Oxidation of alkylboranes

Oxidation of alkylboranes by alkaline hydrogen peroxide produces alcohols. The reaction is essentially quantitative and has been successfully applied to a wide variety of alkylboranes (Equations B2.3-5). It is important to note that the stereochemistry of the carbon atom attached to the boron atom is retained in this conversion of a carbon-boron bond to a carbon-oxygen bond (Equation B2.5). 

An attempt to distinguish the two by lsO labeling was frustrated by exchange in the perrhenate product. However, the authors favored the latter mechanism, based on the observation that the observed rate constants suggested that kA or /cMto had to be near the diffusion-controlled limit, coupled with the known higher nucleophilicity of the hydroperoxide vs hydroxide anions.37 The formation of the new C—O bond in this case is thus reminiscent of the oxidation of alkylboranes with hydrogen peroxide.38... 

Oxidation of alkenylboranes with alkaline hydrogen peroxide is an important pathway to aldehydes and ketones (equation 13). Care must be taken to inhibit hydrolysis of the alkenylborane to the corresponding alkene and hence buffered conditions are frequently us. The reaction toloates the same wide range of functionality as the oxidation of alkylboranes. 

You might recall a somewhat related mechanistic rearrangement, the migration of a group from boron to oxygen during the oxidation of alkylboranes with basic hydrogen peroxide. 

The overall stereochemistry of hydroboration—protonolysis, therefore, is syn (hke that of the oxidation of alkylboranes). 

Alkaline hydrogen peroxide is used for epoxidation of electron-deficient alkenes such as acrylic acids, and also for oxidation of alkylboranes to alcohols, the second step of hydroboration-oxidation. 

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). 

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 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... 

The second reaction (Equation 11 -3) achieves the synthesis of a primary alcohol by the oxidation of the alkylborane with hydrogen peroxide in basic solution. Starting with a 1-alkene, one can prepare a primary alcohol in two steps ... 

Aqueous chromic acid has been used to oxidize alkylboranes derived from cyclic alkenes to ketones. For example, hydroboration and oxidation of 1-methylcyclohexene converts it into 2-methylcyclohexanone (Equation B2.10). 

The BH3 THF reagent is the form of borane commonly used in organic reactions. BH3 adds to the double bond of an alkene to give an alkylborane. Basic hydrogen peroxide oxidizes the alkylborane to an alcohol. In effect, hydroboration-oxidation converts alkenes to alcohols by adding water across the double bond, with anti-Markovnikov orientation. 

The chlorination of trialkylboianes has not been well studied. The chlorination of MesB at -95 C yields ClCH2BMe2. The oxidation of dialkoxy(a-phenylthio)alkylboranes with NCS has been referred to in Section 4.1.8 (equation SO). The reaction of thionyl chloride with PhSCH2B(OR>2 yields PhSCH2Cl. 0>... 

---