Substituent Effects on C-13 Chemical Shifts of Aromatic Carbons in Biphenyl Type Lignin Model Compounds

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Abstract
Simple 5–5 type lignin model compounds have been synthesized from dehydro-divanillin and its ether derivatives by decarbonylation, hydrogenation, and reduction with metal hydride. The chemical shifts for the aromatic carbons of these compounds were assigned, and the substituent effects have been elucidated from 13C NMR spectra of guaiacyl-type monomeric and 5–5 type lignin model compounds. In addition, evaluation of the observed values of substituent chemical shift (SCS) for the aromatic carbons leads to formulation of a generalized SCS additivity rule for the aromatic carbons in 5–5 type substructures. The rule is complementary to the similar rule formulated earlier for the β-0−4 and β-5 type lignin substructures. The rule can be used for the estimation of the chemical shifts of aromatic carbons in lignin model compounds and lignin preparations with reasonable accuracy. On the basis of the observed 13C NMR spectral data for aromatic