Preparation of Optically Active Chiral Phosphines

The absolute configuration of P-chiral phosphines can be determined by special x-ray diffraction techniques (Chapter 14.2), 

Among the many methods of synthesis of chiral phosphines now developed are  

Electrolytic reduction of a phosphoninm salt as (13.57). This proceeds stereo- specifically with retention of configuration and yields of 80-90% are attainable. Lead or mercnry cathodes are used at 70-90°C with typically 24 V at 2-6 A. 

Reduction of an optically active phosphine oxide. The choice of reducing agent determines whether inversion or retention of confignration occurs. The use of Si2Cl( , for example, produces an inversion of configuration, whereas PhSiH3 usually results in retention. 

The production of optically active phosphines and phosphine oxides may be achieved by scheme (13.68) starting with ( ) menthol and a phosphinous chloride. The components of the diastereoiso-meric mixture of phosphinates can be separated by fractional crystallisation and then processed as indicated in the scheme. Other methods of synthesis of optically active phosphinates have been devised [24]. 

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