Geraniol, allylic oxidation

Fretz, H., Woggon, W-D. and Voges, R. (1989) The allylic oxidation of geraniol catalysed by cytochrome P450 proceeding with retention of configuration. Helv. Chim. Acta, 72, 391-400. 

The first step in the biosynthesis is an allylic oxidation of geraniol (6) to give 10-hydroxygeraniol (52), which is then further oxidized to 53. This dialdehyde serves as the substrate for the key cyclization step to iridodial (54). Mechanistically this reaction is supposed to proceed via a substrate-enzyme iminium species (please see the mechanism in Scheme 6.11), a type of activation that we will see... 

Allylic P-unsaturated,alcohols (geraniol, 3-phenyl-2-propenol) were efficiently oxidised to the aldehydes by fran.y-Ru(0)2(TMP)/(lutidine-iV-oxide)/CgHg [586]. As fran.y-Ru(0)2(TMP)/(Cl2pyN0)/CgHy24h it oxidised cyclohexanol to cyclohexanone [587]. 

It is noteworthy that allylic alcohols are oxidized to products with retained configuration of the olefinic bond. Geraniol and nerol were oxidized to the corresponding ( )-and (Z)-a-enals, respectively. As expected, primary alcohols were oxidized faster than secondary ones with the RuCl2(PPh3)3/BTSP system with relative rates from 20-40 1. The new system was applicable for the selective oxidation of primary-secondary diols... 

Oxovanadium(V) and oxomolybdenum(VI) were incorporated into crosslinked polystyrene resins functionalized with iminodiacetic acid or diethylenetriamine derivatives 921 The polymer complexes were used as catalysts in the oxidation of olefins with f-butylhydroperoxide. Vanadium(V) complexes promote the epoxidation of allylic alcohols in a highly regioselective manner, e.g., 2,3-epoxide was obtained in 98 % selectivity from e-geraniol at 80 °C. The catalytic activity of the vanadium(V) complexes is generally higher than that of the molybdenium(VI) complexes in the oxidation of allylic alcohols, whereas an opposed trend holds for the epoxidation of cyclohexene. 

For the first time, catalytic activity of lanthanide isopropoxides was detected in the TBHP (tert-butylhydroperoxide) assisted oxidation of allylic alcohols to epoxyalcohols (Eq. 26) [231], For example epoxy geraniol was obtained in up to 96% yield by using YbfOiPr in the presence of molecular sieves (4 A) [232]. 

Such a simple mechanistic proposal accomodated the observation that highly activated, benzylic alcohols were good substrates due to the enhanced lability of their a-hydrogen atoms. In contrast, aliphatic alcohols are far less reactive towards H-radical abstraction and, accordingly, poor conversions should ensue. However, it was rather disturbing to note that allylic alcohols, such as geraniol and nerol, displayed poor reactivity in this system. Furthermore, it was observed that the aerobic oxidation of aliphatic alcohols invariably resulted in the rapid formation of a green copper(II) salt, with concomitant deactivation of the catalyst. 

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