Stepwise versus Concerted Reductive Elimination Mechanisms in the Carbon-Iodide Bond Formation of (DPEphos)RhMeI2 Complex

Reductive elimination is the key bond formation process of organometallic reactions. Goldberg and co-workers recently revealed an unprecedented competition of parallel stepwise reductive elimination pathways for the carbon-iodide bond formation of (DPEphos)RhMeI2 complex. To understand the controlling factors that differentiate the concerted and stepwise pathways, we performed density functional theory (DFT) calculations to elucidate the mechanistic details. The competing stepwise pathways were identified as the anionic and zwitterionic stepwise pathways. The anionic pathway involves the direct S(N)2 attack of the external iodide anion to the methyl group, leading to the observed carbon-iodide bond formation. Alternatively, heterolytic Rh-I bond cleavage generates the cationic (DPEphos)RhMer intermediate, and the subsequent S(N)2 attack of the iodide anion to the methyl group occurs via the zwitterionic transition state. In comparison with the stepwise reductive elimination pathways, the classic concerted pathways require significantly higher barriers. This is due to the energy penalty associated with the orientation change of the methyl group during the classic three-centered reductive elimination. The energy required for this orientation change is highly related to the hybrization of carbon; thus, the selectivity for the stepwise reductive elimination pathways can be switched if the C(sp(2)) or C(sp) group participates in the carbon-iodide bond formation.