Page 378 - Advanced Organic Chemistry Part B - Reactions & Synthesis

350 simple cycloalkenes give low enantioselectivity (5–30%). Interestingly, vinyl ethers
exhibit good enantioselectivity for both the E- and Z-isomers. 201
CHAPTER 4
Electrophilic Additions CH 3 O OH CH 3 O CH O OH
to Carbon-Carbon 1) Ipc BH CH O 3 1) Ipc BH CH 3
3
2
Multiple Bonds 2
–
CH 3 CH 3 2) H 2O , OH CH 3 CH 3 CH 3 2) H 2O , OH CH 3 CH 3
–
2
2
72% yield 77% yield
> 97% e.e. 90% e.e.
Monoisocampheylborane IpcBH can be prepared in enantiomerically pure form
2
by separation of a TMEDA adduct. 202 When this monoalkylborane reacts with a
prochiral alkene, one of the diastereomeric products is normally formed in excess and
can be obtained in high enantiomeric purity by an appropriate separation. 203 Oxidation
of the borane then provides the corresponding alcohol having the enantiomeric purity
achieved for the borane.

BH 2 R 3 R 1 H R 3 R 1 H R 3 R 1
+ C C IpcB C C H or IpcB C C H
H R 2 H R 2 H R 2

As oxidation also converts the original chiral terpene-derived group to an alcohol,
it is not directly reusable as a chiral auxiliary. Although this is not a problem with
inexpensive materials, the overall efficiency of generation of enantiomerically pure
product is improved by procedures that can regenerate the original terpene. This can
be done by heating the dialkylborane intermediate with acetaldehyde. The -pinene is
released and a diethoxyborane is produced. 204

Me
CH CH CH
CH 3 3 3 3
BH 2 H CH CH O
+ IpcB 3 (C 2 5 2 +
H O) B

The usual oxidation conditions then convert this boronate ester to an alcohol. 205
The corresponding haloboranes are also useful for enantioselective hydrobo-
ration. Isopinocampheylchloroborane can achieve 45–80% e.e. with representative
alkenes. 206 The corresponding bromoborane achieves 65–85% enantioselectivity with

simple alkenes when used at −78 C. 207

201 D. Murali, B. Singaram, and H. C. Brown, Tetrahedron: Asymmetry, 11, 4831 (2000).
202
H. C. Brown, J. R. Schwier, and B. Singaram, J. Org. Chem., 43, 4395 (1978); H. C. Brown,
A. K. Mandal, N. M. Yoon, B. Singaram, J. R. Schwier, and P. K. Jadhav, J. Org. Chem., 47, 5069
(1982).
203 H. C. Brown and B. Singaram, J. Am. Chem. Soc., 106, 1797 (1984); H. C. Brown, P. K. Jadhav, and
A. K. Mandal, J. Org. Chem., 47, 5074 (1982).
204
H. C. Brown, B. Singaram, and T. E. Cole, J. Am. Chem. Soc., 107, 460 (1985); H. C. Brown, T. Imai,
M. C. Desai, and B. Singaram, J. Am. Chem. Soc., 107, 4980 (1985).
205 D. S. Matteson and K. M. Sadhu, J. Am. Chem. Soc., 105, 2077 (1983).
206 U. P. Dhokte, S. V. Kulkarni, and H. C. Brown, J. Org. Chem., 61, 5140 (1996).
207
U. P. Dhokte and H. C. Brown, Tetrahedron Lett., 37, 9021 (1996).