Limonene, selective hydrogenation

Bregeault and co-workers have reported supporting [HP04 W0(02)2 2]2 species on resins and silica (Table 4.6).64 Amberlyst A26 was the macro-reticular resin used. The PW2 species was supported onto dehydrated porous silica. The catalysts were found to be highly selective for the epoxidation of limonene by hydrogen peroxide. 

However, in this section, the total synthesis of yingzhaosu A, the lead compound of a particular class of antimalarial 1,2-dioxocins, is reported. The synthesis involves eight steps and a 7.3% overall yield starting from (A)-limonene (Scheme 64). Besides the TOCO procedure that allowed the formation of five bonds in one step, the most intriguing steps involved the selective hydrogenation of a C-C double bond in the presence of a peroxide and an aldehyde functionalities (step vi) and the stereoselective reduction of the side-chain carbonyl with (R)-CBS catalyst (step viii). Last but not least, the old classical fractional recrystallization allowed the separation of yingzhaosu A from its C-14 epimer and saved two synthetic steps <2005JOC3618>. 

The selective hydrogenation of the disubstituted double bond of limonene (52) took place over a platinum catalyst at 60°C and 3 atmospheres pressure (Eqn. 15.33). 5 1,5-Undecadiene was hydrogenated to 5-undecene with 78% selectivity at 97% conversion over a Pt/A 2eolite catalyst that was treated with diphenyldiethoxysilane. 9... 

Like (- -)-3-carene, (+)-limonene (11) is a readily available natural homochiral feedstock and so, in principle, could be a useful starting material for ( )-menthol synthesis. At least one route has been reported [206, 230]. As shown in Fig. 8.37, the synthesis starts with selective hydrogenation of the disubstituted double bond and epoxidation of the trisubstituted one to give the epoxide (181). Hydrolysis and selective acylation of the... 

Which of the double bonds of limonene is hydrogenated at the faster rate Comment on the likelihood that selective hydrogenation may occur. 

Several generalities can be formulated regarding selective reduction of polyolefins. Usually the least hindered double bond is hydrogenated pre ferentially (123), and, with steric hindrance about equal, the most strained bond will be reduced first. Exocyclic olefins are reduced more easily than those in the ring (R)-(+ )-Limonene, 190 g, was shaken with W-4 Raney nickel (12 g) under hydrogen at atmospheric pressure. After 31.9 1 of hydrogen had been absorbed, the solution was filtered. Essentially, pure (R)-( -i- )-carvomenthene was obtained in 96% yield (58). 

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]. 

Metal-assisted reductions with NaBFLt can be used to hydrogenate various functional groups41,42. The Co2+-NaBH4 system selectively reduces limonene at the less substituted double bond300 though W-4 Raney Ni proved to be more effective301 (equation 21). 

Extension of the above oxidation studies to alkenes such as limonene gave a complex mixture of products that resulted from all possible ene reactions to the trisubstituted double bond (Fig. 30) [165], However, use of NaY zeolite as the microreactor and in the presence of a small amount of pyridine, the photosensitized oxidation of the alkenes is regioselective, yielding only the cis and trans products that result from hydrogen abstraction from the least hindered allylic carbon center. These studies illustrated that a microreactor can provide unprecedented opportunities to conduct selective oxidation of olefins. 

Myrcene and a-terpinene contain conjugated double bonds and are not as reactive as limonene and a-pinene. The product mixture is complex besides the hydrogenated products and the alcohols, undefined high boiling point products also occur [42]. The two conjugated terpenes were also studied with rhodium phosphine catalysts. Within 7 h, 96% of a-terpinene reacts to aldehydes with high selectivity for the product shown in Scheme 13 [43]. 

Catalytic hydrogenation can be used for the selective reduction of a carbon-carbon double bond in the presence of other functional groups such as a carbonyl group or an aromatic ring. Selective reduction of one double bond in (R)-limonene (6.7), which contains two double bonds, gives (R)-carvomenthene (6.8) by hydrogenation over Ni metal. 

There are some interesting examples of selective ozonolysis in the terpene field. Limonene is ozonized at the 8,9- double bond in preference to the 1- double bond. This is indicated by the fact that the amount of formaldehyde found is almost equal to the amount of ozone introduced, up to 1 mole. In like manner, terpinolene yields acetone in an amount nearly equal to the ozone passed, up to 1 mole. -Pinene should add ozone readily and form formaldehyde and nopinone on ozonolysis. Practically none of these products can be obtained by ordinary ozonolysis techniques. The hydrogens alpha to the double bond, with probable additional activation from the general strain of the system, are so active to peroxidation by oxygen that little ozonide is formed, because of the large excess of oxygen present. 

Acid zeolite catalysts offer a very good alternative for the clean synthesis of these sulfur-containing substances. A suitable feedstock is the 4-isopropenyl-l-methyl-1-cyclohexene. In the presence of a commercial P-zeolite (25) hydrogen sulfide is added to the autoclave at a reaction temperature 50 °C at a pressure of 17 bar. The conversion of limonene is 65.1% and the selectivity to 1-p-menthene-8-thiol is 43.9%. These are very promising results and they can be improved by using a commercial H-US-Y zeolite which rendered a conversion of 76.8% and a selectivity of 64.3%. ... 

The ease of reduction of an alkene decreases with the degree of substitution of the double bond, and this sometimes allows selective reduction of one double bond in a molecule which contains several. For example, limonene 7 can be converted into p-menthene (by reduction of the terminal alkene) in almost quantitative yield by hydrogenation over platinum oxide if the reaction is stopped after absorption of one molar equivalent of hydrogen. In contrast, the isomeric diene 8, in which both double bonds are disubstituted, gives only the completely reduced product (7.7). 

An additional methodology for the selective reduction of unsaturated acyclic and cyclic carbonyl compounds is composed of refluxing for 15-45 minutes a mixture of limonene and the enone substrate in the presence of 10% Pd/C. For example, the reduction of /3-octalone afforded the cis isomer in 83% selectivity, which is comparable to the results obtained with hydrogen (Scheme 3 and 5) and ammonium formate (Scheme 19). The high yields and selectivity as well as the no need for an acid or basic medium makes this method very convenient. 

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