| Abstract: | Seven polymer-supported Pd(II) catalysts have been prepared employing polymers carrying nitrile (cyanomethyl) ligands. Five of these involve polybenzimidazole backbones, one a polystyrene skeleton and the last a polyacrylonitrile backbone. The supported complexes have been used with CuCl2 co-catalyst to oxidise dec-1-ene primarily to the methylketone under normal Wacker oxidation conditions. In some instances the supported complexes are more active than the (CH3CN)2PdCl2 model. The most active species is a highly rigid - cyanomethylated polybenzimidazole and at least some of these metal centres may be coordinatively unsaturated. This catalyst also displays remarkable thermo-oxidative stability up to ～400°C. The effect of solvent, temperature and co-catalyst ratio have been examined. The polymer-supported species remain very active at high temperature (～120°C) and require addition of no hydrochloric acid to avoid irreversible precipitation of Pd(O) species. This is a complete contrast to homogeneous PdCl2 which is rapidly deactivated under similar conditions with copious Pd black formation. Pd(O) complexes immobilised on the polymer seem to be “site isolated” and unable to aggregate. Re-oxidation therefore remains facile. The polymer-supported species have been recycled seven times. An initial fall in activity levels after 3 cycles and thereafter remains essentially constant. Appreciable Pd leaching also occurs in the first reaction but is rapidly arrested. After ～6 cycles Pd loss is only ～1 ppm per cycle. Following use of CuCl2 co-catalyst in the first cycle, no additional Cu(II) needs to be added, and sufficient co-catalyst appears to be carried through with the isolated polymer-supported Pd(II) species. 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 in the literature this is significant. |
