CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

IIIIII. .11 DIIASSTEREOSSELECTIIVE SSYNTHESSIISS OFF CHIIRAL CARBIINOLSS VIIA BII- -
METALLIIC REDOX SSTRATEGIIESS
IIIIII. .11. .11 IInnt trroodduucct tioonn
Metal-mediated transformations are among the most powerful tools in synthetic
organic chemistry. 100 In particular, addition of organometallic compounds to aldehydes and
ketones have been extensively studied. Amongst various methods of organometallic
addition to carbonyls, the Barbier type reactions 101 are operationally simple as they allow
in-situ formation of the active organometallic species. The effectiveness of metals as the
mediator for such a reaction is closely related to the location of the metals in the periodic
table and can be tentatively rationalized via inner-sphere and outer-sphere single-electron
transfer processes. Directly or indirectly, their utility depends on their propensity to
undergo electron transfer and/or to form coordination compounds with appropriate
reactivity and geometry. 102 However, in spite of the many applications, the mechanism of
the metal mediated Barbier reactions is not well-understood. 103 Involvement of a radical
pathway, or a discrete allylmetal species have been speculated in the literature. 104 Recent
computational and NMR studies indicate that Barbier reactions with indium, tin and
antimony metals proceed directly through nucleophilic addition, whereas that with
magnesium proceeds via a single electron transfer process. Nevertheless, issues regarding
the nature of the organometallic species and the involvement of single electron-transfer
processes, remain unanswered.
From this perspective, there is a scope for exploring the potential of different
metals in varied physical states to participate in the Barbier type allylation of carbonyls.
However, preparation of the activated metal to form the required active organometallic
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