Hydrolysis hydroboration-oxidation

Tab. 3.1 Regioselectivity for the Hydration (via Hydroboration/Oxidation/Hydrolysis) of Ethylene Derivatives that Contain More Alkyl Substituents at Than at Ca...

The hydroboration/oxidation/hydrolysis of trisubstituted alkenes also takes place as a cis-addition. The reaction equation from Figure 3.25 shows this using 1-methylcyclo-hexene as... 

Fig. 3.16. cis-l Iy drat ion of an olefin via the reaction sequence hydroboration/ oxidation/hydrolysis. 

Fig. 3.17. Mechanistic details of the hydroboration/oxidation/ hydrolysis sequence of Figure 3.16.

The hydroboration/oxidation/hydrolysis of trisubstituted olefins also takes place as a cis addition. The reaction equation from Figure 3.19 shows this using 1-methylcyclo-hexene as an example. 9-BBN adds to both sides of its C=C double bond at the same reaction rate. In the energy profile this fact means that the activation barriers for both modes of addition are equally high. The reason for this equality is that the associated transition states are enantiomorphic. They thus have the same energy—just as enan-tiomorphic molecules. 

The regioselectivity of the solvomercuration/reduction sequence is complementary to that of hydroboration/ oxidation/hydrolysis. 

Isopropyl-2-phenylcyclobutanol (29) was obtained by hydroboration-oxidation of 4-iso-propyl-l-phenylcyclobutene.38 In a synthetically useful procedure, hydrolysis of 1,2-bis(phenylsulfanyl)cyclobutene at room temperature in a solution of copper(IT) chloride and titanium(IV) chloride in acetic acid gave 2-(phenylsulfanyl)cyclobutanone (30) in 85% yield.39... 

The hydroboration of alkenes, in which the Ca=C is not symmetrically substituted can lead to constitutionally isomeric trialkylhoranes. This is because the new C—B bond can form either at the Ca or at the (A of the Ca=C double bond. In the oxidation/hydrolysis sequence that follows, constitutionally isomeric alcohols are produced. In one of them, the OH group binds to Ca and in the other it binds to C. If only one constitutional isomer of the trialkylbo-rane and consequently only one constitutional isomer of the alcohol is to he produced, the hydroboration step must take place regioselectively. Whether regioselectivity occurs is determined by steric and electronic effects. 

The hydroboration of unsymmetrical alkenes thus gives monoalkylboranes (addition of H—BH2), dialkylboranes (addition of H—BHR), or trialkylboranes (addition of H—BR2), which are typical anti-Markovnikov products. Therefore, the reaction sequence hydrobora-tion/oxidation/hydrolysis brings about the anti-Markovnikov addition of H20 to unsymmetrically substituted alkenes. 

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. 

Tetrahydrofuranyl (THF) ethers are reported to be useful alternatives to tetrahydropyranyl (THP) ethers for the protection of alcohols (and thiols). THF ethers are easily formed, and are more sensitive to acidic hydrolysis than are THP ethers. We are warned that hydroboration-oxidation or peracid oxidation of simple unsaturated compounds containing a hydroxy-group protected as its THP ether has led to violent detonations. The danger comes from the ready formation of peroxides from THP ethers, and presumably also from THF ethers these peroxides, though very sensitive, are not easily removed by ordinary chemical methods. 

An attractive preparative procedure involves the oxidation of alcohols formed by hydroboration. Metallation of diethyl r-methoxy-4-tluorobenzylphosphonate at low temperature in THF and alkylation of the carbanion produced thereby with allyl bromide gives the required homoallylphos-phonate. Elaboration of the double bond by a sequence involving, hydroboration, basic hydrolysis, and oxidation of the resulting alcohol provides the desired diethyl l -methoxy-] -(3-oxopropyl)-4-fluorobenzylphosphonate (Scheme 5.37). ... 

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