C-13 NMR SPECTROSCOPIC STUDIES OF PHENYLCOUMARAN AND 1,2-DIARYLPROPANE TYPE LIGNIN MODEL COMPOUNDS .2. SUBSTITUENT EFFECTS ON C-13 CHEMICAL-SHIFTS OF AROMATIC CARBONS

A number of phenylcoumaran (cyclic beta-5') and 1,2-diarylpropane (noncyclic beta-5') type lignin model compounds have been synthesized as described in the preceding paper. C-13 NMR spectra of these compounds were examined and C-13 chemical shifts for aromatic carbons of these compounds were reassignes on the basis of the results from 2D H-1-H-1, H-1-C-13 and C-13-C-13 chemical shift correlation spectroscopic studies and the observed substituent effects on the aromatic carbons of these compounds. A substituent chemical shift (SCS) additivity rule for aromatic carbons in cyclic beta-5' type substructures was elucidated on the basis of the observed SCS values for the aromatic carbons of the model compounds. The SCS additivity rule is compatible with the generalized SCS rule elucidated earlier for aromatic carbons in beta-0-4' and 5-5' type substructures. In addition, chemical shifts of aromatic carbons in c-alpha-reduced, noncyclic beta-5' type 1,2-di-arylpropane and 2-aryl-3-methylcoumarone derivatives were assigned on the basis of their C-13 NMR spectral data.