CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

are consistent with our previous results with the bimetallic protocols and can be
rationalized by the Felkin-Anh model.
The 3,4-diastereoselectivity of the reaction involving γ-substituted (Me or C 6 H 13 )
allylic metal reagents and aldehydes may, in principle, involve a series of cyclic transition
states, wherein the allylmetal reagent coordinates intramolecularly to the oxygen atom of
the carbonyl group (Fig. III.2.1). Formation of TS1 and/or TS2 leads to the syn-adduct,
while the anti-product is obtained from TS3 and/or TS4. Given that no other
intramolecular chelation operates for this reaction, the transition state TS3 leading to the
3,4-anti-adduct is most favourable from the steric point of view, because both R and R'
(cyclohexylidene) groups adopt equatorial positions in it, and the allylic-metal exists as the
E-form, irrespective of the geometry of the starting bromide. 137 The stereoselectivity is
considered to be the outcome of the orientation of R group. These explain the preferential
formation of 69c in the Ga-mediated crotylation of 1. On the other hand, the preferential
formation of 3,4-syn product 69b in the Bi-mediated crotylation reaction suggested an
extensive conversion of the initially formed E-allylbismuth species to the corresponding Z-
isomer. However, the effect of the metal and solvent on the diastereoselectivity of the
reactivity is unclear. A large number of other factors such as nature of the metal, 138
counterion-dependence, aggregation state of the nucleophiles 139 may be involved in
dictating this diastereoselectivity and any more detailed rationalization would be, at best,
speculative.
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