Oxidation of an Organoborane

Step 1 Hydrogen peroxide is converted to its anion in basic solution  

Step 2 Anion of hydrogen peroxide acts as a nucleophile, attacking boron and forming an oxygen-boron bond  

Step 3 Carbon migrates from boron to oxygen, displacing hydroxide ion. Carbon migrates with the pair of electrons in the carbon-boron bond these become the electrons in the carbon-oxygen bond. 

Transition state for migration of carbon from boron to oxygen 

Step 4 Hydrolysis cleaves the boron-oxygen bond, yielding the alcohol  

---

Oxidation of an Organoborane 249 6.11 Free-Radical Polymerization of Ethylene 264... 

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

In addition to the oxymercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H. C. Brown and cailed hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

A useful transformation in synthesis is the reaction of an organoborane to form a carbon-carbon bond. One method to achieve this is by addition of carbon monoxide, followed by oxidation, which can be directed to give primary, secondary or tertiary alcohols, aldehydes or ketones under appropriate conditions. On heating, many... 

The addition of boron and hydrogen across the carbon-carbon double bond appears to occur by stereospecific syn addition. The replacement of the C—B bond by a C—OH bond in the oxidation step is also stereospecific. The mechanism proposed for the oxidation of one of the carbon-boron bonds is shown in equations 9.47 through 9.51, so the configuration of the carbon-boron bond is retained in the product. As an example, hydroboration-oxidation of 1-methylcyclopentene (50, equation 9.52) followed by oxidation of the organoborane (51) gave only the trans-2-methylcyclohexanol (52). 

Organoboranes are important in organic synthesis as reactive intermediates. Reactions have been developed by which the boron atom may be replaced by a wide variety of functional groups, such as —H, —OH, —NH2, —Br, —1, and —COOH.The present experiment demonstrates the conversion of an organoborane to an alcohol by oxidation with alkaline hydrogen peroxide. It is not necessary to isolate the organoborane prior to its oxidation. This simplification is particularly fortuitous in this case, since most alkylboranes, when not in solution, are pyrophoric (spontaneously flammable in air). 

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

The combination of hydroboration and oxidation leads to the overall hydration of an alkene. Notice, however, that water is not a reactant. The hydrogen that becomes bonded to carbon comes from the organoborane, and the hydroxyl group from hydrogen peroxide. 

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. 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Treatment of [4.S ]-4-/-butyl-l-mcthylcyclohcxcnc with borane-THF followed by oxidation with F Ch-NaOH gives a mixture of [ 1.S , 2.S ,5.S ]-5-/-butyl-2-methylcyclohcxan-1 -ol and [ 1 A,2A,5.S -5-/-butyl-2-methylcyclohexan-l-ol and no other diastereomers. There are two steps in this process—addition to give an organoborane and oxidation which cleaves the carbon-boron bond to an alcohol. 

The conversion of an alkene into the corresponding ketone may be effected by means of a convenient sequence which involves hydroboration followed by oxidation with chromic acid of the resulting organoborane (cf. Section 5.4.3,... 

Organoboranes may be oxidized by hydrogen peroxide in an acidic medium. The mechanism of oxidation of alkyldihydroxyboranes in such conditions has been studied and the protonated transition state (1) proposed, as compared with transition state (2) for the reaction in an alkaline medium. ... 

---