Deprotonation of Other Substrates

The optically pure borane was prepared in four steps starting from R)-a-pinene and BH3 SMc2. The alkylated products 140 were obtained with yields in the range 77-92%. The best ee was 74%, obtained in the reaction with acetophenone. The functionalised borane could be cleaved by a conventional treatment with DABCO. 

The metallation of 141 with an excess of -BuLi produced a dianion whose electrophilic quenching afforded ort/zo-functionalised phosphinamides 142. The disastereomeric excess of 142 varied from 33 to 66%, the best result being with E = Ph3SiCl. In all the cases, the epimers could be separated by column chromatography. 

Monoalkylation of 147 was achieved using one equivalent of potassium hydroxide and 10% of L, which served as chiral phase-transfer catalyst. Unfortunately, 148 was obtained in 17% ee only. 

Although the desymmetrisation of te/-t-butyldimethylphosphine borane and the analogous sulfide has been widely used, the application of the same chemistry to other substrates, especially cyclic phosphines, is still relatively undeveloped and further advances are likely to appear. 

The use of (+)-sparteine surrogates and the catalytic version of the protocol (now using substoichiometric amounts of chiral diamine) are very exciting extensions that undoubtedly will see further progress, for example in order to decrease the amounts of chiral inductor required for the reaction to work efficiently. Furthermore, the generalisation of this catalytic variant of the method to the synthesis of more phosphines remains to be seen. 

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