Three-Component Diazaphospholium Synthesis Enables P(V)-to-P(III) Conversion and Ylide-Mediated CO2 and CS2 Activation

Abstract

Diazaphospholium triflate salts 3[OTf]₂ were synthesized through a three-component (1+2+2)-cycloaddition of the imidazoliumyl-subsituted phosphenium surrogate [L_C‑P(OTf)Ph]⁺ cation (L_C = 4,5-dimethyl-1,3-diisopropylimidazolium) with nitriles and N-benzylideneaniline. These P(V) heterocycles display pronounced base-dependent reactivity. Treatment with 4-dimethylaminopyridine (DMAP) triggers a 1,2-phenyl migration and a formal P(V)-to-P(III) conversion, affording cationic DMAP adducts that can be further transformed into azaphospholes bearing P–O, P–C, and P–N bonds. Key intermediates and products were isolated and fully characterized, and the proposed mechanism is supported by DFT calculations. In contrast, reaction of diazaphospholium triflate 3[OTf]₂ with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), followed by deprotonation, furnishes a phosphorus ylide featuring a strained, spirocyclic phosphorane motif. This species exhibits not only classical Wittig-type reactivity but also unusual CO₂ and CS₂ activation, leading to oxazinedione- or thiazinedithione-type products alongside the corresponding phosphole chalcogenides.