Sydnonmethide – fast vergessene Mesoionen als Vorläufermoleküle von anionischen N‐heterocyclischen Carbenen

Abstract

Wir beschreiben Sydnonmethide, von denen bisher nur ein einziges Beispiel in der Literatur erwähnt worden war. Sydnonmethide können zu Anionen deprotoniert werden, die als π‐elektronenreiche anionische N‐heterocyclische Carbene formulierbar sind. Schwefel‐ und Selenaddukte konnten wir als ihre Methylether stabilisieren sowie Quecksilber‐, Gold‐ und Rhodiumkomplexe von Sydnonmethiden herstellen. Die Anionen von Sydnonmethiden gehen zudem C‐C‐Kupplungsreaktionen mit 1‐Fluor‐4‐iodbenzol unter Pd(PPh3)4‐ und CuBr‐Katalyse ein. Zur Abschätzung der elektronischen Eigenschaften dieser anionischen N‐heterocyclischen Carbene bestimmten wir die 77Se‐NMR‐Resonanzfrequenzen und die 1JC4‐Se‐Kopplungskonstanten der Selen‐Addukte sowie die 1JC4‐H‐Kopplungskonstanten der mesoionischen Vorläufermoleküle. Im Vergleich zu klassischen N‐heterocyclischen Carbenen erscheint das Carben‐Kohlenstoffatom des Sydnonmethid‐Anions 3 j in den bei −40 °C gemessenen 13C‐NMR‐Spektren mit δ=155.2 ppm bei extrem hohem Feld.

Sydnone Methides: Intermediates between Mesoionic Compounds and Mesoionic N‐Heterocyclic Olefins

“…New derivatives of sydnone methides were prepared starting from the sydnones 1 a – d which were first converted into their O ‐triflylsydnones by treatment with Tf 2 O, and then subjected to a reaction with the anions of malodinitrile (Z=CN), methyl 2‐cyanoacetate (Z=COOMe), 3‐oxobutanenitrile (Z=COMe), and 2‐(methylsulfonyl)acetonitrile (Z=SO 2 Me) to give the target molecules 2 a – k , respectively (Scheme 3). The methylcarbonyl sydnone methides 2 j , k form a new class of derivatives which, in contrast to the other sydnone methides described here and earlier, [27] are reddish in color. In the 1 H NMR spectra, the protons in the 4‐position show resonance frequencies at δ=9.13 ppm ( 2 k ) and δ=9.08 ppm ( 2 j ), which are significantly shifted downfield in comparison to, for example, those of the cyclohexyl derivatives 2 c (δ=8.54 ppm) or 2 h (δ=8.41 ppm).…”

“…In continuation of our interest in mesomeric betaines, [24] N‐heterocyclic carbenes [25] and the intersection of these two substances classes, we recently became interested in sydnone methides (“sydnone methanides”, “sydnone methylides”), which are members of an hitherto almost unknown class of compounds [27] . They are the carbon analoga of the well‐known sydnones and sydnone imines which possess exocyclic O‐ and N‐substituents, respectively.…”

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

“…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.…”

“…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 coefficients of the HOMO, which are π-orbitals, on the carbene carbon atom (Figure 4 ). The characteristic NHC σ-orbital is HOMO-1 with large coefficients on the carbene carbon atom.…”