Hydrogenolysis palladium

Reduction of benzene derivatives carrying oxygen or nitrogen functions in ben-zylic positions is complicated by the easy hydrogenolysis of such groups, particularly over palladium catalysts. Preferential reduction of the benzene ring in these compounds is best achieved with ruthenium or rhodium catalysts, which can be used under mild conditions. For example, mandelic acid is readily converted into the cyclohexyl derivative 29 over rhodium-alumina, whereas with palladium, hydrogenolysis to phenylacetic acid is the main reaction (7.18)... 

The above sequence was extended to a preparation of 6-epicorynoline. Performic acid oxidation of deoxycorynolone, followed by treatment with alkali, afforded a mixture of diols 3, epimeric at C-5. Lithium aluminum hydride reduction of diols 3, followed by palladium hydrogenolysis of the benzylic C-5 hydroxyl, gave rise to 6-epicorynoline. It will be noted that in this instance performic acid oxidation, followed by hydrolysis, furnished C-6 beta hydroxyl derivatives. ... 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Hydrogenolysis of aryl and alkenyl halides and triflates proceeds by the treatment with various hydride sources. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Several boro hydrides are used for this purpose[680], Deuteration of aromatic rings is possible by the reaction of aryl chlorides with NaBD4681]. 

The best way to make pyrimidine in quantity is from 1,1,3,3-tetraethoxypropane (or other such acetal of malondialdehyde) and formamide, by either a continuous (58CB2832) or a batch process (57CB942). Other practical ways to make small amounts in the laboratory are thermal decarboxylation of pyrimidine-4,6-dicarboxylic acid (744), prepared by oxidation of 4,6-dimethylpyrimidine (59JCS525), or hydrogenolysis of 2,4-dichloropyrimidine over palladium-charcoal in the presence of magnesium oxide (53JCS1646). 

Esters and amides are quite resistant to hydrogenation under almost all conditions so their presence is not expected to cause difficulties. Alkyl ethers and ketals are generally resistant to hydrogenolysis but benzyl ethers are readily cleaved, particularly over palladium or Raney nickel catalysts. ... 

Over palladium this cleavage occurs in preference to the saturation of a 5,6-double bond. " The use of platinum in acetic acid allows saturation of the A -olefin in (36) without hydrogenolysis of the 22,23-dibromides. 

Debenzylation of the benzyloxy groups in 156 and 158 to 157 and 159 respectively was achieved, without affecting the aromaticity of the system, by catalytic hydrogenolysis in the presence of palladium-on-charcoal (86TL3127 89JHC991) (Scheme 61). 

Preparation of the substituted piperazine required for sul-falene (114) starts with bromination of 2-aminopiperazine to give the dihalide (150). Displacement of halogen by sodium methoxide proceeds regioselectively at the more reactive 3 position to give 151. Hydrogenolysis over palladium on charcoal gives the desired intermediate (152). 

In general, the hydrogenolysis product is also favored by an acidic medium, as illustrated in the hydrogenation over 5° palladium-on-carbon of acetophenone to the hydrogenation product phenylethanol and to the hydrogenolysis product ethylbenzene, with various additives present (S3). 

Another example is the hydrogenation of the homoallylic eompound 4-methyl-3-cyclohexenyl ethyl ether to a mixture of 4-methylcyclohexyl ethyl ether and methylcyclohexane. The extent of hydrogenolysis depends on both the isomerizing and the hydrogenolyzing tendencies of the catalysts. With unsupported metals in ethanol, the percent hydrogenolysis decreased in the order palladium (62.6%), rhodium (23 6%), platinum (7.1%), iridium (3.9%), ruthenium (3.0%) (S3). 

The order for decreasing hydrogenolysis of allylic functions appears to be Pd > Pt Rh = Ru. Hydrogenolysis of allylic functions over palladium may... 

Hydrogenation of 2,5-diacetoxy-2,5-dimethyl-3-hexyne 10 over 0% palladium-on-carbon is exceptionally complex. Seven different products are formed together with acetic acid. All are hydrogenolysis products arising from the initially formed 2,5-diacetoxy-2,5-dimethyl-3-hexene 11. One of these, 2,5-dimethyl-2-acetoxy-4-hexene 12 forms in as much as 4S yield. 

In ketones existing largely as the enol, palladium may prove effective in minimizing hydrogenolysis, Hydrogen was added slereoselectively to the least hindered side of L-ascorbic add (3) to give L-gulono-1,4-lactone (4)... 

Hydrogenation of carbonyls, or incipient carbonyls such as phenols (86), in lower alcohol solvents may result in the formation of ethers. The ether arises through formation of acetals or ketals with subsequent hydrogenolysis. The reaction has been made the basis of certain ether syntheses (45,97). Reaction of alcohols with carbonyls may be promoted by trace contamination, such as iron in platinum oxide (22,53), but it is also a property of the hydrogenation catalyst itself. So strong is the tendency of palladium-hydrogen to promote acetal formation that acetals may form even in basic media (61). 

Hydrogenolysis of aromatic carbonyls occurs mainly by conversion to the benzyl alcohol and its subsequent loss. If hydrogenolysis is desired, the usual catalyst is palladium 38). Hydrogenolysis is facilitated by polar solvent and by acid (55). For instance, hydrogenation of 3,3-dicarbethoxy-5,8-dimethoxy-l-tetralone (5) over 5% Pd-on-C gave 6 quantitatively 54) when hydrogen absorption ceased spontaneously. 

An unusual by-product was obtained in small yield in palladium-catalyzed reduction of 2-amino-4,5-dimethoxyindanone hydrochloride, The reduction was done in two stages first, a rapid absorption of 1 mol of hydrogen at 38 C to give the amino alcohol, and then a much slower reduction in the presence of HCIO4 at 70 "C. The rearranged by-product was shown to arise from attack of acid on the amino alcohol (50), Resistance to hydrogenolysis is characteristic of / -amino aromatic alcohols (56), a fact that makes reduction of aromatic oximino ketones to amino benzyl alcohols a useful synthetic reaction. 

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