Hydrogenolysis aromatic acids

Hydrogenolysis and reduction of esters or lactones to hydrocarbons are difficult. The reaction is easy with esters of aromatic acids and pyridinol or quinolinol ... 

The cyclization of aromatic acids (intramolecular acylation) leads to aromatic ketones this reaction is used to construct rings B and C in the synthesis of equilenin and estrone. The keto group formed by acylation is subsequently either eliminated by hydrogenolysis (Schemes 8, 14, 26, 28, 52, 79), or is used for the introduction of new side chains (Schemes 1,6, 78). There is a well-known method for the simultaneous formation of rings B and C by the cyclization of chlorides of (3 -dianisyladipic acids (75) (Scheme 52). In this process, the stereochemistry of the initial acid (meso or dl) determines the stereochemistry of the end product (76) (trans orcis). 

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. 

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]. 

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