Unexpected opposite stereochemistries and different mechanisms of nucleophilic substitution reactions of homochiral tert-butylphenylthiophosphinoyl chloride and bromide

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Reactions of ethylmercapto (ethylsulfanyl) anion with homochiral (+)-(S)-tert-butylphenylphosphinochloridothionate 2 and (–)-(S)-tert-butylphenylphosphinobromidothionate 3 proceed with inversion and retention at phosphorus, respectively, as a consequence of different reaction mechanisms; the stereoretentive reaction with bromide 3, in contrast to the direct nucleophilic substitution at phosphorus in chloride 2, is a two-step process involving in the first step nucleophilic attack of EtS– at the electropositive bromine atom.