Eugenol-isoeugenol isomerization

Synthesis from Eugenol. The sodium or potassium salt of eugenol is isomerized to isoeugenol by heating. Isomerization can also be carried out catalytically in the presence of ruthenium [164] or rhodium [165] compounds. 

Since isoeugenol is the important intermediate in the vanillin production from eugenol, the isomerization of eugenol attracted a considerable amount of attention. The first report on the isomerization reaction appeared in 1891, in which Tiemann reported 50% conversion in 24 hours using an ethanolic KOH solution at 80°C [13]. 

Eugenol is a natural product available from a variety of essential oils (cinnamon-tree or pimentos leaves). Its isomerization (Eq. 49) into isoeugenol, the starting material for synthetic vanillin, is rather difficult and proceeds in modest yields under relatively harsh conditions. It can, however, be very efficiently prepared by use of 2.2 molar equivalents of base and catalytic (5 %) amounts of Aliquat in the absence of solvent. 

Eugenol undergoes MW-assisted isomerization to isoeugenol under solvent-free condition in the presence of potassium tert-butoxide, t-BuOK, and a catalytic amount of phase transfer reagent [34]. 

Keywords eugenol KOH, phase transfer catalyst, microwave irradiation, isomerization, isoeugenol... 

Another occasionally useful transformation of allylphenols is the isomerization to propenylphenols by strong alkali, as in the well-known isomerization of eugenol to isoeugenol. For example, 2-methoxy-6-allylphenol (LXXI) is changed to the propenyl compound LXXII by... 

Peterson et al (1933) applied this method to study the kinetic of eugenol isomerization by chromatographic and spectroscopic techniques [14]. Lampman et al (1977) used the same alkaline solution method run in dimethyl sulfoxide, and obtained 80% conversion after 2 hours reaction at temperatures higher than 170°C [7]. Andrieux et al (1977) obtained 92% isoeugenol with 2 hours of reaction time using RhCU as a catalyst run at 20 oC [15]. Isoeugenol could also be obtained by adding KOH and hexa-decyl-tributylphosphonium bromide to clove oil at 150°C [16]. 

The substrate-directed regiochemistry can ako be overwritten by a change of the temperature. Thus, in the hydroformylation of eugenol [176] and isoeugenol [177], respectively, with an unmodified Rh catalyst with increasing temperature, also the two less usual aldehydes were formed (Table 6.2). This result can be rationalized by a prior isomerization of the double bond at elevated temperatures. 

---