Comparison of intermediates of tryptophan, tyrosine and their dipeptide induced by UV light and SO. 4 –

The chemical processes of tryptophan (Trp), tyrosine (Tyr) and a dipeptide Trp-Tyr, which are induced by UV radiation and one-electron oxidation of SO^. ₄ ^–, have been investigated in aqueous solution by KrF (248 nm) laser flash photolysis. On the basis of optical studies, the photoionization of Trp and Tyr produces the tryptophan indolyl radical and tyrosine phenoxyl radical, respectively, and these are different from the intermediates resulting from interaction of Trp and Tyr with SO^. ₄ ^–. In the case of Trp, SO^. ₄ ^– would attack the indole moiety to produce a C(2)-yl sulphate radical adduct, and Tyr is oxidized to produce mainly the corresponding one-electron oxidized radical, which deprotonates rapidly to form the phenoxyl radical in neutral solution, and a possible sulphate radical adduct. From transient absorption spectra of photoionization of Trp-Tyr, an intramolecular electron transfer, Trp/N^.-Tyr Trp-Tyr/O^., has been observed, but there was no observation of the process of one-electron oxidation of Trp-Tyr by SO^. ₄ ^–.     

Studies of the polymerization of diallyl compounds. XII. Cyclocopolymerization of dimethallyl phthalate with vinyl acetate

Dimethallyl phthalate was copolymerized with vinyl acetate at 60°C with the use of benzoyl peroxide as an initiator. The rate and degree of copolymerization increased with an increase in the mole fraction of vinyl acetate. The residual unsaturation of the copolymer was nearly constant, regardless of the feed molar ratio. The monomer reactivity ratios (MRR) were obtained on the basis of the copolymer composition equation in which the intramolecular cyclization reaction was considered: γ₁ = 1.08 (MRR of the uncyclized radical), γ₂ = 0.99 (MRR of vinyl acetate radical), γ_c = 0.73 (MRR of the cyclized radical). The difference between γ₁ and γ_c is discussed.     

Formation of Cyclic Ketals from Hydroxyalkyl Enol Ethers,a stereoelectronically controlled endo-trig-cyclization process

Acid-catalyzed cyclic ketal formation vs. hydrolysis of a series of hydroxyalkyl cyclic enol ethers in the presence of 1 equiv. of H₂O, and acid-catalyzed cyclic-ketal formation (same ketals as above) vs. methanolysis of a series of mixed pent-4-enyl hydroxyalkyl ketals with N-bromosuccinimide in the presence of 4 equiv. of MeOH led to the same result: the intramolecular cyclization processes occur at similar rates as the intermolecular H₂O or MeOH attacks independently of the size of the rings formed (five-, six-, or seven-membered), by cyclizations. These results can be explained by the facts that, due to stereoelectronic effects which impose a torsional strain to the sp² hybridized O-atom, the cyclization activation enthalpy decreases, as the length of the hydroxyalkyl chain increase (ease of cyclization: 7 > 6 > 5), whereas the entropy factor favors the cyclization in the reverse fashion (ease of cyclization: 5 > 6 > 7). The various reaction pathways have been examined using the semi-empirical Hamiltonian AM1, and the results obtained confirm that large-ring formation is enthalpically much favored over the cyclization processes leading to small rings (ease of cyclization: 7 > 6 > 5).     

Propagation Mechanism of Radical Copolymerization of Sulfur Dioxide and Vinyl Chloride

Radical copolymerization of sulfur dioxide and vinyl chloride (VC) has been studied by the comparison of the composition of copolymers obtaining from different reaction conditions, i.e., reaction temperatures, feed compositions, and total monomer concentrations. The composition of VC in copolymer is independent of comonomer composition except at high concentration of VC in feed; it increases with increasing reaction temperature or decreasing total monomer concentration. At lower temperature, the composition of copolymer becomes independent of total monomer concentration. The overall rate of polymerization is proportional to [VC]^1,7 and [SO₂]^0.5. These results were compared with those obtained in our previous study on the SO₂-styrene copolymerization. A propagation mechanism for radical copolymerization of SO₂ and VC is also proposed.     

Lewis acid mediated highly regioselective intramolecular cyclization for the synthesis of β-lapachone

A highly regioselective intramolecular cyclization of lapachol mediated by Lewis acids including NbCl₅, AlCl₃, and FeCl₃ was developed for synthesizing β-lapachone in excellent yields without any formation of the isomer α-lapachone. This procedure was efficient, mild, and easily scalable that avoided using highly hazardous concd H₂SO₄. In the case of ZrCl₄ the cyclization was found to give α-lapachone as the main product. A possible mechanism for the Lewis acid mediated cyclization was also discussed.     

A radical addition and cyclization relay promoted by Mn(OAc)_3·2H_2O:Synthesis of 1,2-oxaphospholoindoles and mechanistic study

Novel and efficient Mn(OAc)_3·2 H_2 O promoted radical addition-[4+1] cyclization relay of 3-indolymethanols and phosphites was disclosed,which afforded 1,2-oxaphospholoindole derivatives in moderate to good yields.Based on the experimental and computational studies,a mechanism involving radical addition and intramolecular cyclization cascade was proposed.     

Formation of dimer radical cation of 1,1-dianisylethylene through two-step hole transfer processes during TiO2 photocatalytic reactions

The formation of the 1,1-dianisylethylene dimer radical cation ((DAE)₂⁺) during TiO₂ photocatalytic reactions was successfully investigated by the transient absorption measurement with time-resolved diffuse reflectance spectroscopy. This reaction was explained in terms of the cascade hole transfer processes from the photo-generated holes in the TiO₂ particles to free DAE (DAE_free) in the bulk solution mediated by the radical cation of p-phenylbenzoic acid adsorbed on the TiO₂ surface (PBA_ads⁺).     

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

The metalloradical activation of ortho‐benzallylaryl N‐tosyl hydrazones with [Co(TPP)] (TPP=tetraphenylporphyrin) as the catalyst enabled the controlled exploitation of the single‐electron reactivity of the redox non‐innocent carbene intermediate. This method offers a novel route to prepare eight‐membered rings, using base metal catalysis to construct a series of unique dibenzocyclooctenes through selective C_carbene?C_aryl cyclization. The desired eight‐membered‐ring products were obtained in good to excellent yields. A large variety of aromatic substituents are tolerated. The proposed reaction mechanism involves intramolecular hydrogen atom transfer (HAT) to Co^III–carbene radical intermediates followed by dissociation of an ortho‐quinodimethane that undergoes 8π cyclization. The mechanism is supported by DFT calculations, and the presence of radical‐type intermediates was confirmed by trapping experiments.     

1,2-Shift of a Carboxyl Group in a Wagner-Meerwein Rearrangement

On treatment with HSO₃F in SO₂C1F at 0°, 3-hydroxy-2,2-dimethyl-3-phenyl-propionic acid ( 1a ) is transformed into 2-phenyl-3-methyl-2-butenoic acid ( 2a ) (isolated yield: 40–44%). Using monolabelled [3-¹³C]- 1a ( 1a *) and doubly labelled [1,3-¹³C₂]- 1a ( 1a **), the migration of HOOC (or a mechanistically equivalent group) was proved; a cross experiment established the intramolecular character of the rearrangement. By following the reaction at low temperature in an NMR. spectrometer, the formation of intermediates and side products was demonstrated.     

Reactions of sulfur halides with unsaturated compounds. 26. Reactions of sulfur dichloride with tricyclo[4.2.2.02,5]deca-3,7-dienes and tricyclo[4.2.2.02,5]deca-3,7,9-trienes. Synthesis of compounds with a novel thiaskeletal structure

Addition of sulfur dichloride to tricyclo[4.2.2.0^{2, 5}]deca-3,7-dienes and tricyclo[4.2.2.0^2,5] deca-3,7,9-trienes is accompanied by intramolecular cyclization to give high yields of adducts with the novel thiaskeletal 7-thiatetracyclo[4.2.2.0^4,8.0^5,10]undecane structure. In the reaction of SCl₂ with the 2,3-endo, endo-diester (V), intramolecular cyclization is accompanied by lactonization, to give quantitative yields of the thiaskeletal chlorolactone (X). The structure of the latter compounds was established by x-ray diffraction examination.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 858–864, April, 1990.For previous commmunication see [1].     

Synthesis of 3,4-disubstituted 2(1H)-quinolinones via intramolecular Friedel-Crafts reaction of N-arylamides of Baylis-Hillman adducts

3,4-Disubstituted 2(1H)-quinolinones were synthesized starting from the Baylis-Hillman adducts via the following sequential processes: (i) hydrolysis of the Baylis-Hillman adduct to acid, (ii) EDC coupling with anilines, (iii) H₂SO₄-assisted intramolecular Friedel-Crafts cyclization, and the final (iv) DBU-mediated isomerization.     

Electrolytically generated sulfur dioxide anion radical S2O·−4, its absorption coefficient and some of its decay reactions

The electrochemical reduction of SO₂ in dimethylformamide at Pt electrodes finally leads to the red species S₃O^2?₆ via the blue complex S₂O^·?₄. The UV-VIS absorption coefficients are determined: ?(S₂O^·?₄) = (224 ± 25) × 10⁵ cm² mol^?1; ?(S₃O^2?₆) = (0.64 + 0.07) × 10⁵ cm² mol^?1. A calculation of the complexing constant of SO₂ with free SO^·?₂ radical based on potential shifts confirms this constant to be in the range of 200–700 1 mol^?1.Two potentiometric titration methods (viz: with allylbromide and tetraethylammoniumtribromide) for analysis of electrolytically generated S^III-oxo-anions in DMF are presented. Reactions of those anions with aromatic aldehydes and trials for trapping of possibly formed SO are described.     

Highly efficient one-pot synthesis of 1-substituted-1,2,3,4-tetrahydropyrazino[1,2-a]indoles

A practical and general one-pot synthesis of 1-substituted-10-methyl-1,2,3,4-tetrahydropyrazino[1,2-a]indoles is described. The approach uses 2-(3-methyl-1H-indol-1-yl) ethylamine, benzotriazole and aldehydes in the presence of catalytic amount of acid catalysts (AlCl₃, ZnCl₂, ZnBr₂, p-TsOH, CH₃SO₃H) and proceeds in high yields via iminium cation intramolecular cyclization. The mechanism of the observed intramolecular cyclization reaction has been investigated theoretically by means of PM3 semiempirical method and results were consistent with the experimental results.     

Polysulfonylamine. XXXVII. Darstellung von Quecksilberdimesylamiden. Kristall- und Molekülstrukturen von Hg[N(SO2CH3)2]2, Hg[{N(SO2CH3)2}2(DMSO)2] und Hg[{N(SO2CH3)2}2(HMPA)]

Polysulfonyl Amines. XXXVII. Preparation of Mercury Dimesylamides. Crystal and Molecular Structures of Hg[N(SO₂CH₃)₂]₂, Hg[{N(SO₂CH₃)₂}₂(DMSO)₂], and Hg[{N(SO₂CH₃)₂}₂(HMPA)] Hg[N(SO₂CH₃)₂]₂ ( 1 ) and Hg₂[N(SO₂CH₃)₂]₂ ( 2 a ) are formed as colourless, sparingly soluble precipitates when solutions of Hg(NO₃)₂ or Hg₂(NO₃)₂ in dilute nitric acid are added to an aqueous HN(SO₂CH₃)₂ solution. By a similar reaction, Hg₂[N(SO₂C₆H₄ ? Cl? 4)₂]₂ is obtained. 1 forms isolable complexes of composition Hg[N(SO₂CH₃)₂]₂ · 2 L with L = dimethyl sulfoxide (complex 3 a ), acetonitrile, dimethyl formamide, pyridine or 1,10-phenanthroline and a (1/1) complex Hg[N(SO₂CH₃)₂]₂ · HMPA ( 4 ) with hexamethyl phosphoramide. Attempted complexation of 2 a with some of these ligands induced formation of Hg⁰ and the corresponding Hg^II complexes. Crystallographic data (at -95°C) are for 1: space group 14₁/a, a = 990.7(2), c = 2897.7(8) pm, V = 2.844 nm³, Z = 8, D_x = 2.545Mgm^?3; for 4a: space group P1 , a = 767.8(2), b = 859.2(2), c = 925.2(2)pm α = 68.44(2), β = 86.68(2), γ = 76.24(2)°, V = 0.551nm³, Z = 1, D_x = 2.113 Mgm^?3; for 4: space group P2₁/c, a = 1041.3(3), b = 1545.4(3), c = 1542.5(3) pm, β = 100.30(2)°, V = 2.474nm³, Z = 4, D_x = 1.944Mgm³. The three compounds form molecular crystals. The molecular structures contain a linear or approximately linear, covalent NHgN moiety; the Hg? N distances and N? Hg? N angles are 206.7(4) pm and 176.3(2)° for 1, 207.2(2) pm and 180.0° for 3a, 205.7(4)/206.7(4) pm and 170.5(1)° for 4. In the complexes 3a and 4, the 0-ligands are bonded to the Hg atoms perpendicularly to the N? Hg? N axes, leading in 3a to a square-planar trans-(N₂O₂) coordination with Hg? 0 261.2(2) pm and N? Hg? O 92.3(1)/87.7(1)°, in 4 to a slightly distorted T-shaped (N₂O) geometry with Hg? 0 246.2(4)pm and N? Hg? 0 96.7(1)/92.0(1)°. In all three structures, the primary coordination is extended to a severely distorted (N₂O₄) hexacoordination by the appropriate number of secondary, inter- and/or intramolecular Hg…?0 inter-actions (0 atoms from sulfonyl groups, Hg…?O distances in the range 280—300pm). The intramolecular Hg…?O interactions give rise to nearly planar four-membered [HgNSO] rings. The molecule of 1 has a two-fold axis through the bisector of the N? Hg? N angle, the molecule of 3a an inversion center at the Hg atom. The molecule of 4 has no symmetry.     

Cyclopolymerization of perfluoroisopropenyl vinylacetate

To develop the polymerization exploiting the interconversion of fluorinated carbon radical to hydrocarbon radical, the radical cyclopolymerization of perfluoroisopropenyl vinylacetate [CF₂?C(CF₃)OCOCH₂CH?CH₂] (FIA) was investigated to afford a polymer possessing mainly five‐membered ring structure with bimodal molecular weight distribution having 1 × 10⁵ as the higher molecular weight. This may be the first example wherein the cyclopolymerization between usual allyl group and fluorinated vinyl group is performed. The degree of cyclization was between 70 and 80% determined by ¹⁹F NMR of as‐polymerized products. The polymer preparation from perfluoroisopropenyl group, which shows scarce homopolymerization reactivity was accomplished. The mechanism that the addition of hydrocarbon radical to perfluoroisopropenyl group to produce fluorinated carbon radical followed by the intramolecular addition reaction onto allyl group to form five‐membered ring is proposed. The hydrolysis of the FIA polymer afforded a polymer possessing hydrophobic fluoroalkyl group with hydrophilic hydroxyl and carboxylic acid groups. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3220–3232, 2006     

Synthesis of 6,6-dimethyltricyclo[5.4.0.02,8]undecane-2,9-diol for (ent-)longipinane-type sesquiterpenoids using two types of radical cyclization reactions

A synthetic route to 6,6-dimethyltricyclo[5.4.0.0^2,8]undecane-2,9-diol, a key precursor to (ent-)longipinane-type sesquiterpenoids, is described. This unique core common to (ent-)longipinanes was constructed using two types of intramolecular radical cyclization reactions, namely, intramolecular coupling of an acid chloride and an alkyl iodide mediated by SmI₂, TBAI and HMPA, and the coupling of a ketone and an epoxide mediated by Cp₂TiI₂ and SmI₂.     

Gasification characteristics of coal char under CO2 atmosphere

The char gasification characteristics and the composition of evolved gases in a CO₂ environment have been studied using a thermogravimetric analyzer (TG) coupled with a mass spectrometer. Three types of coal char were studied: lignite (TXL), sub-bituminous (PRB), and bituminous (KYB). TG results showed that the reactivities of TXL and PRB were higher than that of KYB, and the reactivity of TXL was higher than that of PRB. The characterization of the chars implied that the mineral content in the char plays an important role in the reactivity and that the surface area and pore volume may accelerate the reactivity of chars. The evolved gases from three chars were mainly CO and SO₂. SO₂ was slightly delayed by CO during gasification of TXL and PRB chars, but for KYB, SO₂ and CO formed in the same temperature range, but at higher temperatures compared with TXL and PRB. The CO production of KYB was the best, 0.98 mg mg^?1; and SO₂ was the least, 0.031 mg mg^?1. PRB and TXL chars had similar CO production, but SO₂ in TXL was higher.     

Side-chain oxidation of some phenyl-substituted alcohols in aqueous solution: Formation and fragmentation of radical-cations

The reactions of the hydroxyl radical (at low pH) and SO₄^−. have been employed to oxidize a number of phenyl-substituted alcohols in aqueous solution; ESR spectroscopy has been employed to study directly the radicals formed by fragmentation of first-formed radical-cations. Examples of deprotonation (to give benzyl radicals), C_α−C_β bond scission (e.g. PhCH₂CH₂OH to PhCH₂ ^.) and longer-range fragmentation (e.g. PhCH₂CH₂CMe₂OH to PhCH₂CH₂ ^.) are described and discussed: evidence is obtained for intramolecular nucleophilic attack as a route for overall electron-transfer from side-chain to aromatic ring. Solvation of the proton and of oxygen-conjugated carbonium ions is thought, at least in part, to account for differences between fragmentations induced in the gaseous and aqueous phases.     

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO₄.3H₂O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO⁻₃ radical and two pairs of lines (A₁, A₂) and (B₁, B₂) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO⁻₃ indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T₁ and T₂) of SO⁻₃ have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO⁻₄ radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO⁻₃ and chemical inequivalence of two types of SO⁻₄ radicals. The principal g-values of the SO⁻₄ radical were evaluated to be: g₁ = 2.007, g₂ = 2.011, g₃ = 2.014 for species A and g₁ = 2.008, g₂ = 2.012, g₃ = 2.015 for species B. The low microsymmetry of the SO²⁻₄ ion in the lattice seems to promote the radiation damage.     

Rate constants for hydrogen abstraction reactions of the sulfate radical,SO4−. Alcohols

Rate constants have been determined for the reactions of SO₄^? with a series of alcohols, including hydrated formaldehyde. The SO₄^? radical was produced by the laser-flash photolysis of persulfate, S₂O₈^2?. Rate constants for the reactions of SO₄^? with alcohols range from 1.0 × 10⁷ for methanol to 3.4 × 10⁸ M^?1 s^?1 for 1-octanol. Rate constants for the reactions of SO₄^? with deuterated methanol and ethanol are lower by about a factor of 2.5. For methanol, ethanol, and 2-propanol, the temperature dependence of the rate constant was determined over the range 10–45°C.