Deoxygenation of chalcogen oxides EO2 (E = S, Se) with phospha-Wittig reagents

Abstract

In here we present the deoxygenation of the chalcogen oxides EO₂ (E = S, Se) with R-P(PMe₃), so-called phospha-Wittig reagents. The reaction of DABSO (DABCO·2SO₂) with R-P(PMe₃) (R = Mes*, 2,4,6-tBu₃-C₆H₂; ^MesTer, 2,6-(2,4,6-Me₃-C₆H₂)₂-C₆H₃) resulted in the formation of thiadiphosphiranes (RP)₂S (1:R), while selenadiphosphiranes (RP)₂Se (2:R) were afforded with SeO₂, both accompanied by the formation of OPMe₃. Utilizing the sterically more encumbered ^DipTer-P(PMe₃) (^DipTer = 2,6-(2,6-iPr₂-C₆H₃)₂-C₆H₃) a different selectivity was observed and (^DipTerP)₂Se (2:^DipTer) along with [Se(μ-P^DipTer)]₂ (3:^DipTer) were isolated as the Se-containing species in the reaction with SeO₂. Interestingly, the reaction with DABSO (or with equimolar ratios of SeO₂ at elevated temperatures) gave rise to the formation of the OPMe₃-stabilized dioxophosphorane (phosphinidene dioxide) ^DipTerP(O)₂-OPMe₃ (4:^DipTer) as the main product. This contrasting reactivity can be rationalized by two potential pathways in the reaction with EO₂: (i) a Wittig-type pathway and (ii) a pathway involving oxygenation of the phospha-Wittig reagents and release of SO. Thus, phospha-Wittig reagents are shown to be useful synthetic tools for the metal-free deoxygenation of EO₂ (E = S, Se).