Heterocycle Syntheses with Anionic N‐Heterocyclic Carbenes: Ring Transformations of Sydnone Imine Anions

Abstract

Sydnone imines were deprotonated at the C4 position to give lithium‐stabilized anions, which proved to be stable for several months in solution. These anions are elements of the intersection of the substance classes of N‐heterocyclic carbenes and mesomeric betaines. They can be formulated as anionic N‐heterocyclic carbenes. The negative charge translates into high‐field shifts of the 13C NMR resonance frequencies of C4 in comparison to other NHCs. Similar to the chemistry of N‐heterocyclic carbenes, reactions with isocyanates and isothiocyanates gave 4‐(N‐carbamoyl)‐benzoylsydnone imines in trapping reactions. As a result of ring transformations involving the characteristic π‐architecture of the mesoionic framework, imidazolidine‐2,4‐dithiones, imidazolin‐2,4‐diones and 1,2,4‐triazol‐5‐ones were formed depending on the substitution pattern of the starting materials and the reaction conditions.

Cycloadditions of anionic N-heterocyclic carbenes of sydnone imines

“…19 In sydnone imines, the exocyclic C-N bond is usually slightly longer than the C=N double bond of imines. 20 The characteristic dipole type of conjugated mesomeric betaines can also be dissected from the mesomeric structures of sydnone derivatives (Scheme 3). Indeed, sydnones 1 are known to be versatile masked 1,3-dipoles in cycloadditions.…”

“…Upon deprotonation with suitable bases, anions of sydnones 19,30 as well as of sydnone imines 31 are formed which can be represented as anionic N-heterocyclic carbenes (Scheme 4). 19,20 These undergo numerous trapping reactions (S, 32,33 B, 33 Se, 34 Au, 34 Pd 34 ). Nucleophilic ring transformations have also been reported.…”

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

“…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.…”

“…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.…”