Olefin isomerization by iridium pincer catalysts. experimental evidence for an η 3-allyl pathway and an unconventional mechanism predicted by DFT calculations

Soumik Biswas, Zheng Huang, Yuriy Choliy, David Y. Wang, Maurice Brookhart, Karsten Krogh-Jespersen, Alan S. Goldman

Research output: Contribution to journalArticle

75 Citations (Scopus)

Abstract

The isomerization of olefins by complexes of the pincer-ligated iridium species ( tBuPCP)Ir ( tBuPCP = κ 3-C 6H 3-2,6-(CH 2P tBu 2) 2) and ( tBuPOCOP)Ir ( tBuPOCOP = κ 3-C 6H 3-2,6-(OP tBu 2) 2) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH 2, are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the "hydride addition pathway"); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η 2-olefin) species undergo isomerization via the formation of (pincer)Ir(η 3-allyl)(H) intermediates; one example of such a species, ( tBuPOCOP) Ir(η 3-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ( tBuPOCOP)Ir(η 2-propene). Surprisingly, the DFT calculations indicate that the conversion of the η 2-olefin complex to the η 3-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η 1-allyl hydride which "closes" to give the η 3-allyl hydride. Subsequently, the η 3-allyl group "opens" in the opposite sense to give a new η 1-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.

Original languageEnglish
Pages (from-to)13276-13295
Number of pages20
JournalJournal of the American Chemical Society
Volume134
Issue number32
DOIs
Publication statusPublished - Aug 15 2012

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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