Theoretical Investigations of the Influence of Pressure on the Selectivity of the Michael Addition of Diphenylmethaneamine to Stereogenic Crotonates

• Published in: Journal of organic chemistry 1999-06, Vol.64 (13), p.4725-4732
• Main Authors: Dumas, F, Fressigné, C, Langlet, J, Giessner-Prettre, C
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo9825308

SIBFA (sum of interactions between fragments ab initio computed) molecular mechanics systematics has been modified to take into account the effect of pressure on intra- and intermolecular energies. The van der Waals radii are related to the pressure, using Bridgman results on the variation of crystal volume, on one hand, and the relation between the volume of an atom and its van der Waals radius on the other. This procedure produces a decrease of the volume of the systems considered. The modified systematics is used for the study of the conformation at 1 atm and 15 kbar of two stereogenic crotonates and of the complexes formed by these two molecules with the diphenylmethaneamine and the three solvent molecules present in the experiment. The results obtained show that in the case of NMECC 1a the diastereoselectivity induced by high pressure and by the presence of methanol proceeds from an important stabilization of the pro-R reactive complex in which the crotonate has a stacked-transoid conformation. This stabilization is mainly due to intermolecular interactions. In the case of the second crotonate considered, NMCC 1b, our results indicate that pressure induces a stabilization of the pro-R and pro-S complexes having the axial conformation for which the reaction exhibits little diastereoselectivity in qualitative agreement with experimental data. This study tends to show that it is possible to account theoretically for the influence of pressure on molecular conformation and/or complex structure, using a molecular mechanics method that is able to take into account the variation of volumes of the different entities present in the system studied.