Halogen transfer reactions

Halogen Transfer Reactions from bis-Amino Halonium Ions to Acceptor Olefins Mechanism and Strategies for Chiral Halogenation... 

The higher halides MCI5 and MBrs show much less aftinity for Cl and Br than does MF5 for F. Halogen transfer reactions are nevertheless observed, mostly when these MX5 halides are allowed to react with AX (A+ = alkali metals, NH4, NR4, PPlq, AsPlq), thus giving A+MXe. The octahedral symmetry of the anions is favorable for... 

These kinetics data are consistent with a preequilibrium dissociation of dmf from the molybdenum center to form a reactive five-coordinate species that rapidly reduces the Fe(III) center via an inner sphere (halogen transfer) reaction. Other one-electron atom transfer reactions are known in oxo-molybdenum chemistry (262). An innersphere (atom transfer) mechanism is not a viable model for intramolecular transfer in sulfite oxidase because in the enzyme the Mo and Fe centers are almost certainly held too far apart by the protein framework. Moreover, the 65-type heme center of sulfite oxidase is six-coordinate with axial histidine ligands from the protein and hence cannot participate in atom transfer reactions. 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

Since Si—halogen bonds are considerably stronger than Si-H bonds, the converse to the carbon analogues, the halogen transfer reactions of silyl radicals are fast. This is illustrated by the reactions between alkyl halides and trichlorosilane initiated by irradiation from a medium-pressure mercury arc [376—378]... 

The second publication is a review article by Ingold [390] on rate coefficients for free radical reactions in solution which includes comprehensive coverage of radical—molecule reactions. Metathetical reactions are usually referred to as Sn 2 reactions, i.e. substitution, homolytic and bimolecular, by organic chemists. Most quantitative kinetic studies of this class of solution reactions involve H atom transfer but halogen-transfer reactions have also been studied. 

Electron- and halogen-transfer reactions are important in a number of reactions... 

We would expect to see manifestations of CSB on reactivity for cases that involve cleavage of CS bonds. Our studies [81] showed that one of these manifestations is the computational [82-86] and experimental [87] results that halogen transfer reactions (and especially of fluorine), Eq. (13a), have much larger barriers (by >20 kcal/mol for X=F) than the corresponding hydrogen transfer processes, Eq. (13b) ... 

---