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.