Carbon Iodine Research Articles

Synthesis and solid state structures of increasingly sterically crowded 1,4-diiodo-2,3,5,6-tetraarylbenzenes: a new series of bulky benzenes and aryls

A series of very bulky 1,4-diiodo-2,3,5,6-tetraarylbenzenes I2Ar4C6 (Ar = 4-tBuC6H4, 1; Ar = 3,5-tBu2C6H3, 2; Ar = 2,4,6-Me3C6H2, 3) have been prepared by a rapid, convenient procedure involving sequential reaction of hexabromobenzene with the appropriate Grignard reagent in excess, followed by addition of excess elemental iodine. The new materials 2 and 3 were isolated in 18 and 28% yields, respectively. The solid state structures of 1–3 have been investigated by single crystal X-ray techniques. Interesting distortions in the structures of these hexasubstituted benzenes are observed upon the increasing steric pressures in 1–3. For example, while the structures of 1 and 2 contain central planar benzene rings, steric interactions in 3 produce a non-planar central benzene ring. Regardless of these factors, the carbon–iodine bond lengths in 1–3 are essentially constant (2.108–2.118 Å).

Electrochemical reduction of benzyl iodide, benzal bromide, and benzal chlorobromide at carbon cathodes in the presence of nitric oxide in acetonitrile

Cyclic voltammetry (CV) and controlled-potential electrolysis have been employed to investigate the electrochemical reductions of benzyl iodide, benzal bromide, and benzal chlorobromide at glassy carbon cathodes in the presence of nitric oxide (NO) in acetonitrile containing tetramethylammonium tetrafluoroborate. One irreversible cyclic voltammetric wave, due to cleavage of the carboniodine bond, is observed for the reduction of benzyl iodide, whereas cyclic voltammograms for the reductions of benzal bromide and benzal chlorobromide exhibit two cathodic waves, arising from sequential cleavage of carbonhalogen bonds. At appropriate potentials, bulk electrolyses of the three starting compounds involve, respectively, the generation of benzyl, bromophenylmethyl, and chlorophenylmethyl radicals which can couple with NO, and the products include benzaldehyde oxime, benzonitrile, and O-benzyl benzaldehyde oxime, as well as toluene, bibenzyl, and cis- and trans-stilbene. Mechanisms for the formation of the various products are discussed and, using CV, we have estimated the concentration of NO in acetonitrile under the extant experimental conditions.

Adsorption of iodobenzene (C 6H 5I) on Au(1 1 1) surfaces and production of biphenyl (C 6H 5–C 6H 5)

Thermal decomposition of alkyl halides has been widely used for generating adlayers of hydrocarbon species or fragments on metal surfaces. We investigated the possibility of using the thermal dissociation of iodobenzene (C 6H 5I) as a method for generating chemisorbed phenyl groups bound to Au(1 1 1) surfaces. Iodobenzene desorbs molecularly in three peaks at 290–308, 212 and 188 K in temperature programmed description (TPD) and partially decomposes at 200–250 K to form coadsorbed biphenyl and iodine on the surface. Biphenyl species produced from C 6H 5I decomposition, or from gas-phase biphenyl dosing, are chemisorbed reversibly and molecularly, with the molecular plane parallel to the surface. These strongly chemisorbed species do not desorb until 400 K. Iodine desorption at 720 K leaves a clean Au(1 1 1) surface following TPD of C 6H 5I adlayers. Carbon–iodine (C–I) bond dissociation to produce phenyl groups (C 6H 5) occurs at a higher temperature than that required for phenyl–phenyl coupling reactions. Thus, we do not detect the formation of adsorbed, isolated phenyl groups from iodobenzene thermal dissociation on the Au(1 1 1) surface.

Addition-insertion-elimination reactions of O(3P) with halogenated iodoalkanes producing HF(v) and HCl(v)

The reaction of CH2ICF3 and other fluorinated or chlorinated iodoalkanes with O(3P), generated by microwave discharge of O2 or 193 nm photolysis of SO2, produces vibrationally excited HF(v) or HCl(v), as observed by steady state or time-resolved Fourier transform infrared (FTIR) emission spectroscopy. This process occurs even in competition with possible pathways to form HOI or IO products. The proposed mechanism is an addition–insertion–elimination process. The nascent vibrational distribution of the HF(v) produced from O+CH2ICF3 is determined to be 0.58±0.10, 0.29±0.08, and 0.12±0.03 for v=1, 2, and 3, respectively, with an upper bound of 0.04 from a few observed lines of v=4. The monotonically decreasing vibrational distribution suggests a reaction involving HF(v) elimination from an intermediate complex. There are a number of possible single or multistep kinetic pathways that could produce HF(v) under these conditions. To determine the predominant pathway that produces the observed HF(v), the dependence of the time-resolved HF(v) emission signal on reactant concentrations is measured and compared with kinetics simulations. The results suggest a single step mechanism involving initial O(3P) attack on the iodine of the CH2ICF3, in a manner similar to the start of the reaction of O(3P) with C2H5I that produces HOI. This is followed by insertion of the oxygen atom into the carbon–iodine bond of the CH2ICF3, producing an activated complex with sufficient energy to eliminate HF(v).

Cathodic reduction of N-(2-iodophenyl)-N-alkylcinnamides: a novel sequential electrochemical radical cyclisation and hydroxylation

In recent years, intramolecular aryl radical cyclisation has emerged as a useful route for the synthesis of benzannulated heterocycles and carbocycles. The aryl radicals are generated in situ from aryl halides (iodides or bromides) with tributylstannyl hydride–AIBN, SmI2, Co(I) or under photochemical conditions. The present work envisages the generation of aryl radicals by cathodic reduction of the carbon–iodine bond of N-(2-iodophenyl)-N-alkylcinnamides and their intramolecular cyclisation. The cathodic reduction of N-(2-iodophenyl)-N-alkylcinnamides under deaerated conditions in DMF gave 1-alkyl-3-benzylindolin-2-ones regioselectively and in the presence of oxygen yielded surprisingly 1-alkyl-3-hydroxy-3-benzylindolin-2-ones. Both these products were formed by a 5-exo-trig process in good yields. A mechanism for the formation of the products has been proposed through the use of cyclic voltammetry, coulometry and controlled-potential electrolysis as well as deuterium labelling.

Photolysis of diaryliodonium salts (UV/Vis, EPR and GC/MS investigations)

The photodecomposition of commercial lipophilic diaryliodonium hexafluoroantimonate SarCat ® SR-1012 was investigated in acetonitrile (ACN) and ACN/H 2O mixed solvents by means of UV/Vis spectroscopy, an EPR in situ spin trapping technique and GC/MS analysis. The application of the spin traps nitrosodurene (ND) and 5,5-dimethyl-1-pyrroline N-oxide gave evidence for the production of radical intermediates corresponding to the breaking of the carbon–iodine bond, in addition to carbon–oxygen bonds in SR-1012 aryl substitution. The photolytic products analyzed in aprotic acetonitrile were iodobenzene, acetanilide, biphenyl and C 12H 25CH(OH)CH 3, while in ACN/H 2O (1:1, vol.) solution iodobenzene, biphenyl, C 12H 25CH(OH)CH 2OC 6H 4OH and hydroxylated biphenyls were identified. The influence of oxygen on the product distribution in both solvent systems was inconsequential. The proton concentrations photogenerated in the irradiated SR-1012 ACN/H 2O solutions were quantified spectrophotometrically using bromophenol blue indicator.

Photochemical substitution of polyhalogenothiophene and halogenothiazole derivatives †

The irradiation of 2,3-diodo-5-nitrothiophene in the presence of aromatic and heteroaromatic compounds gave the corresponding 2-aryl derivatives in high yields. The irradiation of 2,4-diiodo-5-nitrothiophene under the same conditions gave the corresponding 2-aryl derivatives in low yields. The observed difference in the reactivity can be explained on the basis of the hypothesis that the homolytic cleavage of the carbon–iodine bond occurred in a π,σ* triplet state. Computational results showed that the lowest triplet state of the 2,3-diiodo isomer is π,σ*, while that of the 2,4-isomer is π,π*. The irradiation of 2-bromo-5-nitrothiazole in the presence of benzene or indene gave the corresponding 2-bromo-5-arylthiazole. This behaviour can be explained by considering that the lowest excited triplet state cannot allow the cleavage of the carbon–bromine bond thus electron transfer occurs and leads to the substitution of the nitro group. The photochemical substitution reactions on 2,3-diiodo-5-nitrothiophene can be carried out in large scale using a new flow reactor using a PFTE pipe.

Direct and catalyzed electrochemical syntheses in microemulsions

A series of electrochemical reductions were carried out in microemulsions with a view to determining the utility of these novel fluids for preparative electrochemistry. Direct reduction of nitrobenzene in a microemulsion of didodecyldimethylammonium bromide (DDAB), hexane, and water gave azoxybenzene as the product. With HCl in the water phase, azoxybenzene and azobenzene were produced. This, together with spectroscopic results, demonstrates that nitrobenzene resides in a polar region of low proton availability. Reduction of aryl iodides and aromatic hydrocarbons in microemulsions resulted in clean reductive cleavage of the carbon–iodine bond, followed by reduction of the aromatic rings. Doubly electrocatalytic reduction of benzal chloride to toluene was also accomplished by using various cobalt complexes in a DDAB microemulsion. Cyclization of a cyclohexenyl bromoacetal ( 24) gave an unsaturated bicyclic product ( 25) at low potentials with simultaneous irradiation by visible light using a 0.2 ratio of vitamin B 12 to reactant in a cetyltrimethylammonium bromide (CTAB) microemulsion and in organic solvents. Using a 0.01 ratio of catalyst to reactant and −1.8 V, production of the corresponding saturated bicyclic product ( 26) was found in the microemulsion, but not in organic solvents. Results suggest new possibilities for synthetic pathway control using microemulsions.

Silver(I) tetraiodoethylene complexes with twisted olefin moiety

The reactions of silver perchlorate and tetraiodoethylene in different solvents, namely, benzene and toluene, isolated two silver(I)–iodocarbon complexes, [Ag(C 2I 4)(C 6H 6) 2(ClO 4)] ( 1) and [Ag(C 2I 4)(ClO 4)] ( 2). Both compounds contain intact iodoalkenes which coordinate via σ-donation of a halogen lone pair and retain their carbon–iodine bonds. Owing to the participation of the benzene molecules in coordination, complex 1 is found to be a discrete monomer in which the five-coordinate geometry of the silver ion is comprised of two benzene molecules, one C 2I 4 group and one perchlorate ion. In contrast, the unsaturated coordination environment of the metal ion in 2 is filled by the second iodocarbon group leading to a two-dimensional framework. The coordinated tetraiodoethylene molecules involve severe twisting of the CC double bond, causing the CC stretching band to move to a lower frequency.

Ultrafast electron diffraction and direct observation of transient structures in a chemical reaction.

Ultrafast electron diffraction is a unique method for the studies of structural changes of complex molecular systems. In this contribution, we report direct ultrafast electron diffraction study of the evolution of short-lived intermediates in the course of a chemical change. Specifically, we observe the transient intermediate in the elimination reaction of 1,2-diiodotetrafluoroethane (C2F4I2) to produce the corresponding ethylene derivative by the breakage of two carbon-iodine, C---I, bonds. The evolution of the ground-state intermediate (C2F4I radical) is directly revealed in the population change of a single chemical bond, namely the second C---I bond. The elimination of two iodine atoms was shown to be nonconcerted, with reaction time of the second C---I bond breakage being 17 +/- 2 ps. The structure of the short-lived C2F4I radical is more favorable to the classical radical structure than to the bridged radical structure. This leap in our ability to record structural changes on the ps and shorter time scales bodes well for many future applications in complex molecular systems.

Inter- and intramolecular electron transfer in the complex OC···ICl determined from iodine and chlorine nuclear quadrupole hyperfine structure in its rotational spectrum

The ground-state rotational spectra of three isotopomers O12C···I35Cl, O12C···I37Cl and O13C···I35Cl of a linear (or quasi-linear) complex formed by carbon monoxide and iodine monochloride were observed by using a pulsed-nozzle, Fourier transform microwave spectrometer. The spectroscopic constants B0, DJ, χaa(I), χaa(Cl) and Mbb(I) were determined for each isotopomer. Simple models were used to interpret the spectroscopic constants to give details of the geometry, binding strength and electric charge redistribution in OC···ICl. It was concluded that the nuclei lie in the order OC···ICl with the distance r(C···I)=3.011(1) A, that the quadratic intermolecular stretching force constant kσ=8.00(3) N m-1, and that, when CO achieves its equilibrium position in the complex, the changes in the iodine and chlorine nuclear quadrupole coupling constants of ICl are consistent with the transfer of a fraction δ1=0.025(2) of an electronic charge from C to I and an intramolecular transfer of a fraction δ2=0.048(2) from I to Cl. A comparison of the r(C···X), kσ and δ2 values for the series of complexes OC···XY, where XY=ClF, Cl2, BrCl, Br2 and ICl is presented. The order of kσ is ICl>ClF>BrCl>Br2>Cl2, while the order of δ2 is Cl2∽ClF<Br2∽BrCl<ICl. It is deduced that ICl is ‘snub-nosed’, i.e. the I atom has a smaller van der Waals radius along the ICl internuclear axis than perpendicular to it.

Synthesis of 4-O- and 6-O-(2′-iodoethyl)-d-glucose

O -Allylation of 1,2,3,6-tetra- O -acetyl- d -glucopyranose followed by an ozonation/reduction sequence gave the 4-hydroxyethyl derivative. This hydroxyethyl substituent was also introduced at C-6, starting from 1,2:3,5-bis( O -methylidene)- α - d -glucofuranose using an alkylation/reduction sequence. These 4- and 6- O -hydroxyethyl derivatives were then converted to the title compounds by iodination followed by deprotection. Noteworthy is the particular stability of the carbon–iodine bond in these ethers, a prerequisite for their potential use in Single Photon Emitted Computed Tomography medical imaging (SPECT).

A tandem radical macrocyclisation-transannular cyclisation approach towards the taxanes

Separate treatment of the iodotrienedione 19 and the iododienynedione 38 with Bu3SnH–AIBN produces the corresponding taxane ring systems 25 (25–30%) and 56 (50–60%) respectively by way of tandem radical macrocyclisation-radical transannular cyclisations. By contrast the analogous iodopolyenones 61, 63, 65a, 66a, 86a and 87a, together with the corresponding iodoenynones 39a and 59 failed to undergo similar tandem radical reactions, and instead gave products resulting from direct reduction of the carbon–iodine bonds in these substrates. The structures of the taxane analogues 25 and 56 followed from analysis of their NMR spectroscopic data and comparison with similar NMR data for related taxoids described in the literature. The stereochemistry of 56 was secured from an X-ray crystal structure determination of the 1,5-diol 57 produced from reduction of 56 with DIBAL.

Asymmetric Synthesis of α'-Silylated α-Iodo Ketones via Iodination with Trifluoroiodomethane

The diastereo- and enantioselective syntheses of various α'-t-butyldimethylsilyl-α-iodo ketones 4a-h is described. The carbon iodine bond formation is achieved using trifluoroiodomethane as the electrophilic iodination reagent. The iodo ketones 4 are obtained in good yields and with excellent diastereo- and enantiomeric excesses (de,ee ≥ 98%).

Cyclization and related reactions of iodoethanol on Ag(110)

The adsorption and reaction of 2-iodoethanol (IEtOH) on a clean Ag(110) surface were studied under ultrahigh vacuum conditions using temperature programmed deposition (TPD). Under these conditions alcohols are typically unreactive on the clean Ag(110) surface. Replacement of one of the β hydrogens with a weakly bound (53 kcal/mol) iodine atom, however, opens a channel for IEtOH decomposition by initial carbon–iodine scission. TPD results indicate that there are two major reaction channels at 263 and 340 K for IEtOH decomposition on the clean Ag(110) surface. The reaction intermediates that give rise to these two pathways were deduced based upon the products observed, the overall product stoichiometry, and the desorption temperatures of the products. The 263 K reaction channel has an overall stoichiometry of C2H5O, corresponding to a hydroxyethyl intermediate. The hydroxyethyl intermediate undergoes a β-hydride elimination and C-O scission, ultimately yielding volatile acetaldehyde, ethylene, water, ethanol, and another surface intermediate that decomposes at 340 K. Unlike unsubstituted alkyl groups on clean Ag(111) and Ag(110) which couple to form alkanes, there is no evidence for hydroxyethyl coupling to give butanediol. Thus, the hydroxyethyl cannot be treated simply as a substituted alkyl group; that is probably due to interactions between oxygen and the surface. The stoichiometry of the products released at 340 K, C2H4O, is suggestive of an oxametallacycle intermediate. The products of this reaction channel are the same as the 263 K channel with the addition of a cyclic ester product, γ-butyrolactone. This species has not previously been synthesized from a C2 precursor in ultrahigh vacuum, and its observation suggests that surface oxametallacycle chemistry may be richer than previously recognized.

Studies towards the taxane ring system via a cascade macrocyclisation–transannulation strategy

Treatment of the ω-iodoacetylene 6 with Bu3SnH–AIBN produces the taxane ring system 9 (45–60%) by way of transannular cyclisation from the macrocyclic vinyl radical intermediate 8, whereas the analogous iodoacetylenes 5a, 11 and 13a, together with the iodopolyenones 12a, 14a and 17, failed to give corresponding taxane systems, i.e. 7, 16 and 19 on similar treatment; instead only products resulting from direct reduction of the carbon–iodine bonds in these substrates were obtained.

Unusual reactions in molecules with crowded functional groups: sulfonamide reduction under oxidizing conditions in camphor derivatives

G. Wagner, R. Herrmann<img border="0" src="https://www.rsc.org/images/entities/char_200a.gif" alt=" " xmlns="http://www.rsc.org/schema/rscart38" /> and A. Schier, J. Chem. Soc., Perkin Trans. 1, 1997, 701 DOI: 10.1039/A605472B

Generation and Reactions of Trifluoroacetimidoyl Radicals

Abstract N-Aryltrifluoroacetimidoyl radicals have been generated by three different methods: the tin-radical promoted deiodination and photochemical homolysis of imidoyl iodides and the thermal homolysis of imidoyl azo-compounds. N-[2-(1-alkynyl)phenyl]trifluoroacetimidoyl iodides have been converted to trifluoromethylated indoles by the tin-radical promoted homolysis of the carbon–iodine bond followed by intramolecular cyclization. The photolysis of trifluoroacetimidoyl iodides enabled both intra- and intermolecular cyclization to indoles and quinolines. Likewise, thermal reactions of N-(2,2,2-trifluoro-1-tritylazoethylidene)anilines provided trifluoromethylated quinolines and indoles.

Nonadiabaticity and intramolecular electronic energy transfer in the photodissociation of 1-bromo-3-iodopropane at 222 nm

The photodissociation of 1-bromo-3-iodopropane (1,3-C3H6BrI) at 222 nm is studied with crossed laser-molecular beam experiments. Irradiation at this wavelength excites an n(Br)→σ*(C–Br) transition which promotes the molecule to an approximately diabatic excited state potential energy surface which is dissociative in the carbon–bromine bond. This surface intersects an approximately diabatic surface of n(I)→σ*(C–I) character at extended C–Br distances; this surface is dissociative in the carbon–iodine bond. Crossings from the surface initially accessed to the intersecting surface correspond to intramolecular excitation transfer from the carbon–bromine to the carbon–iodine bond. The incidence of such transfer and hence of carbon–iodine bond fission depends upon the strength of the off-diagonal potential coupling of the two diabatic states. These experiments test the dependence of the coupling and consequent energy transfer upon the separation distance of the C–Br and C–I chromophores. The data show C–Br fission dominates C–I fission by a ratio of 4:1 and determine the center-of-mass translational energy distributions and angular distributions of these processes. The measured anisotropy parameters are β(C–Br)=1.6±0.4 and β(C–I)=0±0.2. A third photofission process, IBr elimination, also contributes to the observed signal. The results of the study of C–Br and C–I fission are compared to previous studies on similar molecules to understand how the branching depends on the relative positioning of the C–Br and C–I chormophores.

Radio-high performance liquid chromatographic study on radioactive38Cl compounds

Reaction of recoild38Cl atoms with o-dichlorobenzene in the presence of carbon tetrachloride or iodine has been studied by using radio-high performance liquid chromatography. The major products were detected by a 4-channel-wavelengths spectrophotometric detector. The radioactivity of38Cl compounds including minor products was measured with a NaI(T1) scintillation detector. The main products found were38Cl labeled HCl/Cl2, CHCl3, CCl4, o-, p-, m-C6H6Cl2 and polymer, whereas only minor products such as HCl/Cl2, CHCl3, C2Cl6, C6H3Cl3, and polymer were found in the radio-chromatogram. The reaction mechanisms of recoil38Cl atom are briefly described.