Aromatic hydrogenolysis

A dehydrocyclization (incl. aromatization) hydrogenolysis O isomerization. Right The same catalysts after a standard sulphurization procedure. Arrows show results obtained with a physical mixture of Pt/S/ AkOaand Pt/Re/S/ AI2Q3 catalysts. T = 620 K, 1 bar total pressure. 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

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]. 

Aromatic rings are hydrogenated with a variety of catalysts. However, aromatic alkoxy and hydroxyl substituents are susceptible to hydrogenolysis under most conditions used to saturate the ring. Hydrogenolysis does not occur to any appreciable extent with ruthenium catalysts even though high temperatures and pressures are required. Thus, substituted phenols are... 

Catalytic hydrogenation in acetic anhydride-benzene removes the aromatic benzyl ether and forms a monoacetate hydrogenation in ethyl acetate removes the aliphatic benzyl ether to give, after acetylation, the diacetate. Trisubstituted aDcenes can be retained during the hydrogenolysis of a phenolic benzyl ether. ... 

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). 

Hydrogenolysis of the ring of di- and tetra-hydroisoxazole derivatives proceeds, of course, more readily than in the case of the aromatic system of isoxazole. These two reactions are discussed separately below. 

A practical process had earlier been developed for the transformation of chlortetracycline (2) into tetracycline (1) by catalytic hydrogenolysis of the aromatic chloro group. Application of the reaction under suitable conditions to demethylchlortetra-... 

Aldehydes and ketones are similar in their response to hydrogenation catalysis, and an ordering of catalyst activities usually applies to both functions. But the difference between aliphatic and aromatic carbonyls is marked, and preferred catalysts differ. In hydrogenation of aliphatic carbonyls, hydrogenolysis seldom occurs, unless special structural features are present, but with aryl carbonyls either reduction to the alcohol or loss of the hydroxy group can be achieved at will. 

Ruthenium is excellent for hydrogenation of aliphatic carbonyl compounds (92), and it, as well as nickel, is used industrially for conversion of glucose to sorbitol (14,15,29,75,100). Nickel usually requires vigorous conditions unless large amounts of catalyst are used (11,20,27,37,60), or the catalyst is very active, such as W-6 Raney nickel (6). Copper chromite is always used at elevated temperatures and pressures and may be useful if aromatic-ring saturation is to be avoided. Rhodium has given excellent results under mild conditions when other catalysts have failed (4,5,66). It is useful in reduction of aliphatic carbonyls in molecules susceptible to hydrogenolysis. 

Hydrogenolysis of aliphatic carbonyls usually does not occur readily unless certain types of structures prevail (78), but hydrogenolysis of an aromatic carbonyl will occur easily, mostly via an intermediate benzyl alcohol. 

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. 

This result stands in contrast to hydrogenation of 2-oximino-]-indanone (R = H), which stopped spontaneously at the 2-amino-1-indanol stage under similar conditions (43). This latter result accords with the general exp>erience in reduction of aromatic -oximino ketones (35,37 38,39,40). The amino function usually severely inhibits hydrogenolysis of the alcohol. 

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