Boranes, secondary alkyl oxidation

The oxidation of (secondary alkyl)boranes with chromic acid leads to ketones, " and in combination with hydroboration provides a high-yielding route from alkenes to ketones. The reaction gives ketones at... 

Pyridinium chiorochromate (PCC) is a very useful reagent for the oxidation of oiganoboranes to carbonyl compounds in mildly kaline and anhydrous conditions. As well as oxidizing (secondary alkyl)boranes to ketones, PCC oxidizes (primary alkyl)boranes to aldehydes in excellent yields (equation 47). The latter transformation cannot be accomplished with chromic acid. The reagent tolerates the presence of alkene, ester and acetal groups. ... 

Synthesis of secootkiry eokois from l-aOtynes. Dihydroboration at room temperature of a terminal alkyne with either (I) or 9-BBN gives a 1,1-diborylalkane (3) this is treated at 0-5° with I eq. of methyllithium in ether. The product (4) rearranges to (S). An alkyl halide (100 excess) is then added, and the resultant secondary organo-borane (6) is oxidized with alkaline hydrogen peroxide. Secondary alcohols (7) are obtained in 70-85% yield. 

When hydroboration of an olefin is carried out with the intention of oxidizing an initially formed secondary alcohol to a ketone, hydrogen peroxide oxidation can be dispensed with and the alkylborane oxidized directly with chromic acid. Thus Pappo converted a hydroxyl-free A -steroid into a mixture of stereoisomeric alkyl-boranes comparable to (1) and (2), oxidized the mixture with chromic acid, and obtained the 6-keto-5a-steroid in good yield the initially formed 6-keto-5i3-steroid underwent isomerism in the process. H. C. Brown developed an efficient two-phase system for oxidation of either alkylboranes or free alcohols to the ketones using aqueous chromic acid and ether. 

SeeSecondary alcohols can be prqiared in satisfactory yield by reaction of (he anion of this reagent with trialkylboianes in the presence of excess mercuric chloride to induce migration of two alkyl groups. Oxidation (alkaline HzOg) of the intermediate borane compound leads to the alcohol. ... 

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