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 Bu^tNH₂ 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 Bu^tNH₂ 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–Bu^tNH₂ competition experiments the NHPrⁱ/NHBu^t product ratio also increases substantially as the polarity of the solvent decreases. These observations are thought to indicate reaction via a 3-co-ordinate P^v thiometaphosphonimidate intermediate that exists long enough not only to become liberated, but also, in less polar solvents, to experience unsuccessful collisions with Bu^tNH₂ before being transformed into product.