Hydrodehalogenation

The hydrodehalogenation reaction of haloaromatics involved the substitution of halide atoms bound to the ring, with hydrogen. For example, tetrachloroben-zene could be reduced to benzene in 30 minutes, at 50°C, by bubbling H2 at atmospheric pressure in the multiphasic system constituted by isooctane, 50% aqueous KOH, 0.2 molar A336, in the presence of Pd/C (0.02 molar) (Figure 6.18).43

The A336 phase was shown to play a decisive role on kinetics by coating the Pd/C catalyst. In fact, by plotting the rates of the competitive hydrodehalogenation of o-, m-, and p-chloroethylbenzenes as a function of the concentration of A336, a Langmuir isotherm was obtained: the rates increased by increasing A336, until a plateau was reached (Figure 6.19). Such behavior may indicate the formation of a film of A336 on the catalyst surface.44

Different halides were also shown to behave differently under the multiphasic conditions. While bromide was removed faster than chloride, hydrodeiodination became inhibited, likely due to the formation of the ammonium-iodide ion pair (lipophilic and therefore present in the organic phase), which in contact with the Pd/C catalyst poisoned the metal.

The multiphasic system was able to conduct the hydrodehalogenation reaction selectively in the presence of other functional groups prone to reduction. It was therefore possible to transform halogenated aryl ketones into the corresponding aryl ketones, without consecutive carbonyl reduction. This was possible in the multiphasic system—only when A336 was present—and not in the traditional alcoholic solvent systems (Figure 6.20).45 This kind of selectivity could be used for synthetic purposes in the preparation of aryl ketones otherwise not accessible through direct Friedel-Crafts acylation (e.g., Figure 6.21).48

As far as the metal catalyst was concerned, Raney-Ni and Pt/C were also investigated. Raney-Ni proved effective in the hydrodehalogenation reaction of dichloro- and dibromobenzenes with hydrogen in the multiphasic system

Figure 6.18 Hydrodehalogenation of tetrachlorobenzene.

Figure 6.18 Hydrodehalogenation of tetrachlorobenzene.

Figure 6.19 Rate constants for the competitive hydrodehalogenation of o-, m-, and p-chloroethylbenzenes as a function of the concentration of A336, in the multiphasic system.

Figure 6.19 Rate constants for the competitive hydrodehalogenation of o-, m-, and p-chloroethylbenzenes as a function of the concentration of A336, in the multiphasic system.

(isooctane, 50% aqueous KOH, A336). And—contrary to Pd/C, which showed activity in the absence of A336 as well—Raney-Ni functioned only when A336 was present.46,47

Platinum on charcoal provided a further means for directing the selectivity of the multiphasic hydrodehalogenation reactions. For example, the same reaction in Figure 6.20 conducted using Pt/C instead of Pd/C, yielded selectively the dehalo-genated benzylic alcohol (Figure 6.22).49 The same reaction was conducted using

Figure 6.20 Selective reduction of haloaromatic ketones in the multiphasic system using Pd/C.

Figure 6.20 Selective reduction of haloaromatic ketones in the multiphasic system using Pd/C.

Pd/c, a336

ch3 o

ch3 o

ch, aici3, cs2 ci rcoci

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