Fragmentation radical anions

The cofragmentation of a benzylic radical cation and a fragmentable radical anion, such as that of alkyl halides, is another way of obtaining an efficient process [191-193] (Sch. 10). 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

To check the identity and purity of the products obtained in the above reactions it is not sufficient to analyze for the sulfur content since a mixture may incidentally have the same S content. Either X-ray diffraction on single crystals or Raman spectra of powder-like or crystalline samples will help to identify the anion(s) present in the product. However, the most convincing information comes from laser desorption Fourier transform ion cyclotron resonance (FTICR) mass spectra in the negative ion mode (LD mass spectra). It has been demonstrated that pure samples of K2S3 and K2S5 show peaks originating from S radical anions which are of the same size as the dianions in the particular sample no fragment ions of this type were observed [28]. 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Sulfur diimides are prone to fragmentation to produce thionitroso and, subsequently, sulfido and imido ligands.70,131 They are reduced chemically or electrochemically to the corresponding radical anions [S(NR)2], which exhibit five-line (1 2 3 2 1) ESR spectra. 

The reductive dehalogenation of polyfluoroarenes by zinc in aqueous ammonia gave products derived from the removal of one or two halogen atoms. A radical anion is suggested to form initially by direct electron transfer from the zinc to substrate which then fragments. Ceo undergoes single-electron reduction by the electron-rich. 

Chloroadamantanes (149) and (150) reacted with CH2COPh to afford the monosubstitution products (151) and (152) as intermediates, the intramolecular electron-transfer reaction of the radical anion intermediate being a slow process. Product (151) with chlorine in the 1-position reacted further to give (153), whereas (152) with chlorine in the 2-position is unreactive, showing that the 1-position is the more reactive. 1,2-Diiodoadamantane (154) reacted with CH2NO2 to give the monosubstitution products (155) and (156). This implies that the intramolecular electron-transfer reaction of the radical anion is a slow process. The fact that (155) was formed as major product and (156) was the minor product shows that, when (154) accepts an electron, fragmentation occurs faster at the 1-position than the 2-position. 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

Triethylamine as the electron donor was also used by Mattay and co-workers in tandem fragmentation cyclization reactions of a-cyclopropylketones. The initial electron transfer on the ketone moiety is followed by the fast cyclopropyl-carbinyl-homoallyl rearrangement, yielding a distonic radical anion. With an appropriate unsaturated side chain within the molecule both annealated and spi-rocyclic ring systems are accessable in moderate yields (Scheme 41) [62]. 

Scheme 42 Fragmentation and heterocyctization of oxadiazole radical anions.

Phenyl o-radicals generated by reduction of aryl halides can also interact with an intramolecular alkene bond. Ihe method has been developed for the formation of dihydroindoles by reductive cyclization of N-allyl-2-chloroacetanilides. The results indicate the importance of a time interval between electron addition to give a radical-anion and the fragmentation of this species to give the active a-radical, The time interval allows the radical-anion to diffuse away from the electrode surface so that when the a-radical is foimed, it has time to cyclize before it can be reduced at the surface. 

The mechanism of the reaction presumably involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

In all these cases and in contrast to starting a polymerization by initiators, there are no fragments of the starting molecule incorporated in the polymer chain. Consequently, the mechanisms are different to those of radical, anionic, or cationic polymerizations. 

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