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 1JC4‐Se as well as 1JC4‐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.

Pyrazoles in the Intersection of Mesomeric Betaines and N-Heterocyclic Carbenes: Formation of NHC Selenium Adducts of Pyrazolium-4-aminides

“…Mesomeric structures such as 19a-hC, possessing two formal negative charges on the carbene carbon atom, are characteristic of anionic NHCs derived from mesoionic compounds and other conjugated mesomeric betaines. 26 They are reflected in considerable atomic orbital coeffi-…”

“…Some examples are as follows. Sydnones 1 (X = O) 24 as well as sydnone imines (X = NR) 25 and sydnone methides (X = CR 2 ) 26 are examples of conjugated mesomeric betaines (CMB) (Scheme 1). For historic reasons, their five-membered representatives are called mesoionic compounds.…”

“…53 We calculated the CREF values of the pyrazolium betaines 18a-h at the B3LYP/6-311++G** level in accordance to literature. 44a In comparison to the CREF values of other mesomeric betaines as NHC precursors, 54 such as 1,3-dimethylimidazolium-4-olate (CREF = 0.576), 1,3-dimethylimidazolium-4-aminide (CREF = 0.557), the aforementioned 1,3-dimethyl-6-oxo-pyrimidinium-4-olate 3 (CREF = 0.547), and sydnone methides 1 (X = CR 2 ; CREF = 0.534), 26 these values seemed to be very promising (Table 2). We also calculated the CREF value of the pyrazolium salt 17i, which we prepared for comparison.…”

p‐TSA‐Mediated Four‐Component Reaction: One‐Step Access to Mesoionic 1H‐Imidazol‐3‐ium‐4‐olates, Direct NHC Precursors

“…Excellent reviews [26,27] on NHCs emphasized three major applications of these species, such as their coordination to transition metals, [28] coordination to p ‐block elements, [29] and as organocatalysts [30–33] . NHCs have been reported several times, e. g., [24,34–39] and anionic NHCs appear less frequently in the literature [23,40–42] …”

“…[30][31][32][33] NHCs have been reported several times, e. g., [24,[34][35][36][37][38][39] and anionic NHCs appear less frequently in the literature. [23,[40][41][42]…”