Origins of stereoselectivity in the Diels-Alder addition of chiral hydroxyalkyl vinyl ketones to cyclopentadiene: a quantitative computational study

Modest basis set level MP2/6-31G(d,p) calculations on the Diels-Alder addition of S-1-alkyl-1-hydroxy-but-3-en-2-ones (1-hydroxy-1-alkyl methyl vinyl ketones) to cyclopentadiene correctly reproduce the trends in known experimental endo/exo and diastereoface selectivity. B3LYP theoretical results at the same or significantly higher basis set level, on the other hand, do not satisfactorily model observed endo/exo selectivities and are thus unsuitable for quantitative studies. The same is valid also with regard to subtle effects originating from, for example, conformational distributions of reactants. The latter shortcomings are not alleviated by the fact that observed diastereoface selectivities are well-reproduced by DFT calculations. Quantitative computational studies of large cycloaddition systems would require higher basis sets and better account for electron correlation than MP2, such as, for example, CCSD. Presently, however, with 30 or more non-hydrogen atoms, these computations are hardly feasible. We present quantitatively correct stereochemical predictions using a hybrid layered ONIOM computational approach, including the chiral carbon atom and the intramolecular hydrogen bond into a higher level, MP2/6-311G(d,p) or CCSD/6-311G(d,p), layer. Significant computational economy is achieved by taking account of surrounding bulky (alkyl) residues at 6-31G(d) in a low HF theoretical level layer. We conclude that theoretical calculations based on explicit correlated MO treatment of the reaction site are sufficiently reliable for the prediction of both endo/exo and diastereoface selectivity of Diels-Alder addition reactions. This is in line with the understanding of endo/exo selectivity originating from dynamic electron correlation effects of interacting pi fragments and diastereofacial selectivity originating from steric interactions of fragments outside of the Diels-Alder reaction site.