Page 677 - Advanced Organic Chemistry Part B - Reactions & Synthesis

652 Organozinc reagents can also be prepared from trialkylboranes by exchange with
dimethylzinc. 136
CHAPTER 7
Organometallic CH 2 CH 2 ) 2 Zn
H )
Compounds of Group I CH 3 1) HB(C 2 5 2 CH 3
and II Metals (
2) (CH ) Zn
3 2
CH 3 CH 3
This route can be used to prepare enantiomerically enriched organozinc reagents by
asymmetric hydroboration (see Section 4.5.3), followed by exchange with diisopropyl-
zinc. Trisubstituted cycloalkenes such as 2-methyl or 2-phenylcyclohexene give an
enantiomeric purity greater than 95%. The exchange reaction takes place with retention
of configuration. 137

CH 3 CH 3 1) (C H ) BH CH 3
IpcBH 2 2 5 2
2
BHIpc 2) (i-Pr) Zn ZnCH(CH )
3 2
94% e.e.

Exchange with boranes can also be used to prepare alkenylzinc reagents. 138
(CH ) CH 3 (CH 2 2 3
) CH
2 2
[CH (CH ) CH C ] B + (C H ) Zn [CH (CH ) CH C ] Zn
3
2 5 2
3
2
2 3
3
2 3
Alkenylzinc reagents can also be made from alkynes by Cp TiCl -catalyzed hydro-
2
2
zincation (see Section 4.6). 139 The reaction proceeds with high syn stereoselectivity,
and the regioselectivity corresponds to relative carbanion stability.
Ph CH 3
, LiH Ph CH 3
ZnI 2
PhC CCH 3 + H
(Cp) TiCl 2 IZn H ZnI
2
16%
84%
7.3.1.2. Reactions of Organozinc Compounds. Pure organozinc compounds are
relatively unreactive toward addition to carbonyl groups, but the reactions are catalyzed
by both Lewis acids and chelating ligands. When prepared in situ from ZnCl and
2
Grignard reagents, organozinc reagents add to carbonyl compounds to give carbinols. 140
136
F. Langer, J. Waas, and P. Knochel, Tetrahedron Lett., 34, 5261 (1993); L. Schwink and P. Knochel,
Tetrahedron Lett., 35, 9007 (1994); F. Langer, A. Devasagayari, P.-Y. Chavant, and P. Knochel, Synlett,
410 (1994); F. Langer, L. Schwink, A. Devasagayari, P.-Y. Chavant, and P. Knochel, J. Org. Chem.,
61, 8229 (1996).
137 A. Boudier, F. Flachsmann, and P. Knochel, Synlett, 1438 (1998).
138
M. Srebnik, Tetrahedron Lett., 32, 2449 (1991); K. A. Agrios and M. Srebnik, J. Org. Chem., 59, 5468
(1994).
139 Y. Gao, K. Harada, T. Hata, H. Urabe, and F. Sato, J. Org. Chem., 60, 290 (1995).
140
P. R. Jones, W. J. Kauffman, and E. J. Goller, J. Org. Chem., 36, 186 (1971); P. R. Jones, E. J. Goller,
and W. J. Kaufmann, J. Org. Chem., 36, 3311 (1971).