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Ni 0 ‐Catalyzed Cyclotrimerization of 1,3‐Butadiene: A Comprehensive Density Functional Investigation on the Origin of the Selectivity
Author(s) -
Tobisch Sven
Publication year - 2003
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.200390139
Subject(s) - isomerization , catalysis , reductive elimination , chemistry , 1,3 butadiene , allylic rearrangement , selectivity , oxidative coupling of methane , catalytic cycle , oxidative addition , density functional theory , medicinal chemistry , stereochemistry , photochemistry , computational chemistry , organic chemistry
A comprehensive theoretical investigation of the mechanism for the Ni 0 ‐catalyzed cyclotrimerization of 1,3‐butadiene by the [Ni 0 ( η 2 ‐butadiene) 3 ] active catalyst complex is presented by employing a gradient‐corrected DFT method. All critical elementary processes of the catalytic cycle have been scrutinized, namely, oxidative coupling of two butadienes, butadiene insertion into the allyl–Ni II bond, allylic isomerization in both octadienediyl–Ni II and dodecatrienediyl–Ni II species, and reductive elimination under ring closure. For each of these elementary steps several conceivable routes and also the different stereochemical pathways have been probed. The favorable route for oxidative coupling start from the prevalent [Ni 0 ( η 2 ‐butadiene) 3 ] form of the active catalyst through coupling between the terminal non‐coordinated carbon atoms of two reactive η 2 ‐butadiene moieties; this is assisted by an ancillary butadiene in η 2 ‐mode. The initial η 3 , η 1 (C 1 )‐octadienediyl–Ni II product is the active precursor for subsequent butadiene insertion, which preferably takes place into the η 3 ‐allyl–Ni II bond. The insertion is driven by a strong thermodynamic force. Therefore, the dodecatrienediyl–Ni II products, with the most favorable bis( η 3 ‐allyl), Δ ‐ trans isomers in particular, represent a thermodynamic sink. Commencing from a preestablished equilibrium between the various bis( η 3 ‐allyl), Δ ‐ trans forms of the [Ni II (dodecatrienediyl)] complex, the major cyclotrimer products, namely all‐ t ‐CDT, c , c , t ‐CDT and c , t , t ‐CDT, are formed along competing paths by reductive elimination under ring closure, which is shown to be rate‐controlling. The all‐ c ‐CDT‐generating path is completely precluded by both thermodynamic and kinetic factors, giving rise to negligibly populated bis( η 3 ‐allyl), Δ ‐ cis precursor isomers. The regulation of the selectivity of the CDT formation as well as the competition between the two reaction channels for generation of C 12 ‐ and C 8 ‐cycloolefins is elucidated.