Single electron transfer , amines

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

Since amines generally have low oxidation potentials, they are good electron donors in their ground state, and the donor ability is further enhanced by photoexcitation. The chemical consequence of this single electron transfer (SET) is the generation of the amine radical cations (aminium radicals) and an earlier review on the aminium radicals is available1. 

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

Photo-oxidation of l,l-dialkyl-2-arylhydrazines by single-electron transfer with trimethylsilyl cyanide (TMSCN) as cyanide ion source leads to regio- and stereoselective a-hydrazino nitriles. This stereoselective cyanation of hydrazines takes place on the more substituted carbon atom compared with the results obtained with tertiary amines (Scheme 5). 

A water-soluble Cj-symmetrical trisadduct of Cjq showed excellent radical scavenging properties in vitro and in vivo and exhibits remarkable neuro-pro tective properties [7,8]. It is a drug candidate for the prevention of ALS and Parldnsoris disease. Concerning the reaction mechanism, nucleophilic additions and radical additions are closely related and in some cases it is difficult to decide which mechanism actually operates [92]. For example, the first step in the reaction of f-eo with amines is a single electron transfer (SET) from the amine to the fullerene. The resulting amines are finally formed via a complex sequence of radical recombinations, deprotonations and redox reactions [36]. 

U. C. Yoon u. P.S. Mariano, Acc. Chem. Res. 25, 233-240 (1992) . .Mechanistic and Synthetic Aspects of Amine-Enone Single Electron Transfer Photochemistry". 

An analogous single electron transfer producing two-electron reduced products was encountered in Whitten s study of the photoreduction of thioindigo by amines, eq. 99 (304) ... 

During the reaction silver(0) deposits on the sides of the reaction vessel. The sequence of events leading to the azomethine ylide is unclear. However, evidently single-electron transfer (SET) from the amine to silver takes place either prior or subsequent to fluoride-enabled silyl cleavage. This process is repeated with a second equivalent of silver fluoride resulting in the formation of 38 either in free form or more likely as its silver complex. The scope of the method was expanded to the synthesis of bicylic systems exemplified here by tropinone 42 (Scheme 2.11).19 Pandey has also extended the protocol to the synthesis of tricycloalkanes20 and applied it to a total synthesis of the poisonous frog alkaloid epibatidine.21... 

The mechanism by which amines are oxidised by flavoproteins has been an issue of considerable debate in recent years. The debate has been particularly heated in the case of the enzyme monoamine oxidase (Silver-man, 1995). Through the use of a variety of mechanism-based inhibitors and based on studies of nonenzymic mechanisms of amine oxidation, a mechanism for monoamine oxidase in which substrate is initially oxidized by single electron transfer to the enzyme flavin to give an aminium cation... 

The SET (single electron transfer) mechanism can be invoked in the transannular addition of secondary amines to 1,1-diarylalkcncs in the presence of a catalytic amount of butyllithium, sodium naphthalenide or potassium hydroxide52, since the regiochemistry which is observed is opposite to that predicted by assuming the formation of metal amides. 

Simple amines in the presence of Oxone oxidize alkenes to oxiranes. For example, Oxone, pyridine, and a 2-pyrrolidine derivative in a medium of aqueous acetonitrile selectively converts the triene in Equation (72) to a single epoxide. This process also proceeds using noncyclic alkenes. The mechanism is believed to proceed via a single-electron transfer (SET) process involving radical cation intermediates <2000JA8317>. 

A radical cation can be chemically produced by single-electron transfer to commercially available stable amine radical cation salts. Again, the radical ion is more reactive than the neutral species. 

Selenolates such as Na2Se, NaSeH, PhSeNa, etc., and tellurolates such as NapTe, NaTeH, PhTeNa, etc., are excellent nucleophiles and can reduce a variety of functional groups by nucleophilic attack or single electron-transfer. On treatment with alkali metal selenolates (or amine salts of HpSe and PhSeH), reduction or reductive selenation of ketones and aldehydes, C=C reduction of a,/l-unsaturated compounds, and reduction of nitrogen compounds such as nitro compounds occur successfully [118, 176]. Compared with these selenolate anions, the corresponding tellurium compounds are highly reactive not only toward the same substrates but also toward halo compounds such as a-bromo ketones and vic-dibromoalkanes [46, 52, 177]. 

Jeon, Y. T., Lee, C. P, Mariano, P. S., Radical Cyclization Reactions of a Silyl Amine a,P Unsaturated Ketone and Ester Systems Promoted by Single Electron Transfer Photosensitization, J. Am. Chem. Soc. 1991, 113, 8847 8863. 

N,N-Disubstituted O-Alkylhydroxylamines. In the amination with a series of N,N-disubstituted O-methylhydroxylamines, more bulky alkyllithium compounds react more readily (product 4, Eq. 10).85 The small amounts of products 5 are the result of elimination of methanol from the substrate to give the imine followed by addition of RxLi to the latter. Reagents where R2, R2 is H, Me or Me, Me do not react. A single-electron-transfer process involving a nitrogen radical has... 

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