How the electron-deficient Cp ligand facilitates Rh-catalyzed annulations with alkynes

The mechanism and origin of the CpX ligand effects on Rh-catalyzed annulations with alkynes were investigated using DFT calculations and the approach of energy decomposition analysis (EDA). The results reveal that the alkyne migratory insertion is the rate-determining step for the reactions with both acetanilide and 2-phenyl-2-propanol substrates. The higher reactivities with CpE than with Cp* are due to the promotion of alkyne insertion. EDA results reveal that the dominant factors for the acceleration effect of CpE are different depending on the electronic properties of rhodacycle intermediates. The alkyne insertion with the cationic rhodacycle is promoted by CpE due to the stronger stabilizing electronic interactions, including charge transfer, electrostatics, and polarization, between the cationic rhodacycle and alkyne. For the neutral rhodacycle, the higher reactivity for alkyne insertion with CpE is because of having smaller destabilizing Pauli repulsion between the occupied Rh d orbitals and the alkyne π orbital. The origins of differences in these stabilizing and destabilizing intermolecular interactions are further identified.