Palladium/Benzoquinone-Catalyzed Electrochemical Oxidation of Alcohols Under Anaerobic Conditions

Abstract

Primary and secondary alcohols are oxidized to aldehydes and ketones, respectively, under anaerobic conditions in DMF at 80 °C, in the presence of a base and catalytic amounts of Pd(OAc)₂ and p-benzoquinone. The latter oxidizes the transient Pd(0) formed in the catalytic cycle to Pd(II) and p-hydroquinone is re-oxidized electrochemically.

Key words

palladium - benzoquinone - catalysis - electrochemical oxidation - alcohols

References and Notes

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• 21a

  Interestingly, one reviewer has made the observation that compounds 4i and 3h may also be formed in a palladium-catalyzed Heck reaction between a phenyl halide and the allyl alcohol via HPdX elimination and reverse re-addition (see ref. 21b). In that case, the aldehyde 4i is the major product. In our case, the β-H elimination from 5 should also give the major product 4i but the minor 3h (in equilibrium with 4i via 5) is quenched by its Pd/BQ-catalyzed oxidation to 4h. As a result, 4i is fully consumed via the oxidation of 3h.

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General Procedure for Preparative Electrolyses
The electrosynthesis of 2d (entry 14 in Table [1] ) was carried out in a two-compartment air-tight three-electrode cell under argon at 80 °C. The two compartments were separated by a sintered glass disk. The anode was a carbon cloth (ca. 4 cm² surface area). The cathode was a nickel foam (ca. 1 cm² surface area). The reference was a sat. calomel electrode separated from the solution by a bridge filled with a solution of n-Bu₄NBF₄ (0.3 M) in DMF (2 mL). The anodic and cathodic compartments were, respectively, filled with 10 mL of DMF and 2 mL of DMF containing n-Bu₄NBF₄ (0.3 M). Then 250 µL (2 mmol) of alcohol 1d was added to the anodic compartment followed by 276 mg (2 mmol) of K₂CO₃, 21.6 mg (0.2 mmol) of sublimated p-benzoquinone, and 45 mg (0.2 mmol) of Pd(OAc)₂. Afterwards, 343 µL (6 mmol) of AcOH was introduced into the cathodic compartment. The electrolysis was conducted at a controlled potential of +0.75 V, using a Tacussel PJT 35-2 potentiostat. The electrolysis was stopped after 80 min (passage of 1015 cb). After cooling to r.t., the anodic compartment was hydrolyzed with 50 mL of H₂O. After extraction with Et₂O, the organic phase was dried on MgSO₄ and evaporated. The yield of 2d (98% yield) was determined on the crude mixture by ¹H NMR (250 MHz) spectroscopy using CHCl₂CHCl₂ (0.5 mmol) as internal standard added to the crude mixture after workup. The alcohol 1d was not detected on the NMR spectra of the crude mixture. Product 2d was isolated as pure compound by flash chromatography (eluent: PE-EtOAc, 80:20). ¹H NMR (250 MHz, CDCl₃): δ = 3.71 (s, 3 H, OCH₃), 6.84 (d, 2 H, J = 8.7 Hz, o-H relative to OMe), 7.67 (d, 2 H, J = 8.7 Hz, o-H relative to CHO), 9.71 (s, 1 H, CHO). ¹³C NMR (62.89 MHz, CDCl₃): δ = 55.25 (OCH₃), 113.56 (COMe), 114.64 (o-C relative to OMe), 129.95 (CCHO), 131.83 (o-C relative to CHO), 190.69 (C=O). MS (CI, NH₃): m/z = 154 [M + NH₄ ⁺], 137 [M + H⁺].

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In the latter case, 6f is the minor product because the formation of the intermediate allylic alcohol (E)-n-pentyl-CH=CHCH(OH)Me (7f) is less favored than the formation of 3h in which the C=C bond is conjugated with the phenyl group (Scheme [8] ). Moreover, the oxidation of the secondary alcohol 7f became more difficult than the oxidation of the primary alcohol 3h (Scheme [8] ).