Magnesium amide bases chiral

Recently, Henderson has investigated the effect of Lewis base additives such as HMPA in enantioselective deprotonation of ketones mediated by chiral magnesium amide bases. In almost all reactions investigated, the additive HMPA could be replaced by DMPU without any undue effect on either selectivity or conversion (equation 69) ... 

Novel chiral bases introduced recently include the magnesium amide base 7 and analogs reported by Henderson [48] and Knochel s urea base 8 (mono and di-anions) [49]. 

Novel and readily accessible polymer-supported chiral magnesium amide reagents have been prepared and shown to be effective in the asymmetric deprotonation of a series of prochiral cyclohexanones, affording good to excellent conversion and enantiomeric ratio (up to 93 7) the Merrifield-based chiral amine species has been shown to be readily recyclable (Scheme 3.39) [29]. 

Sodium amide, o LDA, l.illMI)S. " NaHMDS, KHMDS, ° sodium and magnesium methoxide or ethoxide, and DBU are less frequently used. The nature of the cation present in the Homer-Wadsworth-Emmons reaction depends on the chosen base and greatly influences the stereoselectivity of the reaction. The chiral lithium 2-aminoalkoxides (17 ,2S) have been used as chiral bases for the enantioselective reaction between diethyl cyanomethylphosphonate and 4-terz-butylcyclohexanone." ... 

In various cases, Koga s bases 75 and 76 performed better with respect to enantioselectivity [77]. The groups of Williard [78] and Majewski etal. [79] introduced - soluble or cross-linked - polymer-supported lithium bases, as illustrated by the chiral deprotonating agent 77. Aside from lithium amides that were used by far most frequently, chiral magnesium bisamide 74 and polymer-supported regents derived thereof were also applied in enantioseiective deprotonations of prochiral ketones [80]. 

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