Radicaloid Substitution

Radicaloid substitution has not been extensively studied in the thiophene series. Molecular orbital calculations indicate that substitution should occur in the a-position. This has been found to be the case in the Gomberg-Bachmann coupling of diazohydroxides with thiophenes which has been used for the preparation of 2-(o-nitro-phenyl) thiophene, 2-(p-toluyl) thiophene, and 2-(p-chloro-phenyl)thiophene. Coupling in the /8-position has been used for the preparation of 1,3-dimethyl-4,5-benzisothionaphthene (148) from 2-amino-tt-(2,5-dimethyl-3-thienyl)cinnamic acid (149). A recent investigation describes the homolytic phenylation of 2- and 3-phenyl- 

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Nucleophilic and Radicaloid Substitution Reactions of Monocyclic Thio-phens. - The rate constants for the reaction of 5-substituted 2-bromo-3-nitrothiophens, in methanol, with various substituted anilines to give 5-substituted iV-(3-nitro-2-thienyl)anilines have been measured at various temperatures, It was found that the sensitivity parameters that were obtained from Hammett and Bronsted correlations were practically independent of the starting system. The kinetics of piperidino-substitution of methyl 2-methoxy-3-nitrothiophen-5-carboxylate and 5-acetyl-2-methoxy-3-nitrothio-phen, in which the methoxy-group is the leaving group, have been investigated and the mechanism has been discussed. 

Nucleophilic and Radicaloid Substitution Reactions of Monocyclic Thiophens. - The kinetics of piperidino- and benzenethiolato-dehalo-genation of some 4-substituted 2,3-dihalogeno-5-nitrothiophens in methanol has been studied at various temperatures. It was found that the activating effect of the 3-halogen-substituent was... 

That is, the act of shifting the single electron from Y to X may occur either with or without free-radical formation. Usually, the concerted non-radicaloid process is energetically favoured. For a more detailed discussion of the various mechanisms of nucleophilic substitution reactions in aliphatic compounds and their solvent dependence, see references [14, 483, 782-785]. 

Cyclopropanes exhibit similar modes of reactivity. [2Dipolar additions with electron-deficient alkenes and electron-donor-substituted cyclopropanes, additions of electron-rich alkenes to electron-deficient cyclopropanes, a number of radicaloid reactions and intramolecular photochemical cycloadditions are known, which may be described by the general scheme H-2 3. 

A more suitable explanation can be suggested for this case, which makes use of the present model. Because at the transition state the substrate acquires a partial radical-anionic character, the radical anion of the substrate should be examined. The positional selectivity will most likely be determined by the location of the unpaired spin population in the model radical anion, toward which the radicaloid nucleophile will be attached in order to complete bond formation. A similar argument was invoked by Kochi (27) to explain the positional selectivities observed in electrophilic aromatic substitution. 

Basically, any reaction involving radicals, or with a radicaloid transition state, can have its selectivity and efficiency affected by captodative substitution. Although the effect is small, even a change of a few kcal mol in the activation energy of a given pathway can have a tremendous effect on selectivity. The present survey has... 

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