Formation of Chlorophosphine Boranes

The widespread use of achiral or racemic chlorophosphines 27 in synthetic phosphorus chemistry (PClPh2 in particular is an ubiquitous diphenylpho-sphinating agent) contrasts with their rare application in P-stereogenic chemistry. The reason is the lack of methods to produce enantioenriched P-stereogenic chlorophosphines and their marked tendency to racemise. Indeed, the first example of enantiomerically enriched chlorophosphine was reported by Ome-lanczuk in 1992, who prepared (5)-P(/-Bu)PhCl in 49% ee by decomposition of an optically emiched thiophosphonium salt. The same compound has been prepared more recently, albeit in low ee, by kinetic resolution.  

The formation of 29 can be explained by a double nucleophihc attack the phosphorus atom to the H and the chloride anion to the phosphorus centre. Other isomers of 29 are very disfavoured due to the high apicophilicity of the chloro substituent. The easy substitution of Cl by Br when reacting 27 with HBr 

Treatment of 8 (derived from (-l-)-ephedrine) with two equivalents of a toluenic solution of HCI (previously titrated) produced 30 in 90% yield, with inversion of configuration at the P atom. Purification by column chromatography, although 

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