Mechanistic Studies of the Ethylene Trimerization Reaction with Chromium-Diphosphine Catalysts: Experimental Evidence for a Mechanism Involving Metallacyclic Intermediates

Article abstract of DOI:10.1021/ja038968t

A system for catalytic trimerization of ethylene utilizing CrCl₃(THF)₃ and a diphosphine ligand PNP^OMe [=(o-MeO-C₆H₄)₂PN(Me)P(o-MeO-C₆H₄)₂] has been investigated. The coordination chemistry of chromium with PNP^OMe has been explored, and (PNP^OMe)CrCl₃ and (PNP^OMe)CrPh₃ (3) have been synthesized by ether displacement from chromium(III) precursors. Salt metathesis of (PNP^OMe)CrCl₃ with o,o′-biphenyldiyl Grignard affords (PNP^OMe)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