Cross-dehydrogenative coupling via C(sp)─H bond activation by PSP-pincer rhodium photocatalysis.

Cross-dehydrogenative coupling (CDC) between two C─H bonds without any oxidant would be an excellent C─C bond-forming reaction, generating H as the only by-product. However, C(sp)─H/C(sp)─H CDC reactions remain rare despite the rapid development of transition metal catalysis, photocatalysis, and electrocatalysis. Here, we describe directing-group- and oxidant-free C(sp)─H/C(sp)─H CDC between arenes and styrenes/furans photocatalyzed by a PSP-pincer Rh complex to afford stilbenes/heterobiaryls under mild conditions (blue light irradiation, near room temperature). The reaction is applicable to a wide range of substrates, and undesired hydrogenation of the styrenes and stilbenes is strongly suppressed. The generated H gas can be used for hydrogenation of alkenes in a connected flask system. Experimental mechanistic studies indicated that the Rh(I)─Ar complex is a key intermediate in the catalytic cycle, and C(sp)─H bond cleavage of the styrenes/furans occurs via oxidative addition in the excited state.