Pinane pyrolysis

Hydrogenation of the less expensive a-pinene gives pinanol, which can be oxidised by air under radical conditions to give the hydroperoxide, which is then reduced to 2-pinanol. Pyrolysis of this alcohol gives 

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Although cycloreversion is the dominant route in 2,7,7-trimethylbicyclo[3.1.1]heptane (pinane) pyrolysis, appropriate substituents on the pinane skeleton can in fact retard cycloreversion, thereby allowing formation of more products through carbon bond rupture and hydrogen transfer.100,102,106 In support of this feature it has been found that the pyrolysis of isoverbanone [(lS,2JR,5 )-2,6,6-trimethylbicyclo[3.1.1]heptan-4-one, 10] at 580°C gives, in addition to unreacted starting material, a mixture of five components, from which the major product m-3-methyl-4-isopropenylcyclohexanone (15) was isolated in 20% yield. The other products are 2,6-dimethylocta-2,7-dien-4-one (12 15%) and 5,7-dimethylocta-l,6-dien-3-one... 

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). 

Pyrolysis of the i j -pinane produces dihydromyrcene (24) (citroneUene) as the major product in 50—60% yield. Fractionation of the cmde product then gives an 87 wt % dihydromyrcene (57). Dihydromyrceno1 (25) produced from the dihydromyrcene is becoming increasingly important as a fragrance material. It has excellent stabiUty and has a powerhil, fresh, lime-like aroma. Hydration of citroneUene using formic acid has become an important commercial method for producing dihydromyrcenol (58). 

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%. 

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... 

Pyrolysis of pinane gives dihydromyrcene. Hydration of the more reactive, trisubstituted double bond gives dihydromyrcenol, as shown in Scheme 4.13. Direct hydration is difficult, so usually a two-stage process is used. The first involves the acid-catalysed addition of chloride, sulfate or acetate (from the corresponding acids), followed... 

Hydrogenation of the less expensive major component of turpentine, a-pinene (9.48), gives pinane (9.49) which can be oxidised by air under radical conditions to give the hydroperoxide (9.50) which is then reduced by hydrogenation to 2-pinanol (9.51). Pyrolysis of this alcohol gives linalool (9.13). This sequence of reactions is shown in Figure 9.16. 

Pyrolysis of a-pinene is the starting point for a number of chemicals. The allo-ocimene product can be hydrochlorinated and then hydrolyzed to give a mixture of tertiary alcohols that has a sweet floral odor for use in soaps and cosmetics. Tetrahydro-a//o-ocimenyl acetate is used in citrus-type colognes for men. Similarly, dihydromyrcenol, prepared in several steps from 3,7-dimethyl-l,6-octadiene (the pyrolysis product of c/5-pinane), is used in formulations needing a lime note. 

Linalool can be obtained from pinene extracts by hydrogenation, subsequent oxidation to the respective hydroperoxides, reduction to the respective alcohols and pyrolysis to linalool (Fig. 16.9). Pinene extracts mostly contain mixtures of a- and yS-pinene. After hydrogenation a mixture of cis- and trans-pinane is obtained. 

Hydrogenation of either a-pinene (20) or (3-pinene (31) gives pinane (21) and pyrolysis of this produces citronellene (87). Any optical activity in the pinane is transferred to the citronellene (88). Typically, the pyrolysis is carried out at 550-600°C and the crude product contains 50-60% citronellene, which can then be purified by fractional distillation. 

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