(TMSM)Zn (1) to cyclohexenone in THF/NMP in
the presence of TMSBr (or TMSCl).
Entry
R
Zinc reagenr 1
Product 5
Yield [%][a,b]
1
2
3
c-Hex
Bu
Ph
p-MeO-C,H,
lc
Id
le
If
5a
5d
83 (76)
69 (52)
82 (45)
87
5e
84
5f
52 (70)[c]
49
4
PivO(CH,),
EtO,C(CHd,
~
Ih
li
5b
5c
5g
~
[a] Yields of isolated analytically pure products. [b] Yields in parentheses refer to
the reaction performed in the presence of TMSCl [c] Yield obtained for the preparation of 1 h by mixing (PivO(CH,),),Zn with 4 (1 :1) and performing the reaction
in the presence of TMSBr.
300 K
4.6
-
T H F : NMP
1
~
315 K
O
R
TMSX (2 equiv)
-20 "C
6
7
320 K
8
+
-1.1
-1.2
-1.3 -1.4
-6
Figure 1. 'H NMR spectra (500 MHz) for the region of methylene protons of the
TMSM groups of 1 b and 4.
Angew. Chem. In!. Ed. Engl. 1997, 36, No. 13/14
The addition of a functionalized primary zinc compound such
as 1h is best performed by preparing this mixed zinc derivative
with method (3) by mixing equimolar amounts of (TMSM),Zn
(4) and (PivO(CH,),),Zn and performing the reaction in the
presence of TMSBr (Table 1, entry 6). Remarkably, the enantioselective addition of a mixed zinc reagent to benzaldehyde
can also be performed. Thus, mixing Pent,Zn with 4 provides
the mixed zinc species Pent(TMSM)Zn (1 j, 1.8 equiv). Its addition to benzaldehyde (1 equiv) in the presence of (lR,2R)-1,2bis(trifluoromethanesu1fonamido)cyclohexane (6, 15 mol
and Ti(OiPr), (0.6 equiv) at -20 "C for 9 h gives the desired
chiral alcohol 7 in 92 YOyield and 97 YOee. The new procedure
requires significantly less zinc reagent (1.8 pentyl groups compared to 4- 6 pentyl groups for previousiy described reaction
conditions[31)and opens the way to the enantioselective transfer
of highly valuable R groups to aldehydes (Scheme 3).
In summary we have developed three methods to prepare
mixed p-silyl diorganozinc compounds (1). These novel zinc
reagents were characterized by NMR studies. The TMSM
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1497
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6: 15 mol%
"
Pent(TMSM)Zn+ PhCHO
NHTf
Me
Ti(0Pr)d (0.6 equiv)
li
EtZO, -20 "C
7 : 92 %. 97 % ee
Scheme 3. Enantioselective addition of Pent(TMSM)Zn (lj) to benzaldehyde.
group plays the role of nontransferable ligand. The zinc reagents
1 undergo efficient 1,4-additions in NMP/THF solvent mixtures
and substantially improve the highly enantioselective addition
to aldehydes.
Experimental Section
Typical procedure for the 1,Caddition of 1 to enones (example of Sa): lc
(1.0equiv). prepared by the reaction of c-HexZnI with (TMSM)Li ( - 80°C to
-3O"C, 0.5 h), was treated at -20°C in THF/NMP (2-1) with cyclohexenone
(about 0.7-0.8 equiv) for 5 h in the presence of trimethylsilyl chloride (TMSCI,
2.0 equiv) or TMSBr (2.0 equiv). The yield of 5a was 76% and 83%, respectively.
Example of S c : A dried and argon-flushed, three-necked, 50 mL flask was charged
with TMSMZnI (12mmo1, prepared by zinc insertion from TMSMI (2.5g,
12 mmol) and zinc dust (2.6 g, 40 mmol)) in T HF (6 mL). Phenylhthium (12 mmol)
was slowly added at - 50 "C, followed after 30 min of stirring by NMP (2 mL).
cyclohexenone (0.75 g, 8 mmol), and TMSBr (2.4 g, 16 mmol) at - SO "C. The reaction mixture was allowed to warm slowly to room temperature and then stirred for
32 h. After diluting with THF (20 mL), quenching with HCI solution (lo%, 20 mL)
and stirring for 15 min, the reaction mixture was worked up as usual. The crude
product was purified by flash chromatography (hexanesl diethyl ether 9: 1) to yield
pure 5c (1.15g, 82%) as a colorless oil. For less reactive aryl or primary alkyl
(TMSM)zinc, the reaction mixture was stirred at 25 "C for 12 h.
Typical procedure for the enantioselective addition of 1 to an aldehyde (example of
the preparation of 7): A dried and argon-flushed 50 mL Schlenk flask was charged
with (1 R, 2R)-1,2-bis(trifluoromethanesulfonamido)cyclohexane (6, 0.38 mmol,
15 mol%),Ti(OiPr),(0.45 mL, 1.5 mmol,0,6equiv),anddiethylether(3 mL). This
catalyst solution was cooled to -20 "C. Meanwhile Pent& (0.45 g. 2.1 mmol,
0.9 equiv) and TMSMJn (0.56 g, 2.3 mmol, 0.9 equiv) were mixed at 25 "C in
another Schlenk flask. The resulting Pent(TMSM)Zn lj (1.8 equiv) was slowly
added to the catalyst 6.After 10 min, benzaldehyde (0.26 g, 2.45 mmol, 1 equiv) was
added. The reaction mixture was stirred at -20°C for 9 hand worked up as usual.
The crude product was purified by flash chromatography (hexanes/dierhyl ether
2: 1) to yield analytically pure 7 (0.40 g, 92 %) with 97 % ee (HPLC, Daicel, Chiracel
OD, heptane/isopropyl alcohol 97:3). uio -36.4 ( c = 4.2, CHCl,).
Received: January 7, 1996 [Z9972IE]
German version: Angew. Chem. 1997,109, 1603-1605
Keywords: additions
troscopy zinc
-
.
asymmetric synthesis
*
NMR spec-
[l] a) P. Knochel, R. D. Singer, Chem. Rev. 1993, 2117-2188, b) P. Knochel,
Synleft 1995,393-403.
121 a) P. Knochel, M P. C. Yeh, S. C. Berk, J. Talbert, J. Org. Chem. 1988,53,
2390-2392; b) S. C. Berk, P. Knochel, M. C. P. Yeh, &id. 1988,53, 57895791; c) L. Zhu, R M. Wehmeyer, R. D. Rieke, ibid. 1991,56,1445-1453.
[3] a) M. J. Rozema, S. Achyutha Rao, P. Knochel, 1 Org. Chem. 1992,57,19561958; b) W. Brieden, C. Eisenberg, M. J. Rozema, TefrahedronLeft. 1993,34,
5881-5884; c) H. Takahashi, T. Kawakita, M. Ohno, M. Yoshioka, S
Kobayashi, Tetrahedron 1992, 48, 5691 -5700; d) B. Schmidt, D. Seebach,
Angew. Chem. 1991, 103, 100-102; Angeew. Chem. Ind. Ed. Engl. 1991,30.
99-101; e) D. Seebach, A. K Beck, B. Schmidt, Y. M. Wang, Tefrahedron
1994,SO, 4363-4384; f) P. Knochel, Chemtracfs: Org. Chem 1995,8, 205221.
[4] C. K. Reddy, A. Devasagayaraj, P. Knochel, TefrahedronL e f f .1995,37,44954498.
[ 5 ] a) S. H. Bertz, M. Eriksson, G. Miao, J. P. Snyder, J. Am. Chem. Soc. 1996,118,
10906-10907; b) M. Srebnik, Tetrahedron Left. 1991,32,2449-2452.
[6] Compound 4 was prepared by the reaction of (TMSM)MgBr with ZnBr, in
diethyl ether, filtration under argon, and subsequent distillation of 4
(b.p. = 41 " C a t 0.1 mmHg).
[7] W Willker, D. Leibfritz, R. Kerssebaum, W. Bermel, Magn. Reson. Chem.
1993,31,287-292.
1498
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[8] The corresponding cross-peak between the a-proton of the iPr group and the
3-carbon of the TMSM group was not observable due to coupling of the
a-proton with the iPr methyl groups. Attempts to acquire a selectively decoupled HMBC spectrum failed to reveal the weak couplings across the zinc center.
[9] Due to the chemical shift dependence on temperature, Av changes as a function
of temperature. Values for Av at the coalescence temperatures were extrapolated for both the ethyl (lb)and isopropyl (la) groups from low temperature
chemical shift data. Since Av is quite similar in the primary and secondary
cases, the observed difference in fluxional behavior arises from a difference in
chemical behavior.
[lo] M. Bergdahl, E.-L. Lindstedt, M. Nilsson, T. Olsson, Tetrahedron 1989,45,
535-543.
Gas-Phase Reaction of Tetraborane(10) with
Allene : The Fluxional auachno-1-Carbapentaborane(l0) Isomeric System and Derivatives
1,2- and 1,3-Me2-1-CB,H,; Analogies in
l-CB,H,,, MeB,H,,, and B5HT0**
Mark A. Fox, Robert Greatrex,* Matthias Hofmann,
Paul von R. Schleyer,* and Robert E. Williams
Dedicated to Professor Walter Siebert
on the occasion of his 60th birthday
Unsaturated compounds such as alkynes,['. a l k e n e ~ , [ ~ . ~ l
and enynesr5]react in the gas phase with boranes, for example
B,H,,, to give a variety of remarkable products, often in which
the C-C bonds have been cleaved.L6]We now report the identification of a fluxional monocarbapentaborane isomeric system,
arachno-1,3- (1 a) and arachno-1,2-Me2-l-CB,H, (1 b) from the
quenched reaction of B,H,, with H,C=C=CH, (Scheme 1).
Whereas this reaction is also a superior synthetic route to the
arachno-carbapentaborane 1-Me-2,5-pCH,-1-CB4H, (Za),"]
there was no sign of the basket-like ethanotetraborane derivative 2,4-(MeCHCH,)B,H, (3), which is the main product under
hot-cold conditions.[71The present investigation also clarifies
earlier findings on the isoelectronic compounds CB,H,,[8* 91
and B,H;0.[93 lo]
In 1973 Matteson and Mattschei obtained a compound,
believed to be CB,H,, (m/z = 65, M - I),[*] by the reduction
of tris(dichlorobory1)methane with lithium borohydride. Although noting an "infra red spectrum somewhat resembling
that of pentaborane(1 I)", the authors did not assign a structure.
In 1976 Williams predicted that the arachno constitution 4
should be preferred for CB,H,, (the parent compound of l a
and 1 b, see Figure 1).I9] Although in the absence of bridge[*] Dr. R Greatrex, Dr. M. A. Fox
School of Chemistry
University of Leeds
Leeds LS29JT (UK)
Fax: Int code +(113)233-6565
e-mail: r greatrex(ichem.leeds.ac.uk
Prof. Dr. P. von R. Schleyer, Dr. M. Hofmann
Computer-Chemie-Centrum des Instituts fur Organische Chemie
der Universitit Erlangen-Nurnberg
Henkestrasse 42, D-91054 Erlangen (Germany)
Fax: Int code +(9131)85-9132
e-mail : pvrs@organik.uni-erlangen.de
Dr. R. E. Williams
Loker Hydrocarbon Research Institute
University of Southern California, Los Angeles (USA)
[**I This work was supported by the Englneering and Physical Sciences Research
Council, the Royal Society, the Deutsche Forschungsgemeinschaft, and the
Fonds der Chemischen Industrie.
0570-0833/97/3613-1498S 1 7 . 5 0 t .50/0
Angew. Chem. Int. Ed. Engl. 1997,36, No. 13/14
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