Borane complexes, asymmetric deprotonation

It is well known that certain diamines increase the reactivity of organolithium compounds by complexation to the Li atom. Consequently, with the appropriate chiral diamines it should be possible to perform catalytic asymmetric deprotonations. In the original report of Evans and co-workers it is mentioned that the enantioselection can be maintained with only 0.7 equivalents of ( )-sparteine, with no further details. These ideas have been recently exploited for t-butyldimethylphosphine borane and the analogous sulfide with sparteine and its surrogates. It can be understood with the catalytic cycle of Scheme 5.54, reported by O Brien and co-workers. 

Both dimethylphenylphosphine-borane (107) and -sulfide (108) are enantio-selectively deprotonated by a lithiumalkyl (—)-sparteine complex as demonstrated by subsequent reaction with electrophiles to give products with e.e. values of 80-87% (Scheme 8). Oxidative coupling of (109) in the presence of copper(II) pivalate gives the (S. S)-isomer (110) as the major product. Asymmetric metalla-tion and silylation of diphenylphosphinyl ferrocene (111) using the chiral lithium amide base derived from di(l-methylbenzyl)amine has been reported to give an... 

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