Extremely Base‐Resistant Organic Phosphazenium Cations

Abstract

A series of peralkylated polyaminophosphazenium cations exhibiting extraordinary base resistance under phase-transfer conditions were efficiently synthesized from readily available starting materials. Their half lives under these conditions exceed those of the most stable conventional organic cations by factors of up to 3000.

New Lipophilic 2‐Amino‐N,N′‐dialkyl‐4,5‐dimethylimidazolium Cations: Synthesis, Structure, Properties, and Outstanding Thermal Stability in Alkaline Media

“…These stabilities are far superior to those of conventional organic cations, and comparable to those of the most stable peralkylated polyaminophosphazenium cations. [4] The neopentyl-substituted cations 1 b + and 2 b + and the P 5 -phosphazenium cation [4,11] proved to be the most stable of the studied series. They did not show tangible signs of decomposition in a refluxing biphasic mixture of PhCl and 50 % aqueous KOH (Table 1, entries 2, 5, and 7).…”

“…[4,11] The enhanced stability may be attributed to accommodation of the positive charge by an electronically stabilized aromatic imidazolium moiety, with bulky Nalkyl groups providing additional steric protection. Susceptibility to Hofmann degradation is strongly diminished due to the low charge density on the endocyclic nitrogen atoms or may be completely precluded, as in the case of the tetraneopentyl cation 1 b + .…”

“…; uncorrected): Wagner & Munz PolyTherm A apparatus; elemental analyses: PE 2400 Series II CHN analyzer. NMR spectra were measured from solutions in CDCl 3 on a Bruker Avance 400 spectrometer at 300 K unless stated otherwise: 1 H (400 MHz) referenced to internal standard SiMe 4 in CDCl 3 and C 6 D 6 or to residual solvent peaks [24] in [D 6 ]DMSO and CD 3 CN; 13 C (100.6 MHz) referenced to d-solvent signals; [24] 31 P (162 MHz) referenced to internal standard (MeO) 3 PO (d = 3.0 ppm in CDCl 3 or 3.5 ppm in [D 6 ]DMSO); [25] 19 F NMR (376.5 MHz) referenced to internal standard PhCF 3 (d = À63.7 ppm), [26] the signals of BF 4 À of the imidazolium salts appeared at À155 AE 0.5 ppm as two singlets due to 10 …”

“…[3,4] For this reason, the design and synthesis of stable organic cations has become an important objective with regard to phase-transfer methodology. [4,5] The organic phosphazenium cations developed by Schwesinger et al [4,6] set new benchmarks in terms of thermal stability and base resistance, and hence they attracted considerable interest from industrial laboratories.…”

“…The reaction mixture was stirred for 24 h at ambient temperature. Aqueous workup as described in GP 5 furnished the pure product 3·BF 4 Stability tests of cations 1 + -3 + and P 5 + under phase-transfer conditions: Method A: A 50 % aqueous KOH solution (1.5 mL) was added to a Schlenk tube containing a solution of the corresponding tetrafluoroborate salt (0.50 mmol) in chlorobenzene (1.5 mL). As a quantitative NMR standard, dibenzo- [18]crown-6 or [18]crown-6 was chosen, in order to avoid overlaps of the NMR signals with those of the cations to be tested.…”