Application of chemically induced dynamic nuclear polarization to a study of the decomposition of unsymmetric azo compounds

Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction

We report the experimental reassessment of the widely admitted concerted reduction mechanism for diazonium electroreduction. Ultrafast cyclic voltammetry was exploited to demonstrate the existence of a stepwise pathway, and real-time spectroelectrochemistry experiments allowed visualization of the spectral signature of an evolution product of the phenyldiazenyl radical intermediate. Unambiguous identification of the diazenyl species was achieved by radical trapping followed by X-ray structure resolution. The electrochemical generation of this transient under intermediate energetic conditions calls into question our comprehension of the layer structuration when surface modification is achieved via the diazonium electrografting technique as this azo-containing intermediate could be responsible for the systematic presence of azo bridges in nanometric films.

A perspective on free radical autoxidation: the physical organic chemistry of polyunsaturated fatty acid and sterol peroxidation

This Perspective describes advances from the author's laboratory on the free radical reactions of organic compounds with molecular oxygen. Polyunsaturated fatty acids (PUFAs) and sterols are particularly prone to undergo radical chain oxidation, and evidence suggests that this process, known as lipid peroxidation, occurs in vivo under a variety of conditions that are the result of an oxidative stress. Cyclic peroxides, hydroperoxides, and epoxy alcohols are major products formed from peroxidation, and the basic mechanisms of product formation are now reasonably well understood. These mechanisms include reversible addition of oxygen to carbon radicals, rearrangement and cyclization of allyl and pentadienyl peroxyl radicals, and homolytic substitution of carbon radicals on the peroxide bond. A physical organic approach to the problem of free radicals in biology and medicine is highlighted in this Perspective with stereochemical, kinetic, and extrathermodynamic probes applied to the study of mechanism. A radical clock permits the determination of free radical propagation rate constants, and 7-dehydrocholesterol, the immediate biosynthetic precursor of cholesterol, is found by this clock to be one of the most oxidizable lipids known. The consequences of the extreme reactivity of 7-dehydrocholesterol on human health is the focus of a current research theme in the author's laboratory.

Oxidation of 1-(4′-substituted phenyl)-2-(4-methylphenyl)diazene — Benzylic substitution vs. C–azo bond breaking

The low oxidation potential of 1-phenyl-2-(4-methylphenyl)diazene 4′-substituted with a dimethylamino or methoxy group (10 and 11) allows the easy single electron transfer with radical cation generation in the presence of FeCl3. At the same time, the stabilizing effect of the positive charge exerted by 4-dimethylaminophenyl > 4-methoxy groups on the intermediate carbocation favors the hydrogen atom elimination in comparison with proton loss. Whereas benzylic substitution towards di- and triarylmethane derivatives takes place starting from diazene 11, the oxidation of diazene 10 leads mainly to products of C–azo breaking. This difference is well-supported by the idea that the electron, in the first step of oxidation, is extracted from the azo and (or) from the amino nitrogen atom.Key words: diazenes, oxidation, single electron transfer, benzylic substitution, C–azo bond breaking.