Mechanistic Studies of Olefin and Alkyne Trimerization with Chromium Catalysts: Deuterium Labeling and Studies of Regiochemistry Using a Model Chromacyclopentane Complex

Abstract

A system for catalytic trimerization of ethylene utilizing chromium(III) precursors supported by diphosphine ligand PNP^(O4) = (o-MeO−C_6H_4)_2PN(Me)P(o-MeO−C_6H_4)_2 has been investigated. The mechanism of the olefin trimerization reaction was examined using deuterium labeling and studies of reactions with α-olefins and internal olefins. A well-defined chromium precursor utilized in this studies is Cr(PNP^(O4))(o,o'-biphenyldiyl)Br. A cationic species, obtained by halide abstraction with NaB[C_6H_3(CF_3)_2]_4, is required for catalytic turnover to generate 1-hexene from ethylene. The initiation byproduct is vinylbiphenyl; this is formed even without activation by halide abstraction. Trimerization of 2-butyne is accomplished by the same cationic system but not by the neutral species. Catalytic trimerization, with various (PNP^(O4))Cr precursors, of a 1:1 mixture of C_2D_4 and C_2H_4 gives isotopologs of 1-hexene without H/D scrambling (C_6D_(12), C_6D_8H_4, C_6D_4H_8, and C_6H_(12) in a 1:3:3:1 ratio). The lack of crossover supports a mechanism involving metallacyclic intermediates. Using a SHOP catalyst to perform the oligomerization of a 1:1 mixture of C_2D_4 and C_2H_4 leads to the generation of a broader distribution of 1-hexene isotopologs, consistent with a Cossee-type mechanism for 1-hexene formation. The ethylene trimerization reaction was further studied by the reaction of trans-, cis-, and gem-ethylene-d_2 upon activation of Cr(PNP^(O4))(o,o'-biphenyldiyl)Br with NaB[C_6H_3(CF_3)_2]_4. The trimerization of cis- and trans-ethylene-d_2 generates 1-hexene isotopomers having terminal CDH groups, with an isotope effect of 3.1(1) and 4.1(1), respectively. These results are consistent with reductive elimination of 1-hexene from a putative Cr(H)[(CH_2)_4CH═CH_2] occurring much faster than a hydride 2,1-insertion or with concerted 1-hexene formation from a chromacycloheptane via a 3,7-H shift. The trimerization of gem-ethylene-d2 has an isotope effect of 1.3(1), consistent with irreversible formation of a chromacycloheptane intermediate on route to 1-hexene formation. Reactions of olefins with a model of a chromacyclopentane were investigated starting from Cr(PNP^(O4))(o,o'-biphenyldiyl)Br. α-Olefins react with cationic biphenyldiyl chromium species to generate products from 1,2-insertion. A study of the reaction of 2-butenes indicated that β-H elimination occurs preferentially from the ring CH rather than exo-CH bond in the metallacycloheptane intermediates. A study of cotrimerization of ethylene with propylene correlates with these findings of regioselectivity. Competition experiments with mixtures of two olefins indicate that the relative insertion rates generally decrease with increasing size of the olefins.

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