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-d7 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, α: > 99%D, β: 92%D, and γ: 98%D). DHP was prepared solely from coniferyl alcohol-d7, and its NMR spectra were compared with those from coniferyl alcohol. The results indicated that: (1) the deuterium atoms at methoxy group, α- and β-positions were highly retained even in the DHP, and their signals were effectively suppressed; (2) a part of the deuterium at γ-position was replaced with H through the reaction; (3) meanwhile, the formation of γ-CH2 was negligible. This study demonstrated that coniferyl alcohol-d7 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.