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

摘要

The pincer-iridium fragment (~(iPr)PCP)Ir (~RPCP = k~3-2,6-C_6H_3(CH_2PR_2)_2) has been found to catalyze the dehydrogenase coupling of vinyl arenes to afford predominantly (E,E)-l,4-diaryl-l,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., fert-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-asyl positions, to give an iridium (III) metal-loindene 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(sp~2)—H bonds.