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.