Role of amine modifiers in the epoxidation of allylic alcohols with a TiO2-SiO2 aerogel

A detailed study of the epoxidation of 3-methyl-2-cyclohexen-1-ol with tert-butylhydroperoxide revealed that the poor performance of a 20 wt% TiO2-80 wt% SiO2 aerogel was due to nonoxidative consumption of the allylic alcohol. Epoxide selectivities could be improved remarkably and acid-catalyzed side reactions suppressed by addition of small amounts of aliphatic, cycloaliphatic, or aromatic amines. The best modifier was N, N-dimethylbutylamine. Amine (1 mol%) enhanced the epoxide selectivities, related to the reactant or peroxide, from 3 to 99% and 35 to 100%, respectively. Kinetic investigations uncovered how the chemical structure and the amount of various amines influence the complex network of redox- and acid-catalyzed reactions during allylic alcohol epoxidations. The stability of amines was studied under oxidizing reaction conditions. The method of amine addition was applied also to the epoxidation of other linear and cyclic allylic alcohols and 2-hexene. The scope of this method seems to be limited to epoxidation of allylic alcohols. A model for the interaction of allylic alcohol, amine, and peroxide with the Ti active site is proposed, which can interpret the enhanced selectivity and suppressed activity in the presence of amines or other bases. (C) 1999 Academic Press.