Transition metal peroxides nucleophilic substrates

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

The electronic absorption spectrum of the cation-radical of thiophene itself has been observed following low-temperature y-radiolysis of the heterocycle in a Freon matrix.The radical has also been implicated in the oxidation of thiophene by dibenzoyl peroxide it is believed to be formed at the contact of certain transition metal layer-silicates with thiophene.The anodic oxidation of 2,5-dimethylthiophene has been studied by Japanese workers who found strong evidence for the formation of the cation-radical as the primary oxidation product.In the presence of strong nucleophiles such as cyanide ion, the cation-radical undergoes nucleophilic attack before further oxidation. In the presence of more basic species such as acetate ion, the cation-radical is deprotonated to give a thienylmethyl radical which undergoes further reaction. The results were compared with similar observations for the oxidation of 2,5-dimethylfuran. Czech workers have also studied the anodic oxidation of substituted thiophenes. This work has focused on the preparative value of anodic oxidations in acidified methanol. Cation-radical formation is implied for the primary step, but the value of the method lies in the fact that sulfur is ultimately eliminated from the substrate and functionalized y-dicarbonyl compounds result. 

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