Limonene metallation

Towards the end of this section it may be worthwhile to point out some new reactions with high-valent metals and TBHP. The first is a pyridinium dichromate PDC-TBHP system134. Nonsubstituted or alkyl-substituted conjugated dienes, such as 1,3-cyclooctadiene (87) and others (also linear dienes), yield keto allyl peroxides 88 (equation 18), whereas phenyl-substituted dienes such as 1,4-diphenylbutadiene (89) gave diketo compounds, 90 (equation 19). In further research into a GIF-type system135 with iron and TBHP, limonene gave a mixture of products with carvone as the major product. The mechanism is thought to proceed initially by formation of a Fe(V)-carbon... 

Both uncalcined and calcined LDHs have also been shown to be effective supports for noble metal catalysts [18-25]. For example, palladium supported on Cu/Mg/Al LDHs has been used in the liquid phase oxidation of limonene [24], and on calcined Mg/Al LDHs for the one-pot synthesis of 4-methyl-2-pentanone (methyl isobutyl ketone) from acetone and hydrogen at atmospheric pressure [25]. In the latter case, the performance depends on the interplay between the acid-base and hydrogenation properties. More recently. 

The volatility of ethyl acetate requires minimal exposure when preparing standards. The same is true for limonene. Limonene should be stored in a metal canister, and the headspace should be flushed with nitrogen after usage. 

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). 

Tanabe el al. studied in detail the catalytic action and properties of metal sulfates most of the sulfates showed the maximum acidity and activity by calcination at temperatures below 500°C, with respect to the surface acidity and the acid-catalyzed reaction (118, 119). Other acid-catalyzed reactions were studied with the FeS04 catalyst together with measurement of the surface acidity of the catalyst the substance calcined at 700°C showed the maximum acidity at Ho s 1.5 and proved to be the most active for the polymerization of isobutyl vinyl ether, the isomerization of d-limonene oxide, and the dehydration of 2-propanol (120-122). It is of interest that the catalyst calcined at a slightly higher temperature, 750°C, was completely inactive and zero in acidity in spite of the remarkable activity and acidity when heat treated at 700°C. 

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. 

The selectivity pattern of d° transition metal catalyzed epoxidations is much less readily understood. In the molybdenum-catalyzed epoxidation of (S)-limonene (Table 2, entries 1 and 2) the cis selectivity could perhaps be explained by a directing effect due to -coordinating of the second double bond to molybdenum. Such a selectivity is completely missing in the analogous tungsten-catalyzed reaction of (S)-limonene (Table 2, entry 4) in the absence of a second double bond as, for example, in 3/(-acetoxy-5-cholestene (Table 4) reactions with both metals afford similar diastereomeric ratios. 

Metalation of limoaeae. Crawford el a . report that limonene (1) undergoes selective metalation at C,o on treatment with the 1 1 complex of n-butyllithium and TMEDA to afford the 2-substituted allyllithium species represented by (2). The metalation is carried out by allowing a mixture of 2 eq. of limonene and 1 eq. of the complex... 

This new selective metalation reaction is useful for synthesis of 10-substituted limo-nene derivatives. The excess limonene is readily separated from the product by fractional di.stillation. Thus metalation of (-i-)-limonene (3) followed by reaction with carbon dioxide and then esterification affords the 3,v-unsaturated ester (5) in 19% yield. Isomerization with sodium methoxide in methanol converts (5) into the conjugated isomer (6). 

This selective metalation has been used for the synthesis of several bisabolane sesquiterpenes. Thus the sesquiterpene (- )-J -bisabolene (7) can be synthesized in one step from metalated ( —)-limonene by reaction with I-bromo-3-methyl-2-butene. Two... 

Crawford has reported a synthesis of the monocyclic diterpenc nr-artcmiscnc (15), in which selective metalation is used in two of (he four steps. Thus ()-limonene (3) is converted into the metalated derivative (9) reaction of (9) with paraformaldehyde... 

Metal atoms such as lithium, and first row transition metal atoms, have been studied for deoxygenation of epoxides. Limonene oxide is converted in high yield to the corresponding alkene upon treatment with lithium and biphenyl in DME (equation 44). ... 

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. 

Metalation can be conducted with both ( + )-(R)-limonene (3) and ( —)-(S)-limonene (4), both of which are commercially available. 

Besides pyridine-containing polystyrene and pol)q5ropylene resins, polybenzimidazole has been employed as support for nickel(II) acetylacetonate [94]. The nickel-loaded polymer was shown to be an efficient catalyst for the epoxidation of (S)-(—)-limonene, a-pinene, and 1-octene using isobutyraldehyde/02 as coreac-tant/oxidant. However, significant metal leaching from the support associated with a loss of activity upon recycling was reported. It was shown that the reaction is heterogeneously catalyzed, and leached metal species did not contribute to the catal)d ic activity. 

A simple access to 9-oxygenated menthanes (which ought to have appeared in Vol. 4) is by oxidation of the anion 607, obtained by metalation of limonene with butyllithium in V,V,V, V -tetramethylethylenediamine, when optically active l,8-menthadien-9-ol (608) was obtained. An alternative involved addition of phenyl sulfide to 607, followed treatment with trimethyl phosphite in methanol. 

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