Phosphoric acids, enantioselection allylboration

Originally, enantiosdective allylboration was developed using chiral allylbo-ranes and allyl boronates. These reactions require multistep preparahons of chiral reagents that are used in stoichiometric amoimts, and are therefore impractical. Recently, catalytic asymmetric allylborations were developed. These reactions can apply either chiral Lewis bases or BBonsted acids as the catalysts, hi particular, chiral BlNOL-phosphoric acids were demonstrated to provide high optical yields in the enantioselective allylboration reaction between allylboronate 1 and aldehydes. For example, the catalytic asymmetric allylboration of benzaldehyde 2 proceeded quantitatively yielding the corresponding homoallyl alcohol 3 with 98% ee ( heme 3.1). 

The mechanism of enantioselection was examined computationally by two groups. In both studies, it was suggested that the catalyst does not change dramatically the six-membered transition state of the allylboration reaction (Figure 3.1), but decreases its activation barrier via specific coordination of the chiral phosphoric acid to one of the oxygen atoms of the boronate ring.i -i ... 

Allylborations of aldehydes have included enantioselective reactions with pinacol allylboronates, catalysed by l,l -spirobiindane-7,7 -diol (SPlNOL)-based phosphoric acids, and with chiral fi-(3,3-difluoroallyl)diisopinocampheylborane to form 1,1-difluorinated homoallylic alcohols with 91-97% ee ... 

The SPINOL-based phosphoric acid (157) has been found to be a general, highly enantioselective catalyst for asymmetric allylboration of aldehydes with pinacol allylboronates (156). Excellent enantioselectivities have been obtained for different types of aldehydes including aromatic... 

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See also in sourсe #XX -- [ , , ]

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