Limonene dehydrogenation

The carbanions thus produced in situ have only a transient existence because of the more acidic materials present, such as the ketones, acids, and condensation products produced. However, they are effective for a reaction with a relatively large driving force, such as limonene dehydrogenation. 

A quite different limonene conversion is the dehydrogenation into p-cymene, thus giving a green aromatic. Pd-catalysts give yields of >95% [18]. p-Cymene can be oxidized to the hydroperoxide, which can be rearranged to p-cresol, a valuable chemical. 

Dehydrogenation of dipentene (d,l-limonene) and 4-vinylcyclohexene has been studied using both the lithium-ethylenediamine catalyst system (4) and the sodium-alumina catalyst system (12). The aromatic compounds p-cymene and ethylbenzene are produced by both types of catalyst however, while vinylcyclohexene yields ethylbenzene rapidly at 0° over a sodium on alumina catalyst, dipentene yields mainly mixed dienes at 0 but at 25 is also rapidly dehydrogenated to p-cymene (12). 

Limonene is a liquid with lemon-like odor. It is a reactive compound oxidation often yields more than one product. Dehydrogenation leads to / -cymene. Limonene can be converted into cyclic terpene alcohols by hydrohalogenation, followed by hydrolysis. Nitrosyl chloride adds selectively to the endocyclic double bond this reaction is utilized in the manufacture of (—)-carvone from (+)-limonene (see p. 61). 

The metal functions can be elegantly combined with the acidic functions of the zeolitic support to obtain a very effective bifunctional catalyst. For example the selective isomerisation followed by dehydrogenation of limonene to give p-cymene (Scheme 24) can be carried out in one step over a multifunctionalised zeolite [213]. With an acidic boron zeolite (Si/B= 21) 21% selectivity to p-cymene was obtained at 100% conversion. Addition of 3 wt% Pd increased the selectivity to 70% at the same conversion. Further addition of Ce (1.5 wt% Pd, 3.5 wt% Ce) to the metal loaded zeolite led to 87% selectivity. 

Scheme 24. Isomerisation of limonene, followed by dehydrogenation to give...

Pines found that potassium t-butoxide when heated to decomposition temperatures (250-300°) catalyzes the dehydrogenation of hydroaromatic hydrocarbons. For example, d-limonene and the alkoxide were sealed in an autoclave, the air was CH, ipHj... 

Typical reaction in this category is the conversion of limonene to P-cymene over Pd-H-borosilicalite where isomerization is followed by dehydrogenation. Also many reactions mentioned here, particularly catalyzed by transition metal ion or bimeallic, are bifunctional zeolitic reactions. 

The second approach to the conversion of limonene to p-cymene is the use of a purely hydrogenation/dehydrogenation catalyst under reducing conditions. For this process Pd supported on a low-acidic silica carrier turned out to be the most appropriate catalyst [18]. Figure 2 illustrates the possible reaction pathway. 

Using zeolite-supported, Ce-promoted Pd catalysts brought enhanced dehydrogenation activity during a-limonene conversion. Thereby, they used preferably weakly acidic boron-pentasil zeolites and obtained p-cymene yields around 85%. 

Conversion over Pd-modified H-ZSM-5 - Dehydrogenation of a-limonene to cymenes is only obtained in the presence of strong acidic sites of H-ZSM-5... 

The conversion of a-pinene over the Ce-promoted, zeolite-supported Pd catalysts proceeds via an acid-catalysed ring opening of the bicyclic terpene to a monocyclic terpene, e.g. a-limonene. This is followed by dehydrogenation, possibly via isomerization to a-terpinene or y-terpinene. 1,8-Cineole is dehydrated on the acid sites to p-menthadiene prior to dehydrogenation to p-cymene on the Pd sites of the catalyst. The conversion of all reactants is complete during the test run of 8 h. The results are quite similar to a-limonene conversion, as expected from the reaction pathway via p-menthenes and p-menthadienes. 

The "right half of the sesquiterpene (+)-p-selinene (as drawn below) includes (R)-(+)-limonene as a substructure. Retrosynthetic disconnection to (if)-(+)-limonene leads to the intermediate carbenium ions la and lb via 15-nor-ll-eudesmen-4-one (carbonyl alkenylation) and 15-nor-13-chloro-2-eudesmen-4-one (dehydrogenation, protective masking of the double bond in the side chain). These carbenium ions arise from (if)-9-chloro-p-menth-l-ene and the acylium ion Ic (synthone) originating from 3-butenoic acid as reagent (synthetic equivalent). (i )-/7-Menth-l-en-9-ol, on its part obtained by hydroboration and oxidation of (if)-(-l-)-limonene, turns out to be the precursor of the chloromenthene. 

Limonene (16) can also serve as the starting material for a series of further transformations to obtain such important oxygenated monocyclic monoterpenes like menthol (38) or others as shown in Scheme 6.7. These are further impressive examples how nature can anploy rather simple molecules like 16 that contain at least two activated sites for further enzyme-catalyzed transformations to get access to a very diverse assembly of different metabohtes. Hereby, allyUc oxidations and subsequent (stereoselective) redox state manipulations as well as isomerization reactions are the main chemical transformations. Further dehydrogenation reactions of either limonene or advanced oxygenated monocyclic terpenes can also lead to an aromatization giving access to a small group of aromatic compounds (e.g., p-cymene (39) or thymol (40)) that are not biosynthesized by more common pathways like the shikimic acid pathway [1,3]. 

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