Mechanistic Studies of Ethylene and α-Olefin Co-Oligomerization Catalyzed by Chromium−PNP Complexes

摘要

To explore the possibility of producing a narrow distribution of mid- to long-chain hydrocarbons fromudethylene as a chemical feedstock, co-oligomerization ofudethylene and linear α-olefins (LAOs) was investigated, usinguda previously reported chromium complex, [CrCl_3(PNP^(OMe))]ud(1, where PNP^(OMe) = N,N-bis(bis(o-methoxyphenyl)-udphosphino)methylamine). Activation of 1 by treatment withudmodified methylaluminoxane (MMAO) in the presence ofudethylene and 1-hexene afforded mostly C_6 and C_(10) alkeneudproducts. The identities of the C_(10) isomers, assigned byuddetailed gas chromatographic and mass spectrometric analyses, strongly support a mechanism that involves five- and seven-membered metallacyclic intermediates comprised of ethylene and LAO units. Using 1-heptene as a mechanistic probe, it was established that 1-hexene formation from ethylene is competitive with formation of ethylene/LAO cotrimers and that cotrimers derived from one ethylene and two LAO molecules are also generated. Complex 1/MMAO is also capable of converting 1-hexene to C_(12) dimers and C_(18) trimers, albeit with poor efficiency. The mechanistic implications of these studies are discussed andudcompared to previous reports of olefin cotrimerization.