Advanced procedure for the preparation of cis-1,2-dialkylcyclopropanols -: Modified ate complex mechanism for titanium-mediated cyclopropanation of carboxylic esters with Grignard reagents

A procedure for the preparation of cis-1,2-dialkylcyclopropanols by titanium(IV) alkoxide-mediated cyclopropanation of carboxylic esters with Grignard reagents, involving the addition of 1.5 equiv. of a higher homologue of ethylmagnesium halide to a mixture of 1 equiv. of carboxylic ester, 1 equiv. of titanium(IV) isopropoxide, and 1.5 equiv. of methylmagnesium halide in ether or tetrahydrofuran at room temperature, has been elaborated. This procedure minimizes the formation of secondary alcohol side products with chromatographic retention factors close to those of the cis-1,2-disubstituted cyclopropanols. Inhibitory action of carboxylic esters toward the reduction of titanium(IV) isopropoxide with Grignard reagents was observed. This observation, along with some other data, allowed us to suggest a modified ate complex mechanism for the cyclopropanation, proceeding via the corresponding octahedral titanium intermediates. In the context of this mechanism, a suitable explanation for the necessity to use an additional equivalent of Grignard reagent in a stoichiometric version of the reaction was found and experimentally verified. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).