Hydration anti-Markownikoff

A mixture of d- and l- hexoses also results from the hydroboration of these 5-enes. Hydroboration results in anti-Markownikoff, cw-hydration of the double bond and the amount of each hexose formed varies according to the nature of the substituent groups. For example, hydroboration (23) of methyl 6-deoxy-a-D-ryZo-hex-5-enopyranose (3) affords methyl a-D-glucopyranoside and methyl / -L-idopyranoside in the ratio of 1 2.5 respectively whereas hydroboration of the fris-trimethylsilyl ether of 3 afforded them in the ratio 1 0.6 respectively. The hydroboration method can be used to achieve specific labelling of hexoses with tritium methyl-/ -L-idopyranoside[5-H3] and methyl a-D-glucopyranoside [5-H3] were thus prepared (23). Similarly, hydroboration of the D-Zt/ro-hex-5-eno derivative (14) with diborane-H3 followed by removal of the isopropyli-dene group, afforded methyl a-D-mannopyranoside [5-H3] and methyl / -L-gulopyranoside [5-H3] in the ratio of 1 2 respectively (23). 

Aminophenols from anilines, 35, 2 Anhydrides of aliphatic dibasic acids, Friedel-Crafts reaction with, 5, 5 Anion-assisted sigmatropic rearrangements, 43, 2 Anthracene homologs, synthesis of, 1, 6 Anti-Markownikoff hydration of alkenes, 13, 1... 

The R—0—B bonds are hydrolysed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of B-to-0 migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least substituted carbon of a double bond results in the alcohol being formed with regiochemistry which is complementary to that observed in the case of direct hydration or oxymercuration, that is, anti-Markownikoff. 138... 

Like alkenes, the double bonds of ,/3-unsaturated acids can be brominated, hydroxylated, hydrated, and hydrobrominated, although the reactions often are relatively slow. In the addition of unsymmetrical reagents the direction of addition is opposite to that observed for alkenes (anti-Markownikoff). Thus propenoic (acrylic) acid adds hydrogen bromide and water to form 3-bromo-and 3-hydroxypropanoic acids ... 

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. 

An alternative sequence involves the hydroboration of an alkene (Scheme 2.10c). This reaction has the overall effect of producing the anti-Markownikoff hydration of the double bond. A further characteristic of this pathway is that there is a cis relationship between the added hydrogen derived from the borane and the hydroxyl group. 

The synthetic value of the reaction lies in the modification of these organoboranes. The commonest reaction involves the decomposition of the borane by alkaline hydrogen peroxide. The highly nucleophilic hydroperoxide anion attacks the electron-deficient boron with the formation of an ate complex. Rearrangement of this leads to the formation of a borate ester which then undergoes hydrolysis to an alcohol in which an oxygen atom has replaced the boron (Scheme 3.15). The overall outcome of this reaction is the anti-Markownikoff hydration of the double bond. The regiochemistry is the reverse of the acid-catalysed hydration of an alkene. The overall addition of water takes place in a cis manner on the less-hindered face of the double bond. 

The tri-2-phenylpropylborane formed can be oxidized and hydrolyzed to 2-phenylpropanol-l, thus representing [as with aluminum alkyls (146)] an overall anti-Markownikoff hydration of a-olefins. Besides employing trialkylboranes as a source of boron hydrides, similar additions have been accomplished by heating olefins with boron hydride-amine complexes. As the latter are not as oxygen-sensitive, they are easier to handle (73) ... 

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