Adams’ catalyst hydrogenolysis

Aldehydes are easily hydrogenated to alcohols but ketones are more difficult to reduce because of steric hindrance. Hydrogenolysis is a problem with the catalytic reduction of carbonyls, particularly when linked to aromatic systems. Pd and H2 reduce alkenes faster than carbonyls. Metal catalyst Pt is commonly used for the reduction of carbonyls. For example, the Adams catalyst (Pt02) reduces 2-naphthaldehyde (6.31) to 6.32 in 80% when used with FeCls as a promoter. When excess of the promoter is used the product is 2-methylnaphthalene (6.33), which is also obtained by the reduction of 6.31 with Pd on BaS04 and H2. 

A platinum-based catalyst, such as Adams catalyst, is an alternative to nickel and palladium, and is more active. For example, when the reaction in equation (24) is carried out in the presence of a plati-nium catalyst, the nitro group is also reduced. The hydrogenolysis of 2-butene oxide and propylene oxide proceeds at a slightly lower temperature than when using nickel, and for propylene oxide the... 

Catalytic hydrogenation of fluoro- and difluOTO-m- and tran -butenedioic acids and their esters frequently affords products only of hydrogenolysis, especially when platinum oxide (Adams catalyst) is used. Hydrogenolysis is favored in polar solvents, and is found to be more effective when rhodium or nickel rather than palladium are used as the catalysts (Table 2). Hydrogenolysis takes place prior to the saturation of the double bonds fluoro- and 2,3-difluoro-succinic acids do not suffer any loss of fluorine under the same reaction conditions. ... 

Prior to this publication the syntheses of racemic brazilin (ref. 115) and of racemic haematoxylin (ref. 116) were described by an alternative methodology. In the approach to brazilin the dihydroxybenzylidene derivative (J, Scheme 32) was methylated to form the trimethyl ether (I R = Me) and the analogous tribenzyl ether (I R = Bn) was synthesised from appropriate starting materials. (I R = Me was then epoxidised by treatment with hydrogen peroxide in alkaline solution. From treatment with sodium borohydride and then catalytic hydrogenation in acetic acid in the presence of Adams catalyst a mixture of dihydroxy products resulted. By mild cyclisation with perchloric acid, racemic brazilin trimethyl ether (T, R = Me) was derived. From the tribenzyl ether (I R = Bn) by a similar series of reactions, racemic brazilin tribenzyl ether (T R = Bn) was obtained and from hydrogenolysis, racemic brazilin was derived. The process is depicted in Scheme 36. 

To obtain simple, well-characterized derivatives, the epimeric isohumulones have been transformed into the epimeric humulinic acids (see 8.4.) and further into the epimeric dihydrodeoxohumulinic acids (cis form 72, Fig. 37 trans form 73, Fig. 37). This last transformation is achieved by hydrogenolysis of the humulinic acids (26). The yield can be increased to 80-85% by carrying out the reaction in a 20%-solution of the humulinic acids in acetic acid with 5% Adams catalyst at 55°C during 6-8 h. The optical activity is fully retained. The epimeric compounds 72 and 73 are separated by CCD in the two-phase system ether aqueous buffer pH 7.65 after 100 transfers. The cis isomer 72 has a K-value of 1.36 and a melting point of 179°C. The UV absorption... 

Hydrogenolysis of the humulinic acids using Adams catalyst at elevated temperatures in acidic medium, as described for the preparation of the dihydrodeoxohumulinic acids (see 8.4.3.1.3.). produces a complex mixture (51,58). Separation is achieved by gas chromatography (column of 3 m, filled with 12% SE-30 on Chromosorb W 60-80 temperature programmation from 120°C to 200°C at a rate of 2 C.min ) and identification of the individual components occurs by GC-MS. 

The well-known Adams platinum oxide can be prepared conveniently by the procedure of Adams et al. (2). Platinum oxides prepared in this way usually contain some traces of sodium, which in certain reactions can have an adverse effect. The sodium can be removed by washing with dilute acid (53). The Nishimuri catalyst (30% Pt, 70% Rh oxides) can be prepared by the same procedure as for platinum oxide or with variations from platinum and rhodium salts (64,65,66). This catalyst has much merit. It is usually most useful when hydrogenolysis is to be avoided (67,85,86). 

Adams platinum than over 5% Rh-Al203. Resorcinol and hydroquinone hydrogenate faster than catechol over both platinum and rhodium catalysts. In particular, it is noted that, over platinum, resorcinol and hydroquinone hydrogenate as fast as or even faster than phenol, and, at the same time, the proportions of hydrogenolysis with these di-hydroxybenzenes are much greater than with phenol and catechol. 

Platinum catalysts, which usually tend to cause extensive hydrogenolysis, may be used for the selective hydrogenation of benzyl-type alcohols in the presence of a proper base. For example, benzyl alcohol can be hydrogenated to cyclohexane-methanol almost quantitatively over Adams platinum oxide in ethanol with addition of a small amount of acetic acid (eq. 11.36).160... 

Oxazolidines (53) are readily formed from aldehydes or ketones and ethanolamines they can be hydrolyzed with ease and show reactions that might be expected of the imino alcohol intermediate (54). Among these are the addition of Grignard reagentsand catalytic hydrogenolysis of the C—O Ixjnd (equation 28).This reaction is exothermic over Adam s catalyst in methanol but slower in acetic acid. Nickel and copper chromite are also effective but at higher temperatures and pressures,as is the case with palladium. The same cleavage occurs with LAH (unassisted)and with the borane-THF complex. ... 

Phosphorylation. Calderon used hydrogenolysis with Adam s catalyst for conversion of (4) to (5).2a... 

When the earbonyl of ketones or aldehydes is eonjugated to aromatie systems, hydrogenolysis produets are often the major isolated products. Catalytic hydrogenation of 2-naphthaldehyde (395) with palladium on harium sulfate, gave only 2-methylnaphthalene (397).When the Adam s catalyst (Pt02) was used, with ferric chloride (FeCly) as a promoter, an 80% yield of alcohol 396 was obtained although addition of excess promoter induced further hydrogenolysis to 397. 

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