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DOI: 10.1055/sos-SD-025-00010

Podlech, J.Science of Synthesis, (20072543.

Oxygen has long been recognized as a desirable oxidizing agent for the formation of aldehydes from primary alcohols. However, although this method is very useful for the oxidation of benzylic and allylic alcohols, it gives somewhat lower yields with primary alcohols and this limits its use. Instead, a multitude of reagents are used as the actual oxidizing species and, once spent, they are regenerated in situ by oxygen (Scheme 20).[‌118‌,‌130‌] Thus, palladium(II) acetate and pyridine in the presence of oxygen oxidizes 8-(tert-butyldimethylsiloxy)octan-1-ol [27, R1=(CH2)7OTBDMS] to the corresponding 8-siloxylated octanal 28 [R1=(CH2)7OTBDMS] in 92% yield; here, the addition of 3-Å molecular sieves is essential.[‌131‌,‌132‌] A slightly elevated pressure of oxygen (2atm) plus a polyoxometalate (H5PV2Mo10O40) catalyst and 2,2,6,6-tetramethylpiperidin-1-oxyl are used for the oxidation of octan-1-ol itself into octanal [28, R1=(CH2)7Me].[‌133‌] The yield is >98%. Similarly, a combination of oxygen at atmospheric pressure, manganese(II) and cobalt(II) nitrate, and 2,2,6,6-tetramethylpiperidin-1-oxyl is suitable for the oxidation of heptan-1-ol in acetic acid into heptanal in 97% yield.[‌134‌] Somewhat lower yields (8590%) for the oxidation of octan-1-ol to octanal are achieved with oxygen/dichlorotris(triphenylphosphine)ruthenium, either together with 2,2,6,6-tetramethylpiperidin-1-oxyl[‌135‌,‌136‌] or with hydroquinone.[‌137‌] Oxygen and a combination of copper(I) chloride, 1,10-phenanthroline, potassium carbonate, and di-tert-butyl azodicarboxylate can be used for the oxidation of (S)-N,N-dibenzylvalinol [27, R1=CH(iPr)NBn2] into the corresponding enantiomerically pure aldehyde 28 [R1=CH(iPr)NBn2] in 91% yield (Scheme 20).[‌138‌] Alternatively, 2,2,6,6-tetramethylpiperidin-1-oxyl can be used, together with copper(I) chloride.[‌21‌] Supported 2,2,6,6-tetramethylpiperidin-1-oxyl has also been used together with various metal salts[‌139‌] or sodium hypochlorite.[‌140‌] Unfortunately, as hardly any of these methods have been applied to the oxidation of more than one nonconjugated primary alcohol, their general value is hard to assess.

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