Stereoselective Additions with Chiral Allylboronates

Hoffmann and co-workers reported in 1978 the first examples of chiral allylboronates [61]. These reagents were assembled from rigid camphor-derived 1,2-diols, providing different allylation reagents of generic structure 72 and 73 [62]. Although 

Cyclic derivatives 75 lead to higher levels of enantioselectivity but their preparation require more effort than for the simpler reagents 74 [104,105]. Corey and co-workers reported the bis (sulfonamide) derivatives of type 76, which provide very high enantioselectivity ( 95% ee) in the case of unsubstituted allyl and 2-substituted reagents [30]. The recent advent of the Lewis acid-catalyzed manifold (Section 6.3.1.2) opened new doors for enantioselective aUylborations and motivated the reexamination of several chiral auxiliary systems. In this perspective, reagents 73, one of the two classes 

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The use of optically pure a-chloroalkyl boronic esters as electrophiles, e.g. the dicyclohexyl boronate 18 obtained from Matteson s elegant methodology [41], lends access to a-alkyl substituted allylboronates (e.g., 19) with very high diastereomeric purity by way of a net inversion of configuration (Equation 9) [42]. Likewise, alkenylmetal fragments react with chiral dichloromethylboronate 20 to afford optically pure a-chloroallylboronates such as 21 (Equation 10) [43]. Subsequent addition of these a-substituted reagents to aldehydes is highly stereoselective. Furthermore, the chloride... 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

The addition to 1,3-dienes afforded a new class of allylboron compounds. The platinum(0)-phosphine catalysts stereoselectively yielded or-1,4-addition products 131233 234 and 133216 235 for 2,3-disubstituted butadiene, 1,3-cyclohex-adiene, and 1,3-pentadiene by Txr-coordination of a diene to a platinum catalyst (Equations (30) and (31)). In contrast, phosphine-free Pt(dba)2 resulted in the selective formation of a 1,2-addition product 134216 for 1,3-pentadiene (Equation (31)). The corresponding chiral allyl boronates were synthesized by diboration of dienes with 123 or 124.234 235... 

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