Hydroboration-oxidation stereochemistry

A second aspect of hydroboration-oxidation concerns its stereochemistry As illustrated for the case of 1 methylcyclopentene H and OH add to the same face of the double bond... 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

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. 

What product will result from hydroboration/oxidation of 7-methylcyclo-pentene with deuterated borane, BD3 Show both the stereochemistry (spatial arrangement) and the regiochemistry (orientation) of the product. 

There are other stereospecific olefin addition processes which occur with cis or syn stereochemistry. Common examples include catalytic hydrogenation, hydroboration/oxidation, and dihydroxylation using osmium tetroxide. The stereospecificity of these syn additions requires that die facial properties of the olefinic bond be maintained throughout die addition process and that both new bonds are formed to the same face of the olefin. This is normally accomplished by a concerted syn addition to the n system. 

The hydroboration-oxidation of a-pinene gives the product shown. Carefully explain the regiochemistry and the stereochemistry of this reaction. 

Yields refer to chromatographically homogeneous material. bYields are for three steps methylenation, RCM (35-40 mol% of 7) and hydroboration-oxidative work-up. Stereochemistry at C-l and C-2 determined by acetylation and analysis of //-2 coupling constant in H NMR. dA fair amount of unreacted mono-1,6-ester was isolated from the reaction mixture. cIn this case, the RCM reaction was stopped early, the (bis)C-glycal was isolated and purified (48%, unoptimized) and then subjected to hydroboration (66%, unoptimized). 

The stereochemistry of the hydroboration-oxidation of 1-methylcyclopentene is shown next. Boron and hydrogen add to the same face of the double bond (syn) to form a trialkylborane. Oxidation of the trialkylborane replaces boron with a hydroxyl group in the same stereochemical position. The product is frracemic mixture is expected because a chiral product is formed from achiral reagents. 

The reaction of 1,2-dimethylcyclopentene illustrates the stereochemistry of the synthesis hydroboration-oxidation involves overall syn addition. 

Problem 15.10 The stereochemistry of hydroboration-oxidation is the net result of the stereochemistry of the two steps, and is consistent with either of two combinations of stereochemistry for the individual steps. What aic these two combinations ... 

The following alkene undergoes hydroboration/oxidation to yield a single product rather than a mixture. Explain the result, and draw the product showing its stereochemistry. 

The hydration of alkenes via hydroboration-oxidation, developed by Herbert C. Brown (1912-2004 Nobel Prize, 1979) and coworkers, provides a valuable tool for the synthesis of a wide variety of alcohols of predictable regio- and stereochemistry. 

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