DFT Study on the Rhodium(II)-Catalyzed C-H Functionalization of Indoles: Enol versus Oxocarbenium Ylide

The reaction of carbene insertion into the C-H bond of indoole catalyzed by the Rh-2(HCO2)(4) complex has been investigated in detail through, DFT calculations. Results indicate that the indole can proceed via a nucleophilic attack at the carbene first to generate a carbonium ylide intermediate followed by a [1,4]-proton transfer to give a free enol. In this case, the final C-H insertion product can be Obtained through the self-catalyzed [1,3]-proton shift of enol. Alternatively, the nucleophilic attack can also involve the concerted formations of C-O and C-C bonds to produce an oxocarbonium ylide intermediate. The subsequent [1,2]-proton shift catalyzed by a molecule of enol is also energetically feasible to give the C-H insertion product. It indicates the coexistence of two distinct pathways for the C-H insertion reaction of indole. However, the ratio between them can be regulated by the substituents on both carbenoid and indole. For instance, the enol pathway is always dominant for the phenyl-substituted carbenoid. However, the ratio of the two pathways becomes comparable for the ethyl-substituted carbenoid. The reason is mainly associated with the flexibility of the Rh=C bond of the carbenoid, which plays an important role in determining the approach of indole to the carbenoid. This finding is also useful to understand the reaction mechanisms for the related [3 + 2] annulation and the three-component reactions of indole.