Oxygen substituent effects in the pyrolysis of phenethyl phenyl ethers

The dominant structural linkage in the abundant renewable energy resource, lignin, is the arylglycerol-β-aryl ether commonly referred to as the β-O-4 linkage. The simplest representation of this linkage, unadorned by substituents, is phenethyl phenyl ether (PhCH₂CH₂OPh; PPE). Our prior detailed kinetic studies of the pyrolysis of PPE at 330-425 °C showed that a free-radical chain mechanism is involved that cycles through radicals formed at both the α- and β-carbons. The previously unrecognized competing path involving rearrangement of the β-carbon radical by an O,C-phenyl shift accounts for ca. 25% of the products. In this paper, we explore the effect of aromatic hydroxy and methoxy substituents on both the rate and product selectivity for the pyrolysis of these more complex lignin model compounds. The pyrolysis reactions are conducted in biphenyl solvent at 345 °C and low conversions to minimize secondary reactions. The pyrolysis rates were found to vary substantially as a function of the substitution pattern. The rearrangement path involving the β-carbon radical and O,C-phenyl shift was found to remain important in all the molecules investigated, and the selectivity for this path also showed a dependence on the substitution pattern.