A density functional theory study of the mechanisms of addition of transition metal oxides ReO3L (L=Cl-, O-, OCH3, CH3) to substituted ketenes

摘要

Ketenes are excellent precursors for catalytic asymmetric reactions, creating chiral centers mainly through addition across their C=C bonds. Density functional theory (DFT) calculations at the MO6/LACVP* and B3LYP/LACVP* levels of theory were employed in a systematic investigation of the peri-, chemo-and regio-selectivity of the addition of transition metal oxo complexes of the type ReO3L (L=Cl, O-, OCH3, CH3) to substituted ketenes O=C=C(CH3) (X) [X=CH3, H, CN, Ph] with the aim of elucidating the effects of substituents on the mechanism of the reactions. The [2 + 2] addition pathway across the C=C or C=O (depending on the ligand) is the most preferred in the reactions of dimethyl ketene with all the metal complexes studied. The [2 + 2] pathway is also the most preferred in the reactions of ReO3Cl with all the substituted ketenes studied except when X=Cl. Thus of all the reactions studied, it is only the reaction of ReO3Cl with O=C=C(CH3)(Cl) that prefers the [3 + 2] addition pathway. Reactions of dimethyl ketene with ReO3L favors addition across C=O bonds of the ketene when L=O- and CH3 but favors addition across C=C bonds when L=OCH3 and Cl. In the reactions of ReO3Cl with substituted ketenes, addition across C=O bonds is favored only when X=H while addition across C=C bonds is favored when X=CH3, Cl, Ph, CN. The reactions of dimethyl ketene with ReO3L will most likely lead to the formation of an ester precursor in each case. A zwitterionic intermediate is formed in the reactions except in the reactions of ReO4-. The order in the activation energies of the reactions of dimethyl ketenes with the metal complexes ReO3L with respect to changing ligand L is O- < CH3O- < Cl- < CH3 while the order in reaction energies is CH3 < CH3O- < O- < Cl-. For the reactions of substituted ketenes with ReO3Cl, the order in activation barriers is CH3 < Ph < CN < Cl < H while the reaction energies follow the order Cl < CH3 < H < Ph < CN. In the reactions of dimethyl ketenes with ReO3L, the trend in the selectivity of the reactions with respect to ligand L is Cl- < CH3O- < CH3 < O- while the trend in selectivity is CH3 < CN < Cl < Ph in the reactions of ReO3Cl with substituted ketenes. It is seen that reactions involving a change in oxidation state of metal from the reactant to product have high activation barriers while reactions that do not involve a change in oxidation state have low activation barriers. For both [3 + 2] and [2 + 2] additions, low activation barriers are obtained when the substituent on the ketene is electron-donating while high activation barriers are obtained when the substituent is electron-withdrawing.