TEMPO Plus Co-Oxidant

Mechanism + Description

The oxonium ion generated by the oxidation of the TEMPO radical is the oxidant. Used catalytically, a stoichiometric oxidant like bleach or O2 (air) is used to drive the catalytic cycle. The oxidation to aldehyde is rapid. The oxonium ion will only oxidize the aldehyde hydrate, so for substrates with a negligible degree of hydration, a second oxidant – usually NaOCl2 – is added.

General comments

A common terminal oxidant is bleach (NaOCl), which is often employed with a bromide or borate co-catalyst. Reactions in water of bi-phasic reactions are often helped by the addition of a phase transfer catalyst. Ring opening of the oxonium ion and deoxygenation of the TEMPO catalyst can limit catalytic efficiency. In some cases, other catalytic oxidants like copper salts or laccase enzymes are added to promote the TEMPO oxidation, especially when air is used as the stoichiometric oxidant. Because of its hydrophobic nature and steric hindrance, TEMPO oxidations can be kinetically slow. A number of related, but much more reactive nitroxyl radical catalysts like ABNO,AZADO and 1-methylAZADO are becoming more commercially available. While these are currently much more expensive than TEMPO on a mole basis, they are often several of orders more reactive and can be dosed at much lower levels than TEMPO. A number of solid-supported TEMPO or TEMPO analogues are available commercially. These can facilitate downstream processing, provide for reusable catalysts, and facilitate chemistry in flow reactors. Considerations should be given to their preparation and lifespan (number of reuses before depletion of results) to make an informed decision on whether it is a better option than the standalone/homogeneous reagents.

Green Review

Atom efficiency (by-products Mwt)
Generally good if optimized for charge of TEMPO or other catalyst. Mainly driven by stoichiometric/terminal oxidant. NaOCl gives NaCl as a by-product. Air will give H₂O₂ or H₂O as a by-product. Atom efficacy can be poor with high Ml. Wt. oxidants like oxone or high valent iodine reagents.
Safety Concerns
All TEMPO oxidations are exothermic and may present delayed exotherms. Compatibility of NaOCl with other reaction components needs to be considered.
Toxicity and environmental/aquatic impact
Though toxicity and environmental/aquatic impacts are generally low when used catalytically, the major concerns arise from the co-oxidant. Nitroxyl radicals like TEMPO and the hydroxylamine intermediates in the oxidation cycle give positive structural alerts as potential genotoxic impurities (PGI).

Strohmeyer, H. E.; Sluggett, G. W. Determination and Control of TEMPO, a Potentially Genotoxic Free Radical Reagent Used in the Synthesis of Filibuvir. J. Pharm. Biomed. Anal. 2012, 62 (25), 216– 219.
Cost, availability & sustainable feedstocks
The cost of TEMPO has fallen over time and is now available in bulk. Other analogues are less commercially available and much more expensive, but could show far greater activity. The skeleton of TEMPO comes from acetone and ammonia.
Sustainable implications
With good optimization of catalyst loading and a low molecular weight terminal oxidant like NaOCl, this oxidation is a good choice. The major concern would be the solvent used. Many initial publications used dichloromethane, but later work has shown that more sustainable solvents can be used (see references).