CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

R
H
H
R
R'
H
O
M
H
R
O
M
R'
R
O
M
H
H
O
M
H
R'
H
R'
TS1 TS2 TS3 TS4
Fig. III.2.1. Cyclic transition states for metal mediated crotylation of 1.
It is worth mentioning that earlier, better stereocontrol in crotylation was
accomplished with reagents such as crotyltrifluorosilane, 140 and crotyltrifluoroborates 141
that need to be synthesized. In that light, the present approach, using inexpensive and
commercially available reagents such as crotyl bromide, [bmim][Br] and Ga, and the
widely used chiral template, 1 provides a simple and convenient strategy for asymmetric
crotylation. Incidentally, this is the first application of a hydrophilic RTIL in the Barbier
type allylation. Amongst the imidazole-based RTILs, [bmim][Br] is least expensive and
possess negligible vapour pressure, providing additional advantages for the process. In
addition, the present study also demonstrated that by judicious choice of the mediating
metals viz. Ga and Bi, the reaction can be directed to produce 69c and 69b with excellent
diastereoselectivity.
Overall, we disclosed that by careful choice of metal and solvent, the
stereochemical profile of the Barbier type crotylation of the aldehyde 1 can be tuned. Of
particular interest was the combination of Ga/[bmim][Br] and Bi/[bmim][Br], which
ensured a very fast reaction with stoichiometric amounts of the reagents, without any
additional metal activation (chemical, thermal or ultrasonic). The reaction provided
functionalized chiral homoallylic alcohols with high yield and diastereoselectivity. Also,
this provided an example of the Barbier type allylation in a hydrophilic RTIL.
118