Reaction of 1,3,5-tri-tert-butylbenzene with 2,4,6,8-tetraiodo-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione

Abstract

Reactions of 1,3,5-tri-tert-butylbenzene with 2, 4,6,8-tetraiodo-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione in acetic and trifluoroacetic acids involve substitution of one or two tert-butyl groups in the aromatic ring with formation of mono-, di-, and triiodo-substituted derivatives. No iodo derivatives are formed in acetonitrile.Iodo derivatives of trialkylbenzenes are convenient models for studying steric effects of substituents and deformations of the benzene ring. Direct iodination of trimethylbenzene (mesitylene) readily occurs with formation of iodo derivatives in high yield, whereas introduction of iodine into benzene ring containing tert-butyl groups involves some specificity. Monosubstituted tert-butylbenzene undergoes smooth iodination [1-6], while iodination of di-tert-butylbenzenes is not selective, and it leads to the formation of a considerable amount of ipso-substitution products [1,5]. This is especially typical of 1,4-di-tert-butylbenzene [1,5]. Direct iodination of 1,3,5-tri-tert-butylbenzene (I) was not studied.tert-Butyl groups in compound I can be replaced even in the bromination process, in particular in acid medium. According to [7], the bromination of I with acetyl hypobromite generated in situ by reaction of bromine with silver perchlorate in acetic acid yields not only 1,3,5-tri-tert-butyl-2-bromobenzene but also acetoxylation product, 2,4,6-tri-tert-butylphenyl acetate and products of replacement of one tert-butyl group by the halogen atom or acetoxy group, the substrate conversion being 55%. Even more complex mixture of compounds, including ipso-substitution and diarylation products, was obtained in the bromination of I with 2,4,6,8-tetrabromo-2,4,6,8-tetraazabicyclo-[3.3.0]octane-3,7-dione [8].The calculated (PM3) enthalpies of electrophilic (S E ) and ipso-iodination (S i ) of compound I, ΔH = 500.90 and -258.95 kJ/mol, respectively, indicate that ipso-iodination is thermodynamically more favorable than electrophilic iodination. We examined the behavior of compound I in reaction with 2,4,6,8-tetraiodo-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione (II) in acetonitrile and organic acids, acetic (in the presence of sulfuric) and trifluoroacetic. We previously showed that tetraiodoglycoluril II is a soft iodinating agent which readily reacts with polyalkylbenzenes in the temperature range from 0 to 20°C [9].