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

The pincer-iridium fragment (iPrPCP)Ir (RPCP = κ3-2,6-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 β-vinyl and ortho-aryl positions, to give an iridium­(III) metallo­indene intermediate; this intermediate then adds a β-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 β-vinyl and ortho-methyl C–H bonds affords dimethyl­indene, demonstrating that the dehydrogenative coupling is not limited to C­(sp2)–H bonds.