Nucleophilic substrates, oxidation

This also opens a question about the nature of the active species in, for example, oxidative nucleophilic reactions is it peroxo or superoxo form which nucleophilically attacks an organic substrate These are challenging questions which motivate future investigations. 

Spin traps can act as one-electron oxidizers. This property is even more pronounced in the interactions of traps with anion-radicals. Traps can block the ion-radical pathway. In other words, they inhibit the whole reaction, including the ion-radical step. This can be explained by both the oxidation of substrate anion-radical and chain termination due to oxidation of product anion-radical. An illustrative example is the inhibition of nucleophilic substitution of 2-chloroquinoxaline by the radical trap bis(tcrt-butyl)nitrone (Carver et al. 1982). 

Dimesityldioxirane, a crystalline derivative, has been isolated by Sander and colleagues and subjected to X-ray analysis. The microwave and X-ray data both suggest that dioxiranes have an atypically long 0—0 bond in excess of 1.5 A. Those factors that determine the stability of dioxiranes are not yet completely understood but what is known today will be addressed in this review. A series of achiral, and more recently chiral oxygen atom transfer reagents, have been adapted to very selective applications in the preparation of complex epoxides and related products of oxidation. A detailed history and survey of the rather remarkable evolution of dioxirane chemistry and their numerous synthetic applications is presented in Chapter 14 of this volume by Adam and Cong-Gui Zhao. Our objective in this part of the review is to first provide a detailed theoretical description of the electronic nature of dioxiranes and then to describe the nuances of the mechanism of oxygen atom transfer to a variety of nucleophilic substrates. 

Bis(trimethylsilyl)monoperoxysulfate 6 is also an excellent agent for oxygen transfer to nucleophilic substrates such as alkenes and heteroatoms. Compound 6 could oxidize alkenes such as 1-methylcyclohexene and fraw5-/3-methylstyrene, producing 2-methyl-cyclohexanone and benzyl methyl ketone, respectively, in high yield, most likely via the... 

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 second correlation shows that peroxo complexes with k ax values below 400 nm, stretching freqnencies below 900 cm and O-NMR chemical shifts for peroxo moiety below 600 ppm are more efficient oxidants of nucleophilic substrates. 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

Alkyl hydroperoxides can oxidize a variety of other nucleophilic substrates in the presence of d° metal catalysts. Thus molybdenum and vanadium catalysts have been used for the selective oxidation of tertiary amines to the corresponding JV-oxides (equations 79 and 80).225,254... 

Homolytic catalysis is observed with both organometallic and coordination complexes. It is involved in a wide variety of metal-mediated transformations, often in competition with electrophilic or nucleophilic catalysis [11], For example, many metal-catalyzed oxidations involve substrate activation by homolytic catalysis (Eq. 5) [12], Similarly, oxidative additions (Eq. 6) and dioxygen activation (Eq. 7) can proceed via two-step homolytic mechanisms. 

A similar distinction between a system with pre-electrolysis with only one electrode (in this case anodic) process, and a system with simultaneous anodic and cathodic processes (in which anode and cathode are on opposite walls of a microchannel so that each liquid is only in contact with the desired electrode potential, analogous to the fuel cell configurations discussed above) was made by Horii et al. (2008) in their work on the in situ generation of carbocations for nucleophilic reactions. The carbocation is formed at the anode, and the reaction with the nucleophile is either downstream (in the pre-electrolysis case) or after diffusion across the liquid-liquid interface (in the case with both electrodes present at opposite walls). The concept was used for the anodic substitution of cyclic carbamates with allyltrimethylsilane, with moderate to good conversion yields without the need for low-temperature conditions. The advantages of the approach as claimed by the authors are efficient nucleophilic reactions in a single-pass operation, selective oxidation of substrates without oxidation of nucleophile, stabilization of cationic intermediates at ambient temperatures, by the use of ionic liquids as reaction media, and effective trapping of unstable cationic intermediates with a nucleophile. 

Catechols and Hydroquinones. Just as quinones are ideal examples of electrophilic substrates, their fully reduced form (catechols and hydroquinones) illustrates the electrochemical oxidation of aromatic nucleophilic substrates (Lewis bases). Figure 12.3a, b illustrates the oxidation of 3,5-di+m-butyl-catechol (DTBCH2) via an irreversible two-electron process (ECEC) to give the o-quinone (DTBQ) 12... 

Electronic Effects. Singlet oxygen is an electrophilic oxidant that exhibits a clear preference for reactions with nucleophilic substrates. This preference is strikingly evident in a comparison of the rates constants for ene reactions of simple methyl substituted alkenes 2,3-dimethyl-2-butene (A = 2.2 x 107M-1s-1) [19] reacts more than 30 times faster than the tri-substituted alkene 2-methyl-2-butene (k = 7.2 x 105 M-1s-1) [19] and more than 500 times faster than the di-substituted alkene Z-2-butene ( = 4.8 x 104M-1s-1) [19]. The practical implications of these electronic effects are... 

In contrast, reaction of 2-nitrobenzo[. ]thiophene with -butylamine under the same conditions did not lead to any ring-opened product. Instead, the substrate underwent an oxidative nucleophilic substitution reaction. 

The basic concept underlying alkylation reactions of aromatics is the formation of a stabilized carbocation able to attack nucleophilic substrates. Hydrocarbon cracking and hydrocracking, alkane isomerization, and olefin alkylation are important processes based on related alkane carbocation chemistry in the production of various types of hydrocarbons such as the branched ones for high octane gasolines. Zeolites and metal oxides are the preferred catalysts. 

They were interpreted by assuming the formation of a or-complex only. On the basis of Colonna s work with 139 (X=SCH3) it is obvious that radical pair formation is observable only if the nucleophilic substrate has an extremely low oxidation potential. 

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