Mechanism of halogen-catalyzed Mukaiyama aldol reactions: concerted or stepwise?

The halogen-catalyzed (I2, Br2, and Cl2) Mukaiyama aldol (MA) reactions were investigated by ab initio MO calculations. The halogen-catalyzed MA reaction between a trihydrosilyl enol ether and formaldehyde favors a concerted pathway. In sharp contrast, the I2-catalyzed reaction between 1-phenyl-1-(trimethylsilyloxy)ethylene and benzaldehyde favors a stepwise mechanism. The nature of the substituent strongly influences the type of mechanism involved.