The scope of the enantioselective Mukaiyama-Michael reactions catalyzed by trans-2,5-diphenylpyrrolidine has been expanded to include both α- and β-substituted enals. However, the rationalization of the observed enantioselectivity is far from obvious since the catalyst is not very sterically hindered. DFT calculations were carried out to rationalize the observed stereoselectivities. Transition states of the C-C bond formation between iminium intermediates and silyloxyfurans were located and their relative energies were used to estimate the stereoselectivity data. We find excellent agreement between the predicted and observed stereoselectivities. The analysis of intermolecular forces reveals that the enantioselectivity is mostly due to stabilizing noncovalent interactions between the reacting partners, not due to steric hindrance. The role of attractive noncovalent interactions in enantioselective catalysis may be underappreciated.
Keywords: DFT calculations; enantioselectivity; noncovalent interactions; organocatalysis; transition states.
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