Oxidations in Monoterpene Chemistry

Linalool and citral are important building blocks for the synthesis of isoprenoid natural products, e.g. vitamins A, E and K, carotenoids and a broad variety of fragrances. In view of changing starting materials in future decades due to limited amounts of fossil resomces, pinenes might be interesting somces for the fine chemical industry to produce linalool via pinane-2-hydroperoxide. The synthesis of linalool and citral from natural somces is discussed herein. 

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. 

Pinanes were oxidised by applying a gas flow of air or oxygen in the presence of a catalyst, or without a catalyst. In both cases cis-pinane is more reactive than trans-pinane because of the steric effect of the gem-dimethyl group, which hinders the free radical attack on the tertiary C-H bond in the 2-position. 

The catalyst applied was Co(OAc)2/Mn(OAc)2/NH4Br (12.8/1.5/5.0 mol%). The authors oxidised a mixture of cis- and trans-pinane and needed only one catalytic step from pinane to pinanol. The formation of cis-pinanol is favoured over trans-pinanol because the attack of molecular oxygen at the 2-pinanyl radical occurs trans to the gem-dimethyl group due to steric reasons. The best conditions gave 17% conversion and selectivity of 54% to cis-pinanol and 17% to trans-pinanol. Without catalyst and applying the same reaction conditions 17% conversion and selectivity of 52% to cis-pinanol and 10% to trans-pinanol were obtained. The difference in cis/trans selectivity is probably due to the interaction of the transition metal with the 2-pinanyl radical. 

When Co(OAc)2 or Mn(OAc)2 were used alone as catalysts and less oxygen was applied (instead of pure oxygen, air was appUed and a solvent with a lower oxygen dissolution capacity was used) the reaction stopped at pinane-2-hydroperoxide. The authors used enriched cis-pinane ( 96%) and reported 37% as the highest yield to pinane-2-hydroperoxide. 

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