Redox-induced umpolung of transition metal carbenes.

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes. Herein, we report an umpolung of a nucleophilic palladium carbene complex, [{PC(sp2)P} tBuPd(PMe3)] ({PC(sp2)P} tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene), realized by successive one-electron oxidations that generated a cationic carbene complex, [{PC(sp2)P} tBuPdI]+, via a carbene radical, [{PC˙(sp2)P} tBuPdI]. An EPR spectroscopic study of [{PC˙(sp2)P} tBuPdI] indicated the presence of a ligand-centered radical, also supported by the results of reactions with 9,10-dihydroanthracene and PhSSPh. The cationic carbene complex shows electrophilic behavior toward nucleophiles such as NaH, p TolNHLi, PhONa, and PMe3, resulting from an inversion of the electronic character of the Pd-Ccarbene bond in [{PC(sp2)P} tBuPd(PMe3)]. The redox induced umpolung is reversible and unprecedented.