Phospholane Boranes, Sulfides and Oxides

Reaction of meso-115 with a chiral lithium amide at very low temperature (100 °C) furnished, after addition of several electrophiles, the chiral products 116 in good yields and optical purities (Table 5.19). 

Only the diastereomer maintaining the syn arrangement of the three phenyl rings was formed, in eomparable yields and rather similar enantioselectivities of around 85% ee irrespeetive of the electrophile employed. The optical purity of 

Some of the enantiopure phospholane oxides 116 were used for further transformations. For example, they could be isomerised by treatment with base for several hours in a protic solvent to afford 117 or subjected to a second deprotonation to produce 118 as a single isomer. Attempts to prepare Z)w(phospholanes) by double nucleophilic attack of the anions derived from 116 were unsuccessful. Some of the phospholane oxides were reduced with tri-chlorosilane/pyridine to afford the free phospholane with retention of configuration. The same strategy was applied to 1-phenylphospholane oxide but low yields of racemic products were obtained. This suggests that substrate 115 is probably a special case due to the formation of a more stable benzylic carbanion.  

In this case the carbanion was quenched with dry ice in order to prepare the a-carboxylated phospholane borane 123. This compound was obtained as a 

The enantioselective deprotonation of 128 and oxidative addition of the carbanion with Cu(II) afforded TangPhos disulfide as a mixture of diaster-eomers where the chiral C2-symmetric product was the major compound (66% de, 95% ee). A single recrystallisation afforded the desired optically pure TangPhos sulfide in 20% yield. The desulfuration was performed with hexa-chlorodisilane to yield the very air-sensitive Tangphos diphosphine in 88% yield. The synthesis of a family of phospholane-oxazoline ligands started with the preparation of carboxylic acid 129 by the proeedure in 40% yield after recrystallisation. The crude product was obtained in relatively low optical 

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