Coumaraz-2-on-4-ylidene: Ambiphilic N-Heterocyclic Carbenes with a Tunable Electronic Structure

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of an ambiphilic N-heterocyclic carbene, having electronic properties that can be fine-tuned, is reported. The N-carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the ⁷⁷ Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main-group and transition-metal chemistry.