Solvent-dependent stereoselectivity in the non-stereospecific reactions of the diastereoisomers of a phosphonamidothioic chloride with tert-butylamine. A pointer to the lifetime of the thiometaphosphonimidate intermediate
Abstract
The two diastereoisomers of N-[(S)-α-phenylethyl]-P-tert-butylphosphonamidothioic chloride (9) give the same mixture of the diastereoisomers of the phosphonamidothioic diamide product (11) with PriNH2 or ButNH2 in dilute solution, i.e. reaction is completely non-stereospecific. The ratio of the diastereoisomers in the product differs significantly from 50:50, i.e. reaction is appreciably stereoselective. With ButNH2 the Stereoselectivity increases markedly on going from a solvent of high polarity (MeCN or Me2CO) to one of low polarity (CHCl3 or cyclohexane), but with PriNH2 it changes relatively little. In PriNH2–ButNH2 competition experiments the NHPri/NHBut product ratio also increases substantially as the polarity of the solvent decreases. These observations are thought to indicate reaction via a 3-co-ordinate Pv thiometaphosphonimidate intermediate that exists long enough not only to become liberated, but also, in less polar solvents, to experience unsuccessful collisions with ButNH2 before being transformed into product.