Environmentally Friendly and Highly Efficient Co(OAc)₂-Catalyzed Aerobic Oxidation to Access 2,6-Di-Electron-Donating Group Substituted 4-Hydroxybenzaldehydes

Abstract

A highly efficient and green aerobic oxidation has been developed for selectively preparing a series of valuable 2,6-dialkyl-, dialkoxyl-, and alkoxylalkyl-substituted 4-hydroxybenzaldehydes from corresponding 4-cresols in good to excellent yields, using a catalytic system of Co(OAc)₂ · 4H₂O (1.0 mol%)–NaOH (1.0 equiv)–O₂ (1.0 atm) in aqueous ethylene glycol (EG/H₂O = 20/1, v/v) at 50 °C. Furthermore, a plausible mechanism was proposed for the direct oxyfunctionalization of the aromatic methyl group into the aldehyde group.

GRAPHICAL ABSTRACT

Keywords:

• Aerobic oxidation
• Co(OAc)₂
• ethylene glycol
• 4-hydroxybenzaldehydes
• oxyfunctionalization

Notes

^a Reaction conditions: substrate 1a (5.0 mmol), cobalt source (n ₁ mol%), sodium hydroxide (n ₂ equiv), and solvent (5.0 mL), O₂ (1.0 atm), 50 °C for 18 h.

^b Isolated yield.

^c Reaction performed at 40 °C.

^d Reaction time of 24 h.

^e Reaction time of 16 h, EG/H₂O = 5.0 mL/0.1 mL.

^f Reaction time of 12 h, EG/H₂O = 5.0 mL/0.25 mL.

^g Reaction time of 6 h, EG/H₂O = 5.0 mL/0.75 mL.

^a Reaction conditions: substrate 1 (5.0 mmol), Co(OAc)₂ · 4H₂O (0.05 mmol, 12 mg), sodium hydroxide (5.0 mmol), EG/H₂O (5.0 mL/0.25 mL), O₂ (1.0 atm), 50 °C for 12 h.

^b Isolated yield.