(S)-Proline-catalyzed nitro-Michael reactions: towards a better understanding of the catalytic mechanism and enantioselectivity.

(S)-Proline-catalyzed nitro-Michael additions of aldehydes and ketones to β-nitrostyrene were investigated computationally (MP2/6-311+G**//M06-2X/6-31G**). Contrary to what is usually assumed in organocatalysis, the lowest-energy transition states of proline-catalyzed nitro-Michael reactions do not necessarily involve the carboxylic acid group of the proline moiety directing the incoming nitroalkene to the same face through hydrogen bonding. For the aldehyde substrates examined, the TS leading to the major (R,S) product was found to involve the anti-enamine and nitroalkene approaching from the opposite face of the carboxyl group. In the case of ketone substrates, the lowest-energy TSs leading to both enantiomeric products are characterized by the absence of hydrogen bonds and s-cis conformation of the carboxyl group, which functions as an electron donor to stablize the developing iminium. When both hydrogen bonded and non-hydrogen bonded types of TSs are considered, the calculated enantioselectivities for Michael additions of aldehyde and ketone substrates are in good agreement with experimental findings.