Alkylborane oxidation

DIMETHYL-3-PHENYLPRO-PIONALDEHYDE, 54, 46 Alkylboranes, oxidation, 52, 59 synthesis, 52, 59 Alkyl bromides, from alcohols, 54, 66... 

Hydroboration is the addition of borane (BH3) to an aikene, forming an alkylborane. Oxidation converts the C - B bond of the alkylborane to a C - O bond. 

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. 

Hydroboration-Oxidation An alkene reacts with BHsTHF or diborane to produce an alkylborane. Oxidation and hydrolysis of the alkylborane with hydrogen peroxide and base yield an alcohol (Section 8.6). 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

Deuterioboration of 5a-cholest-2-ene (171), followed by oxidation of the alkylborane intermediate with hydrogen peroxide in the presence of sodium hydroxide, illustrates the application of this method for the preparation of c/5-deuterium labeled alcohols.(For the preparation of tra 5 -deuterium labeled alcohols see section VII-A.) The predominant reaction product is 2a-di-5a-cholestan-3a-ol (172, 1.03 D/mole) which is accompanied by 3a-di-5a-cholestan-2a-ol (173) and other minor products." ... 

The alkylboranes obtained by the hydroboration reaction are versatile intermediates for further transformations. The most important transformation is the oxidation to yield alcohols 17 it is usually carried out by treatment with hydroperoxide in alkaline solution. The group R migrates from boron to oxygen with retention of configuration ... 

The alkylborane is oxidized by the addition of 32 ml of 3 A sodium hydroxide followed by 32 ml of 30% hydrogen peroxide to the stirred solution maintained at 30-50° (water bath), and the stirring is continued at the temperature for 1 hour. 

The alkylborane is then oxidized by the addition of 150 ml of a 15% solution of hydrogen peroxide, while the pH of the reaction mixture is maintained at 7-8 by the simultaneous addition of 3 Asodium hydroxide, the process being carried out at ice-bath temperature. The reaction mixture is neutralized and subjected to steam distillation. The distillate is extracted with ether, and the extract is dried over anhydrous magnesium sulfate. After removal of the ether, distillation yields 18.0 g (70%) of n-octanal, bp 83-85733 mm. 

Hydrolioration (Section 7.5) Addition of borane (BH i) or an alkylborane to an alkene. The resultant trialkyliiorane products are useful synthetic intermediates that can be oxidized to yield alcohols. 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

The alkylborane produced in the hydroboration, without isolation, are oxidized... 

The syn steroselectivity was based on the assumption that the oxidation step—the transformation of the carbon-boron bond to carbon-oxygen bond—took place with retention of configuration. More recent NMR studies of alkylboranes formed in hydroborating labeled alkenes indeed confirmed the validity of the earlier conclusion.351... 

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