Mechanistic studies of the ethylene trimerization reaction with chromium-diphosphine catalysts: Experimental evidence for a mechanism involving metallacyclic intermediates

A system for catalytic trimerization of ethylene utilizing CrCl3(THF)3 and a diphosphine ligand PNPOMe [=(o-MeO-C6H4)2PN(Me)P(o-MeO-C6H4)2] has been investigated. The coordination chemistry of chromium with PNPOMe has been explored, and (PNPOMe)CrCl3 and (PNPOMe)CrPh3 (3) have been synthesized by ether displacement from chromium(III) precursors. Salt metathesis of (PNPOMe)CrCl3 with o,o′-biphenyldiyl Grignard affords (PNPOMe)Cr(o,o′-biphenyldiyl)Br (4). Activation of 3 with H(Et2O)2B[C6H3(CF3)2]4 or 4 with NaB[C6H3(CF3)2]4 generates a catalytic system and trimerizes a 1:1 mixture of C2D4 and C2H4 to give isotopomers of 1-hexene without H/D scrambling (C6D12, C6D8H4, C6D4H8 C6H12a 1:3:3:1 ratio). The lack of crossover supports a mechanism involving metallacyclic intermediates. The mechanism of the ethylene trimerization reaction has also been studied by the reaction of trans-, cis-, and gem-ethylene-d2 with 4 upon activation with NaB[C6H3(CF3)2]4. Copyright © 2004 American Chemical Society.