Mysterious Decomposition of Alkoxyphosphonium Chlorides: Postulated Involvement of the HCl2 Anion and Its Capture in the Solid State.

P-Alkoxyphosphonium (AP) chlorides were generated by reacting P-chlorophosphonium chlorides with alcohols. Their well-known spontaneous Arbuzov-type collapse leading to phosphine oxides was studied and its rate found to be dependent on a number of factors in an unexpected fashion: it is inversely proportional to the initial concentration and it shows strong dependence on the acidity of the media but is not very sensitive to the presence of base. To explain these observations, we evoke a self-inhibition model with the formation of the less nucleophilic hydrodichloride anion HCl2 in solution. Detailed analysis of the kinetic data yields the association constant (K=3×102  m-1 ) of the putative HCl2 species in chloroform. Experimental observations for the collapse of highly enriched diastereomeric alkoxyphosphonium (DAP) chlorides are fully analogous to the achiral AP also implying the involvement of HCl2 anions. Moreover, crystallisation of a highly enriched DAP salt derived from (-)-menthol furnished, for the first time, crystals of individual (RP )-DAP hydrodichloride as confirmed by X-ray diffractometry. Importantly, the P-configuration and detailed conformation of the DAP moiety is in good agreement with DFT-level computational results. The thermal collapse of (RP )-DAP⋅HCl2 proceeds with complete retention of the P-configuration furnishing the phosphine oxide of exceptional enantiomeric purity.