Strukturuntersuchungen zum System 1,3‐Dialkoxy‐2‐aza‐allenium‐Ionen/1,3‐Dialkoxy‐2‐aza‐allyl‐Kationen

Abstract

CationsThe title system (I/II) is experimentally accessible by regiospecific alkylation at the carbonyl oxygen of alkyl N-acylimidates 4 using txialkyloxonium salts. According to an X-ray investigation (3a), the salts 3 form a compromise structure between both isomeric forms, the alkoxy substituents being in exo positions in s-cis-conformations (C -N -C-bond angle 133.0", torsional angle 70.4'). Quantum mechanical calculations (3-21G ab initio) for the parent substance N-formylformimidic acid 6 a predict preferred nitrogen protonation giving 7a. Oxygen protonation leading to the allenic struNure 8a is disfavoured by 11.4 kcal/mol. The allenic-(8a) and the ally1 structure (9a) differ by only ca. 5 kcal/mol and indicate therefore enormous structural flexibility of the title system. ' H NMR spectra support the existence of chiral isomers of 3a at low temperatures; at higher temperatures rapid equilibration takes place.Im Rahmen unserer Untersuchungen zur Strukturchemie von 2-ha-alleniumIonen und 2-A~a-allyl-Kationen~~~) und zum Gleichgewicht zwischen diesen beiden isomeren, reaktiven Zwischenstufen konnten wir zeigen, dal3 aromatisch substituierte Derivate bevorzugt in der linearen, allenartigen Struktur ~orliegen~.~); Elektronendonorsubstituenten, z. B. eine Alkoxygruppe4. 5), stabilisieren die gewin-0 VCH Verlagsgesellschaft mbH, D

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

“…The iminium ion 3 ؉ exhibits a longer ''normal'' bond length (Xray: 1.29Å ; RHF/6-31G*: 1.281 Å ; B3LYP/6-31G*: 1.308 Å ), which is close to the standard data for CϭN bonds [18,20] involving an sp 2 -hybridized nitrogen atom. An analogue of the third system 2a ϩ [diphenyl instead of bis(p-tolyl)] with its two electron-donating ethoxy groups was subjected to an X-ray diffraction study some time ago; [11] the CϭN bond length obtained was 1.30 Å , (RHF/6-31G* 1.294 Å ; B3LYP/6-31G* 1.308 Å ), slightly longer than in 3…”

“…- [11] in the solid state was already known. Preliminary experiments have indicated that this system is indeed relatively unreactive, which limits considerably the choice of suitable nucleophiles for kinetic studies.…”

Heterocations in Superacid Systems

“…DFT calculations, AIM analysis, and Boys localized orbitals indicate that the long C(2)-F interaction is a dative bond and the unsymmetrical 188b structure must also contribute. [425][426][427][428][429][430][431] They are well-characterized and used widely in dipolar cycloadditions. The Si-containing ring system 194 analogous to 188 has been reported by M€ uller and co-workers.…”

“…422 The silyl cation is stabilized by intramolecular interactions with the F atoms increasing the coordination number of Si and, consequently, Si becomes considerably shielded. 427 The 2-azaallyl structure 200 of the parent tetrahydroxy model compound (C 2 symmetry) was shown by quantum mechanical calculations to be 20 kcal mol À1 lower in energy than the corresponding 2-azoniaallene. Characteristic data for cation 194 are as follows: SiÀF bond lengths ¼ 1.…”

Alkynylcarbenium ions and related unsaturated cations

“…The alkynyldihydroxycarbenium ions RC:C ± C + (OH) 2 (R = H, Me, Ph) arising upon protonation of the corresponding propiolic acids by FSO 3 H in liquid SO 2 (778 8C) or in SO 2 ClF (7120 8C) have been detected by NMR spectroscopy. 102 A fairly comprehensive study of the 13 C NMR spectra of a series of phenylethynylcarbenium ions (together with benzyl-and naphthylcarbenium cations) has been reported by Olah et al 103 They analysed in detail the degree of positive charge delocalisation as a function of the nature of the substituents at the carbenium centre. In particular, the 13 C NMR spectra of protonated carbonyl compounds 113 were discussed.…”