Figure 6.21 Chloride as directing-protecting group in Friedel-Crafts acylations.
different catalyst-philic phases, in order to understand their influence. In particular, different ammonium salts—triethylbenzyl- and (tributylbenzyl)ammonium; and polyethylene glycols such as Brij 35, 52, 56, 58—were used. A correlation between increased hydrophilicity and chemoselectivity toward the benzylic alcohol was proposed.
The study using Pt/C was further extended by modifying the amount of KOH. The base also influenced the selectivity by reducing the amount from 50% to less than 5% in the aqueous phase, the final pH of the reaction mixture remaining between 8 and 13. In this range, the preferred product returned to be the ketone (Figure 6.23).50
Kinetic studies performed on model compounds were aimed at understanding the effect of different parameters on the selectivity. They showed that selectivity was achieved only when A336 was present. In fact, in the absence of A336 and of the base the hydrodehalogenation of p-chloroacetophenone proceeded all the way to ethylcyclohexane in the biphasic aqueous-organic system. When A336 was added, selectivity was reversed—chloride was removed first—and the selective dehalogenated benzyl alcohol was obtained.51
The multiphasic hydrodehalogenation reaction proved attractive from an environmental perspective. It is well known that halogenated organic compounds are dangerous for the environment and for human health; according to the Stockholm convention, many of the banned compounds are classified as persistent organic pollutants (POPs) that belong in fact to this category. Examples of such toxics are polychlorobiphenyls (PCBs), chlorophenols, chlorobenzenes, poly-chlorodibenzodioxins and -dibenzofurans (PCDDs and PCDFs, respectively, also
Figure 6.22 Selective reduction of haloaromatic ketones in the multiphasic system using Pt/C.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.