CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

The integration of the signal at δ 2.01 reached a maximum (52.3%) in ~ 2.75 h, and
remained steady up to 3 h. In spite of a very marginal decline in its intensity thereafter, a
significant amount of the allyl-In species remained up to 5 h. Since the allylation was over
in ~4 h, the reaction could be carried out without using excess of the reagents.
Interestingly, the maximum concentration of the active species II formed in [bmim][Br]
was significantly higher than that of I in H 2 O. During the reaction with 59a, complete
depletion of the doublet (δ 2.01) confirmed II as the active organometallic species, as
reported previously. 77b
Recently Law et al. reported 77a generation of both I and II in the reaction between
In and allyl bromide in [bpy][BF 4 ]. They also suggested the possibility of the
transformation of I to II, which, however, is slowed down when the RTIL contains a
higher amount (>8.5 mol%) of its precursor, [bpy][Br]. Our results of the exclusive
formation of II in [bmim][Br] (resembling [bpy][Br]) was most striking, and revealed a
different operative mechanism in this medium. To confirm this, we also followed the
reaction between In (1.5 mmol) and allyl bromide (1.5 mmol) separately in neat
[bmim][BF 4 ], and [bmim][PF 6 ], as well as in the presence of [bmim][Br] (10 and 20%),
using 1 H NMR experiments. Amongst the various combinations, only the combination of
[bmim][BF 4 ] + [bmim][Br] (20%) produced the more downfield doublet (δ 2.01, J = 8.2
Hz) after 3 h, although appreciable signal was not detected up to 2 h. The maximum
intensity of the signal (50.4%) was similar to that observed in [bmim][Br] alone. No such
1 H NMR resonances were, however, visible in [bmim][PF 6 ] even in the presence of
[bmim][Br]. These results confirmed that unlike the previous observations, 77a the RTIL,
[bmim][Br] is essential for generating the species II.
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