Sulfur, mercury, and boron adducts of sydnone imine derived anionic N-heterocyclic carbenes

The sydnone imines (5-benzoylimino)-3-(2-methoxyphenyl)-sydnone imine and molsidomine were deprotonated at C4 to give sydnone imine anions which can be represented as anionic N-heterocyclic carbenes, respectively. Trapping reactions with sulfur gave unstable sydnone imine sulfides which were stabilized by the formation of methyl thioethers, methyl sulfoxides, gold complexes [(PPh3)Au–S-sydnone imine] and a bis(ligand) mercury(ii) complex. The latter possesses a tetrahedral coordination of the mercury central atom to the sulfur atoms with the N6 nitrogen atoms coordinating as neutral ligands. Water converted the molsidomine anion into ethyl(2-morpholino-2-thioxoacetyl)carbamate. Mercury(ii)chloride and triphenylborane were employed to trap the sydnone imine carbenes as mercury complexes as well as BPh3 adducts.


Introduction
Mesomeric betaines (MB) can exclusively be represented by several zwitterionic canonical forms in which the positive and negative charges are delocalized within a common p-electron system. They have proven to be versatile starting materials for the generation of N-heterocyclic carbenes (NHC) as well as of their anionic derivatives 1 which play important roles in synthesis and catalysis. According to a recent classication which is based on a matrix-connectivity analysis, ve distinct classes of mesomeric betaines can be differentiated. 2 As examples, conjugated (CMB), cross-conjugated (CCMB) and pseudocross-conjugated mesomeric betaines (PCCMB) differ in their charge distribution with respect to the canonical formulae. 3 In addition, characteristic dipole types of each class can be dissected from the mesomeric structures. 3 Conjugated and cross-conjugated mesomeric betaines differ also in their frontier orbital proles. 4 Not unexpectedly, the distinct types of mesomeric betaines show different chemical behaviours which is also reected in their potential transformations into distinct types of N-heterocyclic carbenes. 1,5 Thus, pseudo-crossconjugated mesomeric betaines such as imidazolium-2carboxylate 1 are by far the most widely applied betaines for the generation of NHCs 2, because they decarboxylate thermally under relatively mild conditions. 6 Pyrazolium-3-carboxylates, 7 indazolium-3-carboxylates, 8 and pyridinium-2-carboxylates 9 react similarly to give the corresponding NHCs in situ, respectively. In general, the extrusion of heterocumulenes from pseudo-cross-conjugated mesomeric betaines is a valuable tool to generate NHCs. By contrast, cross-conjugated mesomeric betaines (CCMB) such as pyrazolium-4-carboxylates 3 require harsh reaction conditions for the formation of remote Nheterocyclic carbenes 4 so that this approach is not useful from a synthetic point of view. 10 Some mesomeric betaines undergo reactions via their formal tautomers which are Nheterocyclic carbenes. The CMB 5 and the CCMB 7 are examples. Thus, nitron 5 reacts with sulfur to give a urea derivative as formal trapping product of the carbene 6. 11 According to a computational study on relative stabilities of mesoionic and N-heterocyclic carbene tautomers in dependence on substituent effects, nitron 5 is by 2.3 kcal mol À1 more stable than its carbene 6. 12 Similarly, betaine 7 tautomerizes to give carbene 8. 13 Deprotonation of mesomeric betaines such as 9 results in the formation of anionic N-heterocyclic carbenes 10 (Scheme 1). 14 Scheme 1 Examples of mesomeric betaine -N-heterocyclic carbene transformations.
Sydnones (Z ¼ O) and their derivatives such as sydnone imines (Z ¼ NR) and sydnone methides (Z ¼ CR 2 ) belong to the class of conjugated mesomeric betaines (CMB). Although the canonical structure I is the most common, the resonance structures II and III indicate that the exocyclic C-X bond corresponds to a carbonyl group for X ¼ O which is in well agreement to results of X-ray analyses as well as vibrational spectroscopy. 15 Sydnones and their derivatives possess the dipole type IV/V which is characteristic of the class of conjugated mesomeric betaines (CMB) (Scheme 2). 3 The anions of sydnones and sydnone imines can be represented as anionic normal NHCs and abnormal NHCs. In the resonance forms, the delocalization of the negative charge of sydnone and sydnone imine anions include the site of deprotonation, i.e. C4 (VI), which is a starred (active) position according to a connectivity analysis (VII). 2 Thus, anions of sydnone derivatives combine the features of N-heterocyclic carbenes due to their s lone pair and of conjugated mesomeric betaines due to their parchitecture. Consequently, the highest occupied molecular orbitals (HOMO) are p-orbitals which display large atomic orbital coefficients on C4 (Scheme 3).
The sydnone imine anion 12 (Z ¼ NR), usually stabilized by Li + , can be deuterated (13) and trapped by selenium, followed by methylation to give 14. 16 It forms palladium as well as gold complexes such as 15 and 16. 16 Iminium salts of formimidate are able to formylate the sydnone imine anions to yield 17, 17 and the treatment with aldehydes give alcohols such as 18. 18 Trapping with isocyanates give amides (19), 19 and treatment with chlorodiphenylphosphane give phosphorus adducts like 20. 20 Some cross-coupling reactions, catalyzed by Pd(PPh 3 ) 4 and copper salts, to yield 21 and 22 were also described. 18 In addition to that, rearrangements of 12 have been reported. 19 In continuation of our studies directed toward the chemistry of mesomeric betaines and their conversions into Nheterocyclic carbenes, we describe here trapping reactions of sydnone imine anions with sulfur, boron, and mercury.

Results and discussion
The sydnone imine anions 12a,b, generated on treatment of the sydnone imines 23a and 23b ("molsidomine") with lithium (trimethylsilyl)amide at rt, can be trapped by sulfur to give the corresponding suldes 24a,b which could not be isolated (Scheme 5). Thiols of sydnone imines are very rare. 21 Methylation by modied literature procedures yielded the stable 21,22 sydnone imine thioethers 25a,b in acceptable yields. We were able to oxidize the thioethers 25a,b with m-chloroperoxybenzoic acid to give the sulfoxides 26a,b. Stabilization of the suldes as gold complexes was accomplished on treatment of freshly prepared samples of the suldes with chloro(triphenylphosphine)gold(I) which resulted in the formation of the complexes 27a,b. These complexes supplement the complexes 16 (Scheme 4) in which the gold is directly attached to C4 of the sydnone imines and which we described earlier. 16 Although the complexes 27a,b are stable enough to survive purication by column chromatography, gold complexes with the transition metal directly bound to the C4 carbene carbon atom are far more stable. The suldes were also stabilized as mercury complexes 28a,b which were formed on exposure of the suldes with mercury(II)chloride. Metal-stabilized sydnone suldes are rare. To the best of our knowledge, only one tin complex has been described so far. 23 Single crystals of the gold complex 27a were obtained by slow crystallization at À20 C in a CHCl 3 -EtOAc mixture (Fig. 1). It is noteworthy to point out the angle P1-Au1-S1 is 168.824 (19) which is slightly bent towards the N6 nitrogen atom and not linear.
Single crystals of the mercury complex 28a were obtained by slow evaporation of a saturated solution in chloroform and diethyl ether. The complex crystallized in an orthorhombic space group. Crystal data show a bis(ligand) mercury(II) complex with tetrahedral coordination of the mercury central atom with the N6 nitrogen atoms coordinating as neutral ligands (Fig. 2). Furthermore, the N6 coordination has a major inuence on the C4-S1-metal angle. While the C4-S1-Au1 angle of complex 27a adopts a value of 101.65 (7) , the corresponding angle of the bis(ligand) mercury(II) complex 28a has a value of 94.02(7) (C34-S31-Au1 95.39 (7) ).
Scheme 2 Features of sydnone derivatives. The bond between the exocyclic substituent and the sydnone imine is known to be very stable under a variety of reaction conditions. However, the molsidomine derivative 24b surprisingly underwent an intramolecular rearrangement of the morpholinyl group and subsequent nitrogen cleavage for which the depicted mechanism is suggested (Scheme 6). Under analogous reaction conditions, 24a decomposed. Sydnone imine cleavages to form unsaturated nitriles such as N-morpholinoformimidoyl cyanide are known. 24 They occur, however, when the exocyclic nitrogen atom is not substituted by electron-withdrawing groups. In these cases the N-N bond connecting the morpholine group and the sydnone imine remain intact.
We were able to obtain single crystals for an X-ray structure analysis by slow evaporation of 29 from a saturated solution in ethyl acetate. The structure was conrmed via single crystal Xray analysis (Fig. 3).   In comparison to the sydnone imine sulde mercury(II) complexes we furthermore prepared the transition metal complexes with mercury directly attached to the C4 carbene carbon atom in high yields. These complexes show very high stability towards water, air, bases and higher temperatures. Crystal data show a tetrahedral coordination of the mercury(II) atom (Fig. 4). In comparison to the complex 28a where coordination is observed by the S1 and N6 atom (forming a vemembered ring), the complex 30a shows coordination by the C4 and O7 atom (forming a six-membered ring).
Moreover, we managed to undergo covalent bond formation with triphenyl borane, resulting in a new mesomeric betaine structure in which the negative charge is in the borate substituent (Scheme 7).
The structure was conrmed by a single crystal X-ray analysis (Fig. 5). The molecular drawing shows a tetrahedral arrangement of the covalently bound boron atom. The C4-B22 bond length (165.2(3) pm) (C104-B122 165.6(3) pm) is in agreement with the Ph 4 B À bond length (164.3 pm) presented in the literature. 25

Experimental
The reactions were carried out under an atmosphere of nitrogen in oven-dried glassware. Nuclear magnetic resonance (NMR) spectra were obtained with a Bruker Avance 400 and Bruker Avance III 600 MHz. 1 H NMR spectra were measured at 400 MHz or 600 MHz and 13 C NMR spectra were measured at 100 MHz or 150 MHz, with the solvent peak or tetramethylsilane used as the internal reference. Multiplicities are described by using the following abbreviations: s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quadruplet, and m ¼ multiplet, and the signal orientations in DEPT experiments were described as follows: o ¼ no signal; + ¼ up (CH, CH 3 ); À ¼ down (CH 2 ). ATR-IR spectra were obtained on a Bruker Alpha in the range of 400 to 4000 cm À1 . Melting points were measured by differential scanning calorimetry (DSC) using a DSC 6 apparatus (Perkin-Elmer). The HR-MS spectra were measured on a Bruker Daltonik Tesla-Fourier transform-ion cyclotron resonance mass spectrometer with electrospray ionisation. Yields are not optimized.

Crystal structure determinations
The single-crystal X-ray diffraction study were carried out on a Bruker D8 Venture diffractometer with Photon100 (29) or PhotonII detector (27a, 28a, 30a, 31a) at 123(2) K using Cu-Ka radiation (l ¼ 1.54178Å) (29, 31a) or Mo-Ka radiation (l ¼ 0.71073Å) (27a, 28a, 30a). Dual space/intrinsic methods (27a, 28a, 30a, 31a) (SHELXT) 26 or direct methods (29) (SHELXS-97) 27 were used for structure solution and renement was carried out using SHELXL-2014 (full-matrix least-squares on F 2 ). 26 Hydrogen atoms were localized by difference electron density determination and rened using a riding model (in 29 H(N) free). Semi-empirical absorption corrections and extinction  correction (for 28a, 29) were applied. In 31a renement with the listed atoms show in one void residual electron density due to a heavily disordered diethylether which could not be rened with split atoms. Therefore the option "SQUEEZE" of the program package PLATON 28 was used to create a hkl le taking into account the residual electron density in the void areas. Therefore the atoms list and unit card do not agree (see cif-.le for details).