Dehalogenation, substitutive electrophilic

When excess alkene is employed, the addition of the hypohalite to the alkene takes place more readily than the above-mentioned substitutive electrophilic dehalogenation reaction. Based on the structures of stereoisomers obtained from the reactions of hypochlorite with cis- and trans-CHF=CHF and trans-CHC =CllC a regio- and stereospecific syn-addition mechanism was suggested (equation 76). Methyl and trifluoromethyl esters of... 

A paper concerned with the synthesis of pyridazino[3,4-fe]indoles 18 included a study of various conversions of 4,5-dichloro-2-methylpyridazin-3-one 17 including nucleophilic substitutions, Suzuki reactions and electrophilic substitution (nitration), combined with reductive dehalogenation, and usefully summarised previous work <06T121>. 

The cathodic substitution of halides has been applied in the synthesis of antiinflammatory agents. Arylpropionic acids are formed by reductive dehalogenation of the corresponding benzylic halides in the presence of carbon dioxide as electrophile [22] ... 

It is observed that a halogen para to the carbonyl moiety of 2(l//)-pyrazinones is extremely inert towards nucleophilic displacement, whereas an ortho halogen is displaced relatively readily. However, use of butyllithium brings about lithio-dehalogenation in 5-bromo- or 5-iodo-2(l//)-pyrazinones (104), which are prepared from the corresponding 2(l//)-pyrazinones (103) with NBS or NIS, respectively <91H(32)2407>. Electrophilic reagents with these lithio intermediates (105) then afford the 5-substituted 2(l//)-pyrazinones (106 Scheme 24). 

It seems unlikely that the influence of the metal is of an electrophilic nature, inducing a carbonium-ion mechanism, as the products of substitution and dehydrohalogenation are never found simultaneously. More plausibly the metal donates electrons, and carbanion or concerted processes with carbanion character rather than radical processes are usually favoured. Radical additions occur very easily to olefinic double bonds and l-bromo-l,2-diphenylethane undergoes radical bromination with N-bromosuccinimide without accompanying dehalogenation. ... 

The mechanisms of dehalogenations have been reviewed by Miller and in a series of papers , the stereoselectivity of the dehalogenation of the stilbene dibromides with a wide variety of reagents has been discussed. The meso-stilbene dibromide always eliminates to give the thermodynamically more stable alkene, namely tra 5-stilbene which is product of apparent a t/-elimina-tion. However, the J/-stilbene dibromide gives both cis- and rm i-stilbenes, and the ratio of these products can provide useful mechanistic information. One-electron reductants, such as chromous ion, give rise to intermediate radical formation in which rotation about the Ca-Cg bond allows thermodynamic control of the reaction. Two-electron reductants, such as iodide ion in dimethyl formamide, induce highly stereoselective a i-elimination. In protic solvents, carbonium ion intermediates were proposed to explain the trend towards thermodynamic control. Miller has proposed a reaction mechanism which embraces elimination, substitution, and electrophilic addition to alkenes. 

Pyridine itself can be converted into 3-nitropyridine only inefficiently by direct nitration even with vigorous conditions, however a pair of methyl groups facilitate electrophilic substitution sufficiently to allow nitration to compete with side-chain oxidation. Steric or/and inductive inhibition of A-nitration allows C-substitution using nitronium tetrafluoroborate, an example is 2,6-dichloropy-ridine dehalogenation of 2,6-dichloro-3-nitropyridine provides a practicable preparation of 3-nitropyridine. ... 

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