5 years ago

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

Catalytic Dehydrogenative C–C Coupling by a Pincer-Ligated Iridium Complex
William D. Jones, William W. Brennessel, Miles Wilklow-Marnell, Karsten Krogh-Jespersen, Bo Li, Tian Zhou, Alan S. Goldman, Thomas J. Emge
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.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b03433

DOI: 10.1021/jacs.7b03433

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