Experimental and Theoretical Mechanistic Investigation of the Iridium-Catalyzed Dehydrogenative Decarbonylation of Primary Alcohols

The
mechanism for the iridium–BINAP catalyzed dehydrogenative
decarbonylation of primary alcohols with the liberation of molecular
hydrogen and carbon monoxide was studied experimentally and computationally.
The reaction takes place by tandem catalysis through two catalytic
cycles involving dehydrogenation of the alcohol and decarbonylation
of the resulting aldehyde. The square planar complex IrCl­(CO)­(rac-BINAP) was isolated from the reaction between [Ir­(cod)­Cl]2, rac-BINAP, and benzyl alcohol. The complex
was catalytically active and applied in the study of the individual
steps in the catalytic cycles. One carbon monoxide ligand was shown
to remain coordinated to iridium throughout the reaction, and release
of carbon monoxide was suggested to occur from a dicarbonyl complex.
IrH2Cl­(CO)­(rac-BINAP) was also synthesized
and detected in the dehydrogenation of benzyl alcohol. In the same
experiment, IrHCl2(CO)­(rac-BINAP) was
detected from the release of HCl in the dehydrogenation and subsequent
reaction with IrCl­(CO)­(rac-BINAP). This indicated
a substitution of chloride with the alcohol to form a square planar
iridium alkoxo complex that could undergo a β-hydride elimination.
A KIE of 1.0 was determined for the decarbonylation and 1.42 for the
overall reaction. Electron rich benzyl alcohols were converted faster
than electron poor alcohols, but no electronic effect was found when
comparing aldehydes of different electronic character. The lack of
electronic and kinetic isotope effects implies a rate-determining
phosphine dissociation for the decarbonylation of aldehydes.