The mechanism of enantioselective control of an organocatalyst with central and axial chiral elements

The mechanism of the enantioselective control of an organocatalyst with central and axial chiral elements in the Michael addition of 2,4-pentandione to a nitroalkene is investigated using density functional theory (DFT) calculations. Two enantioselective channels are characterized in detail. Enantioselectivity is determined in the C-C bond coupling and the proton transfer is identified as the energetic bottleneck. Generally, the level of enantioselectivity of the catalysts depends on the geometrical match or mismatch of two asymmetric elements. The 'closed' geometry of a catalyst makes the cooperation of two chiralities possible, so that the central and axial chiralities work together to enhance the enantioselective control. The 'open' structure of catalyst makes cooperation of the two asymmetric elements impossible, so that its enantioselectivity dominated only by one type of chirality is decreased.