Hydrogenation of Aromatic Nuclei

This area is likely to become increasingly important if coal, which contains polyaromatic compounds, is utilized more for production of petrochemicals (c/. coal liquefaction). 

Because of unfavorable thermodynamics, partial reduction of aromatics is exceedingly difficult to achieve. The selective reduction of benzene to cyclohexene by tetrahydroborate can be carried out with the help of dicationic 77 -benzene complex [C Me M(C6H6)]+ (M = Ir, Rh, az = 5 M = Ru, n = 6). 

The overall reaction consists of addition of two hydrides followed by two protons to coordinated benzene. In this last step, the two protons reoxidize the metal and liberate free cyclohexene [28]. The reaction is thus catalytic in the platinum metal even though it is stoicheiometric in hydride and acid. 

The very simple system NaBH4—RhCl3 in hydroxyhc solvents has proved to be useful for the reduction of aromates to the saturated cycles under mild conditions [29]. 

The reaction is compatible with carboxylic acids, esters and amides which remain unaffected by the reduction system. Even ketones are only partially reduced, though olefinic bonds are completely reduced simultaneously. 

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Synthetic procedural details in each step, enroute to polymers llA and IIB in Scheme 1 were essentially the same. Catalytic hydrogenation of aromatic nuclei to give cycloalkyl-diol(3) was achieved by the procedure of Meyer. 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

The behaviour of the frontier electrons was also attributed to a certain type of electron delocalization between the reactant and the reagent 40). A concept of pseudo-n-orbital was introduced by setting up a simplified model, and the electron delocalization between the 71-electron system of aromatic nuclei and the pseudo-orbital was considered to be essential to aromatic substitutions. The pseudo-orbital was assumed to be built up out of the hydrogen atom AO attached to the carbon atom at the reaction center and the AO of the reagent species, and to be occupied by zero, one, and two electrons in electrophilic, radical, and nucleophilic reactions. A theoretical quantity called "superdelocalizability was derived from this model. This quantity will be discussed in detail later in Chap. 6. 

Hydrogenation of the nuclei in aromatic aldehydes is possible by catalytic hydrogenation over noble metals in acetic acid and is rather slow [767]. 

The figures of B. K. Mazumdar et al. shown in parentheses are the revised values computed by these authors after it had been pointed out that at temperatures above 200°C. the sulfur dehydrogenation method they used is capable of removing hydrogen from aromatic nuclei as well as alicyclic rings. The revised figure for a coal of 85% C was 0.16, not 0.23. 

On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically are converted to o- and p-quinonoid structures and oxirane derivatives of quinols (E, D, C, resp., Fig. 1.4). It should be noted that the conversion of aromatic nuclei to o-quinonoid rings is accompanied by loss of the methoxyl group as methanol and that conversion to p-quinonoid groups in many cases leads to displacement of the side chain. 

Alkaline hydrogen peroxide has not found wide use in the oxidation of aromatic nuclei, although hydroxyquinones can be formed from hydroquinones under strongly alkaline conditions (Figure 3.111).470... 

Reductions of aromatic nuclei containing hydroxyl (method 86), carboxyl (method 270), ester (method 304), and amino (method 430) groups are discussed elsewhere. Hydrogenation of 2-methoxynaphthalene over Raney nickel occurs in the ring containing the methoxyl group. ... 

Other conditions for the reduction of the aromatic nucleus are discussed in method 4. The hydrogenation of heterocyclic nuclei is treated in method 554. 

The use of oxidizing agents can lead to the union, not only of aromatic nuclei, but also of aliphatic methyl, methylene, or methine groups, with elimination of hydrogen. Such a reaction is an oxidative dimerization. 

Reduction of the aromatic nuclei contained in catalytic C-9 resins has also been accomplished in the molten state (66). Continuous downward concurrent feeding of molten resin (120°C softening point) and hydrogen to a fixed bed of an alumina supported platinum—mthenium (1.75% Pt—0.25% Ru) catalyst has been shown to reduce approximately 100% of the aromatic nuclei present in the resin. The temperature and pressure required for this process are 295—300°C and 9.8 MPa (lOO kg/cni2), respectively. The extent of hydrogenation was monitored by the percent reduction in the uv absorbance at 274.5 nm. 

A variety of experiments have shown that for bicyclic aromatic nuclei the weight ratio of reactant to catalyst should be 2 1, whereas for monocyclic aromatic nuclei, the reactant to catalyst ratio should be 3 1. For the latter systems, hydrogen absorption is usually complete within 6-8 hours (see Discussion section). 

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