Introduction
Unlike free carbenes, Rh(II)-catalyzed carbenoids often undergo highly regio- and stereoselective intra- and intermolecular insertion into a sigma bond. Although many transition-metal complexes afford carbenoids, only those of Rh(II) have shown general applicability due to their higher selectivity. The pioneering development of Rh(II) acetate by Teyssie and co-workers
[1]
has resulted in highly chemo-, regio- and stereoselective reactions of α-diazocarbonyl compounds via a variety of reactivity modes. Subsequently, an extensive library of successful transformations rapidly evolved, ranging from Rh(II)-catalyzed OH
[2]
and NH
[3]
insertion to cyclopropanation of olefins
[4]
and aromatic systems.
[5]
Rh(II)-catalyzed reactions can often be performed in the presence of water. Because of the high partition of Rh2(OAc)4 in the water phase, the catalyst can be effectively reused,
[6]
permitting the development of a more environmentally benign process. Dirhodium tetraacetate is conveniently prepared by refluxing RhCl3·3H2O and sodium acetate/acetic acid in ethanol and is now also commercially available. The Rh2(OAc)4 core has approximately D
4h symmetry, with the Lewis base adducts H2O in 1 coordinating to the sites trans to the Rh-Rh bond.