CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

Once produced, the species II reacts with the aldehyde to produce the
homoallyloxy-In species (eq. 6). Its hydrolysis by H 2 O (measured as 230 ppm by Karl-
Fischer titrimeter) present in the RTIL, gave the homoallylic alcohol product and InBr 3 (eq.
7). Though we could not isolate [bmim][OH] from the reaction mixture, the increase of pH
from 5.78 to 8.23 indicated its formation. Also, it has been observed that a slight excess
(1.2 eqv.) of allyl bromide is required for the completion of the reaction. This may be
because of slow conversion of allyl bromide to allyl alcohol by [bmim][OH]. Only trace
amount of allyl alcohol could be detected in the reaction mixture, and it indicated its slow
formation.
During the process, the reaction medium becomes alkaline (pH 8.23) due to the
formation of [bmim][OH], which is known to generate the NHC-carbene diradical from
imidazolium ionic liquids. 84 This, eventually, forms a bisimidazolidene moiety which is
reported to be a super electron donor, and is used as a reducing agent in some reactions. 85
We also speculated that such moiety may have been formed in the reaction mixture, and
this moiety, along with the radical [bmim] . , is responsible for the reduction of In +3 to
metallic In. Thus the catalytic cycle is maintained without further addition of In metal.
Thus, a catalytic protocol for allylation of aldehydes, involving use of indium metal
and allyl bromide in [bmim][Br] has been developed. Besides acting as a solvent, the
chosen RTIL also participates chemically to activate the In-metal and helps in regeneration
of In from In(III), produced during the allylation reaction. This makes the process catalytic
with regard to the In-metal. The stability of the active allylmetal species II in the RTIL
during the reaction also ensures the use of stochiometric amount of allyl bromide.
53