Dehydrogenation of ketones by pincer-ligated iridium: Formation and reactivity of novel enone complexes

The transfer dehydrogenation of several ketones by (PCP)IrH₂ (PCP = κ³-C₆H₃-2,6-(CH₂P^tBu₂)₂) (1) has been observed. Catalytic turnover was inhibited in most cases by the formation of stable metallacycles or the O-H oxidative addition of phenolic products. Catalytic transfer dehydrogenation of 3,3-dimethylcyclohexanone was achieved, giving the corresponding α,β-enone. The transfer dehydrogenation reaction of cycloheptanone with 1 was found to generate a surprisingly stable PCP-iridium troponyl hydride (9), which is stabilized by conjugation and possibly represents an unusual bicyclo[5.2.0]troponyliridium metalloaromatic structure. Complex 9 was found to catalyze the dimerization of tropone to give a fused tricyclic dihydrodicycloheptafuranol. A mechanism for this reaction is proposed wherein the coordinated troponyl group nucleophilically attacks a free tropone molecule.