P-oxidation of gem-dicationic phosphines

Abstract

In spite of the general oxophilic character and λ⁵ valence state accessibility of phosphorus centers, the oxidation of phosphanes bearing proximal positive charges remains challenging, in particular for dicationic representatives. Previously described diimidazolo- and diimidazolio-phosphane substrates were first resumed in the P-phenylated series, where the reported absence of coordinating ability of the N,N-dimethylated dication towards a neutral Rh(I) center is here confirmed towards a promoted oxygen atom. By contrast, the P-tert-butylated homologue is shown to undergo P-oxidation to give a stable gem-dicationic phosphine oxide (in 91% yield with m-CPBA). The latter could also be obtained by N,N-dimethylation of the neutral diimidazolo-phosphine oxide precursor. The strict covalent vs. dative character of the N₂C–P bond in dicationic systems (N₂C⁺)₂P(X)R (R = Ph, tBu; X = lone pair, :O) has been investigated by DFT calculations on a model series where (N₂C⁺) = 1,3-dimethyl-1H-imidazolio-2-yl. The relative magnitude of Gibbs free energies of the homolytic and heterolytic dissociation modes shows that diamidinio-phosphines and corresponding P-oxides are not N₂C → P donor–acceptor adducts (as mono(amidinio)phosphines are), but rather true gem-dicationic phosphines and phosphine oxides in the strict sense, respectively.