Enantioselective Additions of Optically Active Allylic Boron Reagents

4 Enantioselective Additions of Optically Active Allylic Boron Reagents 

In 1978, Hoffmann reported enantioselective allylation reactions of aldehydes with a camphor-derived chiral boronate 13 (Equation 1) [55]. Three classes of chiral boron reagents that furnish allylation products with very high levels of enantioselectivity have emerged Brown s diisopinocampheyl or diisocaranyl allyl boranes [35-41], Roush s tartrate-derived boronates [33, 42-48], and Hoffmann s a-chiral boronates [56-61], 

The addition of allyl and crotyl diisopinocampheylboranes 16 (Ipc2BAllyl R[ = R2 = H, Ipc2BCrotyl Rj or R2 = Me) [35-38, 40, 41] and allyldiisocaranyl-boranes 17 (Icr2BAllyl) [37, 39-41] to aldehydes furnishes adducts in a highly enantioselective manner at or below -78 °C (Equation 2). 

Aldehyde Allyl [35] (E)-Crotyl [38] syn/anti 2 98 (Z)-Crotyl [38] synjanti 98 2 Allyl [40] 

The tartrate-derived allyl and crotyl boronates 25 developed by Roush [33, 42-48] represent a convenient and elegant alternative to the terpene-derived boranes described above, as a consequence of their ease of handling and configurational stability (Equation 3). Representative examples are summarized in Table 5.2 [42, 45, 47]. These reagents have also enjoyed widespread applications in diastereoselective, reagent-controlled additions to chiral aldehydes, displaying superb levels of diasteroinduction [43, 44, 48], An explanation for the observed stereoselectivity has been proffered by Roush [32] and supported by ab initio calculations [62] minimization of lone pair/lone pair interactions as well as an attractive interaction between the ester lone pairs and the polarized aldehyde are believed to result in a preference for transition state structure 26. 

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