QM/MM modeling of enantioselective pybox-ruthenium- and box-copper-catalyzed cyclopropanation reactions: scope, performance, and applications to ligand design.

An extensive comparison of full-QM (B3LYP) and QM/MM (B3LYP:UFF) levels of theory has been made for two enantioselective catalytic systems, namely, Pybox-Ru and Box-Cu complexes, in the cyclopropanation of alkenes (ethylene and styrene) with methyl diazoacetate. The geometries of the key reaction intermediates and transition structures calculated at the QM/MM level are generally in satisfactory agreement with full-QM calculated geometries. More importantly, the relative energies calculated at the QM/MM level are in good agreement with those calculated at the full-QM level in all cases. Furthermore, the QM/MM energies are often in better agreement with the stereoselectivity experimentally observed, and this suggests that QM/MM calculations can be superior to full-QM calculations when subtle differences in inter- and intramolecular interactions are important in determining the selectivity, as is the case in enantioselective catalysis. The predictive value of the model presented is validated by the explanation of the unusual enantioselectivity behavior exhibited by a new bis-oxazoline ligand, the stereogenic centers of which are quaternary carbon atoms.