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

Miles Wilklow-Marnell^†§, Bo Li^‡§, Tian Zhou^‡, Karsten Krogh-Jespersen^‡, William W. Brennessel^† , Thomas J. Emge^‡, Alan S. Goldman^*‡ , and William D. Jones^*† 

^† Department of Chemistry, University of Rochester, Rochester, New York 14627, United States

^‡ Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/jacs.7b03433

Abstract

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.