Halogen abstraction studies. IV. Abstraction of iodine by phenyl radicals from 2-substituted iodobenzenes and iodoferrocene

Abstract

The rate of abstraction of iodine by phenyl radicals from a variety of 2-substituted iodobenzenes relative to the rate of abstraction of chlorine from carbon tetrachloride is reported. Qualitatively, it is found that electron-donating substituents retard and electron-withdrawing groups enhance the rate of homolytic C-I bond cleavage. Attempts are made to correlate the observed rates with various ortho-substituent parameters as well as the extended Hammett relationship (eq 6). It is concluded that steric interactions are important in determining the rate of iodine abstraction from 2-substituted iodobenzenes but that there is no need to invoke anchimeric assistance by a neighboring bromo or iodo group. The rates of iodine abstraction from 2-iodobiphenyl and 2-iodo-2'methylthiobiphenyl are comparable, indicating no participation by the sulfide group in the abstraction process. Iodoferrocene is shown to be slightly more reactive to the phenyl radical than iodobenzene indicating a possible weak interaction of the iron atom in the abstraction process. (X) A . R . Lvons and M. C. R . Symons, ibid., 93, 7330 (1971); Chem.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

“…The electronic properties of the halogen-atom donor (R) are also important for the modulation of XAT reactivity. As an example, kinetic studies on the reaction between the heptyl radical and several α-haloesters and diesters demonstrated how the rate for both iodine and bromine abstraction increased with the increased ability of the ester groups to stabilize a partial negative charge in the TS (i.e., going from α-halopropanoates to the corresponding methyl propanoates) (Scheme A). ,, Additional evidence has been provided by Danen’s pioneering work on XAT reactions promoted by Ph•. As shown in Scheme B, abstraction from aryl iodides is accelerated in the case of 2- and 4-iodopyridines but is retarded in the case of 2- and 3-iodothiophenes.…”

“…36 Steric effects have been observed in XAT reactions and were employed to rationalize the increased rate for abstraction on ortho-substituted aryl halides by both aryl and alkyl radicals. 39 As an example, while Ph-I and 1-and 2-iodonaphthalenes are electronically very similar, there is a noticeable acceleration for XAT with Ph• in the case of 1-iodonaphthalene which was explained on the basis of a favorable release of steric strain upon iodine abstraction (Scheme 9A). 38 A similar trend in reactivity was determined in the reaction between aryl bromides and Bu 3 Sn• where highly hindered derivatives also displayed increased reactivity despite being more electron rich (Scheme 9B).…”

“…The development of synthetic strategies based on XAT processes has been largely impacted by understanding the factors which govern this radical reactivity . Fundamental work from Ingold, − Danen, ,,,, Lorand, Scaiano, Lusztyk, Newcomb, Curran, , Chatgilialoglu, , Giese, Crich and Galli has provided extensive knowledge in this respect and many absolute rate constants have been determined. While a detailed discussion of these mechanistic experiments is beyond the scope of the review, a summary of the rate constants for commonly used XAT reagents and alkyl/aryl halides is presented in Table .…”