Pd/TOMPP-catalysed telomerisation of 1,3-butadiene with lignin-type phenols and thermal Claisen rearrangement of linear telomers

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Abstract
The Pd/TOMPP-catalysed (TOMPP = tris(2-methoxyphenyl)phosphine) telomerisation of 1,3-butadiene was studied under solvent- and base-free conditions with phenolic substrates that can be potentially derived from lignin. Large differences in catalytic activity were observed, with reactivity increasing in the order of phenol, p-cresol, guaiacol, creosol and syringol. This reactivity trend can be attributed to the substrates' relative nucleophilicities, as induced by the donating effects of the p-methyl and o-methoxy substituents. The chosen reaction conditions, i.e. temperature, ligand/metal and butadiene/substrate ratios, strongly influenced both the conversion and selectivity of the reaction. Remarkably, the composition of the reaction medium, i.e. the butadiene/substrate ratio, exerted a strong influence on the linear/branched ratio. High conversions and selectivities to the linear products are obtained when excess butadiene is used. The linear telomer products could be readily converted from O-alkylated to C-alkylated phenolics via the thermal Claisen rearrangement. High conversions and selectivities were observed after 2 hours at 200 °C. Branched o-octadienyl phenols were obtained in all cases except for the syringol telomer which gave the linear p-octadienyl product exclusively.

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