Sydnone Methides—A Forgotten Class of Mesoionic Compounds for the Generation of Anionic N‐Heterocyclic Carbenes

Abstract Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π‐electron rich anionic N‐heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C−C coupling reactions with 1‐fluoro‐4‐iodobenzene under Pd(PPh3)4 and CuBr catalysis. 77Se NMR resonance frequencies and 1 J C4‐Se as well as 1 J C4‐H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N‐heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in 13C NMR spectroscopy at −40 °C which is extremely shifted upfield in comparison to classical N‐heterocyclic carbenes.

Abstract: Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as p-electron rich anionic N-heterocyclic carbenes.S ulfur and selenium adducts were stabilized as their methyl ethers,a nd mercury,g old as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo CÀCcoupling reactions with 1-fluoro-4-iodobenzene under Pd(PPh 3 ) 4 and CuBr catalysis. 77 Se NMR resonance frequencies and 1 J C4-Se as well as 1 J C4-H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N-heterocyclic carbenes.The carbene carbon atom of the sydnone methide anion 3j resonates at d = 155.2 ppm in 13 CNMR spectroscopya tÀ40 8 8Cw hich is extremely shifted upfield in comparison to classical N-heterocyclic carbenes.
We first developed areliable synthetic method which lead to as eries of sydnone methides 2a-l (Scheme 2, Table 1). Thus,w es tarted from the sydnones 1a-l which we treated with Tf 2 Ot oo btain am ixture of inseparable bis-sydnone ethers and sydnone 5-triflates which proved to be very sensitive towards minute traces of water. Tr apping with in situ generated malodinitrile anions for the synthesis of 2b-d, methyl 2-cyanoacetate anions for 2e-h,a nd 2-(methylsulfonyl)acetonitrile anions for the preparation of 2i-l gave the desired sydnone methides,respectively.These compounds are stable,b rilliant yellow to orange in color and slightly fluorescent (see Supporting Information for fluorescence spectra). Sydnone methides can be represented by several resonance structures three of which are shown in Scheme 2. It is interesting to note that carbon atom C4 is asite of negative charge according to the rules of resonance as indicated by mesomeric structure 2a-lB,a lthough it is as ite of deprotonation for the formation of anionic N-heterocyclic carbenes. Correspondingly,t he C4 carbon atoms resonate at high field between d = 108.2 ppm (2j)a nd 110.9 ppm (2f)i n 13 CNMR spectroscopy.F urther DFT calculations predict that the E isomer of 2e is by DG vac = 4.6 kJ mol À1 more stable in vacuo than the corresponding Z isomer and that the rotation barrier is DE vac = 82.7 kJ mol À1 (PBE0-d3/ 6-31G**). We also calculated the solvent dependence.I nT HF,d ichloromethane (DCM) and DMSO the rotation barriers decrease to DE THF = 65.8 kJ mol À1 , DE DCM = 65.0 kJ mol À1 ,a nd DE DMSO = 61.6 kJ mol À1 ,r espectively (see Supporting Information). These results are in agreement with the fact that the bonds C5-C6 of 2e and 2i [crystallographic numbering,F igure 1] display aconsiderable double bond character,asdetermined by single crystal X-ray analyses.AsN2resonates considerably more upfield in 15 NN MR spectroscopy than N3, the mesomeric structures 2a-lC can be identified as the most suitable representation for sydnone methides.Inthe elemental cells,t he phenyl ring is twisted by À52.65 (17)8 8 (2e) Table 1). Selected mesomeric structures A-C.Several Li species detectableb y 7 Li NMR due to rapid decomposition of the anions.   (17), C4-C5-C6-S7:1 1.6(2) 8 8. [46] corresponding N-heterocyclic carbenes. [33] We next performed ab ase screening which revealed that deprotonation of the sydnone methides 2a-l can best be accomplished by LiHMDS in THF at À10 8 8C. According to the rules of resonance,t he resulting anions 3a-l can be represented by an umber of canonical forms,a mong those the mesomeric structures of abnormal N-heterocyclic carbenes 3a-lA,a s structures possessing two formal negative charges at C4 3a-lB, and as normal anionic N-heterocyclic carbenes 3a-lC (Scheme 2). Although the anions 3a-l are instable and decompose rapidly even at low temperatures,w es uccessfully generated 3j quantitatively at À50 8 8Cand immediately measured NMR spectra at À40 8 8C. The 1 HNMR spectra show the absence of the proton at C4. In the 13 CNMR spectra the signal of C4 of the precursor shifted considerably from 108.2 ppm (2j)t o 155.2 ppm (3j). Thec arbenesr esonance frequency is thus extremely shifted upfield in comparison to other NHCs.A ll chemical shift differences are summarized in Table S2 (Supporting Information). Moreover,t he mass of the sydnone methide anion 3j was confirmed by high resolution electrospray ionization mass spectrometry in the anion detection mode on spraying acooled in situ prepared sample of 3j.T o gain insight into the electronic properties of the sydnone methide anions we performed DFT calculations (B3LYP/6-311 ++G**). In contrast to 1,3-dimesitylimidazol-2-ylidene and 1,3-dimesitylimidazolidin-2-ylidene which we chose as examples,the highest occupied molecular orbitals (HOMOs) of the sydnone methide carbenes 3a, 3e and 3i are p-orbitals with significant atomic orbital coefficients on C4 as legacy of their origin from mesoionic compounds (Figure 2a nd Supporting Information). Vice versa, the HOMOs-1 display the characteristic geometries of N-heterocyclic carbenes.T heir energies are considerably higher than those of the aforementioned N-heterocyclic carbenes of imidazole,b ut slightly lower than those of N-phenylsydnone (Figure 3). Concerning the substituent effects,t he different electron-withdrawing capacities of the COOMe group (Hammett constants [36] s m = 0.37; s p = 0.45), CN group (s m = 0.56; s p = 0.66) and SO 2 Me group (s m = 0.60; s p = 0.72) correlates with the HOMO energies within the series of sydnone methide anions.
Reaction of the in situ generated anionic sydnone methide carbenes 3a,e,i with sulfur and selenium gave the thioethers 4a-c and selenium ethers 5a-ca fter methylation as stable compounds,r espectively (Scheme 3). The 77 Se NMR reso-nance frequencies of the selenium ethers 5a-c were detected at d = 86 ppm, 101 ppm and 109 ppm, respectively,and these values correspond to those measured for Molsidomine [that is,N -(ethoxycarbonyl)-3-(4-morpholino)sydnone imine; d = 97 ppm]. [37] As expected they are considerably more upfield than those of the selenium ethers of 1,3-dimesitylimidazol-2ylidene and its imidazolidine derivative which are cations and which resonate at d = 199 ppm and 271 ppm, respectively. Tr eatment of 3a,e,i with acetyl chloride (R = Me) and benzoyl chloride (R = Ph) gave 6a-e which slowly reconstitute the corresponding sydnone methides on exposure to water. The acetyl derivative of 6f (Z = SO 2 Me,R= Me) could not be isolated as it is unstable under ambient conditions.T hese reactions correspond to rare trapping reactions of normal [38,39] as well as abnormal imidazolylidenes with acyl chlorides. [39] Mesoionic compounds such as 1,3-dimesitylimidazolium-4olates,h owever,u ndergo electrophilic heteroaromatic substitutions to give these structures. [40] Ther eaction of 3e,f with 0,5 equiv of mercury(II)chloride lead to the predominant formation of the dimeric mercury complexes 7 2 (a,b),w hereas 3-4 equiv of HgCl 2 yielded the monomeric complexes 7a,b from 3b,f.   Table S1).
Chloro(triphenylphosphine)gold(I) converted the sydnone methide carbenes 3c,f,i into the gold(I) complexes 8a-c as yellow solids (Scheme 4). We also reacted the sydnone methide carbenes 3a,f,i with RhCO(PPh 3 )Cl which resulted in the formation of the rhodium complexes 9a-c as pale yellow solids.
Single crystals of the gold complex 8a were subjected to an X-ray analysis (Figure 4). Ther esults show that complex formation does not influence the starting mesoionsgeometry (2a)significantly.
Concerning C À Cc oupling reactions,t reatment of 3a,e,i with CuBr gave in situ generated, non-isolable copper compounds,w hich coupled with 4-fluoro-1-iodobenzene in the presence of Pd(PPh 3 ) 4 in acceptable yields (Scheme 5). These reactions proceed in analogy to those of sydnones. [41] In summary,w ep resent the syntheses,s pectroscopic characterizations,results of calculations [42,43] and single crystal X-ray analyses [44,45] of new stable members of the substance class of syndone methides from which only one single example had been described in 1984. Deprotonation yielded sydnone methide anions which can be formulated by anumber of resonance forms,a mong those mesomeric structures of anionic N-heterocyclic carbenes.T he frontier orbital profile sets these N-heterocyclic carbenes apart from other examples of this class of compounds,a st heir highest occupied molecular orbitals are p-orbitals with considerable atomic orbital coefficients on the carbene carbon atom, and extremely upfield shifted 13 CNMR resonance frequencies of the carbene carbon atom (3j: d = 155.2 ppm). Thes ydnone methide anions can be reacted with sulfur and selenium, respectively,a nd stabilized by S-and Se-methylation. Reaction with acyl chlorides gave sydnone methide ketones,a nd trapping reactions with mercury,g old, and rhodium gave the corresponding complexes.F inally,w ep resented aP d 0 /Cu Icatalyzed C À Cc oupling reaction at C4 of the sydnone methide.O ur results supplement the knowledge about the sydnone family of compounds as well as about anionic Nheterocyclic carbenes.