ORGANOMETALLIC complexes are used to effect a wide range of catalytic transformations in organic synthesis, such as the activation of C-H bonds(1,2). Carbon-carbon bonds, however, are generally unreactive towards transition metals under homogeneous conditions. C-C bond activation by a process of oxidative addition to soluble transition-metal complexes has been limited mostly to stoichiometric (not catalytic) reactions(1,3-7,18), to highly strained substrates such as cyclopropane and cubane(1,8-11) or to chelating ketones(19). Here we present a synthetically useful process of selective C-C bond activation in which the C-C bond adjacent to a carbonyl group is opened by insertion of a soluble rhodium(I) complex. The resulting organometallic intermediate can be transformed to a variety of products in a way that regenerates the rhodium complex. We anticipate that this catalytic scheme will have considerable utility in organic synthesis.