On the Reactivity of Dihydro‐p‐coumaryl Alcohol towards Reductive Processes Catalyzed by Raney Nickel

There are several established approaches for the reductive fractionation of lignocellulose (e.g., “catalytic upstream biorefining” and “lignin‐first” approaches) that lead to a lignin oil product that is composed primarily of dihydro‐p‐monolignols [e.g., 4‐(3‐hydroxypropyl)‐2‐methoxyphenol and 4‐(3‐hydroxypropyl)‐2,6‐dimethoxyphenol]. Although effective catalytic methods have been developed to perform reductive or deoxygenative processes on the lignin oil, the influence of the 3‐hydroxypropyl substituent on catalyst activity has previously been overlooked. Herein, to better understand the reactivity of the depolymerized lignin oil obtained from catalytic upstream biorefining processes, dihydro‐p‐coumaryl alcohol was selected as a model compound. Hydrogenation of this species in the presence of Raney Ni with molecular hydrogen led to ring saturation (100 % selectivity) in the absence of hydrodeoxygenation, whereas under hydrogen‐transfer conditions with 2‐propanol, hydrogenation occurred (≈55 % selectivity) simultaneously with hydrodeoxygenation (≈40 % selectivity). In a broader context, this study sheds light not only on the reactivity of dihydro‐p‐monolignols but also on the intricacies of the catalytic upstream biorefining reaction network in which these species are revealed to be key intermediates in the formation of less‐functionalized p‐alkylphenols. Fifty shades of green: The advent of catalytic upstream biorefining has made available new platform chemicals from lignin. In this Communication, the reactivity of dihydro‐p‐coumaryl alcohol is explored in reductive processes catalyzed by Raney Ni under H2 pressure as well as under H‐transfer conditions. Under H2 pressure, Raney Ni is a selective hydrogenation catalyst. However, under H‐transfer conditions, the catalyst also produces 4‐alkylphenols.