Hydrogenolysis of benzyl alcohols

Pd is the most widely used catalyst for the cleavage of benzylic C—O bonds, and 10% Pd/C is the most popular form of Pd for hydrogenolysis of benzyl alcohols. This catalyst usually contains residual acids, which increase reaction rates. 

For example, 10% Pd/C in CH2Cl2 with catalytic amounts of p-TsOH was used for 56 hours to reduce benzyl alcohol.47 Likewise, the catalytic hydrogenolysis of an epimeric mixture of another benzyl alcohol was performed in dry ethanol using 10% Pd/C and two drops of HC104 for 3 hours at room temperature and atmospheric pressure.48 

In contrast, the Pd-catalyzed hydrogenation of the [l6]-orthocyclophane-1,3-dione was attempted repeatedly for 7 days, over a wide range of H2 pressures (35-45 psi) and temperatures (25-40°C), to produce a similar reduction 

No suppression in hydrogenolysis of the C-O bond was observed over either bimetallic catalysts or Ti02-supported catalysts, indicating involvement of different types of sites than C-C bond hydrogenolysis.58 

Such a hydrogenolysis over 10% Pd/C in ethanol at room temperature and atmospheric pressure gave the desired product in 83% yield.59 

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The mechanisms of most alcohol reductions are obscure. Hydrogenolysis of benzyl alcohols can give inversion or retention of configuration, depending on the catalyst. - ... 

Benzylic alcohols were also converted to hydrocarbons by sodium borohy-dride [621], by chloroalane [622], by borane [623], by zinc [624], and by hy-driodic acid [225,625], generally in good to excellent yields. Hydrogenolysis of benzylic alcohols may be accompanied by dehydration (where feasible) [622],... 

For discussions of the mechanisms of the hydrogenolysis of benzyl alcohols, see Khan McQuillin Jardinc Tetrahedron Lett. 1966, 2649. J. Chem. Soc. C1967, 136 Garbisch Schreader Frankel J. Am. them. Soc. 1967,89, 4233 Mitsui Imaizumi Esashi Bull. Chem. Soc. Jpn. 1970, 43, 2143. 

Hydrogenolysis of benzyl alcohols 0-79 Reduction of benzylic ethers 0-86 Coupling of halides containing aryl groups... 

The hydrogenolysis of benzylic alcohols in many cases can benefit from the use of an acid to provide faster or more satisfactory reaction rates (Scheme 84). ... 

The platinum catalysts are universally used for hydrogenation of almost any type of compound at room temperature and atmospheric or slightly elevated pressures (1-4 atm). They are usually not used for hydrogenolysis of benzyl-type residues, and are completely ineffective in reductions of acids or esters to alcohols. At elevated pressures (70-210atm) platinum oxide converts aromatics to perhydroaromatics at room temperature very rapidly [5]. 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

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]. 

AUylic ethers were reduced by treatment with lithium in ethylamine to alkenes [636]. Benzyl ethers are hydrogenolyzed easily, even more readily than benzyl alcohols [637], 3,5-Bis(benzyloxy)benzyl alcohol gave 3,5-dihydroxy-benzyl alcohol on hydrogenation over palladium on carbon at room temperature and atmospheric pressure in quantitative yield [638. Hydrogenolysis of benzylic ethers can also be achieved by refluxing the ether with cyclohexene (as a source of hydrogen) in the presence of 10% palladium on carbon in the presence of aluminum chloride [639]. 

Most experimental data are reported on the use of Pd-Ti02 catalysts in the hydrogenation. As equation 24 shows, product distribution is considerably affected by the para substituent. The formation of benzyl alcohols is favorable on nonacidic supports while acidic supports promote hydrogenolysis. Hydrogenolysis can also be avoided under strongly acidic conditions in the presence of ethanol. In this case, the product benzyl alcohol readily undergoes dehydration to form benzyl ethyl ether. 

The production of hydrocarbons from aromatic alcohols is most readily explained by the hydrogenolysis of the alcohol, but an alternate possibility should be considered. The formation of an aldehyde and its subsequent decarbonylation under reaction conditions could lead to the hydrocarbon. Both toluene and 2-phenylethanol, the mixture of products secured from benzyl alcohol, may be regarded as derived from phenylacetaldehyde as an intermediate ... 

When pure phenylacetaldehyde was treated under the conditions that resulted in the conversion of benzyl alcohol to toluene and 2-phenyl-ethanol, the phenylacetaldehyde polymerized and no identifiable reaction products were secured. More information on this matter was obtained with the substituted benzyl alcohol, benzhydrol. Benzhydrol, on treatment with synthesis gas, gives an almost quantitative yield of diphenyl-methane. The hydrocarbon may be formed by decarbonylation of the intermediate diphenylacetaldehyde, or by direct hydrogenolysis of the alcohol ... 

Copper-chromium oxide catalyst is effective for the hydrogenation of aldehydes at a temperature of 125-150°C.1 The hydrogenation of benzaldehyde over copper-chromium gives a high yield of benzyl alcohol even at 180°C without hydrogenolysis to give toluene (eq. 5.5).7... 

Scheme 11.9 Hydrogenation and hydrogenolysis pathways of benzyl alcohol and the formation of cyclohexanecarboxaldehyde.

Protection of hydroxyl groups (11, 166).- 3.4-Dimethoxybcnzyl ethers are oxidized by DDQ more readily thanp-methoxybenzyl ethers. Moreover, the dimethoxybenzyl ethers of secondary alcohols can be selectively oxidized in the presence of the corresponding ethers of primary alcohols. Benzyl, p-methoxybenzyl, and 3,4-dimethoxybenzyl ethers all undergo hydrogenolysis catalyzed by Pt/C or Pd/C, but selective hydrogenolysis of benzyl ethers is possible with W-2 Raney Ni. 

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