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New Mechanistic Insights into the Iridium–Phosphanooxazoline‐Catalyzed Hydrogenation of Unfunctionalized Olefins: A DFT and Kinetic Study
Author(s) -
Brandt Peter,
Hedberg Christian,
Andersson Pher G.
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.200390029
Subject(s) - alkene , iridium , catalysis , chemistry , catalytic cycle , hydride , density functional theory , reductive elimination , photochemistry , oxidative addition , computational chemistry , substrate (aquarium) , reaction mechanism , hydrogen , organic chemistry , oceanography , geology
The reaction mechanism of the iridium–phosphanooxazoline‐catalyzed hydrogenation of unfunctionalized olefins has been studied by means of density functional theory calculations (B3LYP) and kinetic experiments. The calculations suggest that the reaction involves an unexpected Ir III –Ir V catalytic cycle facilitated by coordination of a second equivalent of dihydrogen. Thus, in the rate‐determining migratory insertion of the substrate alkene into an iridium–hydride bond, simultaneous oxidative addition of the bound dihydrogen occurs. The kinetic data shows that the reaction is first order with respect to hydrogen pressure. This is interpreted in terms of an endergonic coordination of this second equivalent of dihydrogen, although a rate‐determining step, in which coordinated solvent is replaced by dihydrogen, could not be ruled out. Furthermore, the reaction was found to be zeroth order with respect to the alkene concentration. This correlates well with the calculated exothermicity of substrate coordination, and the catalyst is thus believed to coordinate an alkene in the resting state. On the basis of the proposed catalytic cycle, calculations were performed on a full‐sized system with 88 atoms to assess the appropriateness of the model calculations. These calculations were also used to explain the enantioselectivity exerted by the catalyst.