Polymer-supported Pd(II) Wacker-type catalysts. Part III. Isomerisation of alkene double bond

A cyanomethylated polybenzimidazole coordinated to Pd(II) has been employed as a Wacker-type alkene oxidation catalyst in aqueous ethanol. Starting with alk-1-enes isomerisation to the more thermodynamically stable internal alkenes is very much faster than oxidation. Indeed after only a short time no alk-1-ene is detectable e.g. by nuclear magnetic resonance analysis. Almost certainly, however, traces of the alk-1-ene do exist in equilibrium. Irrespective of whether the starting alkene is oct-1-ene, t-oct-2-ene or t-oct-4-ene the same three products are obtained: octan-2-one, -3-one and -4-one. In the case of oct-1-ene and t-oct-2-ene the composition of the ketone product mixture is very similar, although with t-oct-4-ene a significant increase in the proportion of the 4-one is observed. The major product in all cases however in the 2-one. The latter almost certainly arises from rapid oxidation of a small stationary concentration of alk-1-ene, with shift of the alkene equilibria maintaining the latter. Direct oxidation of the higher alkenes to the higher ketones occurs more slowly, but contrary to other reports this is significant.