Geraniol oxidation

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). 

Most terpene-based citral (5) produced is based on the catalytic oxidative dehydrogenation of nerol (47) and geraniol (48), or by the Oppenauer oxidation of nerol and geraniol (123—125). 

Oxidations and reductions are amongst the most frequent in situ prechromato-graphic reactions they were exploited as early as 1953 by Miller and Kirchner [9]. They characterized citral as an aldehyde by oxidizing it to geranic acid and reducing it to geraniol. Further examples are listed in Table 10. 

However, nerol and geraniol yield on oxidation exactly the same products. 

On oxidation by weak oxidising agents, citral yields geranic acid, CjjHjgOj on reduction it yields geraniol. 

Hdschle B, D Jendrossek (2005) Utilization of geraniol is dependent on molybdenum in Pseudomonas aeruginosa. evidence for different metabolic routes for oxidation of geraniol and citronellol. Microbiology (UK) 151 2277-2283. 

Figure 1 Selective oxidation of 1-octanol, 2-octanol and geraniol to the...

Table 1 Selective oxidation of geraniol using air as oxidant. Conditions 0.085mol geraniol in toluene, 5wt.% loading of catalyst (dry weight), 60°C, 3bar air, 600rpm, 6h.

Table 3 indicates that 5%Pt,l%Bi/C is active for three reaction cycles in the selective oxidation of the chosen alcohols. For primary alcohols the use of water as solvent can promote the aldehyde to carboxylic acid reaction (3). This effect is observed in the selective oxidation of 1-octanol where octanoic acid is formed with 97% selectivity in the first cycle dropping to 81% in the third. In the selective oxidation of geraniol only citral is observed as the oxidation product. The presence of the double bond stabilises the aldehyde even in the presence of... 

In the classical procedures W, the 5-T or D-labeled mevalonate is converted enzymatically to farnesol, which is then oxidized to famesal by liver alcohol dehydrogenase. This enzyme transfers the pro-R hydrogen of C—1 of ethanol or geraniol (or farnesol) to the 4 pro R position of the nicotinamide ring of NAD. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

Figure 11.2 Oxidation of the monoterpene alcohol geraniol to geranial by CPO in the presence of hydrogen peroxide and absence of halide ions.

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