Enol Boranes

A regiospecific synthesis of unsymmetrical /3-diketones is achieved by cycloaddition of nitriles to enol boranes, giving the corresponding boroxazines that readily hydrolyze in acid conditions to /3-diketones (Equation (3)) 47... 

Enol boranes, which may be formed regiospecifically, undergo cydoaddition to acetonitrile to give boroxazines, which are precursors of specifically substituted 1,3 diketones (Scheme 43). ... 

If boranes (K. Utimoto, 1973 H.C. Brown, 1975, 1980 A. Pelter, 1979) are used as donor synthons for the alkylation of a, -unsatarated carbonyl compounds, no enolate anion is formed, and the, 8-position of the C=C bond is the only reaction site. 

Hydrogenation of olefins, enols, or enamines with chiral tVilkinson type catalysts, e.g., Noyort hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

Enolates also result from the deprotonation of ketones 4 by means of dieyclohcxylchloro-borane. As expected, the (A)-enolboranes 5 formed in this way lead to rw/Z-aldols. Remarkably simple induction of diastereoselectivity is achieved in aldol additions with isobutyraldehyde53d< ... 

The (acyloxy)borane complex 9, readily available from tartaric acid derivative 8, also catalyzes aldol additions of silyl enol ethers34 and silylketene acetals3 5 in an enantioselective manner. Thus,. u -ketones 10 and /Thydroxy esters 12 are available34, as well as a-unsubstituted ketones 1135. 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

---