Pinene pyrolysis

The two main disadvantages of this route are the cost of -pinene and the presence of trace amounts of chlorinated materials which must be removed from the product. 

---

Synthesis from (3-Pinene. Pyrolysis of /3-pinene yields myrcene, which is converted into a mixture of predominantly geranyl, neryl, and linalyl chloride by addition of hydrogen chloride in the presence of small amounts of catalyst, e.g., copper(I) chloride and an organic quaternary ammonium salt [29]. After removal of the catalyst, the mixture is reacted with sodium acetate in the presence of a nitrogen base (e.g., triethylamine) and converted to geranyl acetate, neryl acetate, and a small amount of linalyl acetate [30]. 

Figure 6.9.2. Result for a Py-GC/MS analysis of poly(a-pinene). Pyrolysis done from 0.4 mg sample, at 600" C in He, with the separation on a Carbowax type column.

A process of polymerization of isomerized a-pinene or turpentine with vinylbenzenes has been disclosed (105). a-Pinene or turpentine is isomerized by flash pyrolysis at 518 5° C in a hot tube reactor to yield a mixture of predominantly dipentene and i7t-alloocimene... 

In several important cases, new synthetic strategies have been developed into new production schemes. An outstanding example of this is the production of an entire family of terpene derivatives from a-pinene (29), the major component of most turpentines, via linalool (3) (12). Many of these materials had been produced from P-pinene, a lesser component of turpentine, via pyrolysis to myrcene and further chemical processing. The newer method offers greater manufacturing dexibiUty and better economics, and is environmentally friendly in that catalytic air oxidation is used to introduce functionality. 

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). 

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). 

Uses ndReactions. Some of the principal uses for P-pinene are for manufacturing terpene resins and for thermal isomerization (pyrolysis) to myrcene. The resins are made by Lewis acid (usuaUy AlCl ) polymerization of P-pinene, either as a homopolymer or as a copolymer with other terpenes such as limonene. P-Pinene polymerizes much easier than a-pinene and the resins are usehil in pressure-sensitive adhesives, hot-melt adhesives and coatings, and elastomeric sealants. One of the first syntheses of a new fragrance chemical from turpentine sources used formaldehyde with P-pinene in a Prins reaction to produce the alcohol, Nopol (26) (59). 

Dihydromyrcene Manufacture. 2,6-Dimethyl-2,7-octadiene, commonly known as dihydromyrcene (24) or citroneUene, is produced by the pyrolysis of pinane, which can be made by hydrogenation of a- or P-pinene (101). If the pinene starting material is optically active, the product is also optically active (102). The typical temperature for pyrolysis is about 550—600°C and the cmde product contains about 50—60% citroneUene. Efficient fractional distUlation is requited to produce an 87—90% citroneUene product. 

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%. 

Pyrolysis of -pinene results in the triene myrcene, which leads to menthol and its derivatives, on one hand, and the rose alcohols and citral-related chemicals, on the other hand. 

A great deal is already known about the pyrolysis of pinenes," which constitutes a perfect case for the study of cyclobutane cycloreversion reactions. In practice, this avenue was first explored with the hope of obtaining products with commercial value.99 Unfortunately, the application of these reactions to organic synthesis is somewhat restricted, because complex product mixtures cause complications. For the sake of clarity Table 6100 110 outlines only the cycloreversion products and their straightforward secondary derivatives nevertheless, it demonstrates some of the synthetic uses of these thermal cleavage reactions. 

The pyrolysis of pinenes is mechanistically similar to that of cyclobutane, giving initially a 1,4-diradical. Subsequent C-C bond fission of this 1,4-diradical thus generates a diene. The stepwise cycloreversion of 7,7-dimethylbicyclo[3.1.1]heptan-2-one (20) at 600 °C is a good example.100 106... 

Myrcene Manufacture. An important commercial source for mycene is its manufacture by pyrolysis of p-pinene at 550—600°C (87). The thermal isomerization produces a mixture of about 75—77 wt % myrcene, 9% limonene, a small amount of F-limonene [499-97-8] and some decomposition products and dimers. The cmde mixture is usually used without purification for the production of the important alcohols nerol and geraniol. Myrcene may be purified by distillation but every precaution must be taken to prevent polymerization. The use of inhibitors and distillation at reduced pressures and moderate temperatures is recommended. Storage or shipment of myrcene in any purity should also include the addition of a polymerization inhibitor. 

In all examples of the palladium-catalyzed telomerization discussed up till now, the nucleophile (telogen) can be considered renewable. The taxogens used (butadiene, isoprene), however, are still obtained from petrochemical resources, although butadiene could, in principle, also be obtained from renewable resources via the Lebedev process that converts (bio)-ethanol into 1,3-butadiene. Limited attention has been given in this respect to the great family of terpenes, as they provide direct access to renewable dienes for telomerization. In particular, those terpenes industrially available, which are derived mostly from turpentine, form an attractive group of substrates. Behr et al. recently used the renewable 1,3-diene myrcene in the telomerization with diethylamine, for instance [18]. The monoterpene myrcene is easily obtained from (3-pinene, sourced from the crude resin of pines, by pyrolysis, and is currently already used in many different applications. 

Terpenes are obtained either by processing wood in the kraft process in paper production or by collecting resins and turpentine from conifers. The scale of produced terpenoids in comparison with fats and oils is small. Applications for terpenes are in pure form as solvents, as odorous substances, or in dyes. Most terpenoids contain double bonds which are readily available to perform chemical reactions. A widespread component of turpentine is a-pinene, from which many fragrances are produced. A further often-used resource is myrcene, which is obtained by pyrolysis of (3-pinene. Myrcene is an important base chemical to produce, for example, the fragrances nerol and geraniol [7]. 

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... 

Another impressive use of silyl reagents came in the sequence leading to 8-terpineol (167).317 Homologation of 5-methylcyclohex-2-en-l-one catalysed by Fe° complexes led (after appropriate functionalization) to menthone and iso-menthone,318 whereas Pdn-catalysed coupling of 2-bromopropene with 1-methyl-cyclohexa-1,3-diene gave / -mentha-1,3,8-triene directly.319 Treatment of a-pinene with Bza02 and Cu1 salts gave the benzyl derivative of franj-carveol, which could be converted into carvone in 46% overall yield 320 the phenylurethane of pinol formed (168) on pyrolysis.321... 

Hydrogenation of the less expensive a-pinene gives pinane, which can be oxidized by air under radical conditions to give the hydroperoxide, which is then reduced to pinan-2-ol. Pyrolysis of this alcohol gives linalool, as shown in Scheme 4.3. This process is operated by Glidco at their plant in Georgia in the USA. 

The other major producer of synthetic L-menthol is the Japanese company Takasago. They produce about 1000 tonnes per annum using elegant chemistry developed by Noyori (Scheme 4.22). Pyrolysis of / -pinene gives myrcene, to which diethylamine can be added in the presence of a catalytic amount of strong base. This produces N,N-diethylgeranylamine. Isomerization of this with the rhodium 2,2 -(diphenylphosphino)-1,1-binaphthyl (BINAP) complex produces the enamine of citronellal. The elegance of this route stems from the fact... 

---