Evaluation of the reactivity and regioselectivity of superelectrophilic iodinating systems

Abstract

Iodination of o-nitrotoluene in H 2 SO 4 or CF 3 SO 3 H at 0°C with compounds having a nitrogeniodine bond leads to the formation of isomeric mono-and diiodo derivatives whose ratio differs from the statistical value. The reaction of nitrobenzene with 2 equiv of N-I reagents in trifluoromethanesulfonic acid at 0°C takes less than 1 min and yields 79-85% of m-iodonitrobenzene. The electrophilic reactivity of the iodinating agents was estimated by quantum-chemical methods.The rate of electrophilic substitution reactions in the aromatic series and isomeric composition of the products are largely determined by the electrophile nature. In keeping with the Brown selectivity relationship, stronger electrophiles exhibit lower intramolecular selectivity in substitution reactions in the benzene ring [1]. The most reactive electrophiles give rise to a mixture of regioisomers whose ratio approaches statistical value. In the iodination with common iodinating systems, the reactivity and selectivity may be evaluated using the reaction with toluene as an example [2]. However, the procedure based on determination of the ratio of o-and p-iodotoluenes cannot be applied to reactions with superelectrophilic iodinating systems [3][4][5][6], which lead to the formation of a mixture of polyiodotoluenes containing mostly tri-and tetraiodo derivatives. Therefore, to estimate the selectivity of superelectrophilic iodinating systems we compared the compositions of products formed in the iodination of o-nitrotoluene (I) with reagents generated by dissolution of 1,3,4,6-tetraiodoglycoluril (TIG), 1,3-diiodo-5,5-dimethylhydantoin (DIH), and N-iodosuccinimide (NIS) in sulfuric acid [3, 5, 6] (Scheme 1).Traditional methods for the determination of the reactivity and regioselectivity of iodinating systems by analysis of the iodination products of nitroarene I cannot be used, for nitrotoluenes do not undergo iodination by common methods at 0°C. Detailed GC-MS study on the products of reactions of compound I with TIG-H 2 SO 4 , DIH-H 2 SO 4 , and NIS-H 2 SO 4 showed ( Table 1) that the use of stoichiometric amounts of the reagents (calculated on the active iodine) does not ensure complete conversion of I (Table 1, run nos. 1-3), and 30-35% of the initial compound remains unchanged.A similar conversion of other substrates at the same reagent-to-substrate ratio was noted by us previously [3][4][5][6]. Approximately similar selectivities were observed in the iodination with TIG-H 2 SO 4 and DIH-H 2 SO 4 . In the reaction with N-iodosuccinimide, the fraction of 4-iodo-2-nitrotoluene (II) was slightly larger than the fraction of 6-iodo-2-nitrotoluene (III), but the substrate conversion was lower.The reactions in the presence of 2 equiv of the iodinating agents (Table 1, run nos. 4-6) are characterized by appreciably reduced para-selectivity. However,

High-Performance Ammonium Hypoiodite/Oxone Catalysis for Enantioselective Oxidative Dearomatization of Arenols

“…On the other hand, oxidation of 2a under catalytic conditions using 10 mol % of either Bu 4 N + [I 3 ] − or I 2 in the presence of oxone afforded 3a quantitatively (see reactions 7 and 8 in Scheme ). These results suggested that highly reactive I + species such as hypoiodous acid or sulfatyl hypoiodite, [SO 4 I] − , , might be generated from I 2 or [I 3 ] − in the presence of HSO 4 – (for I 2 ) or oxone (for both I 2 and [I 3 ] − ). In contrast to the catalytic conditions, the low conversion of stoichiometric reactions with I 2 in the presence of Bu 4 NHSO 4 or Bu 4 NOH additives might be attributed to the generation of I 3 – , an inert species, from the fast reaction of I 2 or IOH species with I – , which is generated as the reaction progresses (see reactions 2 and 3 in Scheme ).…”

Superelectrophilic Iodination of Deactivated Arenes with Triiodoisocyanuric Acid

“…As an extension of our search for more potent and practical electrophilic halogenation reagents, 11 in the present work we describe our results on the iodination of deactivated aromatic rings using triiodoisocyanuric acid in acidic medium, as the source of superelectrophilic iodine. 12 The iodination of chlorobenzene with TICA in acetonitrile was not observed after 72 hours at room temperature. However changing the solvent to 98% sulfuric acid led to the formation of several polyiodinated products, while using sulfuric acid diluted with acetic acid gave a cleaner reaction; the monoiodinated products were obtained using H 2 SO 4 -AcOH 1:8 (Scheme 1).…”