CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

of the products were modest (48-62%). More importantly, a large excess of metal and allyl
bromide was required for the completion of the reaction. All the reactions in H 2 O
proceeded with a modest diastereoselectivity for the anti-isomer 68b (entries 1, 4 and 7,
Table III.2.1). Surprisingly, the Ga- and Bi-mediated allylations in THF did not take place
at room temperature (entries 5 and 8, Table III.2.1). However, the In-mediated allylation
in THF proceeded with excellent anti-selectivity (entry 2, Table III.2.1). Although the Inand
Bi-mediated allylation in [bmim][Br] proceeded with modest anti-selectivity, use of
Ga for the allylation in [bmim][Br] yielded the anti-isomer 68b with 90%
diastereoselective excess (entry 6, Table III.2.1). In all the cases, the predominant
formation of the anti-isomer indicated that all the reaction took place via the Felkin-Anh
model. 24b,112
In case of crotylation (Scheme III.2.1), we also used two additional metals, Mg
and Sb. In this case also the diastereoselectivity could be tuned by changing the metals,
and solvents. 117 The results are summarized in Table III.2.2. With Mg in ethereal solvents,
practically no selectivity was obtained, and the diastereomers 69a-c were produced in
almost equal amounts (entries 1, 2, Table III.2.2). Use of Sb in aqueous THF or H 2 O gave
a similar diastereoselectivity, but poor yields, and required an additional metal activator
(KF) (entries 3, 4, Table III.2.2). The In-mediated reaction proceeded well in H 2 O, but not
in THF (entries 5-8, Table III.2.2), resulting in preferential formation of 69b and 69c in
~1:1 ratio, as the major products. On the other hand, the Ga-mediated reaction in THF
produced the products in poor yields even under metal activation and resulted in modest
69b/69c selectivity (entry 10, Table III.2.2). A similar stereoselectivity was earlier
observed using Zn metal (Table III.1.2, Chapter III.1.2). The inability of Ga to carry out
113