Boranes reaction with basic hydrogen peroxide

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 ... 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

Hydroboration-oxidation of alkenes preparation of alcohols Addition of water to alkenes by hydroboration-oxidation gives alcohols via anti-Markovnikov addition. This addition is opposite to the acid-catalysed addition of water. Hydrohoration is regioselective and syn stereospecific. In the addition reaction, borane bonds to the less substituted carbon, and hydrogen to the more substituted carbon of the double bond. For example, propene reacts with borane and THF complex, followed by oxidation with basic hydrogen peroxide (H2O2), to yield propanol. 

The reduction is bimolecular and thus the rate is dependent on concentration. Running the reaction neat provides the fastest rates. Usually an excess of Alpine-Borane is used to insure that the reaction does not become excessively slow at the end of the reduction. The excess organoborane may be destroyed by addition of an aldehyde such as Acetaldehyde. The resulting alkoxy-9-BBN may be treated with Ethanolamine to liberate the alcohol and precipitate the majority of the 9-BBN. Any remaining borane impurities may be removed by oxidation with basic Hydrogen Peroxide. 

The hydroboration reaction with alkenes to produce alkylboranes (sec. 5.2) proceeds by a four-center transition state rather than a cationic intermediate. The regiochemistry of the final alkylborane product is controlled by the nonbonded steric interactions of the groups attached to boron (in this case sec-isoamyl from the disiamylborane) and the groups on the alkene. Oxidation with basic hydrogen peroxide converts the borane to the anti-Markovnikov alcohol (2). The difference in regiochemistry between 1 and 2 arose because the mechanism for generating each relied on difference factors. 

Reactions that probe these disconnections are presented in the homework problems. The disconnection diagrams should be read as follows. Alkyl halides are prepared from alkenes, with regioselectivity. Similarly, alcohols are prepared from alkenes with regioselectivity. A chemical reaction involves treatment of an alkene with an acid HX to give the alkyl halide. Another chemical reaction is treatment of an alkene with aqueous acid to give the more substituted alcohol, or with borane and then basic hydrogen peroxide to give the less substituted alcohol. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

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. 

Bicyclic borane 23 gives the products with principally different structures in various chemical reactions (Scheme 2.31) [48-50]. Thus, thermolysis of 23 yields the tricyclic borane 46, oxidation by basic hydrogen peroxide affords the expected products with a cyclooctatriene skeleton 47,48 (in a 5 1 ratio). Methanolysis gives borinic ester 49, whereas the single product of the reaction of 23 with acetone is cyclic borinic ester 50. 

Another method for the addition of the elements of water, H and OH, to an alkene was developed by H. C. Brown, who shared the 1979 Nobel Prize in chemistry for this work. In this reaction the alkene is first allowed to react with a complex of borane (BH3) in tetrahydrofuran (THF). The initial product is then allowed to react with a basic solution of hydrogen peroxide. An example is shown in the following equation ... 

---