1 -phenylphospholane borane

Kobayashi and coworkers have demonstrated that optically active phos-pholane-2-carboxylic acid [n=l,Eq. (33)] and phosphorinane-2-carboxylic acid (n=2) can be prepared by asymmetric carboxylation of the phosphine-borane complexes. High trans selectivity and excellent enantiomeric excesses of the trans isomers were achieved on treatment of 1-phenylphospholane-borane and 1-phenylphosphorinane-borane complexes with s-BuLi-(-)-sparteine and subsequently with CO2. Notably, enantioselectivity was improved up to 92% ee when 1-phenylphospholane-borane was treated with s-BuLi-(-)-sparteine at 25°C, compared with 83% ee at -78°C [87]. 

Better results have been achieved in the enantioselective deprotonation of 1-phenylphospholane borane by Kobayashi and co-workers (Scheme 5.49). ... 

Scheme 5.49 Stereoselective deprotonation/carboxylation of 1-phenylphospholane borane. Only one enantiomer of the trans diastereomer of 123 is represented.

Dephenylation of phosphine-borane complexes. A P-phenylphospholane-borane complex is converted to the P-lithio reagent by treatment with lithium. The latter species are used for the synthesis of chiral ligands. 

The principle has been successfully applied to the differentiation between enantiotopic methylene groups in 1-phenylphospholane- and 1-phenylphosphorinane-borane complexes (199, 200) by Kobayashi and coworkers (equation 46) . The conditions for optimized yields, diastereomeric and enantiomeric ratios of appropriate 2-carboxylic acids 201 or 202 were carefully worked out. For the deprotonation of 199, the application of... 

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