Web Release Date: October 31,
Mechanistic Studies of Olefin and Alkyne Trimerization with Chromium Catalysts: Deuterium Labeling and Studies of Regiochemistry Using a Model Chromacyclopentane Complex
Contribution from the Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125
Received May 16, 2007
Abstract:
A system for catalytic trimerization of ethylene utilizing chromium(III) precursors supported by
diphosphine ligand PNPO4 = (o-MeO-C6H4)2PN(Me)P(o-MeO-C6H4)2 has been investigated. The mechanism of the olefin trimerization reaction was examined using deuterium labeling and studies of reactions
with 
-olefins and internal olefins. A well-defined chromium precursor utilized in this studies is Cr(PNPO4)(o,o'-biphenyldiyl)Br. A cationic species, obtained by halide abstraction with NaB[C6H3(CF3)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 (PNPO4)Cr
precursors, of a 1:1 mixture of C2D4 and C2H4 gives isotopologs of 1-hexene without H/D scrambling (C6D12,
C6D8H4, C6D4H8, and C6H12 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 C2D4
and C2H4 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-d2 upon activation of Cr(PNPO4)(o,o'-biphenyldiyl)Br with NaB[C6H3(CF3)2]4. The trimerization of cis- and trans-ethylene-d2 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)[(CH2)4CH=CH2] 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(PNPO4)(o,o'-biphenyldiyl)Br. -Olefins react with
cationic biphenyldiyl chromium species to generate products from 1,2-insertion. A study of the reaction of
2-butenes indicated that 
-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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