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Mechanism of Reppe's Nickel‐Catalyzed Ethyne Tetramerization to Cyclooctatetraene: A DFT Study
Author(s) -
Straub Bernd F.,
Gollub Caroline
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305666
Subject(s) - cyclooctatetraene , homoleptic , chemistry , isomerization , ligand (biochemistry) , nickel , catalysis , photochemistry , reaction mechanism , crystallography , stereochemistry , metal , molecule , organic chemistry , biochemistry , receptor
In this B3 LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C 2 H 2 ) 3 yields monoethyne nickelacyclopentadiene in the rate‐determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the NiC σ bond. A highly strained monoethyne trans ‐nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis ‐structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(η 2 ‐C 2 H 2 )(η 2 ‐C 8 H 8 ), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, CC bond formation steps at Ni 2 (μ‐cot) fragments have been predicted to feature prohibitively high overall reaction barriers.