Positive halogen compounds

Foucaud A (1983) Positive halogen compounds. In Pattai S, Rappoport Z (eds) The chemistry of functional groups, supplement D. Wiley, New York, pp 441-480... 

The elemental halogens are not the only source of electrophilic halogen atoms, and, for some synthetic purposes, other positive halogen compound may be preferable... 

Walling C, Wagner PJ (1964) Positive halogen compounds. X. Solvent effects in the reactions of f-butoxy radicals. J Am Chem Soc 86 3368-3375... 

The simplest and, unfortunately, perhaps most generally perceived mechanism suggests that the halonium ion, X+, itself is the halogenating agent which is either innately present in the positive halogen compound or formed in situ from such compounds (equation 1). While substantial data now support the existence of X+ in the gas phase1, there is... 

Positive halogen compounds III. NITROGEN-HALOGEN COMPOUNDS... 

Several positive halogen compounds such as NBS, NCS, (dichlotoiodo)benzene 0 hICl2), sulforyl chloride, thionyl chloride, 2,4,4,6-tetrabromocyclohexa-2,S-dienone and diethyl bromomalonate etc. react with Mols to produce initially RSX, which dien reacts widi excess diiol to give disulfides. Tri-... 

Many positive halogen compounds such as NCS," 1-chlorobenzotriazole," and chlo-... 

This positive halogen compound oxidizes alcohols to aldehydes or ketones and hydrazo compounds to azo compounds under mild conditions and in high yields. The oxidant is converted into bcnzo-triazole hydrochloride. It also oxidizes 1-amino-benzotriazole to benzyne (80% yield, 2 moles of oxidant required) and 2-amino-benzotriazole to cis.cis-1,4-dicyanobutadienc-1,3. ... 

Sulfides - sulfoxides. Like /-butyl hypochlorite, this positive-chlorine compound oxidizes sulfides to sulfoxides without formation of sulfones. The reaction is carried out in methanol at —78°. Yields are usually high.2 Note that oxidation of sulfides of the type RSCH2R with positive-halogen compounds results in part or entirely in substitution at the a-methylene group.3... 

Source C. Walling and W. Thaler, Positive Halogen Compounds. III. Allylic Chlorination with t-Butyl Hypochlorite. The Stereochemistry of Allylic Radicals, Journal of the American Chemical Society 83 3877-3884 (1961). With permission. 

Somewhat less clear is the nature of catalysis by halogens and positive halogen compounds, such as I2 [176, 177], NBS [178], tetra-N-butylammonium tribromide [179], and hexamethylenetetramine-bromine complex [180]. These compounds would be expected to react with indoles by halogenations, generating the hydrogen halide as a by-product. Another halogenated catalyst, 2,4,6-trichlorotriazine, is believed to function by generation of HCl in the presence of moisture [181]. 

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