Sydnone anions and abnormal N-heterocyclic carbenes of O -ethylsydnones. Characterizations, calculations and catalyses †

Deprotonated sydnones, which can be represented as anionic N-heterocyclic carbenes, were prepared as Li adducts and compared with deprotonated O -ethylsydnones (5-ethoxy-1,2,3-oxadiazol-4-ylidenes) which belong to the class of abnormal NHCs. The Pd complexes of the sydnone anions (X-ray analysis) as well as of the O -ethylsydnone carbenes proved to be eﬃcient catalysts in aryl couplings of thiophenes.

Of all the 228 theoretically predicted mesoionic compounds 1 sydnones are probably the most prominent. Sydnones (1,2,3oxadiazolium-5-olates) are 5-membered representatives of the class of conjugated mesomeric betaines (CMB) which is one of four distinct categories of this class of compounds. 2 They are not only known as versatile 1,3-dipoles in [2+3]-cycloadditions but also as biologically active compounds. 3 So far the chemistry of sydnones has not been associated with the chemistry and applications of N-heterocyclic carbenes which have developed rapidly during the last two decades. Numerous structural variations of N-heterocyclic carbenes have been described, most of them aiming at enhancing the electron density at the carbene center and thus influencing the s-donor capacity in catalytically active metal complexes. 4 Recently mesomeric betaines including mesoionic compounds, however, have also come into the focus of carbene research. First results of interconversions of mesomeric betaines into N-heterocyclic carbenes and vice versa are summarized in recent review articles. 5 In continuation of our interest in N-heterocyclic carbenes and mesomeric betaines 6 we report here on sydnones as well as on O-ethylsydnones (5-ethoxy-1,2,3-oxadiazolium salts) and deprotonated species derived thereof.
We prepared a series of sydnones 2a-f by cyclization of N-nitroso amino acids 1a-f with acetic anhydride (Scheme 1). Among eight dipolar resonance forms of sydnones, I is the most common. The best single representation which reflects the properties of sydnones, however, appears to be II. 7 Resonance form II is in agreement with carbonyl stretching frequencies of 2a-f between 1728 cm À1 and 1736 cm À1 . Bond length (1.197 Å), calculated from IR spectra of 2a, and bond order (1.99) correspond to an exocyclic CQO double bond which can also be seen in a single crystal X-ray analysis of 2b (ESI †). We calculated the NICS(1) 8 value of 2a (À6.8) which is between the cyclopentadienyl anion (À12.3) and cyclopentane (À2) (DFT LACVP*/B3LYP). In summary these values confirm that sydnones are no aromatics.
In contrast to other mesoionic compounds such as nitron 3 9 and imidazolium-4-aminide 4 10 no hints on tautomeric equilibria between sydnones and their N-heterocyclic carbenes can be found in the spectra. DFT calculations revealed that the mesoionic form 2aA is considerably more stable than its tautomer 2aB [DE = +191 kJ mol À1 ] (Scheme 2). Except for few photochromic sydnones, 11 valence isomers such as 2aC are not detectable.
A base screening employing LiOtBu, NaOtBu, KOtBu, KH, LiAlH 4 and nBuLi in a variety of solvents, respectively, which have been used earlier to deprotonate sydnones for further functionalizations at C4, 3,7 caused a decomposition of the sydnones 2a-f, when we tried to isolate the deprotonated species. Fortunately, deprotonation by cyanomethyllithium, freshly prepared from nBuLi and anhydrous MeCN, resulted in the formation of the sydnone anions 5a-f in quantitative yields Scheme 1 a Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstrasse 6, as colorless, moisture-sensitive compounds which proved to be stable under an inert atmosphere for several weeks (Scheme 3). On exposure to water or protic solvents, reprotonation of 5a-f to the sydnones 2a-f occurred, which were recovered quantitatively in all cases. Ethylation of the sydnones 2a-f by triethyloxonium tetrafluoroborate gave the 5-ethoxy-1,2,3-oxadiazolium salts 6a-f which are stable under an inert atmosphere up to approximately 60 1C. As the calculated NICS(1) value of 6a is À8.9 these O-ethylsydnones are slightly more aromatic than the corresponding sydnones. The isolation of the abnormal N-heterocyclic carbenes (aNHC) 7a-f, however, failed, as all attempts to deprotonate the O-ethylsydnones resulted in decomposition products.
Sydnone anions can be represented by several resonance forms, among those representations as anionic abnormal NHCs I and II, and the anionic NHC III. Do sydnone anions bear more than a formal relationship to N-heterocyclic carbenes? As a matter of fact, the calculated bond lengths of C4-C5 (1.451 Å), C5QO (1.227 Å), and N3-C4 (1.353 Å) are best represented by resonance form II. As calculated by us and found IR-spectroscopically, 12 the lithium cation is located between C4 and the exocyclic oxygen of 5a. By contrast, the aNHC 7a is best represented as shown, as the calculated C4-C5 bond (1.390 Å) is shortened in comparison to 5a, whereas the C5-O and O-Et bonds are essentially C sp 2 -O and O-C sp 3 single bonds (1.325 and 1.445 Å, resp.). The ESP partial charges of C4 of the sydnone anion 5a as well as of the aNHC 7a adopt values between the CAAC 9 13 on the one hand, and the aNHC 10 14 on the other (Fig. 1). As a comparison, we also calculated the phenyl derivatives of carbene 11 15 derived from a cross-conjugated mesomeric betaine, the anionic NHCs 12 16 and 14 17 derived from the mesoionic compounds imidazolium-4-aminide and imidazolium-4-olate, respectively, and the NHC imidazole-2ylidene 13. Similar to N-heterocyclic carbenes derived from ylides 18 the calculated HOMO of 5a is a p-orbital with a large coefficient on C4, whereas the HOMO À 1 is perpendicular to the HOMO and contains the lone pair of electrons at C4 (Fig. 2).
As expected the HOMO of the aNHC 7a is mainly a s-type molecular orbital as shown.
Palladium complexes to stabilize the aNHC 7a and to study catalytic activities were prepared as follows. 4-Bromo-Nphenylsydnone 15 formed the palladium complex 16 of the sydnone anion 5a as a mixture of trans and cis isomers (Scheme 4). 19 We were able to perform a single crystal X-ray analysis of trans-16 (ESI †). Ethylation of the bromosydnone 15 under exclusion of moisture resulted in the formation of the new 4-bromo-1,2,3-oxadiazolium salt 17 in good yields, which served as starting material for the preparation of the Pd complex 18 of the aNHC 7a.