Reactivity vs. Stability of Cyclopropenium Substituted Phosphonium Salts

Abstract

The stability vs. reactivity of electrophilic 3‐(triphenylphosphonio)‐cyclopropenium salts towards a neutral nucleophile, such as triphenylphosphane, is reported. Depending on the nature of cyclopropenyl substituents (R), the three‐membered cyclic structure is preserved (R = Ph) or evolves by ring opening to the isomeric linear allene (R = Mes). The respective formation of 1,3‐bis(triphenylphosphonio)‐2,3‐diphenylcyclopropene and 3,3‐bis(triphenylphosphonio)‐1,1‐dimesitylallene products is rationalized on the basis of steric and electrostatic constraints.

N-Cyclopropenio-imidazol-2-ylidene: An N-heterocyclic carbene bearing an N-cationic substituent

“…17 The dicationic imidazolium salt (1•H)(OTf)2 was obtained in 98% isolated yield upon reacting 1-mesitylimidazole with the chlorocyclopropenium triflate (2)(OTf) in presence of sodium triflate as chloride scavenger in THF at 60°C (Scheme 1). 18 The addition of a triflate source was shown crucial since no reaction occurred in its absence. 19 The air stable salt (1•H)(OTf)2 available on a gram scale precipitated along the course of the reaction and was isolated simply by filtration and washings in spectroscopic and analytical pure form.…”

“…Hydrogen atoms, except on C2, have been omitted for clarity. Selected bond lengths (Å) and angles (deg): N2-C4 1.4090(18), C4-C5 1.3581(19), C4-C6 1.360(2), C5-C6 1.4426(19), C5-N3 1.2983(17), C6-N4 1.2965(18), C1-N22-C4-C5 81.15.…”