Density Functional Theory Study of Rh(III)-Catalyzed C–H Activations and Intermolecular Annulations between Benzamide Derivatives and Allenes

Density functional theory has been applied to gain insight into the Cp*Rh­(OAc)₂-catalyzed C–H activation and intermolecular annulation of benzamide derivatives with allenes. The study shows that the reactions proceed in three steps: (1) C–H activation induced by Rh catalyst reacting with benzamide derivatives, (2) carborhodation of allene, and (3) regeneration of Rh catalyst. The results indicate that the N–H deprotonation makes the following C–H activation much easier. The regio- and stereoselectivities of <b>1a</b> (<i>N</i>-pivaloyloxy benzamide)/<b>2a</b> (cyclohexylallene) and <b>1b</b> (<i>N</i>-pivaloyloxy-4-methyl-benzamide)/<b>2b</b> (1,1-dimethyl allene) depend on the allene carborhodation step. The steric hindrance effect is the dominant factor. We also discuss the reaction mechanism of <b>1c</b> (<i>N</i>-methoxy benzamide)<b>/2a</b>. The chemoselectivity between <b>1c/2a</b> is determined by the N–O cleavage step. Replacement of OPiv by OMe leads to loss of the stabilization effect provided by CO in OPiv. Additionally, Cp*Rh­(OAc)­(OPiv) is produced in the Cp*Rh­(OAc)₂ regeneration step, which can work as catalyst as well