CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

The reaction could also be accomplished even with 20 mol% In, although it became slower
under this condition. The protocol was equally effective with both aromatic and aliphatic
aldehydes 1a-1j (Scheme 1, Table 1) furnishing the products in good yields, and
proceeded with complete chemoselectivity furnishing the 1,2-addition product (e. g., 2g)
only with the conjugated aldehyde 1g.
The 1 H NMR spectroscopic studies of the reaction between In and allyl bromide
revealed formations of CH 2 =CHCH 2 -In (I) in H 2 O and CH 2 =CHCH 2 -InBr 2 (II) in THF /
[bmim][Br], as the active allylating agents. However, the species I got easily hydrolyzed,
explaining the requirement of a large excess of the reagents in the aqueous phase
allylation. In contrast, the species II remained stable up to 6 h in [bmim][Br], when the
reaction was complete. The activation of the In metal surface was caused by polarization of
the In electrons to [bmim][Br], as revealed from the upfield shift of imidazole protons (4.2
Hz) as well as carbons (10 Hz) in NMR spectra when In metal was incubated in
[bmim][Br].
Allyl Bromide/ In/ Solvent
RCHO
1a-j
Scheme 1
R
OH
2a-j
Table 1. Reaction course of In-mediated allylation of aldehydes in [bmim][Br] a
Entry Substrate R Aldehyde : Allyl bromide
: Metal (equiv.)
Time
(h)
Product (%)
1 1a C 6 H 5 1 : 1.5 : 0.2 22 2a (84)
2 1b 4-Bromo-C 6 H 4 1 : 1.5 : 0.2 20 2b (81)
3 1c 3-Methoxy-C 6 H 4 1 : 1.5 : 0.2 22 2c (88)
iv