Hydrogenation carvone

Alternatively, hydrogenated carvone can be converted into a polymerizable lactone by Bayer-Villiger oxidation (Scheme 9). Hydrogenation of carvone may yield two cyclic ketones, i.e., dihydrocarvone and carvomenthone, which can be oxidized to the... 

Absorption, metaboHsm, and biological activities of organic compounds are influenced by molecular interactions with asymmetric biomolecules. These interactions, which involve hydrophobic, electrostatic, inductive, dipole—dipole, hydrogen bonding, van der Waals forces, steric hindrance, and inclusion complex formation give rise to enantioselective differentiation (1,2). Within a series of similar stmctures, substantial differences in biological effects, molecular mechanism of action, distribution, or metaboHc events may be observed. Eor example, (R)-carvone [6485-40-1] (1) has the odor of spearrnint whereas (5)-carvone [2244-16-8] (2) has the odor of caraway (3,4). 

By reduction carvone fixes 2 atoms of hydrogen on to the ketonic group, and 2 atoms in the nucleus, with the formation of dihydrocarveol, CijHjgO, whose corresponding ketone, dihydrocarvone, Cj Hj O, exists in small quantities in caraway oil. 

G. Vavon has examined the hydrogenation of carvone, in presenc of platinum black as a catalyst, and shown that it takes place in three entirely distinct phases. Carvone fixes successively three molecules of hydrogen, giving dextro-carvotanacetone, then tetrahydrocarvone, and finally carvomenthol. 

Carvone, a substance responsible for the odor of spearmint, has the following structure. Tell how many hydrogens are bonded to each carbon, and give the molecular formula of carvone. 

Table 3 summarizes the scope and limitation of substrates for this hydrogenation. Complex 5 acts as a highly effective catalyst for functionalized olefins with unprotected amines (the order of activity tertiary > secondary primary), ethers, esters, fluorinated aryl groups, and others [27, 30]. However, in contrast to the reduction of a,p-unsaturated esters decomposition of 5 was observed when a,p-unsaturated ketones (e.g., trans-chalcone, trans-4-hexen-3-one, tra s-4-phenyl-3-buten-2-one, 2-cyclohexanone, carvone) were used (Fig. 3) [30],... 

Equation (81)), while the other two C=C double bonds in the structure are intact. Under the same reaction conditions, the racemic carvone is also resolved kinetically with a KR/KS ratio of 33 1. Asymmetric hydrogenation of a,/Tacetylenic ketones to chiral propargylic alcohols is still unavailable. 

The Wilkinson catalyst reduces external double bonds much faster than the internal ones as in the hydrogenation of carvone (equation l)7. 

The hydrogenation of the monoterpenes (—)- and (-l-)-carvone was studied extensively. Several microorganisms were used in these reductions. They catalyzed the production of all possible stereoisomers, but some of them only in small quantities. The distribution of the products depended on the catalyst applied116. 

Literature examples include one-, two-, and three-step hydrogenation reactions, hi each case, the reactant and product concentrations were monitored, as were intermediates or side reaction products as necessary. Simple peak height or area measurements were sufficient to generate reaction profiles for the reduction of cyclohexene [116] and of l-chloro-2-nitrobenzene [117]. However, for more spectrally complex systems, such as the reduction of carvone and of 2-(4-hydroxyphenyl) propionate, multivariate curve resolution (MCR) was required [117]. 

The preferred industrial method of carvone synthesis utilizes the selective addition of nitrosyl chloride to the endocyclic double bond of limonene. If a lower aliphatic alcohol is used as solvent, limonene nitrosochloride is obtained in high yield. It is converted into carvone oxime by elimination of hydrogen chloride in the presence of a weak base. Acid hydrolysis in the presence of a hydroxylamine acceptor, such as acetone, yields carvone [88]. 

The main renewable resource for L-carvone is spearmint oil (Mentha spicata), which contains up to 75% of this flavour chemical. There also exists a synthetic process for the manufacturing of L-carvone, which is based on (-t)-limonene, which is available as a by-product of the citrus juice industry as a major component of orange peel oil (Scheme 13.4). The synthesis was developed in the nineteenth century and starts with the reaction of (-t)-limonene and nitrosyl chloride, which ensures the asymmetry of the ring. Treatment with base of the nitrosyl chloride adduct results in elimination of hydrogen chloride and rearrangement of the nitrosyl function to an oxime. Acid treatment of the oxime finally results in l-carvone. 

Therefore It seems to be interesting to study such effects in the selective hydrogenation of a poly-unsaturated molecule as carvone. It was reported that the partial hydrogenation of this molecule is very sensitive to different homogeneous catalysts organometallic compounds (refs. 8-10), Zn/OH (ref. 11), NaBH (ref.12), and Zn-NiCl (ref. 13) as examples. 

The purpose of the present work is to study the precursor (metallic chlorides or carbonyl compounds), particle size and support (silica, magnesia and titania) effects in the selective hydrogenation of carvone employing rhodium as active metal. 

Catalytic Experiments. Activities were performed in a 1 liter Parr reactor. A typical experiment was performed as follows at a temperature of 100 °C, 100 mg of the catalyst and 1.5 /. wt of (-)-carvone (Aldrich) in n-hexane solution (100 ml) were Introduced in a high pressure Parr reactor equipped with mechanical stirring and automatic temperature control. Before introducing the hydrogen the system was purged 2 or 3 times with Nz> The total hydrogen pressure was 21 atm. The reaction products were analysed by gas chromatography. NMR and Mass Spectrometry and identified as unreacted carvone, carvotanacetone, carvomenthone and three carvomenthol stereoisomers (axial-equatorial, equatorial-equatorial and equatorial-axial). 

Dispersion, particle size and activity of Rh catalysts for carvone hydrogenation. 

The results of Table 1, show that the preparation method does not affect the metallic dispersion. However, the catalysts prepared in ammoniacal solution have the lowest activity per site, showing that in carvone hydrogenation an important precursor effect in activity is obtained. 

Nevertheless, in the hydrogenation of poly-unsaturated molecules the catalyst effects are more evident in the selectivity patterns, as is shown in Table 2 and 3. The selectivity behavior for the various catalysts, show that Rh/MgO is the most selective for carvotanacetone formation. The addition is mainly limited, in these catalysts, to one hydrogen molecule, although in carvone there are three possible sites at which reduction can occur. 

Stereoisomers selectivity (%) of carvomenthol of carvone hydrogenation on Rh Catalysts. 

Selectivity (54) for carvone hydrogenation on Rh catalysts prepared from carbonyl clusters. 

The following important conclusions emerge from this study (i) precursor effect is exhibited in carvone hydrogenation activity, (ii) the Rh/MgO catalysts results to be more selective towards carvotanacetone formation than... 

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