Hydroboration/oxidation sequence

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

Like alkenes (Sections 7.4 and 7.5), alkynes can be hydrated by either of two methods. Direct addition of water catalyzed by mercury(II) ion yields the Markovnikov product, and indirect addition of water by a hydroboration/ oxidation sequence yields the non-Markovnikov product. 

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

The hydroboration/oxidation sequence does not change the molecular-weight distribution. Gel permeation chromatography (GPC) measurements in dimethyl-formamide (DMF) with the resulting polystyrene-ft-polyalcohol polymers show very similar polydispersity indexes (Table 10.2). Here, the hydroboration/oxidation sequence is clearly superior to the epoxidation reaction, which leads to a... 

For carbosilane dendimers using a repetitive alkenylation/hydroboration/oxidation sequence, see Dendritic Carbosilanes Containing Hydroxy Groups on the Periphery, C. Kim, S. Son, B. Kim,/. Orga-nometallic Chem. 1999, 588, 1-8. 

A hydroboration-oxidation sequence has been described for the desymmetrization of bicyclic hydrazino-alkenes. The use of BDPP as a chiral ligand on Rh provides the desired alcohol in 84% ee, following oxidation of the hydroborated... 

The advantages of a hydroboration-oxidation sequence to prepare alcohols are simplicity of procedure relatively mild reaction conditions high overall yields absence of skeletal rearrangements production of carbinol in which there is an overall cis addition of water to a double bond in a counter-Marknowikoff sense. 

Coupling the hydroboration with a subsequent oxidation of the new formed borane yields a //-Markovnikov alcohols. The hydroboration/oxidation sequence constitutes a powerful method for the regio- and stereoselective synthesis of alcohols. 

Write the structure of the alcohol produced from the hydroboration-oxidation sequence when applied to a particular alkene. 

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. 

This process is sometimes abbreviated to S f2 at silicon to save space. The intermediate is a trigonal bipyramid with negatively charged pentacovalent silicon. It is often omitted in drawings because it is formed slowly and decomposes quickly. This mechanism is similar to nucleophilic substitution at boron except that the intermediate is pentacovalent (Si) rather than tetrahedral (B). The hydrolysis of a boron ester at the end of a hydroboration-oxidation sequence would be an example of an analogous boron reaction. 

A synthesis of a-lipoic acid and derivatives has been reported. The pendant alkene of complex (111) was transformed to a primary alcohol (112) via hydroboration-oxidation sequence. Mitsunobu coupling of (112) with thiobenzoic acids to give (113), followed by desilylation and nucleophilic substitution again employing thiobenzoic acid gave the advanced intermediate (114) (Scheme 169). 

The direction of addition of the B-H bond to atrisubstituted double bond seems to be determined by the addition of the more electronegative hydrogen atom to the more highly substituted carbon atom of the double bond. Thus, with the B H bond assumed to be polarised in the sense B " —the process may be considered in terms of the electronic arguments summarised in the modern interpretation of Markownikoff s rule for ionic additions to double bonds [15]. The outcome of the hydroboration/oxidation sequence corresponds to an "anti-Markownikoff hydration of the double bond, which has many synthetic applications. If steric factors are also important, these of course operate in the same sense, generally favouring attachment of boron to the less-hindered secondary carbon atom. 

Problem 10.30 What alkene can be used to prepare each alcohol as the exclusive product of a two-step hydroboration-oxidation sequence ... 

When an alkene reacts with BH3 in THF solution, rapid addition to the double bond occurs. Since BH3 has three hydrogens, addition occurs three times, and a trialkylborane, R3B, is formed. For example, 1 molar equivalent of BH3 adds to 3 molar equivalents of cyclohexcne to yield tri-cyclohexylborane. When tricyclohexylborane is then treated with aqueous hydrogen peroxide (H2O2I 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. The net effect of the two-step hydroboration/oxidation sequence is hydration of the alkene double bond ... 

The three C-B bonds are broken, -OH groups bond to the three carbons, and 3 equivalents of cyclohexanol are produced. The net effect of the two-step hydroboration/oxidation sequence is hydration of the alkene double bond ... 

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