Continuous‐Flow Palladium‐Catalyzed Synthesis of Cyclohexanones from Phenols using Sodium Formate as a Safe Hydrogen Source

Abstract

We report ap rocedure for the continuous-flow production of cyclohexanone from phenol on the basis of the use of sodium formate as ab iomass-derived sourceo fh ydrogen and Pd/C as an easily accessible catalyst system.T he reactionw orked in water at pH 12.0 at 90 8C. By setting ap acked reactor charged with the Pd/C catalyst (10 wt %) at af low rate of 0.5 mL min À1 , we achieved continuous-flow production of cyclohexanone in high yield with high selectivity and productivity.Phenols, mostly in their polymeric forms in lignin, are the second mostp revalent naturallyo ccurring structural units of renewable biomass on the planet, and they are regarded as possibly valuable materials derivingf rom biomass treatment. Indeed,r emoval of the functional groups presenti nl ignin yields simple aromatic compounds such as phenol,b enzene, toluene,a nd xylene.C onsequently,a pplicationo fp henolsa s commodity chemicals or substrates foru seful transformations is highly desirable. [1] Cyclohexanone is one of the main commoditieso ft he chemicali ndustry feeding the productiono fc aprolactam and adipic acid, whichi nt urn are transformed into Nylon 6, Nylon 6,6, and polyamide resin. [2] Large production of cyclohexanonei sb ased on either the oxidation of cyclohexane under Co catalysis or the partial hydrogenation of phenol in the presence of different transition-metal catalysts (Scheme1). In terms of ad ifferent approach startingf rom phenol, the industrial process towardc yclohexanone comprises an initial reductive step to access cyclohexanol, which is furtherd ehydrogenatedt oc yclohexanonei nahigh-temperature gas-phase Pd-or Pt-catalyzed process. To define milder and more energy efficient one-step processes, the use of selectiveh omogeneous and heterogeneous catalysts hasb een investigated in the liquid phase and has resulted in many successful examples mainly based on single or mixed Pd, [3] Pt, [4] Ru, [5] Rh, [6] andN i [7] catalysts on different supports. [8] Considering the commonly proposed mechanism for the hydrogenation of phenol on supported palladiumc atalysts, reductiono ccurs on the phenolate ion adsorbed on the support in proximity of the metal particles, which are responsible for the activation of hydrogen. The chemical strategy to reduce phenol selectively is based on controlling the desorption of cyclohexanone from the catalyst surface beforef urther hydrogenation leads to cyclohexanol. [9] The use of formic acid derivativesa sahydrogen sourcei n combination with ap alladium catalyst is ap romising, safe, and sustainable alternative to classic methods. Formates are stable and nontoxic, and they can be obtainedf rom the processing of biomass. [10] Formic acid derivatives have also been proposed as hydrogen-storage materials, as they are suitable for safe transportation and handling. [11] Some examples of phenol reductionw ith the use of formic acid, [12] sodium formate, [13,14] and potassium formate [15] under batch conditions were reported recently.A lthough very attractive, some of these protocols feature ...

C2–H Arylation of Indoles Catalyzed by Palladium‐Containing Metal‐Organic‐Framework in γ‐Valerolactone

“…In the last few years, the interaction between the MOF support and the catalytically active metal has been extensively studied because new or enhanced catalytic properties can arise from this combination . In selected cases, improved performance compared with the analogous homogeneous catalysts could be noticed for C−H activation reactions owing to the porosity, large surface area, immobilization, and site isolation that can occur in the heterogeneous MOF catalysts …”

Liquid Organic Hydrogen Carriers (LOHCs) as H‐Source for Bio‐Derived Fuels and Additives Production

“…Among the LOHC-promoted hydrogenation [303][304][305][306] and hydrodeoxygenation [307][308][309][310][311] reactions of lignin-derived phenolic compounds, the hydrodeoxygenation of vanillin (4-hydroxy-3-methoxybenzaldehyde) is particular intriguing for the production of 2-methoxy-4-methylphenol (2-OMe-4-MePhOH), a promising biofuel candidate. Owing to the complexity of the vanillin, efficient catalytic systems, as well as appropriate LOHCs, should be used to afford the desired product saving the aromaticity of the phenol ring (Scheme 11).…”

Sustainable Routes for the Synthesis of Renewable Adipic Acid from Biomass Derivatives

“…[129] Analogously, the catalytic reduction of guaiacol to 2-MCO with formic acid as reducing agent over Pd/C has been also realized. [130] For the following step of the oxidation of 2-MCO to AA, the authors used H 4À can decrease the reactivity of the radical with radical inhibitor. Subsequently, O 2 is added to the carbon atom bound to the methoxide group (step ii).…”

“…For the first step, Pd‐based catalysts have been shown to be effective in selective hydrogenation of phenolic compounds and the conversion of guaiacol to 2‐MCO over Pd/HAP catalyst has been actually reported with 98 % high selectivity [129] . Analogously, the catalytic reduction of guaiacol to 2‐MCO with formic acid as reducing agent over Pd/C has been also realized [130] . For the following step of the oxidation of 2‐MCO to AA, the authors used H 3 PW 12 O 40 to selectively convert 2‐MCO into AA with O 2 as an oxidant in water solvent.…”