Reactivity of phosphonamidic chlorides & phosphinoyl aminimides
The phosphonamidic chlorides ArP(0)(Cl)NMe2 and ArP(0)(Cl)NHBut (Ar = phenyl, o-tolyl, mesityl, 2,4,6-triisopropylphenyl) have been synthesised and characterised. Both types react cleanly with t-butylamine, isopropylamine, and other primary amines in aprotic solvents to give the expected phosphonic diamides, but from the results of rate studies (steric retardation or acceleration) and competitive experiments (high or low discrimination between competing amines) it is clear that the mechanisms differ. For ArP(0)(Cl)NMe2 reaction is always associative SN2(P) but base-induced dissociative elimination-addition processes are important for ArP(0)(Cl )NHBut, especially at higher concentrations of amine. These give rise to reactive metaphosphon- imidate intermediates that display little discrimination and phosphorylate even such poor nucleophiles as diisopropylamine and t-butanol. The results of stereochemical studies with optically active PhP(0)(Cl)NHBut (partially resolved) and ButP(0)(Cl)NHCHMePh (pure diastereoisomers) confirm that while elimination-addition usually proceeds by a preassociation mechanism, the planar metaphosphonimidate intermediate is produced as a free species at low amine concentrations. Part B Reactivity of Phosphinoyl Aminimides The principal photochemical reaction of the phosphinoyl aminimide Ph2P(0)N--+NMe3 in dimethyl sulphoxide or methanol is rearrangement to the aminal Ph2P(0)NHCH2NMe2 which subsequently decomposes to give Ph2P(0)NH2. The phosphinoyl hydrazinium salt Ph2P(0)NMe+NMe3 undergoes a formally similar rearrangement with base to give the aminal Ph2p(0)NMeCH2NMe2. The corresponding benzoyl substrates undergo comparable reactions. To explore the scope and mechanism of these aminal forming rearrangements, the substrates where one or more of the N-methyl groups is replaced by an ethyl group have been synthesised and their reactions examined. For the base-induced 1,2 sigmatropic rearrangement of the hydrazinium salts, insertion into a methyl group, with formation of a methylene aminal, is preferred whereas for the photochemical rearrangement, insertion into an ethyl group, with formation of an ethylidene aminal, is preferred. In the light of these results the mechanisms of the rearrangement are discussed.