1,8‐Disubstituted Xanthylidene‐Based Remote Carbenes: Photolytic Generation and Isolation of Low‐Coordinate Palladium(II) Complex

The isolation and study of carbenes with σ-donating and π-accepting properties stronger than those of N-heterocyclic carbenes (NHCs) have been regarded as one of the big challenges in carbene chemistry. We have investigated the properties of 1,8-disubstituted xanthylidene-based compounds by using two different approaches: The photolysis-induced formation of free carbenes and the synthesis of the corresponding metal–carbene complexes by oxidative addition. The former study revealed that the carbene unsubstituted at the 1- and 8-positions is kinetically more reactive than the parent xanthylidene and that the substituents at the 1- and 8-positions are less effective in generating carbenes by photolysis. However, oxidative addition of palladium(0) to the 1,8-diphenyl-substituted precursor successfully afforded a low-coordinate 14-electron palladium(II)–carbene complex. This unusual complex is stabilized by agnostic interactions involving an adjacent phenyl group. This complex is the second example of a stable cationic 14-electron palladium(II)–carbene complex to have been reported so far.