Insight into the Mechanism and Stereochemistry of the Transformations of Alkyltitanium Ate-Complexes. An Enhanced Enantioselectivity in the Cyclopropanation of the Carboxylic Esters with Titanacyclopropane Reagents

The dependence of the stereoselectivity of the cyclopropanation reaction of gamma,gamma-diphenyl-gamma-butyrolactone and carboxylic esters with alkylmagnesium bromides in the presence of titanium(IV) TADDOLates on the structure of the reactants has been examined in detail. It has been found that the enantioselectivity in the formation of cis-1,2-disubstituted cyclopropanols is a function of the structure of the carboxylic ester alkoxy group and leads, in the case of cyclopropanation of hexafluoroisopropyl alkan-oates, to 84% ee. To rationalize these observations, as well as the NMR data on the structures of the TADDOL-derived alkyltitanium species, a new mechanistic model for the formation of the five-coordinated chiral titanacyclopropane reactive intermediates and their subsequent reorganization in the reactions with esters is proposed. The essential features of this model consist in the generation of configurationally stable TADDOL derived titanacyclopropane species with an apical or equatorial position of the stereogenic carbanion center which is followed by retention or inversion of absolute configuration in course of the displacement of the carboxylic ester alkoxy group.