GEOMETRICAL ASPECTS OF THE ACTIVATION OF KETONES BY LEWIS-ACIDS

The rates of reaction of the TiCl4 Complexes of two diastereomeric 1-(methoxymethyl)-1-propyldecahydronaphthalen-2-ones (1a and 1e) with trimethylsilyl cyanide have been measured for the purpose of determining the preferred complexation geometry for the activation of carbonyls toward nucleophilic addition. Compound 1a incorporates an axial methoxymethyl group which is designed to steer complexation of TiCl4 to the pi-system of the carbonyl, while an equatorial methoxymethyl group in le directs complexation into the plane. Chemical shift changes in the H-1 and C-13 NMR spectra of 1a and 1e as TiCl4 is added are indicative of the formation of 1:1 complexes. In the case of 1a, complex formation involves a conformational change of the cyclohexanone portion of the naphthalenone from a chair to a twist-boat; no such conformational change is observed for 1e. At -45-degrees-C in CH2Cl2, 0.042 M 1e.TiCl4 and 1a-TiCl4 react with 0.38 M Me3SiCN to give cyanohydrin products with pseudo-first-order rate constants of 0.028 and 0.0069 min-1, respectively. These results exclude the possibility that the reaction of ketone-TiCl4 complexes proceeds through a small concentration of a highly reactive pi-complexed intermediate.