CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

(B) Benzylation of aldehydes
Addition of benzyl moiety to aldehydes is an important carbon-carbon bond
forming reactions in organic synthesis. Due to their functional richness, the resultant
homobenzylic alcohols are amenable for different applications in organic synthesis. 119 The
most common procedure to prepare homobenzylic alcohols involves addition of
benzylmagnesium bromide to carbonyls, that requires anhydrous conditions. Besides this,
some other strategies have also been reported to prepare the homobenzylic alcohols. These
include hydroboration of 1-aryl alkenes, 122 solvolyses of sulfonates obtained from bridged
hydrocarbons 123 and regio-selective hydrogenation of aromatic epoxides. 124 In view of
current interest on performing many organic reactions in environmentally friendly aqueous
media, 125 considerable attention has been focused on performing Barbier type additions of
benzyl bromide to aldehydes. This has resulted in several reports where metals such as Cd,
obtained from a tri-metal system, 126a,b Zn in presence of catalytic Ag 126c etc. In a recently
reported approach, silver catalyzed Mn mediated Barbier type benzylation has also been
performed in anhydrous THF. 127
The present work describes a very mild and inexpensive procedure in moist
medium for the metal mediated Barbier type benzylation of aldehydes as an application of
the previously described bimetallic redox strategy. 128 First, we investigated the efficacy of
the present reaction using all four possible combinations between two reducible salts FeCl 3
and CuCl 2 -2H 2 O and two reducing metals, Zn dust and Mg turning. Three classes of
aldehyde substrates viz aliphatic, aromatic and a chiral were chosen, with a view to
understanding the generality of this strategy. In all these heterogeneous reactions, aldehyde
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