Reactions of Phosphine Oxides with Bromophosphoranimines; Synthesis and Unusual Rearrangements of O–Donor Stabilized Phosphoranimine Cations

Reaction of phosphine oxides R3PO [R = Me (1a), Et (1c), iPr (1d) and Ph (1e)], with the bromophosphoranimines BrPR′R″PNSiMe3 [R′ = R″ = Me (2a); R′ = Me, R″ = Ph (2b); R′ = R″ = OCH2CF3 (2c)] in the presence or absence of AgOTf (OTf = CF3SO3) resulted in a rearrangement reaction to give the salts [R3PNPR′R″O−SiMe3]X (X = Br or OTf) ([4]X). Reaction of phosphine oxide 1a with the phosphoranimine BrPMe2NSiPh3 (5) with a sterically encumbered silyl group also resulted in the analogous rearranged product [Me3PNPMe2O-SiPh3]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl substituted phosphine oxide, tBu3PO (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized phosphoranimine cations [tBu3PO·PR′2NSiMe3]+ ([3]+, R′ = Me (d), OCH2CF3 (e)). A mechanism is proposed for the unexpected formation of [4]+ in which the formation of the donor-stabilized adduct [3]+ occurs as the first step.