Addition of Water to Alkenes Hydroboration

Problem 7.8 What alkenes might the following alcohols have been prepared from  

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

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  

Herbert Charles Brown (1912-20D4) was born in London to Ukrainian parents and brought to the United States in 1914. Brown received his PhD. in 1938 from the University of Chicago, taught at Chicago and at Wayne State University, and then became professor of chemistry at Purdue University. The author of more than 1000 scientific papers, he received the 1979 Nobel Prize in chemistry for his work on organoboranes. 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

When an alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs three times and a trialkylboiwie, R3B, is formed. For example, 1 molar equivalent of BH3 adds to 3 molar equivalents of cyclohexene to yield tricyclohexylborane. When tricyclohexylborane is then treated with aqueous hydrogen peroxide (H2O2) in basic solution, an oxidation takes place. The three C-B bonds are broken, -OH groups bond to the three carbons, and 3 equivalents of cyclohexanol are produced. I he net effect of the 

What products would you obtain from reaction of 2,4-dimelhyi-2-pentene with  

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and then apply that knowledge to the specific case you re dealing with. In the present instance, recall that the two methods of 

4-Methyl-2-hexene has a disubstituted double bond. RCH=CHR , and would probably give a mixture of two alcohols with either hydration method since Markovnikov s rule does not apply to symmetrically substituted alkenes. 3- lethyI-3-hexene, however, has a trisubstituted double bond, and would give only the desired product on non-Markovnikov liydralion using the hydroboration/oxidation method. 

Mechanism of alkene hydroboration. The reaction occurs in a single step, in which both C-H and C-B bonds form at the same time and on the same face of the double bond. Electrostatic potential maps show that boron becomes negative in the transition state, as electrons shift from the alkene to boron, but is positive in the product. 

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D. Addition of water to alkenes hydroboration/oxidation (Section 7.5). 

Addition of Water to Alkenes Oxymercuration 239 Addition of Water to Alkenes Hydroboration 242 Addition of Carbenes to Alkenes Cyclopropane Synthesis Reduction of Alkenes Hydrogenation 249... 

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 is defined as the addition of borane or one of its derivatives to a multiple bond. It is an enormously versatile reaction synthetically, developed by H.C. Brown in the 1950s and recognized by his Nobel Prize in 1979. We will be looking specifically at the use of a hydroboration-oxidation sequence to accomplish the stereo- and regiospecific anti-Markovnikov addition of water to alkenes. 

The overall result of the sequence hydroboration -I- oxidation is a regioselective and-Markownikoff-addition of water to an alkene. This reaction is an important method in organic synthesis, since it can be made stereoselective and even enantioselective. 

The overall result of the hydroboration-oxidation sequence is addition of water to an alkene with the opposite regiochemistry to that expected for a conventional acid-catalysed hydration. The usual way to do such a hydration is by oxymercuration-reduction. 

It is important to recognize that this reaction gives products corresponding to anti-Markovnikov addition of water to the carbon-carbon double bond. This behavior evidently results because carbonium ions are not intermediates - rearrangement does not occur in hydroboration. The stereochemistry of this reaction involves syn addition, that is, addition to the double bond is on the same face of the alkene. 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

Hydroboration—oxidation reactions are regioselective the net result of hydroboration—oxidation is anti-Markovnikov addition of water to an alkene. 

At this point in your study of organic chemistry, you know only three ways to synthesize a ketone (1) the addition of water to an alkyne (Section 7.7), (2) hydroboration-oxi tion of an alkyne (Section 7.8), and (3) ozonolysis of an alkene (Section 6.12). Because the target molecule has the same number of carbons as the starting material, we can rule out ozonolysis. Now we know that the precursor molecule must be an alkyne. The alkyne needed to prepare the ketone can be prepared from two successive E2 reactions of a vicinal dihahde, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction, using a bulky base to maximize the elimination product... 

When you know what functional group you want to create, you can try to remember the various ways it can be synthesized. For example, a ketone can be synthesized by the acid-catalyzed addition of water to an alkyne, hydroboration-oxidation of an alkyne, oxidation of a secondary alcohol, and ozonolysis of an alkene. Notice that ozonolysis decreases the number of carbons in a molecule. 

The hydroboration-oxidation product has a hydroxyl group at the CH, site and a hydrogen atom at the ring carbon atom. This process is equivalent to Markovnikov addition of water to an alkene. 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

One of the features that makes the hydrobora ( ion reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration/oxidation of 1-methylcyclopentene yields trans-2-methylcydopentanol. Boron and hydrogen both add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti—with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 

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. 

Hydroboration is mostly used for the conversion of alkenes to alcohols by the cis addition of water with the OH group going to the less substituted end of the alkene. This is clearest with a cyclic trisub-stituted alkene. 

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