The Role of the Cyclopentadienol Ligand in Ir-catalyzed Deoxygenation of Model Lignin Bio-oil Compounds

Efficient conversion of arenols and aryl ethers into deoxygenated aromatics is crucial for the valorization of lignin biomass, yet current catalytic processes tend towards production of cycloalkanes, rather than the valuable aromatics. The Ir (Cp4Ph–OH)(H)2(PPh3) complex presented by Kusumoto and Nozaki was reported to selectively produce aromatics from substituted phenols, but the root of this selectivity has not yet been unearthed.[1] Here we present a DFT study of the catalysis reported by the original authors, predicting that the cyclopentadienol ligand plays no direct role in catalysis, and that the species responsible for producing aromatics is Ir(H)3(PPh3). The most favorable pathway found involves η-4 coordination of the substrate to Ir(H)3(PPh3), followed by a hydride transfer to the ipso carbon of the substrate. An ipso dehydration follows, and the aromatic compound is formed by a second hydride transfer to the ipso carbon. This mechanism is analogous to that proposed by the Martin for homogeneous nickel catalysts by the Martin group.[2] Forming the active species from Ir(Cp4Ph–OH)(H)2(PPh3) is predicted to be the rate-determining step in the deoxygenation process. More easily activated precatalysts could be synthesized and would presumably lead to higher activities while retaining the high selectivity for aromatics. 1: Kusumoto, S.; Nozaki, K., Nat. Commun., 2015, 6, 6296. 2: Cornella, S.; Gómez-Bengoa, E.; Martin, R., J. Am. Chem. Soc., 2013, 135(5):1997-2009