Quantum-chemical study on the relation between thermodynamic parameters in the iodination of benzene with N-I and O-I reagents and pK a values of the corresponding NH and oh acids

Abstract

Thermodynamic parameters of direct iodination of benzene with several iodinating agents were calculated using semiempirical (PM3), ab initio (3-21G**), and DFT (B3LYP/LanL2DZ) methods, as well as in terms of the polarization continuum model (PCM or Tomasi model). A close to linear correlation was found between the calculated thermodynamic parameters (ΔH ≠ , ΔG ≠ ) and pK T and experimental pK a values of acids whose anions are incorporated into iodine-containing intermediates.Methods for direct iodination of aromatic compounds may be divided into two large groups. The first group includes procedures utilizing a combination of molecular iodine and an oxidant, and the set of available oxidants is very wide. Methods of the second group are based on the use of compounds having an iodine-heteroatom bond. Strong electronegativity of the heteroatom in such compounds induces a partial positive charge on the iodine atom, so that its electrophilicity increases. Examples of oxidative iodinating systems are I 2 -HNO 3 -H 2 SO 4 -AcOH, I 2 -oleum, I 2 -CrO 3 , I 2 -HgO, I 2 -HIO 3 , I 2 -HIO 4 , I 2 -KMnO 4 , I 2 -CH 3 CO 3 H, I 2 -PhI(OCOCF 3 ) 2 , etc.[1]. Most of these systems are capable of iodinating not only activated but also some deactivated arenes. In all cases, oxidation of molecular iodine with formation of electrophilic intermediates like I + or I 3 + is assumed.Iodinating reagents possessing iodine-heteroatom bonds include: iodine chloride (ICl), hypoiodites (HOI, CH 3 COOI, CF 3 COOI, HSO 3 OI, TsOI, TfOI), N-iodo imides (N-iodosuccinimide [1], N-iodosaccharin [2], 1,3-diiodo-5,5-dimethylhydantoin [3], tetraiodoglycoluril [4]), and N-iodoacetamide [5]. Iodine chloride, N-iodo imides, and N-iodoacetamide are fairly stable compounds, whereas hypoiodite-like compounds are only postulated as independent iodinating species generated in situ from iodinating reagent and the corresponding acid. It is well known that ICl, N-iodo imides, and N-iodoacetamide are weak iodinating agents, and their activity considerably increases upon addition of strong acids such as sulfuric, trifluoroacetic, trifluoromethanesulfonic, etc. For example, N-iodosuccinimide in one of the strongest acids, trifluoromethanesulfonic acid, is capable of iodinating deactivated arenes, in particular nitrobenzene, at room temperature due to formation of iodine trifluoromethanesulfonate ISO 3 CF 3 as a hypoiodite-like species [6]. Enhancement of the reactivity of N-iodosucinimide was also observed in the presence of CF 3 COOH [7] and H 2 SO 4 [8,9], presumably due to generation in situ of CF 3 COOI and IOSO 3 H. This may be illustrated by Scheme 1 where HY is a protic acid, and IY is a hypoiodite-like species with enhanced electrophilicity. Likewise, 1,3-diiodo-5,5-dimethylhydantoin [9] and tetraiodoglycoluril [10] were reported to behave similarly in concentrated sulfuric acid.Although available experimental data show that the electrophilicity of iodine species generated in strongly acidic medium considerably increases, no quantitative studies were perf...

Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride

“…Control experiments reveal no dark reaction between VDF and all HVICs (X = F and H). Under irradiation, the radical generation in the FRP mechanism is prompted by the photolysis of the weak − I–O bond to produce CX 3 COO • (Scheme , eqs 1 and 1′) . Fast ( k ≈ 10 9 s –1 ) , subsequent decarboxylation affords CX 3 • , which adds to VDF, thereby initiating the polymerization, which propagates as before, with both 1,2- and 2,1-units (eq 2).…”

Iodine Monochloride Facilitated Deglycosylation, Anomerization, and Isomerization of 3-Substituted Thymidine Analogues

“…[5, 6, 26–28] Decomposition of ICl or I 2 to acidic species (i.e. HCl and HI) in the presence of hydroxyl or sulfhydryl groups is well established [10,33–39] and there has been an extensive discussion on whether the cleavage of various protective groups mediated by e.g. I 2 , ICl, or IBr is caused by the intact halogen/interhalogen or by acidic species generated in situ via reaction with hydroxyl or sulfhydryl groups.…”

“…Hypoiodite-type compounds, such as the one postulated in pathways (A), are instable and have only been generated in situ . [39, 40] Such species are supposedly very moisture sensitive and should hydrolyze upon aqueous work-up. [41] …”