Hydroboration alkene

Prochem Maxflo T agitator, 7 739 Prochiral alkenes, hydroboration of, 73 665-666... 

Scheme 7.8. Nickel-catalyzed 1,4-addition, using alkene hydroboration and boron-zinc exchange.

With unsymmetrical alkenes, hydroboration occurs so that boron becomes attached to the less-substituted end of the double bond ... 

You may have noticed that only one of the three alkyl groups of a trialkyl-borane is converted to an aldehyde by the carbonylation-reduction-oxidation sequence. To ensure that carbonylation takes the desired course without wasting the starting alkene, hydroboration is achieved conveniently with a hindered borane, such as 9-BBN, 12. With 12, only the least-hindered alkyl group rearranges in the carbonylation step ... 

The electrophilic addition of hydrogen-bearing boranes to alkenes (hydroboration), now recognized as a vital component of organic synthesis, is covered in Chapter 1.7 of this volume. The boron compounds that do fall under the realm of this chapter are boron bromides. Suzuki and coworkers have demonstrated... 

Dicobalt octacarbonyl, in Pauson—Khand reaction homogeneous catalysis, 11, 340 metal-coupled promoters, 11, 339 non-oxidative promoter-assisted, 11, 338 oxidative promoter-assisted, 11, 337 physical promoters, 11, 339 solid-supported promoters, 11, 339 Dicobalt triple-decker sandwiches, preparation, 3, 14 (+)-Dictamnol, via [5+2]-cycloadditions, 10, 613-614 Dicyclohexylborane, for alkene hydroboration, 9, 150... 

On combination with alkene hydroboration, the resulting two-step process is a very important, widely-used transformation which may be regarded as anft-Markovnikov hydration of the alkene (Equation B2.6). 

Functional Group Transformation Alcohols can be prepared by nucleophilic substitution of alkyl halides, hydrolysis of esters, reduction of carboxylic acids or esters, reduction of aldehydes or ketones, electrophilic addition of alkenes, hydroboration of alkenes, or substitution of ethers. 

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

Why does alkene hydroboration take place with non-Markovnikov rc o-chemiatry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene oddition reactions in that it occurs in a single step without a carlK>cation intemnediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 Cp. 244 j. Since both 0-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

Part of the intense interest in M-B compounds derives from the participation of these species in various catalytic processes.1,2 Scheme 6 depicts proposed catalytic cycles for metal-catalysed alkene hydroboration and alkyne diborylation. Key steps involve the insertion of unsaturated organic substrates into M-B bonds and a key intermediate involved in the formation of product is molecule A in which there are ad jacent M-C and M-B bonds. The ruthenium and osmium boryl complexes described in this section provide models for these steps and intermediates. 

Alkene Hydroboration Method" Ratio (1antijsyn) Yield (%) Ref... 

Alkene Hydroboration/ Oxidation Conditions Alcohol Config. of Product ee (%) or op (%) Determination b Yield (%) Ref... 

Enantioselective Alkene Hydroboration. Substituted styrene derivatives undergo rhodium-catalyzed hydroboration in the presence of a catalytic amount of the title reagent. However, optimal regio- as well as enantioselection is attained by using the corresponding reagent derived from pseudoephedrine (eq 2). ... 

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