Hydroboration of internal alkenes

The procedure represents one of the most convenient methods for the synthesis of optically active alkylboronates. However, the procedure can be of limited use for the hydroboration of styrene derivatives. The optically active organoborates are also obtained by hydroboration of internal alkenes with diisopinocampheylborane, followed by dealkylation of two diisopinocampheyl groups with acetaldehyde [58]. 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

Scheme 15.36 Copper-catalysed hydroboration of internal alkenes.

The reaction follows a different course when applied to alkenylboranes (3) prepared by hydroboration of internal alkynes (equation II). (Z)-Alkenes (4) are obtained and only catalytic amounts of Pd(0Ac)2 are necessary. An added base is not required because THF or acetone can serve as the proton source. ... 

The examples in Equation 16.45 show that the regiochemistry of the metal-catalyzed hydroboration of alkenes contrasts with that of tlie metal-catalyzed hydroboration of vinylarenes. The predominant products from the metal-catalyzed hydroborations of 1-alkenes are terminal boranes. - " These terminal boranes can also be formed from internal alkenes, as shown in Equation 16.46. Similar to hydrosilylation witli Speier s catalyst, the... 

After hydroboration, oxidation of radioiodide is performed with Chloramine-T, forming HO-I, which then adds to the C-B bond. The reactivity of the C-B bonds depends on the position of boron, which can either be formed from a terminal double bond or from an internal alkene. While the former case produces two radioiodinated compounds, the latter forms only one. Therefore, dicyclohexylborane is used for the hydroboration step. Both reactions, hydroboration of the alkene and radioiodination can be performed in one sequence without isolating the intermediately formed trialkylborane. 

The hydroboration of internal allenes with 9-BBN affords the corresponding allylboranes [21]. These allylboranes when treated with water, leading to the formation of predominantly of (Z)-4-(trimethylsilyl)-2-alkenes (Chart 24.4) [22]. 

Figure 3. The asymmetric hydroboration of representative alkenes with 3. Chemicai yields were determined by GC analysis employing an internal standard. With the exception of the a-deuteriostyrene example ( optical rotation), all product ee determinations were performed by NMR analysis of the Mosher esters through comparisons with authentic samples of each alcohol in both racemic and optically pure forms.

Asymmetric hydroboration of internal olefins was developed by Perez, Fernandez and co-workers,with [B(pin)]2 as boration reagent. Ligands 126 provided the best chiral induction (up to 59% ee) (Scheme 13.6). Improved results were obtained by Hoveyda and co-workers using 127 and 128 with acyclic and cyclic internal alkenes. 

The regioselectivity for hydroboration of alkenes with 9-BBN is controlled by steric factors. Selective hydroboration of terminal alkenes in dienes 4, 6, and 8 can be readily achieved in the presence of internal olefins (Scheme 28.2). ... 

This dialkylborane effects hydroboration of a variety of internal alkenes with good enantioselectivity, but terminal alkenes give low enantioselectivity. The main drawback with this reagent is that its preparation requires several steps. 

Long-chain primary alcohols, eg, triacontanol, can be prepared by the hydroboration, isomerization, and oxidation of the corresponding internal alkenes (437). The less thermodynamically stable stereoisomer can be transformed into the more stable one by heating, eg, i j -into /ra/ j -myrtanjiborane (204). 

Tripylborane is an interesting reagent which resembles thexylborane. One of the important uses of thexylborane lies in the synthesis of unsymmetrical thexyldialkylboranes which can then be used in the synthesis of unsymmetrical ketones. However, the reaction is only successful if the alkene used in the first hydroboration step is an internal alkene. Simple terminal alkenes such as 1-hexene react too rapidly with the initially formed thexylmonoalkylborane to allow the reaction to be stopped at that stage. Therefore, mixtures of products result (ref. 27). 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

The hydroboration of terminal and internal alkenes with pinacolborane can be carried out at room temperature in the presence of an iridium(I) catalyst (3 mol.%) formed by the addition of dppm (2 equiv.) to [Ir(cod)Cl]2 (dppm = Ph2PCH2PPh2), a mixture that presumably furnishes [Ir(cod)(dppm)]Cl as the true catalyst precursor. Hydroboration results in the addition of the boron atom to the terminal carbon of 1-alkenes with more than 99% selectivity [18]. 

Unlike the selectivity of the hydroboration of terminal and internal alkenes, which depends on the metal catalysts and hydroboration reagents [20], the boron... 

Pinacolborane (PBH, 537) sluggishly hydroborates alkynes and alkenes. Hydro-boration of alkyne with PBH is catalyzed by hydrozirconocene chloride (HCp2ZrCl) [206], CpNi(Ph3P)Cl and Rh(CO)(Ph3P)2Cl [207] at room temperature. Hydrobora-tion of 4-octene with PBH at room temperature gives either terminal or internal boranes 538 or 539 regioselectively, depending on the catalyst used [207]. PBH is more stable than CBH, and easier to handle. 

Step 1 Hydroboration-oxidation of sterically less hindered terminal alkene. Step 2 Oxymercuration-demercuration of the internal alkene yields the diol G. 

We have seen the relative reactivities of structurally different alkenes towards various dialkylboranes vary over a broad range of differences, e.g., disiamylborane 431 and 9-BBN481 vary in a range of 105. This makes selective hydroboration of the more reactive alkenes possible in the presence of less reactive one. Disubstituted internal (Z)-alkenes are more reactive to disiamylborane than their ( T)-isomers (Eq. 125). Recently, it has been observed that the selectivity is much higher with ThxBHCl SMe2... 

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