Pincer-ligated iridium complexes undergo oxidative addition of the strained C–C bond of biphenylene. The sterically crowded species (tBuPCP)Ir (RPCP = κ3-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)–H bond. Upon heating at 125 °C for 24 h, full conversion to the C–C addition product, (tBuPCP)Ir(2,2’-biphenyl), is observed. The much less crowded (iPrPCP)Ir undergoes relatively rapid C–C addition at room temperature. Based on DFT calculations the large difference in the barriers to C–C addition can be explained in terms of a “tilted” transition state (ca. 50°–60°). The pincer-iridium fragment (iPrPCP)Ir has also 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 one equivalent of ethyl arene formed for each equivalent of diarylbutadiene produced. Alternatively, sacrificial hydrogen acceptors (e.g. t-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 an iridium(III) metalloindene intermediate, which then adds a β-vinyl C-H bond of a second styrene molecule before reductively eliminating product.

C-H and C-C bond cleavage, and catalytic dehydrogenative C-C coupling by iridium-pincer complexes

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.