Photolytic halogenation

We have seen that photolytic halogenation usually leads to a wide variety of products with little or no selectivity. However, the bromination of alkyl bromides gives about 90 per cent substitution on the adjacent carbon. Furthermore, if the adjacent carbon is chiral then it retains its configuration. Suggest a mechanism that may account for these experimental observations. 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

Another very useful photolytic homolysis is that of halogen molecules to yield atoms,... 

A project at the University of Arizona (FEDRIP 1996) will study microbial dehalogenation of several compounds, including chloroform. A major part of the study will focus on the facultative anaerobic bacteria Shewanella putrefaciens sp., which is known to catalyze the transformation of carbon tetrachloride to chloroform and other as yet unidentified products. The organic substrates will also contain metals. It is hoped that the end-products from the biochemical treatment can be subjected to a photolytic finishing process that will completely mineralize any remaining halogenated compounds. 

Organotellurium tri- and dihaUdes undergo a-elimination by oxidative, photolytic or thermal routes, giving the corresponding halides with a selective transference of the halogen at the position where the tellurium moiety was originally attached (/pxo-substitution). 

The time-resolved spontaneous emission from an electronically excited halogen atom X(np5 2Py2) produced in a flash photolysis process in a static system is found experimentally to be described by an over-all first-order kinetic process for a long interval following the photolytic pulse.11 Thus the intensity of emission (Iemm) is given by... 

In the light-induced reactions with diazoalkanes, methylene, the photolytic fragment from diazomethane, may be interposed between the halogen atom in the organic halide and the carbon atom to which it is attached,289 290 or may add to the double bonds in the aromatic ring.58 69... 

The present review article is concerned with the production and utilization of alkoxy radicals generated by photolytic cleavage of the type (X = NO, halogen, etc.)... 

Why and how are nuclei of silver formed by the photolysis What becomes of the other photolytic product, the hole or halogen, and how important is it to photographic sensitivity How do crystal defects and impurities influence sensitivity and latent image formation How can radiation absorbed by dyes at the crystal surface induce formation of silver My intention in this chapter is to review recent experimental and theoretical work bearing on these questions and to offer some conclusions as to what we know, what we think we know, and what can be decided only by future investigations. 

Recombination fluorescence has been used to study the decay of radical ion pahs generated photolytically.288 Simulation of quantum beats caused by hyperfine interaction in the R and R+ enable the values of hfc to be determined for very short-lived species. In the case of one R excellent agreement with the value of hfc as determined by ESR is reported. The primary reaction in the photolysis of 1-arylalkyl radicals (128) is the heterolytic cleavage of the -halogen (X), generating the radical cation (129).289... 

Typical radical reactions are substitution and addition reactions as shown below (Scheme b). A typical substitution reaction is the halogenation of methane with chlorine gas under photolytic conditions, and generally available chlorohydrocarbons are prepared by this method. The chlorination reaction proceeds through a chain pathway via the initiation step, propagation step, and termination step as shown below (Scheme 1.1). 

In some ylides photolytic conditions were necessary for their transylidation [30]. The conversion of iodonium ylides into a-halogeno derivatives of the parent carbonyl compound (or other precursor) with hydrogen halides is normally effected directly, without isolation of their iodonium salts. A similar reaction with halogens leads to the formation of a,a-bis halogenated products [31]. The reaction of pyridines with the non-isolable PhI=C(CN)2 is of interest, since it permits the ready transfer of the C(CN)2 functionality to the nitrogen of pyridine, quinoline, etc. the yields here were generally moderate but in some cases the products could not be obtained using other dicyanocarbene precursors [32],... 

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

The photolytic or thermal decomposition of 0-acyl thiohydroxamates in halogen donor solvents such as tetrachloromethane or bromotrichloromethane constitutes the most wide-ranging and generally iplic-... 

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