Allyl alcohols, hydrogenolysis hydrogenations

The hydrogenation of unsaturated aldehydes IV can be a complex transformation, as depicted in Scheme 2. Although the desired reactions are normally either the formation of allylic alcohol V, or saturated aldehyde VI, by 1,2 addition of hydrogen across the functional group, 1,4-addition across the conjugated functions can provide the enol, VII. Over-hydrogenation can result either in further saturation or, for allylic alcohols, hydrogenolysis to the alkene VIII (which can, in turn, be further saturated). 

Results of the reduction of unsaturated alcohols depend on the respective positions of the hydroxyl and the double bond. Since the hydroxyl group is fairly resistant to hydrogenolysis by catalytic hydrogenation almost any catalyst working under mild conditions may be used for saturation of the double bond with conservation of the hydroxyl [608]. In addition, sodium in liquid ammonia and lithium in ethylamine reduced double bonds without affecting the hydroxyl in non-allylic alcohols [608]. 

Unsaturated epoxides are reduced preferentially at the double bonds by catalytic hydrogenation. The rate of hydrogenolysis of the epoxides is much lower than that of the addition of hydrogen across the carbon-carbon double bond. In a, -unsaturated epoxides borane attacks the conjugated double bond at -carbon in a cis direction with respect to the epoxide ring and gives allylic alcohols [660], Similar complex reduction of epoxides occurs in a-keto epoxides (p. 126). 

Oxidation with lead dioxide in 5% phosphoric acid yielded acetaldehyde and formaldehyde. Catalytic reduction in the presence of platinum resulted in the absorption of 1.5 mole equivalents of hydrogen and the isolation, by its steam volatility, of nearly 0.6 mole of a-methylbutyric acid. The carbon skeleton of sarracinic acid was thus determined, and the accompanying hydrogenolysis (some nonvolatile acid was also obtained) established that the double bond and hydroxyl constituted an allylic alcohol moiety. Since further data (spectroscopic data would be especially valuable) were not available, Danilova and Kuzovkov (127) were limited to the conclusion that sarracinic acid could be represented by one of three possible structures (CLXIa-c) ... 

Hydrogenation and Deoxygenation. Nickel boride, produced in situ by the action of sodium borohydride on nickel(II) chloride, has been used to carry out a two-step hydrogenolysis of allylic alcohols via reductive cleavage of the trimethylsilyl ethers. Tertiary alcohols may be deoxygenated in reasonable yields by means... 

Reductions - For unsaturated compounds containing oxygen (allylic alcohols and ethers, ynols, epoxides, a,6-unsaturated ketones, aldehydes, acids and epoxides) nidkel boride (prepared from N1 acetate and NaBH ) selectively and quantitatively hydrogenates the C-C double bonds without rearrangements, hydrogenolysis or carbonyl reductions. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

Partial reduction of phenols affords mixtures of allylic and vinylic alcohols. From the generality derived for aliphatic systems, the most hydrogenolysis of this mixture is expected with platinum, palladium, and iridium catalysts, and much less with rhodium and ruthenium, an expectation substantiated in practice. For example, hydrogenation of resorcinol in neutral medium affords 20, 19, and 70% cyclohexanediol over palladium-, platinum-, and rhodium-on-carbon, respectively (29). Many examples attest to the value of rhodium and ruthenium at elevated pressure in avoiding hydrogenolysis. 

Because of the need for initial coordination with the catalyst, only benzylic or allylic C-X bonds can be reduced, but the X can be oxygen as well as nitrogen. We will come back to benzyl groups, and their hydrogenolysis, as a means for temporary protection of amines and alcohols later in the chapter. For the moment, though, we should take a broader look at catalytic hydrogenation as our second (after hydride reduction) important class of reductions. 

Vinyl and allyl groups can be saturated but hydrogenolysis and other undesired reactions may occur. The choice of metal as a catalyst and the solvent affects the results of hydrogenating substituted cyclo-hexenyl ethers. Besides the saturated ether, the cyclohexanone and its diethyl acetal, the cyclohexanol and cyclohexane are formed in ethanol in proportions which vary with the metal. The relative rates of the competing reactions depend on both solvent and metal and yields exceeding 96% of the saturated ether are reported for catalysis by Pd in ethanol, and Ru or Rh in r-butyl alcohol (see Section 3.1.3.3.5). 

Acetylenic carbinols, which are manufactured from aldehydes or ketones and acetylenes, can be partially hydrogenated, with the exception of propargyl alcohol itself. Hydrogenolysis is a minor side reaction unless the hydrogenation is completed to the alkane. Nickel boride (PI or P2) catalysts achieve the saturation of propargyl and allylic compounds in the presence of double bonds without hydrogenolysis, such as in 2 an intermediate in the total synthesis of the sesquiterpene sesquicarene ... 

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