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

Abstract

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 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.

Cyclic (Aryl)(Amido)Carbenes: NHCs with Triplet‐like Reactivity

“…The use of elemental Se instead of S in the aforementioned trapping experiment afforded carbene–selenium adduct 12 a . The 77 Se NMR signal recorded for 12 a ( δ =1204.4 ppm, [D 6 ]acetone; 1267.9 ppm, CDCl 3 ) was found to be downfield when compared to the values reported for the Se adducts of a cyclic (alkyl)(amido)carbene (1179.71 ppm, [D 6 ]acetone), DAC 1 ( δ =847 ppm, [D 6 ]acetone), CAArC 2 ( δ =616 ppm, [D 6 ]acetone), and coumaraz‐2‐on‐4‐ylidene 3 ( δ =1103 ppm, [D 6 ]acetone; δ =1143.3 ppm, CDCl 3 ) . Likewise, the 13 C NMR signal assigned to carbene nucleus in 12 a was found to resonate at a relatively downfield value ( δ =202.8 ppm, CDCl 3 ) when compared to the analogous signals recorded for the Se adducts of DAC 1 ( δ =182.7 ppm, CDCl 3 ) or coumaraz‐2‐on‐4‐ylidene 3 ( δ =197 ppm, CDCl 3 ) .…”

“…Bertrand's cyclic (amino)(aryl)carbenes (CAArCs) ( 2 ) serve as examples of such systems. While a CAArC was calculated to exhibit a lower Δ E ST value when compared to its analogous cyclic (amino)(alkyl)carbene (CAAC) (39.5 kcal mol −1 vs. 47.5 kcal mol −1 , respectively), a similar value was calculated for a DAC (42.3 kcal mol −1 ) . More recently, a coumaraz‐2‐on‐4‐ylidene ( 3 ), which features a carbene nucleus linked to a carbamoyl group, was reported by Song and Lee to exhibit a relatively low Δ E ST value (26.7 kcal mol −1 ) .…”

“…While a CAArC was calculated to exhibit a lower Δ E ST value when compared to its analogous cyclic (amino)(alkyl)carbene (CAAC) (39.5 kcal mol −1 vs. 47.5 kcal mol −1 , respectively), a similar value was calculated for a DAC (42.3 kcal mol −1 ) . More recently, a coumaraz‐2‐on‐4‐ylidene ( 3 ), which features a carbene nucleus linked to a carbamoyl group, was reported by Song and Lee to exhibit a relatively low Δ E ST value (26.7 kcal mol −1 ) . Using a similar level of theory (B3LYP/def2‐TZVP), we calculated a Δ E ST of 19.9 kcal mol −1 for a cyclic (aryl)(amido)carbenes (CArAmC) 4 ( R =2,6‐diisopropylphenyl).…”

Activation of C–F, Si–F, and S–F Bonds by N-Heterocyclic Carbenes and Their Isoelectronic Analogues

“…20 Most prominent examples include cyclic -amino carbenes (CAAC), [21][22][23] cylic (aryl)(amido)carbenes, 24,25 diamidocarbene (DAC), [26][27][28] and coumaraz-2-on-4-ylidene (CONY). 29 Mesoionic (abnormal) carbenes 30,31 are NHCs without an uncharged resonance structure.…”

“…Tacke. 76 Tetra- (29), tri- (27,28), and dicoordinated (30) silylenes undergo a formal insertion into aromatic C-F bonds providing the corresponding Si(IV) compounds as stable solids (Scheme 20). 77 In case of partially fluorinated arenes, competing C-H bond activation is observed with the divalent silylene 30.…”