Cr(VI) Oxidation of Cholesterol—A Kinetic Study Using N-Cetylpicolinium Dichromates, A Class of Novel Phase Transfer Oxidants

Resumo

Kinetic study of cholesterol oxidation has been studied using a series of N-cetylpicolinium dichromates (CPDC), a class of phase transfer oxidants, in acetic acid medium under first order conditions with respect to oxidant. Rate constants were calculated in the temperature range 290–300 K. The kinetics was followed spectrophotometrically; cholest-5-en-3-one is found to be the only oxidation product. Unlike the previously reported lipopathic oxidant containing cetyltrimethylammonium ions, these oxidants show a direct variation of rate with the oxidant concentration ruling out any reversed micellar organization of the oxidant molecules. From the experimental data formation of an unstable cyclic transition state followed by intra-molecular proton transfer has been proposed. Solvent isotope effect for the cholesterol oxidation (\({{{{k}_{{{\text{CHC}}{{{\text{l}}}_{3}}}}}} \mathord{\left/ {\vphantom {{{{k}_{{{\text{CHC}}{{{\text{l}}}_{3}}}}}} {{{k}_{{{\text{CDCC}}{{{\text{l}}}_{3}}}}}}}} \right. \kern-0em} {{{k}_{{{\text{CDCC}}{{{\text{l}}}_{3}}}}}}}\) = 1.52) indicated a carbon-hydrogen cleavage rather than a carbon-carbon cleavage. Variation of solvent polarity is found to impose a remarkable impact on the rate of oxidation: more polar reaction environment favours the oxidation by β-CPDC oxidant to a higher extent, compared to the other two oxidant isomers, α-CPDC and γ-CPDC.