Hydroboration-oxidation of alkenes

The gas phase reaction of ethanol with hydrogen bromide can occur either by elimination or substitution. 

Which product, ethylene or ethyl bromide, will increase relative to the other as the temperature is raised Why  

Acid-catalyzed hydration converts alkenes to alcohols according to Markovnikov s rule. Frequently, however, one needs an alcohol having a structure that corresponds to hydration of an alkene with a regioselectivity opposite to that of Markovnikov s rule. The conversion of 1-decene to 1-decanol is an example of such a transformation. 

The synthetic method used to accomplish this is an indirect one known as hydroboration-oxidation. It was developed by Professor Herbert C. Brown and his coworkers at Purdue University as part of a broad program designed to apply boron-containing reagents to organic chemical synthesis. The number of applications is so large (hydroboration-oxidation is just one of them) and the work so novel that Brown was a corecipient of the 1979 Nobel Prize in Chemistry. 

Hydroboration is a reaction in which a boron hydride, a compound of the type R2BH, adds to a carbon-carbon tt bond. A carbon-hydrogen bond and a carbon-boron bond result. 

You may have noticed that the acid-catalyzed hydration of an alkene and the acid-catalyzed dehydration of an alcohol are the reverse of each other. 

According to Le Chatelier s principle, a system at equilibrium adjusts so as to minimize any stress applied to it. When the concentration of water is increased, the system responds by consuming water. This means that proportionally more alkene is converted to alcohol the position of equilibrium shifts to the right. Thus, when we wish to prepare an alcohol from an alkene, we employ a reaction medium in which the molar concentration of water is high—dilute sulfuric acid, for example. 

In any equilibrium process, the sequence of intermediates and transition states encountered as reactants proceed to products in one direction must also be encountered, and in precisely the reverse order, in the opposite direction. This is called the principle of microscopic reversibility. Just as the reaction 

With sodium hydroxide as the base, boron of the aikyiborane is converted to the water-soiubie and easiiy removed sodium sait of boric acid. 

Hydroboration-oxidation was deveioped by Professor Herbert C. Brown [Nobei Prize in Chemistry (1979)] as part of a broad program designed to appiy boron-containing reagents to organic chemicai synthesis. 

Hydroboration-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 regiochemistry of acid-catalyzed hydration of alkenes to give alcohols is that predicted by Markovnikov s rule (Sec. 10.4) because the more stable intermediate carbocation is preferentially formed. Sometimes it is desirable to add the elements of water across a carbon-carbon Ti-bond in the opposite regiochemical sense to provide the anti-Markovnikov product (see the Historical Highlight at the end of this chapter). In order to accomplish this goal, a process termed hydroboration-oxidation was developed that involves the reaction of an alkene sequentially with diborane, B2H, and basic hydrogen peroxide (Eq. 10.26). 

Eiiborane is a dimer of borane, BH3. The bonding in diborane is unusual because the hydrogen atoms bridge the two boron atoms with the two monomeric BH3 subunits being bound by two-electron, three-center bonds. Because the boron atom in borane possesses an empty p-orbital, borane is a Lewis acid, and it forms stable complexes upon reaction with tetrahydrofuran (THF) and other ethers, which function as Lewis bases, as illustrated by the formation of a borane-THF complex (Eq. 10.27). 

The extent to which dialkyl- and trialkylboranes will be formed is dependent upon steric factors and the degree of substitution on the double bond. 

the regiochemistry of hydroboration is predicted by the same general rule that applies to all electrophilic additions to alkenes The reaction of an electrophile with a carbon-carbon double bond occurs preferentially via the transition state in which a partial positive charge develops on that carbon atom better able to accommodate it. Geometric constraints inherent in the cyclic transition state 60 require that the addition of borane to the alkene proceed so that both the boron and the hydrogen add from the same face of the double bond, a process called sy -addition. 

Moreover, you will discover that the hydroboration of (+)-a-pinene is an example of a stereoselective reaction because only one of several possible stereoisomeric products is formed by addition of the borane reagent to only one face of the TT-bond of 65. 

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Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

Thomson v Click Organic Interactive to use a web-based palette to predict products of the hydroboration/oxidation of alkenes. 

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. 

Hydroboration-oxidation of alkenes 5-18 Addition of organometallic compounds to unsaturated alcohols 5-20 Addition of CH, and H to allylic alcohols... 

The overall reaction is quite complex but involves a rearrangement similar to that described for the hydroboration-oxidation of alkenes (Section 11-6E). The first step is hydroboration of the alkene to a trialkylborane. When the trialkylborane is exposed to carbon monoxide, it reacts (carbonylates) to form a tetracovalent boron, 9 ... 

This section covers the hydroboration-oxidation of alkenes to give alcohols. The author chooses to include this under oxidation since an oxygen atom is introduced into the molecule. This reaction can be performed in a stereocontrolled fashion and it is these methods that are highlighted here. In addition, one similar reductive-oxidation reaction is included, since it is an extremely facile route to benzyl alcohols and a-hydroxyalkanoic acids. 

Table 3. Selected Chiral Alcohols via Catalytic Asymmetric Hydroboration/Oxidation of Alkenes with 1,3,2-Benzodioxaborole...

Hydroboration-oxidation of alkenes is a method that leads to hydration of the double bond with a regioselectivity opposite to Markovnikov s rule. 

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