Mechanistic picture of the redox-neutral C C bond cleavage in 1,3-dilignol lignin model compound catalyzed by [Ru(Cl)(H)(PPh3)3]/triphos

Abstract The cleavage mechanism of the C α C β bond in a 1,3-dilignol catalyzed by [Ru(Cl)(H)(PPh 3 ) 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 C C 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 C α C β bond. This rationalizes the experimental finding that [Ru(Cl)(H)(PPh 3 ) 3 ]/triphos complexes were inactive for cleavage of the C α ‒C β bond cleavage of 1,3-dilignols with any absence of the α -hydroxyl group, γ -hydroxyl group, or C γ H unit.