Abstract
The cleavage mechanism of the CαCβ bond in a 1,3-dilignol catalyzed by [Ru(Cl)(H)(PPh3)3]/triphos has been investigated with the aid of DFT calculations to understand the unique catalytic activity and selectivity of ruthenium complexes towards the redox-neutral deploymerization of lignin model compounds. The proposed mechanism divides the reaction into four stages: coordination of the substrate to the Ru center, dehydrogenation, retro-aldol-type CC bond cleavage, and product and pre-product release with catalyst regeneration. The dehydrogenation process was identified as the bottleneck of the catalytic cycle and the estimated high barrier (29.8 kcal/mol) qualitatively rationalizes the experimental observation that the reactions were carried out under elevated temperature (160 °C). The α- and γ-hydroxyl groups as well as the γ-H in the 1,3-dilignol were shown to play substantial roles and their simultaneous presence is necessary for cleavage of the CαCβ bond. This rationalizes the experimental finding that [Ru(Cl)(H)(PPh3)3]/triphos complexes were inactive for cleavage of the Cα‒Cβ bond cleavage of 1,3-dilignols with any absence of the α-hydroxyl group, γ-hydroxyl group, or CγH unit.
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