A R T I C L E S
Rauniyar et al.
catalytic enantioselective synthesis.11 The difficult challenge that
represents a catalytic allylboration is exemplified by the
instantaneous additions of most allylic boron derivatives at low
temperatures, and as such, effective catalysis is rendered very
difficult because of a significant background reaction to be
competed with (even at -78 °C).12 In this context, the most
convenient achiral allylboron reagent appears to be the air- and
water-stable, nontoxic, and commercially available allylboronic
acid pinacol ester (1a), which has been shown to possess
negligible allyl transfer activity at -78 °C.12 Another key issue
with catalysis of allylboration was the perception that incorpo-
rating Lewis acidic catalysts would interfere with the intrinsic
type I nature of the reaction and turn it into a type II process,
thereby destroying its highly diastereoselective nature. Our
group,13,14 and others15 addressed this issue in 2002. We
reported that Lewis acid catalysts dramatically accelerate the
allylboration of aldehydes and, more importantly, retain the
diastereoselectivity of the reaction. In their report, Miyaura and
co-workers showcased the first example of a catalytic enanti-
oselective and diastereoselective crotylboration of benzaldehyde,
albeit with moderate enantioselectivity (51% ee).15 Subse-
quently, our group garnered significant evidence that pointed
Figure 1. Proposed transition state for the Lewis acid activation of pinacol
allylic boronates (1).
toward an electrophilic activation of the allylic boronate by
coordination of the Lewis acid to one of the oxygens of the
boronic ester in the type I transition state (Figure 1).16 Other
beneficial effects of this new mode of activation have been
observed in the much improved E/Z selectivity of the homoallyl
alcohol products when employing chiral R-substituted allylbo-
ronates as well as the large rate acceleration of deactivated allylic
boronates.17 Until recently, however, all efforts by our group
and others only led to modest levels of enantioselectivities for
the catalytic manifold using chiral Lewis acids. This can be
partially attributed to the sterically crowded nature of the
dioxaborolane in reagents 1, which prevents efficient coordina-
tion of the large chiral Lewis acids (Figure 1).
Following our first report on the Brønsted acid catalyzed
allylboration,18a we unveiled the utility of Yamamoto’s chiral
diol•SnCl4 combined acid catalyst system19 in the enantiose-
lective addition of allylboronic acid pinacol ester to aldehydes.20
We first aimed at optimizing a procedure for the simple
allylation of aliphatic aldehydes, which tend to be the most
difficult substrates with existing catalytic enantioselective al-
lylation methodologies.3e,f Under this first-generation catalyst
system, homoallylic alcohols were obtained in moderate to good
er and excellent dr. Thereafter, we undertook an extensive
optimization of the chiral diol and arrived at a novel one,
hereafter named Vivol (4m), which was found to be very
efficient in the diol•SnCl4 catalyst system for the enantioselective
addition of pinacol allyl- and crotylboronates onto aliphatic
aldehydes.21 With this second-generation catalyst, homoallylic
alcohols are now obtained in very good to excellent er and
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