Catalytic Dehydrogenative C-C Coupling by a Pincer-Ligated Iridium Complex

The pincer-iridium fragment ((PCP)-P-1Pr)Ir ((PCP)-P-R = kappa(3)-2,6-C6H3(CH2PR2)(2)) has been found to catalyze the dehydrogenative coupling of vinyl arenes to afford predominantly (E,E)-1,4-diaryl-1,3-butadienes. The eliminated hydrogen can undergo addition to another molecule of vinyl arene, resulting in an overall disproportionation reaction with 1 equiv of ethyl arene formed for each equivalent of diarylbutadiene produced. Alternatively, sacrificial hydrogen acceptors (e.g., tert-butylethylene) can be added to the solution for this purpose. Diarylbutadienes are isolated in moderate to good yields, up to ca. 90% based on the disproportionation reaction. The results of DFT calculations and experiments with substituted styrenes indicate that the coupling proceeds via double C-H addition of a styrene molecule, at beta-vinyl and ortho-aryl positions, to give an iridium(III) metal-loindene intermediate; this intermediate then adds a beta-vinyl C-H bond of a second styrene molecule before reductively eliminating product. Several metalloindene complexes have been isolated and crystallographically characterized. In accord with the proposed mechanism, substitution at the ortho-aryl positions of the styrene precludes dehydrogenative homocoupling. In the case of 2,4,6-trimethylstyrene, dehydrogenative coupling of beta-vinyl and ortho-methyl C-H bonds affords dimethylindene, demonstrating that the dehydrogenative coupling is not limited to C(sp(2))-H bonds.