DEDICATED TO PROFESSOR HC BROWN ON HIS ... - Sigma-Aldrich

B–O bond. 15 The mechanism involves a
similar six-membered cyclic transition state,
and much success has been achieved with
pinane-based chiral auxiliaries. 16
3. Preparation of Allylboranes
The simplest synthesis of allylboranes
involves the treatment of allylmetals with
borane derivatives, such as haloboranes or
alkoxyboranes (eq 10, 11). 9 Bubnov and
coworkers have reported the synthesis of
bis(allylboranes) from dilithioisobutylene
(eq 12). 17
Allylboronates can be prepared by a
one-carbon homologation (Matteson homologation)
of vinylboronates with halomethyllithium
(eq 13). 18 Treatment of vinyllithiums
eq 6
eq 8
with halomethylboronates can also be used to
prepare “crotyl”boronates stereospecifically
(eq 14). 19
Brown and coworkers have reported a
three-carbon homologation procedure for the
preparation of higher crotylboronates
(Scheme 1). 20
Suzuki and coworkers have shown that
catalytic hydroboration of 1,3-dienes with
catecholborane in the presence of Pd(0)
catalyst results in the formation of
crotylborane derivatives (eq 15). 21 Another
procedure for preparing stereospecific
“crotyl”boranes involves the haloboration of
terminal acetylenes, followed by Negishi
coupling and Matteson homologation
protocols (Scheme 2). 22
VOL. 35, NO. 1 • 2002
eq 7
eq 9
eq 10
eq 11
eq 12
eq 13
Yet another procedure for the preparation
of “crotyl”boranes involves the hydroboration
of substituted allenes. Although
several hydroboration products are possible,
the predominant product observed is the E
“crotyl”borane (eq 16). 23
4. Asymmetric Allylboration
eq 14
Asymmetric allylboranes that react with
aldehydes to produce chiral homoallylic
alcohols have attained considerable
importance in the art of stereoselective
synthesis of highly sophisticated,
conformationally nonrigid systems. 5,8 Here,
not only the enantioselectivity, but also the
diastereoselectivity of the reaction is highly
important. Accordingly, numerous searches
25