Oligomerization of α-Olefins via Chromium Metallacycles

Chromium complexes that oligomerize and polymerize olefins via a metallacycle mechanism represent possible models for the commercially important Phillips catalyst, which is still poorly understood mechanistically. Comparisons between the two might provide insight into the process, and as such the oligomerization of α-olefins (propene to 1-octene) with chromium(III)−bis(carbene)pyridine complexes in combination with MAO has been studied. Linear α-olefins are homo-oligomerized, as well as co-oligomerized with ethylene, via a mechanism most likely involving metallacycles. Homo-oligomerization of α-olefins leads predominantly to head-to-tail dimers with vinylidene unsaturation, while a less favorable coupling leads to linear internal olefin dimers. With shorter chain monomers, trimerization and tetramerization become more significant, albeit still minor processes. The kinetics of 1-octene dimerization were studied and are found to be first-order in chromium but zero-order in 1-octene concentration. The results are interpreted in terms of the likely rate-determining step of the reaction, and comparisons are drawn between the behavior of this system and the heterogeneous Phillips catalyst.