The mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals: VI. The selectivity-determining step on Pt

Abstract

During studies of the hydrogenolysis and isomerization of the 2-Me-oxacycloalkanes, it was found that the less sterically hindered C-O bond undergoes cleavage on a Pt catalyst. From the observed regioselectivity, it could be concluded that the primary C-O bond is split much more quickly than the secondary C-O bond. To prove this assumption the authors compared the rates of hydrogenolysis and isomerization of 2-Me-oxacycloalkanes and the corresponding cis-diMe-oxacycloalkanes under the same experimental conditions. The experimental results show that the secondary C-O bond is cleaved substantially more quickly than the primary C-O bond. Hence, for the 2-Me derivatives it must be assumed that the selectivity-determining step is not the same as the rate-determining step, but precedes this. Since the rate-determining step is probably the cleavage of the C-O bond, the selectivity-determining step may be the adsorption of the C-O bond. The determining factor, therefore, is the preferred adsorption of the primary carbon atom regardless of whether the mechanism is dissociative or associative. 9 references.