The aryl ether bond reactions with H-donor solvents: Guaiacol and tetralin in the presence of catalysts

The effect of homogenous catalysis, Fe and Ru, on the conversion of guaiacol in tetralin to catechol and phenol has been investigated as a model for the behaviour of the aryl-oxy linkage that is found in wood, peat and younger coals. In the absence of catalyst and at low ratios of guaiacol to tetralin, the primary product is catechol. Kinetic analysis has confirmed that the rate constant for this primary and rate determining step is given by an Arrhenius pre-exponential factor of 10(13.8) s(-1) With an activation energy of 215 kJ mol(-1). The activation energy found is in good agreement with those of other investigators and lies between the values proposed for homolytic fission (>240 kJ mol(-1)) and for a concerted or pericyclic reaction (188 kJ mol(-1)). In the presence of catalysts the rate is not changed; however, the yield of a secondary product phenol is increased with both Fe and Ru. Separate experiments confirmed that the selectivity of catechol to phenol conversion was markedly increased in the presence of these catalysts. There is strong evidence for the formation of catecholato-iron complexes and this suggests that in pyrolysis and liquefaction of biomass and young coals there may well be a role for homogeneous catalysts in directing the product slate towards useful intermediate chemicals such as phenols.