Ambiphilicity of ring-expanded N-heterocyclic carbenes

N-heterocyclic carbenes, such as imidazole-2-ylidenes and imidazolin-2-ylidenes, the popular class of singlet carbenes introduced by Arduengo in 1991 have not been shown to be ambiphilic owing to the two σ-withdrawing, π-donating amino groups flanking the carbene centre. However, our experimental data suggest that ring-expanded N-heterocyclic carbenes (RE-NHCs), especially the seven and eight membered rings, are significantly ambiphilic. Our results also show that the steric environment in RE-NHCs can become a determining factor for controlling the E–H bond activation.


Introduction
Thanks to growing efforts in main group chemistry, the activation of enthalpically strong bonds and industrially relevant small molecules is no longer restricted to transition-metals. 1 More than a decade ago, our group discovered that cyclic (alkyl)(amino)carbenes (CAAC-5), 2,3 a class of highly ambiphilic carbenes, could react with carbon monoxide, 4 H 2 , 5 NH 3 (ref.5) and P 4 . 6More recently, it has been shown that CAAC-5s not only activate a variety of bonds (C-H, Si-H, B-H.) 7 but also promote catalytic reactions. 8In comparison, imidazole-2-ylidenes 9 and imidazolin-2-ylidenes,10 the classical N-heterocyclic carbenes (NHC-5s), are much less ambiphilic due to their two p-donating amino substituents.Consequently, they are reluctant to activate small molecules, as illustrated by their lack of reactivity with CO. 11,12 Much less studied than NHC-5s are the so-called ringexpanded N-heterocyclic carbenes (RE-NHCs). 13Herein we compare the ambiphilic nature of NHC-5 with RE-NHCs (−6, 13a −7 (ref.14) and −8 (ref.13c)) and CAAC-5 through DFT calculations and their reactivity with small molecules.

Results and discussion
Compared to NHC-5s, RE-NHCs display a larger N-C-N bond angle (: carb ) which imposes greater steric constraint when used as a ligand for transition metals, a feature used to enhance catalytic activity. 15Arguably less emphasized, is the larger carbene bond angle, which increases the p-character of the lone pair, and thus the energy level of the HOMO. 16Comparatively, the LUMO is less affected since ring expansion does not signicantly disrupt the planarization of the a-amino fragments, which leaves the mesomeric stabilization of the p p orbital by the nitrogen lone pairs nearly identical.
The ambiphilicity of a carbene can be estimated computationally by considering the singlet-triplet gap (DE S-T ) (Scheme 1).As expected, our calculations indicate a correlation between the ring size and ambiphilicity of a carbene.Interestingly, the data also suggests that the ambiphilicity of NHC-7 and NHC-8 approaches that of CAAC-5.
To compare experimentally the ambiphilicity of NHCs with that of CAAC-5, we rst considered the activation of sphybridized CH bonds which has been reported with CAACs, 17 but seldomly described with NHCs (one example has been reported using acetylene gas). 18We rst investigated the reaction of p-tolylacetylene [pK a (DMSO) = 28.8 vs. 25 for acetylene] with NHC-5 at room temperature in benzene solution (Scheme 2).In this case, no reaction was observed within 1 hour.In marked contrast, using CAAC-5 the oxidative addition product 1a was quantitatively obtained within minutes.Under the same conditions a rapid and clean reaction was also observed with NHC-6 13a NHC-7 15 and NHC-8 13c giving adducts 1b-d as shown by characteristic 1 H NMR signals at 6.04, 5.87 and 5.64 ppm, and 13 C NMR signals at 72.3, 73.9 and 77.1 ppm, respectively.The structure of adduct 1c (from NHC-7) was conrmed by X-ray crystallography.Because of the signicant difference in reactivity observed between NHC-5 and the RE-NHCs, we re-evaluated the reaction of NHC-5 with p-tolylacetylene and observed very slow conversion to adduct 1e upon performing the reaction at 80 °C for 4 hours.These initial results prompted us to search for more challenging molecules to activate.Examples of stable carbenes reacting with isonitriles to afford ketenimines are scarce.
They only include the anti-Bredt NHC 19 and diamidocarbenes (DAC) 20 thanks to their enhanced electrophilicity resulting from reduced donation of the nitrogen lone-pair into the empty p-type orbital of the carbene carbon.Curious to probe the reactivity of RE-NHCs, we considered their reactivity and that of NHC-5 or CAAC-5 with adamantyl isocyanide (Scheme 3).CAAC-5 cleanly afforded the ketenimine 2a within minutes, while no reaction occurred with NHC-5 aer 12 hours at room temperature in benzene solution. 21This result contrasts with CAAC-5 which cleanly afforded the ketenimine 2a within minutes.With NHC-6, no reaction was observed even aer 12 hours.However, with NHC-7 and NHC-8, the quantitative formation of compound 2b and 2c, was observed aer 10 minutes, as evidenced by the diagnostic 13 C NMR signal for the central carbon of ketenimines at 216.9 ppm and 211.5 ppm, respectively.We conrmed the structure of compound 2b by Xray crystallography.Interestingly, the solid-state structure of 2b revealed a pronounced bent geometry (C NHC -C-N angle: 158.5°) compared to that of diamido cyclohexylketenimine (C DAC -C-N angle: 173.8°) 20b with a longer C NHC -C ket bond (133.8 pm vs. 129.7 pm for DAC).This observation indicates that NHC-7 is less electrophilic than DAC.
Collectively, the reactions with terminal alkynes and isocyanides suggest that the ambiphilicity of the carbenes is in the order NHC-5 < NHC-6 < NHC-7 < NHC-8 < CAAC-5, which is in agreement with their singlet-triplet gap.To deconvolute these results further, we wondered if RE-NHCs, notwithstanding their lower electrophilicity could compare with CAAC-5 in the activation of ammonia. 5Under 2 atmospheres of NH 3 , no reaction occurred with NHC-5, which was expected since several diaminocarbenes have even been generated in liquid ammonia. 22Scheme 4).In agreement with literature precedent, 5 under the same conditions, CAAC-5 rapidly led to the ammonia adduct 3a.Switching to RE-NHCs, no reaction was observed with NHC-6 despite prolonged reaction time, while NHC-7 led to the clean formation of product 3b with distinctive 1 H and 13 C NMR signals at d = 5.25 ppm and 85.8 ppm, respectively.This result was conrmed by single crystal X-ray diffraction.However, to our surprise, no reaction was observed with NHC-8.
We previously reported that the steric environment of CAAC-5 is a determining factor in controlling the reversibility of E-H bond activation (E = N-H, P-H). 8Compared to CAAC-5 (: carb = 106°), NHC-8 is more sterically constrained around the carbene carbon due to its large N-C NHC -N bond angle (: carb = 122°).13c We hypothesized this could explain its lack of reactivity with ammonia despite favourable electronics.To probe this hypothesis, we prepared the N-Mesityl (-Mes) substituted NHC-8 ( Mes NHC-8) since its steric prole is signicantly smaller than that of the Dipp-substituted NHC-8.This is apparent from the solid state structures, when considering the steric maps (see ESI for details †) and percent buried volumes (%V bur 23 ) around the carbene carbon.NHC-8 (80.1%) compared to Mes NHC-8 (77.2%) which is closer to that of NHC-7 (78.4.%).The larger steric hindrance is also apparent in solution when considering the unusual 77 Se NMR downeld shi of the NHC-8-Se adduct 4a (571.1 ppm) compared to Mes NHC-8-Se adduct 4b (437.9 ppm) (Scheme 5).Indeed, 77 Se NMR is a spectroscopic marker for highlighting non-classical bonding (NCB) interactions between pendant N-Dipp substituents and the selenium atom. 24ote that when comparing the reactivity of N-tolyl and N-Dipp 8membered NHCs with silver chloride, Cavell and co-workers discovered that in very large ring NHCs the steric environment provided by N-Dipp substituents can become so overwhelming that it prevents coordination.13c Having conrmed that Mes NHC-8 is less sterically hindered than NHC-8 and even NHC-7, we evaluated its reactivity towards ammonia.Gratifyingly, rapid formation of the corresponding ammonia adduct was observed when performing the reaction in C 6 D 6 under 2 atmospheres of NH 3 (Scheme 6).To conrm these results, we also investigated the reactivity of the corresponding imidazolium salts with sodium amide which provided the expected adducts via nucleophilic addition of NH 2 − (Scheme 7).
Note that under these conditions, reaction of NHC-8 HBr with NaNH 2 afforded the free NHC-8 and ammonia.Overall, these results suggest that for 8-membered ring NHCs, the activation of ammonia is controlled by steric parameters and possibly reversible.

Conclusions
Imidazole-2-ylidenes 9 and imidazolin-2-ylidenes, 10 the popular class of singlet carbenes introduced by Arduengo, have not proven to be ambiphilic owing to the two s-withdrawing, p-donating amino groups stabilizing the carbene centre.However, our experimental data demonstrate that ring-expanded N-heterocyclic carbenes, NHC-7s and NHC-8s, belong to the class of ambiphilic carbenes.Our results also show that the steric environment in RE-NHCs can become a determining factor for controlling the E-H bond activation.We anticipate these results will have far reaching implications in the design and applications of large ring singlet carbene skeletons.