Isomerization of -a-pinene

Acid-cataly2ed isomerization of a-pinene is carried out by heating with catalysts or other activated clays. Camphene (13) and tricydene... 

Fig. 2. Acid treatment and thermal isomerization of a-pinene where R = CH. The dipentene (15) is 4 -lhnonene (18) is a-pyronene (19) is P-pyronene.

Thermal isomerization of a-pinene, usually at about 450°C, gives a mixture of equal amounts of dipentene (15) and aHoocimene (16) (49,50). Ocimene (17) is produced initially but is unstable and rearranges to aHoocimene, which is subject to cyclization at higher temperatures to produce a- and P-pyronenes (18 and 19). The pyrolysis conditions are usually optimized to give the maximum amount of aHoocimene. Ocimenes can be produced by a technique using shorter contact time and rapid quenching or steam dilution (51). 

ZSM-5 Isomerization of a-pinene Higher activity and yield to products [73]... 

Isomerization of a-pinene epoxide to campholenic aldehyde, an intermediate for perfumery chemicals, has been carried out elegantly with ultra stable Y-zeolite. 

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... 

Acid-catalyzed isomerization of a-pinene is carried out by heating with Ti02 catalysts or other activated days. Camphene (13) and tricydene [508-32-7] (14) are the products obtained in a 4 1 equilibrium mixture in about an 80% yidd (Fig. 2). Tricydene undeigoes reactions identical to those of camphene therefore, the crude material is often used for producing other products. 

Observing a process, scientists and engineers frequently record several variables. For example, (ref. 20) presents concentrations of all species for the thermal isomerization of a-pinene at different time points. These species are ct-pinene (yj), dipentene ( 2) allo-ocimene ( 3), pyronene (y ) and a dimer product (y5). The data are reproduced in Table 1.3. In (ref. 20) a reaction scheme has also been proposed to describe the kinetics of the process. Several years later Box at al. (ref. 21) tried to estimate the rate coefficients of this kinetic model by their multiresponse estimation procedure that will be discussed in Section 3.6. They run into difficulty and realized that the data in Table 1.3 are not independent. There are two kinds of dependencies that may trouble parameter estimation ... 

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. 

Rate constants and activation energies for liquid- and gas-phase isomerization of a-pinene have been determined.310 The activity of metal sulphate monohydrates in isomerizing a-pinene is correlated with the strength of co-ordination of the water of crystallization to the metal ion.3" Pyrolysis of chrysanthanol acetate (217 R = Ac) gives citronellal and the (E)- and (Z)-3,7-dimethylocta-l, 6-dien-l -ol acetates in 20, 28, and 3% yields respectively formation of the enol acetates is consistent with a biradical or a concerted pathway.312 Further work directed towards C-l—C-7 bond pyrolysis of pinane derivatives shows C-l—C-7 C-l—C-6 bond cleavage ratios of 4 51 for (217 R = Ac), 13 22 for (217 R = H), 6 7 for (218 R = H), and 43 35 for (218 R = Me) the expected acyclic and cyclic alcohol, aldehyde, and ketone pyrolysis products are obtained.313 The ene reaction between /3-pinene and methyl... 

The extra-framework aluminium species (EFAL) generated by hydrothermal treatment of zeolite Y have a strong effect on catalytic activity and selectivity upon cracking, isomerization and alkylation reactions of hydrocarbons [14-17]. The effect of EFAL species on the liquid phase isomerization of a-pinene has also been studied [18]. 

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

By far the greatest amount of work in this area has been eentered on the commercially important transformation of a-pinene (1) into, primarily, eamphene (2) and limonene (3) (Scheme 1). The first use of a heterogeneous catalyst for the isomerization of a-pinene was reported by Gurvieh in 1915 [2] and many forms of solid-acid catalyst have subsequently been studied in relation to this reaction. We shall study just a few of these. 

Isomerization of a-pinene over Ti02 kinetics and catalyst optimization... 

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. 

Fio. 10. Isomerization of a-pinene to camphene at 150°. Catalyst NiS04 pinene reacted O camphene produced x acidity at pK = - -3.3. 

In contrast, when (—)-a-pinene was reacted in the presence of an unmodified Rh catalyst, mainly (-l-)-3-formyl-pinane was obtained [69-71]. In addition, two diastereomers of 10-formyl-pinane were formed as a result of the isomerization of a-pinene to the more reactive fl-pinene. When the catalyst was modified with P(O-0-fBuPh)3 as Ugand, the reaction could be run under much milder conditions (80 bar, 100 "C) [72]. 3-Formyl-pinane was obtained as a main aldehyde with almost 100% stereoselectivity for the trans isomer and a mixture of trans- and cis-lO-formylpinanes (about 1 1). When the reaction was conducted in ethanol... 

A study of the preparation of ocimene by isomerization of a-pinene has been reported. a-Azidoacetophenones have been converted to 2-aryl-l,3-oxazole-4-carbaldehydes by rearrangement of the carbon framework on exposure to DMF/POCI3. The rearrangement occurs via alkenyl azides and 2//-azirines. Enamine rearrangement of pyridinium salts to indoles has been studied.Kinetic stabilization and reactivity of a series of singlet 2,2-dialkoxy-l,3-diphenyloctahydropentalene-l,3-diyls enable cis-selective formation of the ring-closed compounds (Scheme 105). ... 

Perhaps the most technically sophisticated area of turpentine utilization is in the small (10%-15% of turpentine consumption) but growing production of flavor and fragrance chemicals. Although small amounts of turpentine components are directly used in flavors and fragrances, most of the turpentine components are converted to other chemicals. Of the turpentine components, )8-pinene is the most important starting material. Its importance is indicated by the development of a )ff-pinene production method based on isomerization of a-pinene the )ff-pinene is recovered from a continuously recycled process steam that has an equilibrium concentration of 95% a-pinene/5% )ff-pinene. 

Usually isomerization proceeds more cleanly over solid bases than over solid acids because of the absence of polymerization for the former. The turnover frequency is probably greater over solid bases. For example, the isomerization of a-pinene and related cyclic compounds having exo-cyclic double bonds efficiently catalyzed by solid bases such as SrO and MgO. ... 

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. 

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