Rhodium-catalyzed C-C coupling reactions: Mechanistic considerations

An alternative mechanism for intramolecular C-C coupling between heterocycles and alkenes with rhodium phosphine catalysts is presented involving oxidative addition, alkene insertion, and reductive elimination (route 2), as described previously for similar group 10 reactions by Cavell and McGuinness. Computational studies indicate that the rate-determining step is associated with reductive elimination of the product and overall barriers indicate this mechanism would be competitive with an alternative involving formation of a carbene complex derived from mechanistic work by Bergman, Ellman, and associates (route 1). Activation of the reacting azole through inclusion of an acid catalyst appears to support the route 2 mechanism. A much lower activation barrier is observed under acidic conditions, a result consistent with that found under experimental conditions.