Menthylphosphinite Boranes and Related Compounds

The sequence reductive cleavage lectrophilic quenching has often been used to prepare products with retention of configuration after a seminal report of Imamoto and co-workers. This is because these transformations are carried out under much milder conditions giving products of higher optical purity. It was found that the best reducing systems in terms of optical purity of the products was lithium naphthalenide and Li/NH3(1). Care has to be taken because it was also found that the anionic tricoordinated phosphorus species generated racemised extensively via pyramidal inversion unless they were 

These reactions often proceed in high yields and enantioselectivities with only a few exceptions (entries 11 and 16). In most of the entries the menthoxy group is substituted either by a hydrogen atom or by methyl group. The products of entries 19-21 are especially interesting because are direct precursors of bidentate phosphines. 

Deboronation of 56 and methylation afforded the phosphonium salt 57, which was stereoselectively reduced and reboronated again. Phosphine borane 58 was obtained in a combined yield of 73% and 97% ee. 

Many of the secondary phosphine boranes obtained with the menthol methodology have been deprotonated (usually with n-BuLi) and quenched with electrophiles leading to a variety of tertiary phosphine boranes (Table 2.13). 

The generated phosphide boranes are extremely nucleophilic and reacted smoothly with activated alkyl halides (entries 3-7), epoxides (entries 1, 2 and 8) and molecules with activated multiple bonds such as ethyl acrylate (entry 9), benzyne (10) or even fullerene (entry 14). Some C2-symmetric diphosphine 

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