A proline mimetic for enantioselective aldol reaction: a quantum chemical study of a catalytic reaction with a sterically hindered l‐prolinamide derivative

The direct aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by sterically hindered l-prolinamide derivative (64 atoms) of the (11S,12S)-9,10-dihydro-9,10-ethanoanthracene-11,12-diamine molecule has been investigated using density functional theory at the B3LYP/6-31G(d) level of theory. The reliability of the B3LYP/6-31G(d) calculations to elucidate the reaction mechanism and estimate activation and reaction energies was confirmed by energy calculation of the net reaction with full geometry optimizations of the reactants and product at the B3LYP/6-311++G(2d,2p) as well as B2PLYP/def2-TZVPP levels with correction to Van der Waals interaction. The calculations reveal that the l-prolinamide derivative catalyzes the reaction according to a multistep enamine mechanism with highly activated C–C bond and/or enamine formation in the proposed mechanism. The final elementary reaction – the C–N bond cleavage in the chiral diol adduct – is accompanied by a very large barrier, which may inhibit further progression of the reaction. The origin of the enantioselectivity and the corresponding reaction paths to a chiral product were unambiguously identified. Copyright © 2012 John Wiley & Sons, Ltd.