Oxidation of organoboranes

The electrophilic character of boron is again evident when we consider the oxidation of organoboranes. In the oxidation phase of the hydroboration-oxidation sequence, as presented in Figure 6.11, the conjugate base of hydrogen peroxide attacks boron. Hydroperoxide ion is formed in an acid-base reaction in step 1 and attacks boron in step 2. The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it. 

This reaction also takes place with retention of configuration => the stereochemistry of the reaction is like that of the oxidation of organoboranes => it can be very useful in introducing deuterium or tritium in a specific way. 

A Simple and Convenient Method for the Oxidation of Organoboranes Using Sodium Perborate Preparation of (+)-Isopinocampheol. 

Conditions that permit oxidation of organoboranes to alcohols using molecular oxygen,139 sodium peroxycarbonate,140 or amine oxides141 as oxidants have also been developed. The reaction with molecular oxygen is particularly effective in perfluoroalkane solvents.142... 

The oxidation of organoboranes is exothermic, and efficient cooling and slow addition are necessary to keep the temperature near CC.5... 

Oxidation of organoboranes (6, 624 8, 507). A new synthesis of acylsilanes (2) involves oxidation of the hydroboration products of alkynylsilanes (I).1 Oxidation with the commercially available dihydrate of trimethylamine N-oxidc is sluggish however, anhydrous reagent2 is satisfactory for this purpose (equation I). 

Oxidation of organoboranes.1 This oxidation is generally conducted with H202 (30%) and 3 N NaOH at 50°. Comparable yields can be obtained by use of this milder oxidant, which is less expensive, more stable, and far safer than H202. The yield of the reaction shown in equation (I) is 35% when H202 is used in place of sodium borate. 

Hydrogen peroxide oxidation of organoboranes is exothermic. Careful, dropwise addition of 30% hydrogen peroxide to the organoborane will provide sufficient heating to maintain a reaction temperature in the 40-50°C range. 

Organoborane complexes have shown a high efficiency for polymerizing vinyl monomers at room temperature as evidenced by numerous investigations developed by Chung et al. [94-97]. The oxidation of organoborane leads to the formation of borane peroxide able to initiate the polymerization ... 

A SIMPLE AND CONVENIENT METHOD FOR THE OXIDATION OF ORGANOBORANES USING SODIUM PERBORATE (+HSOPINOCAMPHEOL... 

SODIUM PERBORATE OXIDATION OF ORGANOBORANES a Alkene Reagent Product Yield(%)b... 

Oxidations of organoboranes involve numerous reagents and several different general mechanisms, most of which parallel those wluch occur for other types of oiganoborane reactions. The mechanisms fall under three broad headings (i) ionic, with a 1,2-shift fiom boron to a heteroatom (equations 1-3) (ii) radical and (iii) electrocyclic. 

Electrocyclic reactions (equation 7) may also lead to oxidation of organoboranes, usually with retention of configuration of the otganyl group. 

Electrochemical oxidation of organoboranes can lead to a variety of products, depending on die natures of the electrodes, solvents and electrolytes. It appears that radicals, R-, are generated and that diese... 

Pyridinium chlorochromate (PCC) is a very useful reagent for the oxidation of organoboranes to carbonyl compounds in mildly alkaline 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 caimot be accomplished with chromic acid. The reagent tolerates the presence of alkene, ester and acetal groups. ... 

Oxidation of organohoranes. This oxidant is superior to chromic acid for oxidation of organoboranes derived from cycloalkenes to the corresponding ketones. For example, cyclohexanone is obtained in 81% yield from cyclohexene. 

General information on the hydroboration reaction to form a C—C bond is given in Section D.1.5.7. and for a C —H bond by reduction in Section D.2.5.2. of this volume, whereas the oxidation of organoboranes is described in Vol. 13/3 a, pp 489-853. This section deals with the formation of C—O bonds, e.g., the synthesis of alcohols by the oxidation of organoboranes bearing the boron atom at the stereogenic center (see Vol. 6/1 a, pp 494-553). 

Direct Oxidation of Organoboranes with Molecular Oxygen. 66... 

The oxidation of organoboranes with molecular oxygen leading to the corresponding alcohols can also be carried out in perfluorinated solvents Eq.(5) [14]. 

---