Molecular Insight into Hydrodeoxygenation of Naphthols: Iridium-Catalyzed Ring Hydrogenation and Substrate-Catalyzed Dehydration

The only molecular precatalysts offering high aromatic selectivity in hydrodeoxygenation (HDO) of phenolic and methyl ether lignin model compounds are hydroxy-tetraphenyl-cyclopentadienyl (CpOH) iridium complexes such as IrCpOH(H)₂PPh₃ (Ir1) [Kusumoto, S.; Nozaki, K. Nat. Commun. 2015, 6, 6296]. Here, we synthesized a variant (Ir1L) in which the CpOH and phosphine moieties are tethered and unlikely to dissociate from iridium. Surprisingly, unlike Ir1, Ir1L neither catalyzes HDO of phenylphenols nor the interconversion between naphthalene and tetralin. The density functional theory-calculated barriers for the corresponding reactions catalyzed by unmodified Ir1 or Ir1L are high (>44 kcal/mol), suggesting that the observed activity of Ir1 in these reactions is due to catalyst initiation. In contrast, intact Ir1 and Ir1L both appear to catalyze HDO of naphthols. Notably, the calculations show that Ir1 and Ir1L both mediate initial ring hydrogenation to 1,2-dihydronaphthol (2H), while only Ir1L can continue hydrogenation to 1,2,3,4-tetrahydronaphthol (4H). The subsequent substrate-catalyzed dehydration of 2H leads directly to naphthalene, whereas that of 4H leads to 1,2-dihydronaphthalene and, via hydrogenation, to tetralin. The calculations are thus consistent with the near-perfect aromatic selectivity observed at short reaction times using Ir1 and the mixture (19:81) of naphthalene and tetralin obtained by using Ir1L.