Boranes alcohols, branched

Alkyl boric acid esters derived from straight-chain alcohols and aryl boric acid esters are stable to relatively high temperatures. Methyl borate is stable to 470°C (11). Trialkoxyboranes from branched-chain alcohols are much less stable, and boranes from tertiary alcohols can even decompose at 100°C (12). Decomposition of branched-chain esters leads to mixtures of olefins, alcohols, and other derivatives. 

Albert S.C. Chun of the Hong Kong Polytechnic University reports (J. Org. Chem. 68 1589, 2003) two important transformations. The three-component (Mannich) condensation of 10 with 11 and 12 proceeds with high diastereoselectivity, to give the amino alcohol 13. Hydroboration of the alkyne 14 followed by transmetalation of the intermediate vinyl borane gives a zinc species, which under catalysis by the easily-prepared 3-naphthol 13 adds to aromatic and branched aldehydes with high . The product allylic alcohols are useful intermediates for organic synthesis. 

Masamune and coworkers reduced unsymmetrical dialkylketones RCOR (R1 = branched alkyl) with boranes 2.14 (R = H), and this reaction is highly enan-tioselective. Asymmetric induction occurs by coordination of the carbonyl group of the ketone to the mesylate 2.14 (R = 0S02Me) which is present in catalytic amounts. This is followed by hydride transfer in a fashion so that steric interactions are minimized [85, 566, 1089, 1098, 1099] (Figure 6.7). The availability of (R,R)- and (5,iS)-2.14 allows the formation of either enantiomeric secondary alcohol. 

---