CeO₂-Supported Pd(II)-on-Au Nanoparticle Catalyst for Aerobic Selective α,β-Desaturation of Carbonyl Compounds Applicable to Cyclohexanones

Abstract

Direct selective desaturation of carbonyl compounds to synthesize α,β-unsaturated carbonyl compounds represents an environmentally benign alternative to classical stepwise procedures. In this study, we designed an ideal CeO2-supported Pd­(II)-on-Au nanoparticle catalyst (Pd/Au/CeO2) and successfully achieved heterogeneously catalyzed selective desaturation of cyclohexanones to cyclohexenones using O2 in air as the oxidant. Besides cyclohexenones, various bioactive enones can also be synthesized from the corresponding saturated ketones under open air conditions in the presence of Pd/Au/CeO2. Preliminary mechanistic studies revealed that α-C–H bond cleavage in the substrates is the turnover-limiting step of this desaturation reaction.

C–H Bond Activation Mechanism by a Pd(II)–(μ-O)–Au(0) Structure Unique to Heterogeneous Catalysts

“…On the other hand, the XANES results for Pd/Au/CeO 2 revealed the formation of Pd(0) species during the reaction. 43 Given our previous report on the dehydrogenative aromatization of cyclohexanones to phenols at 130 °C using a Pd(0)–Au(0) alloy catalyst supported on LDH (Pd–Au/LDH) via aerobic α,β-dehydrogenation of cyclohexanones to 2-cyclohexen-1-ones, 54 , 55 the Pd(0) species formed in Pd/Au/CeO 2 may be the true active species.…”

“… 40 Recently, our group realized heterogeneously catalyzed aerobic α,β-dehydrogenation of saturated ketones including cyclohexanone by using Pd(II) species-on-Au(0) nanoparticles supported on CeO 2 (Pd/Au/CeO 2 ) ( Figure 1 b). 43 This system not only is environmentally benign but also exhibits a turnover frequency (TOF) (for cyclohexanone dehydrogenation) per Pd that exceeded those of previously reported homogeneous Pd catalysts 39 − 41 (122 vs 2–3 h –1 ). Considering their turnover-limiting steps are the same: α-C–H bond activation, 40 , 43 the active site possessed by Pd/Au/CeO 2 is more effective than that of Pd(TFA) 2 (DMSO) 2 for C–H bond activation.…”

“…We previously prepared NaBH 4 -treated Pd and Au hydroxides supported on CeO 2 in an Ar atmosphere (i.e., Pd/Au/CeO 2 -Ar), which had a Pd valence of zero ( Figure S5 ). 43 In the present study, we prepared (i) air-exposed Pd/Au/CeO 2 -Ar (i.e., Pd/Au/CeO 2 -air) and (ii) water-treated Pd/Au/CeO 2 -Ar in an open-air atmosphere (i.e., Pd/Au/CeO 2 -water). Then, the catalytic activities of Pd/Au/CeO 2 , Pd/Au/CeO 2 -water, and Pd/Au/CeO 2 -air for the aerobic α,β-dehydrogenation of 1a to 2a at 70 °C in DMA were compared under an air atmosphere ( Figure 4 ).…”

“…When 1b was dehydrogenated by using Pd/Au/CeO 2 , the phenyl-substituted side was regioselectively dehydrogenated to give 2-phenyl-2-cyclohexen-1-one ( 2b ), while the unsubstituted side was not dehydrogenated to 6-phenyl-2-cyclohexen-1-one ( 2b′ ) ( Figure 10 c). 43 We investigated whether the selectivity of this reaction could be explained by DFT calculations if we assume a reaction mechanism similar to that of cyclohexanone. The activation free energy in the deprotonation step was calculated to be 9.4 kcal/mol for the phenyl-substituted side ( 3 - 1 → 3 - 3 ) in contrast with 20.6 kcal/mol for the nonsubstituted side ( 3 - 1′ → 3 - 3′ ), which is sufficient to explain the regioselectivity in the reaction ( Figure 10 c).…”

“…The Pd/Au/CeO 2 catalyst was prepared according to the method presented in our previous report. 43 CeO 2 (2 g) was added to an aqueous solution (60 mL) of HAuCl 4 (6.25 mM) and K 2 PdCl 4 (0.83 mM), which was generated in situ from a mixture of PdCl 2 (8.9 mg, 0.05 mmol) and KCl (7.5 mg, 0.1 mmol). The slurry was stirred for 15 min, which was followed by dropping of aqueous NaOH (1 M) until the pH reached 10.…”