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Selective Ethylene Oligomerization in the Presence of ZnR2: Synthesis of Terminally-Functionalized Ethylene Oligomers

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journal contribution
posted on 23.08.2010, 00:00 by Kyung-sun Son, Robert M. Waymouth
The selective oligomerization of ethylene to 1-hexene or 1-octene provides a selective route to these valuable α-olefins. Selective trimerization and tetramerization of ethylene with Cr catalysts have been proposed to occur by a mechanism involving metallacycle intermediates. We envisioned that the intermediacy of metallacycles could provide an opportunity to generate new classes of functionalized ethylene oligomers by chain transfer to a transmetalation reagent. Herein, we report the influence of transmetalation agents such as zinc alkyls on the selectivity of ethylene oligomerization with a Cr(PNP)Cl3/MAO system (PNP = Ph2PN(iPr)PPh2). Oligomerization of ethylene with Cr(PNP)Cl3/MAO at 200 psig of ethylene at 25 or 45 °C generates 1-hexene, 1-octene, C10−C22 oligomers, and polyethylene. The addition of ZnR2 increases the productivity and selectivity for C10−C22 oligomers and results in a decrease in the amount and molecular weight of the polyethylene generated. Analysis of the product distribution after quenching with D2O reveals that 1-hexene and 1-octene are unlabeled, but that all higher oligomers are deuteriated alkanes or alkenes, depending on the nature of the zinc alkyl (R = Me, Et, Bu). Mechanistic proposals are presented to explain the formation of differentially functionalized coproducts in the presence of zinc alkyls. These studies reveal that oligomerization in the presence of ZnR2 provides a new route to end-functionalized ethylene oligomers and new insights on the reactivity of metallacycle intermediates in the Cr-catalyzed oligomerization of ethylene.