m-Iodosylbenzoic Acid as a Convenient Recyclable Reagent for Highly ­Efficient RuCl₃-Catalyzed Oxidation of Alcohols to Carbonyl Compounds

Abstract

m-Iodosylbenzoic acid selectively oxidizes primary and secondary alcohols to the respective carbonyl compounds in the presence of RuCl₃ (0.5 mol%) at room temperature in aqueous ­acetonitrile. Separation of pure products is conveniently achieved by scavenging any aryl iodide by ion exchange with IRA-900 (hydroxide form) or by simple extraction of the basic aqueous solution with water. The reduced form of the reagent, m-iodobenzoic acid, can be easily recovered from the ion-exchange resin or from the ­basic aqueous solution by simple acidification with HCl.

Key words

hypervalent iodine - m-iodosobenzoic acid - oxidation - catalysis - scavenger

References and Notes

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11

Improved Procedure for the Preparation of m -Iodosyl-benzoic Acid ( 2): m-Iodobenzoic acid (1; 2.48 g, 10 mmol) was added at 40 °C to a freshly prepared solution of peracetic acid in AcOH [prepared by stirring a mixture of Ac₂O (60 mL) and 35% H₂O₂ (14 mL) at 40 °C for 4 h in dark] and the resulting mixture was stirred for 12 h at ambient temperature in dark. A spontaneous heating of the reaction mixture up to 45 °C and complete dissolution of m-iodobenzoic acid was observed during the first two hours of stirring. The reaction mixture was poured onto ice-water (150 mL), and allowed to crystallize. The precipitate was filtered on a Büchner funnel and washed with ice-water (200 mL). The solid was then dried, first by maintaining suction and then in vacuum, to afford product 2 as a light yellow powder. Yield: 1.90-2.11 g (72-80% yield); mp 168-169 °C. ¹H NMR (200 MHz, CD₃COOD): δ = 7.71 (t, 1 H), 8.35 (d, 1 H), 8.48 (d, 1 H), 8.89 (s, 1 H). Anal. Calcd for C₇H₅IO₃ (263.93): C, 31.84; H, 1.91. Found: C, 31.77; H, 1.84.

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RuCl ₃ -Catalyzed Oxidation of Alcohols to Carbonyl Compounds:
Method A: To a mixture of alcohol (0.05-0.20 mmol) and m-iodosylbenzoic acid (2; 0.075-0.300 mmol, 1.5 equiv) in aq MeCN (MeCN-H₂O, 5:1; 0.5-2 mL) an aqueous solution of RuCl₃ (1.0-4.0 µL of 0.25 M solution; 0.25-1.0 µmol) was added under stirring at r.t. An instantaneous formation of a cotton-like, off-white precipitate was observed. The reaction mixture was stirred for a period of time indicated in Figures [1] and [2] (the reactions were monitored by TLC). Then, CH₂Cl₂ (1.5 mL) and IRA 900 (hydroxide form; 160-620 mg) were added and the mixture was stirred for 5 min. The polymer was removed by filtration and the solution was concentrated under vacuum to afford the NMR-pure carbonyl compound. Most products were additionally identified as 2,4-dinitrophenylhydrazones, prepared by the treatment of reaction mixtures with a standard solution of 2,4-dinitrophenylhydrazine. m-Iodobenzoic acid (1) can be easily regenerated (>95%) from IRA 900 by treatment with aq HCl and reused without additional purification. Method B: m-Iodosylbenzoic acid (2; 40 mg, 0.15 mmol) was added to an aqueous solution of NaHCO₃ (63 mg in 1.0 mL of H₂O) and the mixture was stirred until the formation of a clear solution was observed. Then, the alcohol (0.1 mmol) and an aqueous solution of RuCl₃ (2.0 µL of 0.25 M solution; 0.5 µmol) were added under stirring at r.t. The reaction mixture was stirred for a period of time indicated in Figures [1] and [2] (the reactions were monitored by TLC via disappearance of the alcohol). After completion of the reaction, the solution was extracted with Et₂O. The final NMR-pure products were obtained by evaporation of Et₂O from the extract; most prepared carbonyl compounds were additionally identified as their 2,4-dinitrophenylhydrazones. The aqueous solution that was left after extraction was acidified with HCl, and the white precipitate of m-iodo-benzoic acid was separated by filtration and used for regeneration of reagent 2.

13

In one case we encountered further oxidation to galacturonic acid when 1,2:3,4-di-O-isopropylidene-α-d-galacto-pyranose was employed (Scheme [2] ).

14

This is particularly well demonstrated for the formation of ulose 4r which can undergo facile elimination of methanol to the corresponding enulose.