Synthesis of highly deuterated coniferyl alcohol for silencing of NMR signals in the resulting dehydrogenative polymer

To establish a facile methodology for the elucidation of the lignin chain-growth mechanism, the preparation of monolignol that does not show NMR signals in the dehydrogenative polymer (DHP) was attempted. As a monolignol of which aliphatic moieties were deuterated, coniferyl alcohol-d(7) was successfully synthesized from protocatechualdehyde and malonic acid via the modified Knoevenagel-Doebner reaction and the Luche reduction. The process achieved high to excellent deuteration efficiencies at the aimed positions (i.e., methoxy: > 99%D, alpha: > 99%D, beta: 92%D, and gamma: 98%D). DHP was prepared solely from coniferyl alcohol-d(7), and its NMR spectra were compared with those from coniferyl alcohol. The results indicated that: (1) the deuterium atoms at methoxy group, alpha- and beta-positions were highly retained even in the DHP, and their signals were effectively suppressed; (2) a part of the deuterium at gamma-position was replaced with H through the reaction; (3) meanwhile, the formation of gamma-CH2 was negligible. This study demonstrated that coniferyl alcohol-d(7) could silence the majority of the signals even when converted to DHP. Highly deuterated monolignols can be a unique molecular tool that can differentiate the signals of interest from those derived from monolignols.

Automated summary

By preparing highly deuterated monolignols, a facile methodology for elucidating the lignin chain-growth mechanism was established. This study demonstrated that highly deuterated monolignols can effectively suppress NMR signals and serve as a unique molecular tool to differentiate signals of interest.