Carbon-halogen bond fission

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

Aryl halides have been used with moderate success as photoaffinity reagents and they react in a process initiated by homolytic fission at the carbon-halogen bond (Sharma and Kharash, 1968, Grimshaw and de Silva, 1981). 

Alkyl halides undergo photoreduction on irradiation in methanol or diethyl ether (in competition with nucleophilic substitution), but the mechanism of reduction is not simple homolytic fission of the carbon-halogen bond, followed by hydrogen abstraction.85 Thus, irradiation of (41a) or (41b) in MeOD gives... 

In the fission of a carbon-halogen bond, the dropping mercury electrode can be considered to function as an electron source and the carbon to which the halogen is attached as an electrophilic reaction center. We can then depict a t)q)e of nucleophilic displacement reaction on this carbon atom with electrons acting as the displacing agent. 

Just as in the case ofa-participation, the electrons of the neighbouring 7t-bond interact covalently with a carbon from which a halogen, a tosylate or other leaving group detaches, contributing thereby to the bond fission. This interaction (intramolecular nucleophilic substitution) occurs at the backside of the leaving group. 

Instead, the first step of the reaction with a tertiary halogenoalkane involves ionization of the halogenoalkane through the breaking of its carbon-bromine bond. Tbis is an example of beterolytic fission. The pair of electrons in the bond both end up on the halogen, forming the bromide ion. 

This chapter describes the photochemical C-X bond fission in alkene systems that have halogen atoms bonded to the carbon-carbon double bond. The substitution of halogen atoms in alkenes results in a red shift of the n,n absorption band because of the interaction between the lone pair on the halogen atom and the 7t-orbitals. > However, the UV absorption of aliphatic alkenyl halides, except for the iodides, lies in a region of short wavelength less than 254 nm (Table 11.1). Therefore, the majority of the alkenyl halides studied in these photolyses are the iodides. 

Photochemical C-X fission in aryl halides may be defined as any photochemical process leading to cleavage of a carbon-to-halogen bond. The emphasis in this review is on mechanisms of C-X bond fission, of which three mechanistic reaction types may be identified. 

The electron-withdrawing effect of the halogen, coupled with that of the carbonyl oxygen, leads to a very electron-deficient carbon, and this is not effectively counteracted by the lone pairs on halogens such as chlorine. Consequently, the carbonyl carbon atom is very sensitive to nucleophilic addition to form a tetrahedral intermediate. The collapse of the tetrahedral intermediate with the expulsion of the halide ion, which is a good leaving group, enhances the reactivity of the acyl halides (Scheme 3.64a). The direct fission of the acyl halide C-X bond leads to the formation of an electrophilic acylium ion (Scheme 3.64b). 

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