A Computational Study of the Hofmann Elimination Pathway for the MoritaBaylis-Hillman Reaction Under DABCO Catalysis. Participation of a Bridge-Head Ylide

In comparison to the popular pathway involving proton-transfer via a four-centred cyclic transition state structure, the recently proposed overall lower energy pathway involving proton-transfer via a seven-centred cyclic transition state structure followed by Hofmann elimination for the Me₃N-catalyzed Morita-Baylis-Hillman reaction is applicable to the DABCO-catalyzed reaction equally well. This finding clearly establishes that the zwitterion at the bridge-head in DABCO is well tolerated. Also, the activation free energy of the rate-limiting aldol reaction under DABCO-catalysis is lower than that under Me₃N-catalysis, suggesting that DABCO is likely a better catalyst to achieve faster conversion.