Isomerization of a-pinene oxide

A major drawback of this process is the pollution of water by zinc halides, which cause severe problems in sludge treatment by killing the bacteria. Although ZnBr2 in benzene is well known as an effective homogeneous catalyst with selectivity of ca 85 % for the campholenic aldehyde, many efforts have been made to find a truly heterogeneous system [32]. 

van Bekkum and coworkers found that heteropolyacids, e. g. H4SiWi204o, are very active catalysts for the rearrangement of (5). With benzene as solvent conversion of (5) was complete after 15 min. Selectivity for (6) was, however, low- 

With heterogeneous catalysts the selectivity depends on adsorption effects. Hdl-derich and coworkers found, that some H-US-Y zeolites with many mesopores are suitable catalysts for this reaction [29]. 

As shown in Table 1, a pretreatment of the mother catalyst (A) with dilute acid (0.01 M HCl at 25 °C for 24 h), subsequent washing and calcination at 550 °C gives a better catalyst (B), resulting in major enhancement of activity but without any loss in selectivity for the desired reaction to campholenic aldehyde. The performance of these catalysts, especially the high selectivities observed, seem to result from Lewis acid sites [33]. 

The different performances of catalysts (A) and (B) was elucidated by characterization of these materials [29]. Al and Si MAS NMR showed that after treatment with 0.01 M HCl most of the amorphous silica material is removed from the parent catalyst (A), leaving extra-framework aluminum species also created by the steaming procedure [29]. This can be readily understood, because the solubility of silica is maximum at pH 2 [34]. It is believed that the silica species blocked most of the catalytically active centers, i. e. the highly dispersed Lewis acidic, extra-framework alumina sites, which seem to be partly bonded to the zeo-litic framework of the starting material (A). The EFA species are not, therefore, leached out. 

It was found that the selectivity and conversion are optimal if the pretreatment of the highly dealuminated Y zeolite is pursued at pH = 2. Lower pH values in the pretreatment of the catalyst A cause a decrease of both conversion and selectivity in the catalytic run. It is well known that the application of strong acid treatment (pH 1) leads to removal of framework as well as extra-framework-alumina from dealuminated Y zeolite.  

To elucidate the reason for the better performance of the acid-treated catalyst (B) we examined samples A and B with various analytical methods such as AAS, FTIR, BET, pyridine adsorption, Si and Al solid NMR investigations. 

The A1 and Si NMR measurements showed that after treatment with 0.01 molar HCl most of the amorphous silica-containing material is removed from the parent catalyst A. This can easily be understood since the maximum solubility of silica is reached at pH = 2. Although the improved performance of the treated catalyst cannot be entirely explained by the removal of less active material, i.e. the increase of the number of Lewis acid sites per mass unit, it is believed that these silica species block most of the catalytically active centres, i.e. the highly dispersed Lewis acidic alumina sites in the micro- and mesopores of the parent US-Y zeolite. 

After complete reaction, the catalyst can be re-used without loss of performance. The catalyst can be successfully reactivated by calcination under an air atmosphere at 550 °C. 

A new process for the heterogeneously-catalysed production of campholenic aldehyde from a-pinene oxide has been found which is competitive with the homogeneous ZnBr2 system and gives yields up to 85%. 

---

Pinocarveol has been prepared by the autoxidation of a-pinene,5 by the oxidation of /S-pinene with lead tetraacetate,6 and by isomerization of a-pinene oxide with diisobutylalumi-num,7 lithium aluminum hydride,8 activated alumina,9 potassium ferf-butoxide in dimethylsulfoxide,10 and lithium diethylamide.11 The present method is preferred for the preparation of pinocarveol, since the others give mixtures of products. It also illustrates a general method for converting 1-methylcy-cloalkene oxides into the corresponding exocyclic methylene alcohols.11 The reaction is easy to perform, and the yields are generally high. 

Selective Isomerization of a-Pinene Oxide with Heterogeneous Catalysts... 

Isomerization of a-pinene oxide to campholenic aldehyde is an important and complicated reaction. Side products like those described in Scheme 8.19 can be produced together with the product of interest. Working with a Zn(0Tf)2/Si02 catalyst calcined under N2 at 200°C prior to use, the selectivity was increased to 65% [97]. This increase in selectivity was attributed to loss of Brpnsted acid sites by dehydration of the catalyst surface, which, in turn, reduced the amount of side reactions. Further calcination at 400°C decreased the activity of the catalyst due to decomposition of Zn(OTf)2. 

Acid-catalysed rearrangement of epoxides is another widely used reaction in the fine chemicals industry. Here again the use of solid acid catalysts such as zeolites is proving advantageous. Two examples are shown in Fig. 2.25 the isomerization of rsophorone oxide (Elings et al., 1997) and the conversion of a-pinene oxide to campholenic aldehyde (Holderich et al., 1997 Kunkeler etal., 1998). Both products are fragrance intermediates. 

Alkaline earth metal oxides are active catalysts for double bond isomerization. For example, SrO exhibits high activity and selectivity for the isomerization of a-pinene to /1-pinene 110). MgO and CaO have excellent activities for isomerization of 1-butene and 1,4-pentadiene and, particularly, for isomerization of compounds containing heteroatoms, such as allylamine or 2-propenyl ethers 111-115). Recently... 

High amounts of 24 were also obtained by the use of H-US-Y (96) followed by H-US-Y (96)-HCl. The H-US-Y (96)-HCl used, a modified highly dealuminated ultrastable Y zeolite, was pretreated with diluted acid according to the method described before (28, 31). This zeolitic catalyst, unlike many others, remains active at lower temperatures and also at high loading, as was previously demonstrated in the isomerization of alpha-pinene oxide using this heterogeneous catalyst (28, 31). 

Epoxides can be isomerized to carbonyl compounds. Industrial examples include the isomerization of styrene oxide to phenylacctaldehyde and that of a-pinene oxide to campholenic aldehyde. As Ti-containing zeolites as TS-1 [77J and the Ti-Beta [78] are good catalysts for these isomerizations it is tempting to combine the epoxidation and the isomerization step. Several substituted styrenes have been subjected [79[ to such a two-step one-pot procedure. 

Figure 4. Isomerization products from the acid-catalyzed conversion of a-pinene oxide.

The acid catalysed isomerization of a-pinene proceeds via two types of reactions, one giving bi- and tricyclic products such as camphene, p-pinene, tricyclene, and bornylene and the other giving rise to monocyclic compounds such as dipentene, terpinolene, a-terpinene, y-terpinene and p-cymene [1]. Over solid catalysts such as clays, mineral oxides and inorganic salts,the main product is camphene [2], of particular interest as an intermediate in the synthesis of camphor. Camphor is of value due to its aroma and pharmaceutical properties. 

The production of camphene is usually carried out by isomerization of a-pinene over titanium oxide catalysts [3]. These are prepared by treating titanium oxide with an acid in order to obtain a layer of titanic acid on the surface of the oxide [4]. The reaction was reported as showing zero order [4] or a transition from first order at low conversion to zero order above ca. 30% conversion [5]. 

In view of optimizing the production of camphene it is of interest to have comprehensive kinetic data and models for the isomerization of a-pinene over titanium oxide. The catalyst is previously treated with sulfuric acid. It is of interest to know the effect on catalyst activity and selectivity of varying (i) the amount of sulfuric acid in relation to the amount of oxide (ii) the amount of catalyst in relation to the amount of pinene (iii) the time of catalyst activation (iv) the temperature of catalyst activation. 

Another important use of a-pinene is the hydrogenation to i j -pinane (21). One use of the i j -pinane is based on oxidation to cis- and /n j -pinane hydroperoxide and their subsequent catalytic reduction to cis- and /n j -pinanol (22 and 23) in about an 80 20 ratio (53,54). Pyrolysis of the i j -pinanol is an important route to linalool overall the yield of linalool (3) from a-pinene is about 30%. Linalool can be readily isomerized to nerol and geraniol using an ortho vanadate catalyst (55). Because the isomerization is an equiUbrium process, use of borate esters in the process improves the yield of nerol and geraniol to as high as 90% (56). 

Finding a modei Fig, 1 shows experimental data of a-pinene isomerization at 150° C catalysed by 4% of titanium oxide catalyst (conditions 15 % H2SO4, activation temperature, 135 C activation time, 4 hrs). In this figure the concentrations of some species are lumped together in the way described below. 

The isomerization of styrene oxide to phenylacetaldehyde yields 100% using modified ZSM-5 zeolites, thereby the highest target achieved by catalysis has been fulfilled. A new process as well has been found for the heterogeneously-catalysed production of campholenic aldehyde from a-pinene oxide. By using low reaction temperatures of 0 °C and below in combination with HCl-treated H-US-Y zeolites, up to 85% yield is achieved. This process is competitive with the homogeneous ZnBr2 system. 

In [138] a number of siliea- and alumina-supported Ln203 samples are investigated by means of TPD of ehemisorbed CO2 and pyridine, FTIR of adsorbed pyridine, as well as eatalytie assays of a-pinene isomerization and 2-butanol decomposition. Particular attention is paid to the supported ytterbia systems. In good agreement with the results commented on above, supported rare earth oxide systems... 

Bark beetles of the genus Ips are pests which attack pine and spruce trees. They use ipsdienols as aggregation pheromones, Ips confusus emitting the (5)-(-l-)-, and Ips paraconfusus the (.K)-(-)-enantiomer The beetles receive the myrcenes (section 2.2) occurring in conifers with their food and metabolize them to ipsdienols some evidence for de-novo synthesis by the bugs is also reported. In order to catch the beetles, pheromone traps are supplied with both enantiomers of ipsdienol which are produced from (-l-)-verbenone, a constituent of the Spanish verbena oil (section 2.4.3). This terpenone, also available by oxidation of a-pinene, is isomerized to the enantiomers of 2(10)-pinen-4-one via three steps (reduction, protonation, oxidation). After separation, both enantiomers are reduced by lithiumaluminumhydride. Pyrolytic cycloreversion of the resulting diastereomeric 2(10)-pinen-4-ols provides the enantiomers of ipsdienol... 

An important advantage of hydroboration-oxidation for hydration of alkenes is that rearrangements of the carbon skeleton do not occur, whereas they do in acid-catalyzed hydration. For example, acid-catalyzed hydration of (+)-a-pinene (65) proceeds with skeletal rearrangements to produce a number of isomeric alcohols. 

By far, the most important use of a-pinene is as a feedstock for production of other terpenoids and a wide variety of fragrance ingredients. Some of the more important conversions are shown in Fig. 8.8. a-Pinene undergoes thermal isomerization to ocimene and alloocimene, acid-catalyzed isomerization to camphene, hydration to pine oil/terpineol, and polymerization to terpene resins. Its epoxide is a useful intermediate and hydrogenation with subsequent oxidation leads on to the rose alcohols linalool, nerol, and geraniol. 

Another important process for linalool manufacture is the pyrolysis of i j -pinanol, which is produced from a-pinene. The a-pinene is hydrogenated to (73 -pinane, which is then oxidized to cis- and /n j -pinane hydroperoxide. Catalytic reduction of the hydroperoxides gives cis- and /n j -pinanol, which are then fractionally distilled subsequendy the i j -pinanol is thermally isomerized to linalool. Overall, the yield of linalool from a-pinene is estimated to be about 30%. 

---