Single electron transfer cleavage process

The first step of the reaction involves the dimerization of MPG via single electron transfer process by Ti(III) and the sequential Ti(IV)-catalyzed intramolecular heterolytic cleavage of the dimer, regenerating M PG and the nucleophilic radical (Equations 14.25 and 14.26). 

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... 

Since almost all stable species are even electron and spin paired (molecular oxygen is the obvious exception), radical processes must start either by single electron transfer or by homolytic cleavage. It is important to recognize these initiation steps in order to know when to use the one-electron paths from this chapter rather than the two-electron paths described previously. Half-headed arrows are used to symbolize the movement of one electron. Homolytic cleavage is the simple extension of a bond-stretching vibration (Fig. 11.4). The process is always endothermic, for the barrier must be at least equal to the strength of the bond cleaved. 

A laser flash photolytic study of the reaction between 2,2 -dipyridyl and tryptophan has been described. The primary photochemical step has been demonstrated to be pH independent and involves an electron transfer from the tryptophan to the dipyridyl triplet state. The triplet excited state of some peptide conjugates is produced on irradiation by a nanosecond laser flash. C-C Bond cleavage is the result of irradiation of the pinacols (214) in chloroform. This yields the corresponding aldehydes. The mechanism of the cleavage process has been shown to involve single electron transfer with chloroform as the electron acceptor. A study of intramolecular charge separation in aminophenyl(phenyl)acetylene and A, A-dimethylaminophenyl(phenyl)-acetylene has been reported. ... 

Evidence has been provided for the intermediacy of A-methyl-A-phenylnitre-nium ion in the photolysis of iV-(methylphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate. Substituted aryltropylium ions have been generated by photolysis of a series of 7-methoxycycloheptatrienes, and found to have lifetimes strongly dependent on the donor capacity of the aryl substituent. Photolysis of a mixture of the two pinacols (96 and 97) in acetonitrile resulted in efficient fragmentation of both compounds, by cleavage of the central C-C bonds. This reaction has a quantum yield of 9 1, which suggest a chain process initiated by single electron-transfer quenching of excited (96) by (97). 

The reaction mechanism appears to be similar to those proposed for the alkylation and arylation of C(sp )-H bonds (Schemes 7 and 11). Mechanistic studies indicated that (1) the C-H bond cleavage is reversible and is not a difficult process, even in the case of strong C(sp )-H bonds (2) the oxidation of a Ni(0) species to a Ni(II) species occurs, which is the actual catalytic species, by the Ar-I with the generation of the respective Ar-H and (3) a single-electron transfer (SET) was not involved, based on radical trapping experiments with TEMPO. A proposed mechanism for the reaction is shown in Scheme 20. Coordination of the amide 46 to the... 

---