A stable dirhodium tetracarboxylate carbenoid: crystal structure, bonding analysis, and catalysis.

Transition metal catalyzed reactions of R-diazo ketones (1) are a powerful means to synthesize complex polycyclic organic frameworks.1 In particular, dirhodium(II) carboxylate and carboxamide catalysts mediate a wide range of synthetic transformations such as cyclopropanation,2 C-H and X-H insertion,3,4 aromatic substitution,5 and ylide formation.6 Enantioselective transformations promoted by chiral Rh(II) complexes allow construction of both carbocyclic and heterocyclic systems in optically active form,7-10 and much effort has been directed toward understanding the factors that control both regioand enantioselectivities.7,11,12 The critical intermediate presumed to unify this body of chemistry is an electrophilic rhodium carbenoid in which a metalaxial carbene adopts a linear Rh-Rh-C arrangement (3, R2 ) COR4). A growing number of examples demonstrate that the reactivity of rhodium carbenoids is greatly affected by the electronic nature of the bridging ligand attached to the metal.1 Accordingly, the reactivity-determining Rh-C interaction is regarded as highly polarizable,7,13 although steric14 and conformational effects15 inherent within a particular system can override electronic factors. When the latter operate, the selectivity of the rhodium carbenoid intermediate is less predictable.