Heterolytic Bond Cleavage Research Articles

Alkyl chloride initiators for SET‐LRP of methyl acrylate

AbstractSingle‐electron transfer living radical polymerization (SET‐LRP) proceeds by an outer‐sphere single‐electron transfer mechanism that induces a heterolytic bond cleavage of the initiating and propagating R‐X (where X = Cl, Br, and I) species. Therefore, unlike the homolytic bond cleavage mechanism claimed for ATRP, SET‐LRP is expected to show a small dependence of the nature of the halide group on the apparent rate constant of activation. This means the R‐X with X = Cl, Br, and I must all be efficient initiators for SET‐LRP and no chain transfer must be observed in the case of initiators with X = Br and I. Here, we report the SET‐LRP of methyl acrylate initiated with the alkyl chlorides methyl‐2‐chloropropionate (MCP) and chloroform (CHCl3) and catalyzed by Cu(0)/Me6‐TREN/CuCl2 in DMSO at 25 °C. A combination of kinetic and structural analysis was used to elucidate the MCP and CHCl3 initiating behavior under SET‐LRP conditions, and to demonstrate the very small dependence of the SET‐LRP apparent rate constant of propagation on X while providing polymers with well defined architecture. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4917–4926, 2008

The Effect of Tb and Sm Ions on the Photochromic Behavior of Two Spiropyrans of Benzoxazine Series in Solution

The photochromism of [7′-hydroxy-8′-formyl-3-methyl-4-oxospiro[1,3-benzoxazin-2,2′-[2H-1]benzopyran],SP(I),[7′-hydroxy-8′-formyl-3-benzyl-4-oxospiro[1,3-benzoxazin-2,2′-[2H-1]benzopyran] SP(II) and their coordination with Tb3+and Sm3+ ions have been studied in DMF. UV/vis induced-color development due to heterolytic bond cleavage of SP(I) and SP(II) is greatly influenced by complexation with the lanthanide ions. The irradiation-induced color enhancement due to ring opening and thermal decoloration of the open forms of SP(I), SP(II) follows first-order kinetics. Physical characteristics of the studied systems such as colorability and relaxation time of thermal bleaching parameters were determined. Moreover, light-energy transfer-induced luminescence of lanthanide ions via coordination with the two spirobenzoxazines was monitored.

Cyclopropylmethyl/Cyclobutyl Rearrangements on Surfaces: Evidence for Transient Cation Formation near O-Covered Mo(110)

Rearrangement reactions of C4-alkoxides on O-covered Mo(110) have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclobutoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropylmethoxide during high-temperature annealing. In light of compelling new data, previous results are reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopropylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D2-cyclopropylmethanol was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 x 6)-O. The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered radicals are not known to rearrange in this manner. However, in none of the cases discussed could contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are discussed in the context of heterogeneous hydrocarbon oxidation.

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

Methyl transfer reactions are important in a number of biochemical pathways. An important class of methyltransferases uses the cobalt cofactor cobalamin, which receives a methyl group from an appropriate methyl donor protein to form an intermediate organometallic methyl-Co bond that subsequently is cleaved by a methyl acceptor. Control of the axial ligation state of cobalamin influences both the mode (i.e., homolytic vs heterolytic) and the rate of Co-C bond cleavage. Here we have studied the axial ligation of a corrinoid iron-sulfur protein (CFeSP) that plays a key role in energy generation and cell carbon synthesis by anaerobic microbes, such as methanogenic archaea and acetogenic bacteria. This protein accepts a methyl group from methyltetrahydrofolate forming Me-Co(3+)CFeSP that then donates a methyl cation (Me) from Me-Co(3+)CFeSP to a nickel site on acetyl-CoA synthase. To unambiguously establish the binding scheme of the corrinoid cofactor in the CFeSP, we have combined resonance Raman, magnetic circular dichroism, and EPR spectroscopic methods with computational chemistry. Our results clearly demonstrate that the Me-Co3+ and Co2+ states of the CFeSP have an axial water ligand like the free MeCbi+ and Co(2+)Cbi+ cofactors; however, the Co-OH2 bond length is lengthened by about 0.2 angstroms for the protein-bound cofactor. Elongation of the Co-OH2 bond of the CFeSP-bound cofactor is proposed to make the cobalt center more "Co1+-like", a requirement to facilitate heterolytic Co-C bond cleavage.

Synthesis of Heterobimetallic RuMn Complexes and the Coupling Reactions of Epoxides with Carbon Dioxide Catalyzed by these Complexes

The heterobimetallic complexes [(eta5-C5H5)Ru(CO)(mu-dppm)Mn(CO)4] and [(eta5-C5Me5)Ru(mu-dppm)(mu-CO)2Mn(CO)3] (dppm = bis-diphenylphosphinomethane) have been prepared by reacting the hydridic complexes [(eta5-C5H5)Ru(dppm)H] and [(eta5-C5Me5)Ru(dppm)H], respectively, with the protonic [HMn(CO)5] complex. The bimetallic complexes can also be synthesized through metathetical reactions between [(eta5-C5R5)Ru(dppm)Cl] (R = H or Me) and Li+[Mn(CO)5]-. Although the complexes fail to catalyze the hydrogenation of CO2 to formic acid, they catalyze the coupling reactions of epoxides with carbon dioxide to yield cyclic carbonates. Two possible reaction pathways for the coupling reactions have been proposed. Both routes begin with heterolytic cleavage of the RuMn bond and coordination of an epoxide molecule to the Lewis acidic ruthenium center. In Route I, the Lewis basic manganese center activates the CO2 by forming the metallocarboxylate anion which then ring-opens the epoxide; subsequent ring-closure gives the cyclic carbonate. In Route II, the nucleophilic manganese center ring-opens the ruthenium-attached epoxide to afford an alkoxide intermediate; CO2 insertion into the RuO bond followed by ring-closure yields the product. Density functional calculations at the B3LYP level of theory were carried out to understand the structural and energetic aspects of the two possible reaction pathways. The results of the calculations indicate that Route II is favored over Route I.

Abilities of 2‐arylmethyloxy‐6‐cyanonaphthalene derivatives to initiate radical and cationic photopolymerizations

AbstractAn investigation was undertaken to shed much light on the bond‐cleavage mode (heterolysis vs homolysis) of bichromophoric photoinitiators in styrene (St) and 1,2‐epoxycyclohexane (EpC) and then to develop a highly efficient hybrid‐type photoinitiator. Excited‐state 2‐(9‐anthrylmethyloxy)‐6‐cyanonaphthalene in toluene and 1,2‐dimethoxyethane, which were used instead of St and EpC, respectively, underwent both homolytic ArOCH2 bond cleavage and heterolytic ArOCH2 bond cleavage to give the corresponding radical‐pair and ion‐pair intermediates. The charge‐separated state characteristic of the naphthol chromophore in the singlet excited state was found to play a pivotal role in this heterolytic bond‐cleavage mode. An inspection of the radical (St) and cationic (EpC) photopolymerization behavior observed in the presence of some 2‐arylmethyloxy‐6‐cyanonaphthalenes led us to conclude that the aforementioned 9‐anthrylmethyloxy‐substituted naphthalene derivative is a candidate for a functional hybrid‐type photoinitiator containing no halogen and/or metal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 25–31, 2006

Kinetics Study of the Photocleavage of (Coumarin-4-yl)methyl Esters

Photolabile coumarinylmethyl esters of biomolecules (caged compounds) are new tools for studying spatial and time-dependent aspects of signal transduction in living cells. Herein we describe a fluoresence spectroscopic method for the determination of the rate constants of the photolysis steps of such caged compounds using (6.7-dimethoxycoumarin-4-yl)methyl diethyl phosphate (DMCM-DEP) and sodium (6,7-dimethoxycoumarin-4-yl)methyl sulfate (DMCM-S). DMCM-DEP and DMCM-S are caged compounds which photorelease a proton, the corresponding acid anion, and the strongly fluorescent alcohol DMCM-OH upon excitation. The results of stationary and time-resolved measurements of the photochemistry and the luminescence of both caged compounds indicate that DMCM-OH is produced already during the excitation pulse. The quantitative analysis of the data demonstrates that the first step of the reaction--heterolytic bond cleavage of the coumarinylmethyl ester leading to the ion pair of a DMCM cation and an acid anion--is very fast with a rate constant of k1 approximately 2 x 10(10) s(-1). Recombination of the ion pair occurs with a rate constant of k(rec) approximately 2.3 x 10(9) s(-1) and is about 10 times faster than the competing hydrolysis reaction of the DMCM cation yielding DMCM-OH and a proton. Thus, both caged compounds belong to the fastest phototriggers known.

Potential energy surface discontinuities in local correlation methods.

We have examined the occurrence of discontinuities in bond-breaking potential energy surfaces given by local correlation methods based on the Pulay-Saebø orbital domain approach. Our analysis focuses on three prototypical dissociating systems: the C-F bond in fluoromethane, the C-C bond in singlet, ketene, and the central C-C bond in propadienone. We find that such discontinuities do not occur in cases of homolytic bond cleavage due to the inability of the Pipek-Mezey orbital localization method to separate singlet-coupled charges on distant fragments. However, for heterolytic bond cleavage, such as that observed in singlet ketene and propadienone, discontinuities occur both at stretched geometries and near equilibrium. These discontinuities are usually small, but may be of the same order of magnitude as the localization error in some cases.

Photoelectron spectroscopic and electronic structure studies of CH(2)O bonding and reactivity on ZnO surfaces: steps in the methanol synthesis reaction.

Adsorption of CH(2)O on ZnO(0001) has been investigated using XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and density functional theory (DFT) calculations. CH(2)O is chemisorbed on the (0001) surface at 130 K. Its C1s XPS peak position at 292.7 eV and NEXAFS sigma shape resonance at 302.6 eV are consistent with an eta(1) bound surface geometry. Geometry optimized DFT calculations also indicate that CH(2)O is bound to the Zn(II) site in an eta(1) configuration through its oxygen atom. The variable-energy PES of the eta(1) bound CH(2)O/ZnO(0001) complex exhibits four valence band features at 21.2, 16.4, 13.8, and 10.7 eV below the vacuum level providing an experimental and theoretical description of this surface interaction. Annealing the ZnO(0001)/CH(2)O surface complex to 220 K decomposes the chemisorbed CH(2)O, producing formyl (291.5 eV), methoxide (290.2 eV), and formate (293.6 eV) intermediates. Thus this reaction coordinate involves the conversion of an oxygen bound formaldehyde to a carbon bound formyl species on ZnO(0001). Only formate is formed on the ZnO(100) surface. DFT is used to explore surface intermediates and the transition state in the methanol synthesis reaction (MSR). The bonding interactions of H(2), CO, CH(3)O(-), HCO(-), and trans-HCOH to the ZnO(0001) surface are elucidated using geometry optimization. H(2) was found to be heterolytically cleaved on the ZnO(0001) surface, and carbon monoxide, formyl, and methoxide are calculated to be eta(1) bound. These results are consistent with observed metal oxide surface reactivity where heterolytic bond cleavage is dominant. The oxygen atom in the bound formyl was found to be activated for attack by a proton. This results in the planar eta(1) bound trans-HCOH surface species. The transition state in the gas phase rearrangement of trans-HCOH to formaldehyde was calculated to have a barrier of 31 kcal/mol. The correlation diagram for this rearrangement in the gas phase indicates that configuration interaction at the crossing of two levels helps to lower the barrier. A transition state calculation was also performed for this rearrangement on the ZnO(0001) surface. The surface transition state geometry is significantly different than the gas phase. The surface geometry is no longer planar (23 degrees dihedral angle) and is displaced parallel to the surface. Interaction with the Zn(II) site at the crossing of surface bound CH(2)O and trans-HCOH levels further lowers the barrier to rearrangement relative to gas phase by 9 kcal/mol. The rearrangement of trans-HCOH (carbon bound) to CH(2)O (oxygen bound) on ZnO(0001) was calculated to be the overall barrier of the MSR reaction.

C–N bond formation followed by N–Cl bond breaking. One more and unexpected case of the formation of a hydroxamic group via heterolytic bond cleavage

An unexpected and previously unknown reaction sequence in the interactions of the acyl halides with nitrosobenzenes, which involves carbon–nitrogen bond formation followed by heterolytic nitrogen–chlorine bond cleavage giving the corresponding unsubstituted N-phenylalkylhydroxamic acids (or N-phenylarylhydroxamic acids) and chlorine as the products has been observed. The kinetic and other evidence obtained suggest that the carbon–nitrogen bond formation is the consequence of a nucleophilic interaction of an N-phenylchlorohydroxylamine intermediate, formed in the first reaction step, with the acyl halide in the second step of the complex sequence, which leads to an N-acyl- N-chlorophenylhydroxylamine cation intermediate. The key reaction step involves the interaction of an N-acyl- N-chlorophenylhydroxylamine cation intermediate with chloride ion, which leads to the N–Cl heterolytic bond cleavage and the final formation of the hydroxamic group and a molecule of chlorine.

Adsorption on perfect and reduced surfaces of metal oxides

Two major chemical processes, acidobasic and redox, track the adsorption mechanism on metal oxides. Molecular and dissociative adsorption on stoichiometric surfaces can be understood as acid–base processes. Clean and anhydrous surfaces of metal oxides have two different active sites: cations and anions. Electron-rich molecules or fragments arising from a heterolytic bond cleavage (Lewis bases) react with M n+ , while electron poor ones (Lewis acids) react with O 2−. The MgO and TiO 2 surfaces clearly appear to be predominantly acidic and molecules that do not dissociate generally bind to the metal cation. The electronic structure, insulating character for the stoichiometric surface, is preserved upon adsorption. When the initial system does not favor an energy gap (open-shell adsorbates, defective surfaces), the best adsorption mode is via a redox mechanism that restores the situation of an insulator and the highest oxidation states for all the atoms. For radical adsorption a first solution occurring on irreducible oxides is to couple the electrons and form two opposite ions adsorbed on the two surface sites, as for H 2/MgO, involving an acid–base mechanism. Another possibility occurring on reducible oxide is via an electron transfer to or from the oxide (redox mechanism). The electron transfer occurs from the substrate to the adsorbate for electronegative group (Cl adsorption on O) or the other way around for an electropositive one (NO adsorption on M). The reactivity at surfaces deviating from stoichiometry differs from that on the perfect ones. The difference does not only originate from the modification of the coordination number but also from the electron counting.

Electrospray ionization and liquid secondary ion mass spectrometric study of N-heterocyclic carbenes and their 1,2,4-triazolium salt precursors

N-Heterocyclic carbenes and their 1,2,4-triazolium salt precursors were analyzed by electrospray ionization (ESI) and liquid secondary ion mass spectrometry (LSIMS). It was found that under applied conditions the C5 carbene center is formed from [M+ClO 4] + ion, were M corresponds to the 1,2,4-triazolium dicationic system, by loss of HClO 4 molecule. Further decomposition of formed carbene ions, namely [M−H] + ion, consist in the breaking of triazolium ring and loss bulky substituents from N1 and N4 atoms through the hydrogen transfer to the carbene center. This is in contrast to the [M] 2+ carbene precursor ions for which only the loss of N1 substituent occurred as a simple heterolytic bond cleavage.

Mechanistic insight from activation parameters for the reaction between co-enzyme B12 and cyanide: further evidence that heterolytic Co–C bond cleavage is solvent-assistedElectronic supplementary information (ESI) available: NMR and kinetic data. See http://www.rsc.org/suppdata/dt/b2/b210068a/

The potential involvement of the solvent in heterolytic Co–C bond cleavage for vitamin B12 derivatives has been probed by measuring for the first time activation parameters for heterolytic Co–C bond cleavage. If a water molecule is part of the transition state, negative entropies and volumes of activation should be observed. Conversely, if Co–C bond cleavage is a purely dissociative process, these parameters will be positive. Activation parameters have been determined for the reactions of co-enzyme B12 (5′-deoxyadenosylcobalamin, AdoCbl) and the corresponding cobinamide, 5′-deoxyadenosylcobinamide (AdoCbi+), with cyanide, since previous studies have shown that these reactions proceed at convenient rates, Co–C heterolytic bond cleavage is rate-determining and the reaction proceeds cleanly (that is, negligible homolytic cleavage). In addition, the kinetics of the reaction of AdoCbl with cyanide in aqueous solution were re-investigated at high concentrations of sodium cyanide (up to 3.00 M) at pH 11.0 and I = 5.0 M (NaClO4) using UV-visible and 1H NMR spectroscopy. A significant kinetic saturation was obtained at high CN− concentrations and the limiting rate constant at 25.0 °C was found to be (2.21 ± 0.04) × 10−2 s−1, with an overall formation constant for the (β-Ado)(α-CN)Cbl− intermediate, KCN/KCo, of 0.259 ± 0.007 M−1. ΔH≠, ΔS≠ and ΔV≠ under these conditions were found to be 55 ± 1 kJ mol−1, −98 ± 3 J K−1 mol−1 and −5.7 ± 0.3 cm3 mol−1, respectively. ΔV≠ for the reaction of AdoCbi+ and CN− at pH 11.0 and 35.0 °C was found to be −4.5 ± 0.4 cm3 mol−1, while ΔV≠ for the reaction of AdoCbl and AdoCbi+ with tetrabutylammonium cyanide in 92%DMF–8%D2O at 35 °C was found to be −8.2 ± 0.3 and −8.8 ± 0.7 cm3 mol−1, respectively. The activation parameters data obtained in the present study support the earlier suggestion that the rate determining step for the reaction of AdoCbl and AdoCbi+ with CN− which involves heterolytic cleavage of the Co–C bond of the (β-Ado)(α-CN)Cbl− and (β-Ado)(α-CN)Cbi intermediates, respectively, is a solvent-assisted, β-elimination process, in which the solvent partially protonates the ribosyl oxygen of the intermediate.

Anaerobic microbial and photochemical degradation of 4,4′-dibromodiphenyl ether

The anaerobic microbial and photochemical degradation pathways of 4,4′-dibromodiphenyl ether (BDE15) were examined. BDE15 was reductively debrominated within a fixed-film plug-flow biological reactor at hydraulic retention times of 3.4 and 6.8 h, leading to exclusive production of 4-bromodiphenyl ether (BDE3) and diphenyl ether (DE). A suite of potential BDE15 metabolites arising from reductive debromination, hydroxylation, and methoxylation of the aromatic C–Br and C–H bonds were not observed. Following initial debromination of BDE15, degradation of BDE3 to DE readily occurs, suggesting the rate-limiting step for anaerobic BDE15 degradation is conversion of BDE15 to BDE3. The photochemical degradation of BDE15 was also examined in organic (CH 3CN and CH 3OH) and aqueous (H 2O:CH 3CN; 1:1 v/v) solvent systems at 300 nm. Only photochemically induced reductive debromination was found to occur via homolytic C–Br bond cleavage, with no evidence of C–O bond cleavage or products arising from heterolytic bond cleavage.

Metal-hydride bond activation and metal-metal interaction in dinuclear iron complexes with linking dinitriles: a synthetic, electrochemical, and theoretical study.

The dinuclear iron(II)-hydride complexes [[FeH(dppe)(2)](2)(mu-LL)][BF(4)](2) (LL = NCCH=CHCN (1a), NCC(6)H(4)CN (1b), NCCH(2)CH(2)CN (1c); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and the corresponding mononuclear ones, trans-[FeH(LL)(dppe)(2)][BF(4)] (2a-c) were prepared by treatment of trans-[FeHCl(dppe)(2)], in tetrahydrofuran (thf) and in the presence of Tl[BF(4)], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe(II)/Fe(III) 1a(+) complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H(+)) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF(4)(-), to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P(L) and E(L) ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.

Breaking bonds by mechanical stress: when do electrons decide for the other side?

Using first-principles molecular dynamics, we have simulated reactions that can be induced by mechanical stress in a polymer. We have stretched a small piece of poly(ethylene glycol) (PEG) in water at finite temperature. Both the molecule and the solvent were described quantum mechanically on an equal level. The formation of ions was observed, which corresponds to a heterolytic bond cleavage. We were able to monitor the motion of the electrons during the reactions. Our simulations show that the electron transfer and the breaking of the bond occur almost simultaneously and that both processes are initiated by the approach of a solvent molecule toward the destabilized bond.

DFT studies on the pairing abilities of the one-electron reduced or oxidized adenine–thymine base pair

Using density functional theory (DFT), the hydrogen bonds making up the adenine–thymine (A–T) base pair are found to increase in total energy upon one-electron oxidation or reduction by 10.9 and 13.3 kcal mol−1, respectively. Due to unsymmetric changes in the H-bond lengths, this strengthening affects an expansion of the base pair length (N1′–N9) by ∼0.27 A. In the oxidized pair, A˙+–T, deprotonation from N6, and with the reduced pair, A˙−–T, protonation on N3 or N7 lead to base pairs which have similar base pairing energies as their parent A–T, i.e., the stabilization by the change in oxidation state is annihilated by (de)protonation. The calculated proton affinities of A˙−–T are large enough to explain its protonation by H2O, which involves heterolytic bond cleavage of a water molecule. The N1 protonated electron adduct of A is a powerful H-bond donor; it is able to mismatch with cytosine (−28.9 kcal mol−1). In DNA this could compete with the “legitimate” guanine-cytosine pairing. The pairing abilities of 2-aminopurine, an “unnatural” isomer of A, used as a fluorescent probe in DNA assemblies, are calculated to resemble those of A closely.

Thermolysis of 2,4,6-trichlorophenol chemisorbed on aluminium oxides as example of fly ash mediated surface catalysis reaction in PCDD/PCDF formation

The influence of aluminium cation as a strong electrophilic centre on the thermolysis of chlorophenols chemisorbed on Al(OH) 3 surface was investigated. If thermolysis is carried out at 300 °C the spontaneous rupture of the bond between aluminium and oxygen of phenol takes place in the temperature range of 260–280 °C. The thermolysis of chlorophenoxy aluminium compounds occurs through homolytic and heterolytic bond cleavage. In the case of heterolytic cleavage the leaving chlorophenoxy anion causes a simultaneous formation of the aluminium cation, which is the driving force for the rearrangement of the unstable intermediate. By homolytic cleavage of the Al–O bond the chlorophenoxy radical is formed. The isolation of reaction products of the thermolysis of the system Al(OH) 3/2,4,6-trichlorophenol gave five isomers of dimeric compounds of resonance stabilised 2,4,6-trichlorophenoxy radical. The compounds are stable in nonaqueous, aprotic solution, but they are very sensitive to acid catalysis. They quickly transform into aromatic hydroxydiphenyl ethers. The process of dechlorination and aromatisation of cyclohexadienone dimers gives PCDD/PCDF.

Modeling the active sites in metalloenzymes 5. The heterolytic bond cleavage of H(2) in the [NiFe] hydrogenase of desulfovibrio gigas by a nucleophilic addition mechanism.

The H(2) activation catalyzed by an Fe(II)-Ni(III) model of the [NiFe] hydrogenase of Desulfovibrio gigas has been investigated by density functional theory (DFT/B3LYP) calculations on the neutral and anionic active site complexes, [(CO)(CN)(2)Fe(mu-SH)(2)Ni(SH)(SH(2))](0) and [(CO)(CN)(2)Fe(mu-SH)(2)Ni(SH)(2)](-). The results suggest that the reaction proceeds by a nucleophilic addition mechanism that cleaves the H-H bond heterolytically. The terminal cysteine residue Cys530 in the [NiFe] hydrogenase active site of the D. gigas enzyme plays a crucial role in the catalytic process by accepting the proton. The active site is constructed to provide access by this cysteine residue, and this role explains the change in activity observed when this cysteine is replaced by a selenocysteine. Furthermore, the optimized geometry of the transition state in the model bears a striking resemblance to the geometry of the active site as determined by X-ray crystallography.

Meta-Benzyne Reacts as an Electrophile

Examination of gas-phase reactions of various nucleophiles with meta-benzyne analogues that carry a positively charged substituent has revealed that the meta-benzyne moiety is susceptible to nucleophilic attack. Addition of nucleophiles to the meta-benzyne moiety occurs rapidly without energy-demanding uncoupling of the singlet biradicals' formally unpaired electrons. The resulting zwitterionic intermediate may undergo fragmentation either by a homolytic bond cleavage to yield net-radical type products or by a heterolytic bond cleavage that generally leads to replacement of the original charged group with the incoming nucleophile. The intermediate was characterized experimentally and computationally and found to be a low-energy species. The nonradical reactivity reported here is common for meta-benzynes with positively charged substituents, and is predicted also to dominate the chemistry of anionic and neutral meta-benzynes.