Iron(II)-Catalyzed Biomimetic Aerobic Oxidation of Alcohols

Abstract

We report the first Fe^II -catalyzed biomimetic aerobic oxidation of alcohols. The principle of this oxidation, which involves several electron-transfer steps, is reminiscent of biological oxidation in the respiratory chain. The electron transfer from the alcohol to molecular oxygen occurs with the aid of three coupled catalytic redox systems, leading to a low-energy pathway. An iron transfer-hydrogenation complex was utilized as a substrate-selective dehydrogenation catalyst, along with an electron-rich quinone and an oxygen-activating Co(salen)-type complex as electron-transfer mediators. Various primary and secondary alcohols were oxidized in air to the corresponding aldehydes or ketones with this method in good to excellent yields.

Keywords: aerobic oxidation; biomimetic reactions; electron transfer; homogeneous catalysis; iron.

Scheme 1

Principle for oxidation with O₂ or H₂O₂ through the use of ETMs. ETM=electron transfer mediator, SSRC=substrate selective redox catalyst.

Scheme 2

Activation of iron tricarbonyl complex (I) and Shvo's catalyst (III).

Scheme 3

The biomimetic oxidation approach using an iron catalyst as SSRC.

Figure 1

Iron tricarbonyl complex VII (DMPh=3,5‐dimethylphenyl).

Scheme 4

Substrate scope. General reaction conditions: The reaction was conducted under air at 100 °C with 0.5 mmol of 1, 0.05 mmol of VII, 0.05 mmol of TMANO, 0.2 mmol DMBQ, 0.02 mmol of VI, 0.5 mmol of K₂CO₃, and 1.5 mL of anisole. [a] Yield of isolated product. [b] NMR yield determined by using 1,3,5‐trimethoxybenzene as internal standard. [c] 0.1 mmol (20 mol %) of VII used.