The hydroxylation of alkenes

Hydroxylation of an alkene may be carried out using osmium tetroxide in an inert solvent (e.g. ether or dioxane), whereupon a cyclic osmate ester is formed. This undergoes hydrolytic cleavage under reducing conditions (e.g. aqueous sodium sulphite) to give the 1,2-diol resulting from a ds-hydroxylation process.71 

Alternatively and more conveniently this ds-hydroxylation process can be effected using only catalytic amounts of osmium tetroxide together with hydrogen peroxide, which cleaves the first formed osmate ester to the diol and regenerates the osmium tetroxide. The reaction is carried out in t-butyl alcohol and is illustrated by the conversion of cyclohexene into ds-cyclohexane-l,2-diol (Expt 5.47). 

Other variations in this hydroxylation procedure include the use of t-butyl hydroperoxide72 in place of hydrogen peroxide, and the use of PTC conditions.73 

Alkaline potassium permanganate also effects the ds-hydroxylation of an alkene. A useful procedure involves the hydroxylation reagent cetyltrimethyl-ammonium permanganate in dichloromethane or in aqueous t-butyl alcohol.74 The preparation of the reagent and the two alternative general procedures are given in the cognate preparation to Expt 5.47. 

2-diol arising from a trans-hydroxylation process is formed from an alkene by way of an intermediate epoxide which is subjected to a ring-opening reaction and hydrolysis. The epoxides may be isolated when the alkene is reacted with perbenzoic acid or m-chloroperbenzoic acid (Section 4.2.56, p. 457) in a solvent such as chloroform or dichloromethane the preparation of epoxides by this method and by other important procedures are discussed and illustrated 

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Thus by suitable choice of reagent, the hydroxylation of alkenes can be made stereoselectively SYN or ANTI at will. 

The hydroxylation of alkenes to diols with potassium osmate-oxidant mixtures has been described, with either hydrogen peroxide or sodium chlorate as the oxidant. The sodium chlorate method is not applicable where the diol is to be distilled from the mixture, because of the danger of explosive oxidation of the product diol by the chlorate. 

Another familiar example of the reaction proceeding in cis fashion is the hydroxylation of alkenes which occurs by osmium tetroxide or by the action of KMn04. 

This is an alternative scheme for the hydroxylation of alkenes (see Section 11-7C). However, the overall stereochemistry is opposite to that in permanganate hydroxylation. For instance, cyclopentene gives trans-1,2-cyclopen-tanediol. First the oxirane forms by suprafacial addition and then undergoes ring opening to give the trans product ... 

This is supplied as the solid in sealed ampoules or as a solution in water or 2-methylpropan-2-ol (t-butyl alcohol). It must be handled in a fume cupboard. It is extremely irritating and toxic and constitutes a severe eye injury hazard. The solution in t-butyl alcohol (Expt 5.47) must be prepared and dispensed in an efficient fume cupboard, with the added protection of gloves and goggles. This solution is reasonably stable (e.g. the decomposition after one month is about 20%), provided that no 2-methylprop-l-ene arising from the t-butyl alcohol is present as an impurity. Formation of black coloidal osmium, which can catalyse the decomposition of hydrogen peroxide, for example in the hydroxylation of alkenes, is rapid. 

Acetaldehyde has been prepared in part (a). Recalling that vicinal diols are available from the hydroxylation of alkenes, 1,2-ethanediol may be prepared by the sequence... 

Alkenes and Potassium Osmate. Sodium chlorate should not be used in the hydroxylation of alkenes to diols in the presence of potassium osmate if the diol is to be distilled from the reaction mixture explosive oxidation may occur.3... 

The enzymes reductively activate dioxygen using NADPH as an electron source. One oxygen atom is then reduced to water and the other atom is transferred to a substrate, resulting in the hydroxylation of alkenes and arenes, the epoxidation of alkenes and the formation of N-oxides and S-oxides from amino and sulphur compounds. Other P-450 reactions include N-dealkylation, O-dealkylation and reductase-like dehalogenation of halocarbons. Typical P-450 reactions are summarised in Ihble 5-4. 

The term glycol generally means a 1,2-diol, or vicinal diol, with its two hydroxyl groups on adjacent carbon atoms. Glycols are usually synthesized by the hydroxylation of alkenes, using peroxyacids, osmium tetroxide, or potassium permanganate (Section 8-14). 

Vanadium pentoxide, VjOj (mp 690 °C), is one of the strongest catalysts used for dehydrogenations [478] and oxidations with air or oxygen 479], especially in the gas phase and at very high temperatures. It also catalyzes the hydroxylation of alkenes with aqueous hydrogen peroxide 34]. 

The same oxidizing agents and conditions used for the hydroxylation of alkenes and cycloalkenes are applied to unsaturated alcohols, ethers, aldehydes, ketones, and acids and their derivatives. 

Electrophilic Addition.—The hydroxylation of alkenes, including several steroids, with osmium tetroxide has been reviewed. Addition of HOBr to the 19-functionalized-5a-cholest-6-enes (13) gave mixtures of the corresponding bromohydrins (14) and the 6,19-epoxides (15) whereas the analogous B-homo-compounds (16) gave only the 6,19-epoxides (17). Similar effects were noted for the related HBr- and HC104-catalysed opening of the 6a,7a-epoxides and it was observed that the predominant attack by the 19-0 atom [5(0)" attack] in the B-homo-series lay in the possibility of its linear approach with the C-6—Br or C-6—O bond. The reactions of chromyl chloride and chromyl fluoride with steroidal alkenes and dienes have been reported " and it was observed that the... 

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