Hydrogenolysis Birch reduction

Bamford-Stevens decomposition of tosylhy-drazones, 351 p-Benzoquinone, 308 Benzyl ether hydrogenolysis, 139 Benzyl thioenol ethers, 87 Birch reduction, 11, 49, 50 Birch reduction of estrone methyl ether diethyl ketal, 51... 

The methods which are available for the protection of the aromatic carbonyl group are similar to those for the aliphatic and alicyclic analogues (Section 5.8.8, p. 623). It should be noted however, that when a cyclic acetal is used as the protecting group, a Birch reduction (Section 7.5) on the protected compound usually results in hydrogenolysis of the protecting acetal. 

The Birch reduction of 3,4,5-trimethoxybenzoic add 17a b (cognate preparation in Expt 7.19) is of interest in that the product is 1,4-dihydro-3,5-dimethoxy-benzoic add (46) thus revealing that in this reduction the 4-methoxy group is removed by hydrogenolysis. The product possesses two methyl vinyl ether residues which are labile under aqueous acidic conditions to yield the saturated diketone, 3,5-diketocyclohexanecarboxylic add (47). 

Catalytic hydrogenolysis using Pd—C, Pd(OH)2 or Pd(OAc)2 is the most commonly employed method for the removal of benzyl ethers, and yields are often quantitative. Cyclohexene, cyclohexadiene, formic acid and ammonium formate can also be used as hydrogen sources rather than hydrogen. Benzyl ethers can also be removed by Birch reduction with lithium or sodium dissolved in liquid ammonia, but this procedure is not often applied in carbohydrate chemistry. 

In a new asymmetric synthesis of chiral 1,4-diols, the dioxocane 179 was transformed into diols 180 and 181 by either the Birch reduction or catalytic hydrogenolysis, respectively (Equation 40) <1996TL2245> (cf. Equation (46), Section 14.06.6.6). 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

Benzylic amines are resistant to hydrogenolysis under Birch reduction conditions, unless the amine is quaternary, as in the Emde reaction. A means of preserving an aryl carbonyl group is therefore to protect it as an aminal derivative. An illustration is provided by the preparation of aldehyde (213), an important flavor constituent of cumin, as outlined in Scheme 47." ... 

General problems, as compared to the formation of 0-glycosides, are the incompatibility between catalytic hydrogenolysis and sulfur functions, which complicates the use of benzyl ethers as protecting groups, although Birch reduction might be an alternative, and the easy formation of disulfides from thiols, irrespective of if they are used as donors or acceptors. 

Allylic and benzylic heteroatom substituents such as -OR, -SR, and halogens undergo concomitant hydrogenolysis during Birch reduction. However, benzylic -OH groups are converted to alkoxides, and the resultant electron-rich -CHjO moiety resists further reduction. 

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