Tertiary alcohols hydrogenolysis

Hydrogenolysis of tert-alcohols.1 2 3 This deoxygenation can be effected with Raney Ni slurry (Aldrich 50% slurry in water) that has been washed eight times with distilled water and twice with 1-propanol. Thus reaction of a tertiary alcohol with washed Raney Ni for a short time yields a mixture of alkenes that furnish a single alkane on hydrogenation catalyzed by Pd/C. 

Notes This is often used as a protecting group for alcohols, where it is observed that primary alcohols form more readily than secondary hydroxy groups, which in turn are more reactive than tertiary alcohols. As with most benzylic ethers, this protecting group can be removed by hydrogenolysis over Pd or by metal-ammonia reduction.1 Examples ... 

In contrast, the unexpected hydrogenolysis of a tertiary alcohol was observed during the desulfurization either of 1,3-dithiane derivatives or of their monosulfoxides, as exemplified in Scheme 3. It is intriguing that even deactivated W-2 Raney Ni did not improve the result, but a low temperature did allow convenient desulfurization with a minimum amount of side product. 

Hydrogenolysis of dialkylperoxides or epidioxides (cyclic peroxides) also takes place over these same catalysts under mild conditions. If tertiary alcohols are the products, running the reaction in acid media can lead to dehydration. Deactivated catalysts are also needed if the product contains an allyl alcohol that could be cleaved by the more active catalysts (Eqn. 20.72). 6.I87... 

Elevated temperature is usually needed for the hydrogenolysis of C-O bonds in alcohols [12]. Hydrogenolysis of primary alcohols can be deseribed as an 8 2-like reaction involving hydride attack on the carbon, whereas transformation of a tertiary alcohol is closer to an S jl-type reaetion [2,13]. Tertiary alcohols can be hydrogenolyzed much faster than secondary. Alcohols are readily converted to the corresponding 0-alkylisoureas. Hydrogenolysis of these compounds leads to hydrocarbons (Scheme 1) [14]. 

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... 

Because the hydrogenolysis of the phenylmethyl (benzyl) ether in the final step occurs under neutral conditions, the tertiary alcohol function survives untouched. A tertiary butyl ether would have been a worse choice as a protecting group, because cleavage of its carbon-oxygen bond would have required acid (Section 9-8), which may cause dehydration (Section 9-2). 

The stoichiometry determines the ratios of lithium aluminum hydride to other compounds to be reduced. Esters or tertiary amides treated with one hydride equivalent (one fourth of a molecule) of lithium aluminum hydride are reduced to the stage of aldehydes (or their nitrogen analogs). In order to reduce an ester to the corresponding alcohol, two hydride equivalents, i.e. 0.5 mol of lithium aluminum hydride, is needed since, after the reduction of the carbonyl, hydrogenolysis requires one more hydride equivalent. 

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