Addition ephedrine-borane

The first attempt to use a chiral ligand to modify borane was Kagan s attempt at enantioselective reduction of acetophenone using amphetamine-borane and desoxy-ephedrine-borane in 1969 [18]. However, both reagents afforded 1-phenyl ethanol in <5% ee. The most successful borane-derived reagents are oxazaborolidines, introduced by Hirao in 1981, developed by Itsuno, and further developed by Corey several years later (reviews [19,20]). Figure 7.2 illustrates several of the Hirao-Itsuno and Corey oxazaborolidines that have been evaluated to date. All of these examples are derived from amino acids by reduction or Grignard addition. Hirao... 

Rh-H / -elimination step in step E gives the vinyl-borane which then dissociates to generate the first intermediate. This cycle explains the production of equimolar quantities of alkane and -vinylborane with unreactive R2BH derived from ephedrines with catecholborane there is an additional shunt pathway — step C in competition with step E leading to the primary regio-isomer of alkylborane. 

This can be done using either (-)- or (+)-ephedrine in the synthesis of 7 or, more interestingly, reverting the order of addition of the organolithium reagents from one of the precursors. An example is found in Table 4.4 (entries 1 and 12) in the synthesis of both enantiomers of PAMP BH3 from (4-)-7 (derived from (—)-ephedrine). Although promising, this approach has rarely been used, i.e. in most cases only one of the enantiomers of the phosphine borane has been obtained. 

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