Homolytic Substitution at the Sulfur Atom as a Tool for Organic Synthesis

The use of intramolecular homolytic substitution at the sulfur atom by aryl and vinyl radicals, as an alternative to the use of alkyl halides, and chalcogenides as radical precursors in organic synthesis is reviewed.     

煤大分子结构的电子自旋共振谱表征

应用ＥＳＲ技术对一变质系列煤的自由基特性进行了实验研究，探讨了煤中自由基的起源、性质和数量的变化，并将其与煤的大分子结构相关联。结果表明，煤自由基特性在煤化过程中的规律性较好。     

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e_aq⁻) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK_a values of ketylanion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.     

Photoinduced Interactions Between Oxidized and Reduced Lipoic Acid and Riboflavin (Vitamin B2)

As a powerful natural antioxidant, lipoic acid (LipSS) and its reduced form dihydrolipoic acid (DHLA) exert significant antioxidant activities in vivo and in vitro by deactivation of reactive oxygen and nitrogen species (ROS and RNS). In this study the riboflavin (RF, vitamin B₂) sensitized UVA and visible‐light irradiation of LipSS and DHLA was studied employing continuous irradiation, fluorescence spectroscopy, and laser flash photolysis (LFP). Our results indicate that LipSS and DHLA quench both the singlet state (¹RF*) and the triplet state (³RF*) of RF by electron transfer to produce the riboflavin semiquinone radical (RFH^.) and the radical cation of LipSS and DHLA, respectively. The radical cation of DHLA is rapidly deprotonated twice to yield a reducing species; the radical anion of LipSS (LipSS^.?). When D₂O was used as solvent, it was confirmed that the reaction of LipSS with ³RF* consists of a simple electron‐transfer step, while loss of hydrogen occurs in the case of DHLA oxidation. Due to the strong absorption of RFH^. and the riboflavin ground state, the absorption of the radical cation and the radical anion of LipSS can not be observed directly by LFP. N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) and N,N,N′,N′‐tetramethyl benzidine (TMB) were added as probes to the system. In the case of LipSS, the addition resulted in the formation of the radical cation of TMPD or TMB by quenching of the LipSS radical cation. If DHLA is the reducing substrate, no formation of probe radical cation is observed. This confirms that LipSS^.+ is produced by riboflavin photosensitization, and that there is no oxidizing species produced after DHLA oxidization.     

Antioxidant and Antiradical Activity of Hydroxy‐Substituted 4‐Thiaflavanes

The antioxidant activities of several hydroxy‐substituted 4‐thiaflavanes, compounds 1 – 3 , were determined by measuring their ability of inhibiting the autoxidation of styrene or cumene. On this basis, the role played by the number and position of OH groups and by the oxidation state of the S‐atom was quantified and rationalized. With these data, it should be possible to optimize the structural features of these ‘double‐faced’ antioxidants for structure? activity‐relationship studies. A comparison between the kinetic data (k_inh) reported in this paper and the previously reported values of the antiradical activities (SC₅₀), measured by the DPPH^. bleaching method, for 1 – 3 is made (Table).     

An EPR and ENDOR investigation of a series of diazabutadiene-group 13 complexes

Paramagnetic diazabutadienegallium(II or III) complexes, [(Ar-DAB)2Ga] and [{(Ar-DAB*)GaX}2] (X = Br or I; Ar-DAB = {N(Ar)C(H)}2, Ar = 2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, [K(tmeda)][:Ga(Ar-DAB)], with either "GaI" or [MoBr2(CO)2(PPh3)2]. A related InIII complex, [(Ar-DAB*)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, [(Ar-DAB(.))MX(2)(thf)(n)] (M = Al, Ga or In; X = Cl or I; n = 0 or 1) and [(Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a = 2.85 G), Ga (a = 17-25 G) or In (a = 26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert-butyl groups in the gallium complex, [(tBu-DAB*)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, [(Ar-DAB*)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa approximately 1.4 G for [(tBu-DAB*)GaI2] and aGa approximately 25 G for [(Ar-DAB*)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, [{(Ar-DAB*)GaBr}2], [{(Ar-DAB*)GaI}2], [{(tBu-DAB*)GaI}2] and [{(Ar-DAB*)InCl}2], revealed doublet ground states, indicating that the Ga-Ga and In-In bonds prevent dipole-dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, [(Ar-BIAN*)GaI2] (Ar-BIAN = bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert-butyl protons were obtained by ENDOR spectroscopy. In [(Ar-DAB*)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time.     

Thermodynamic and kinetic characteristics of intermediates of electrode reactions: A comparative investigation of a number of alkylaryl and alkyl halide radicals by the laser photoemission methods

A comparative investigation of thermodynamic and kinetic properties of a number of alkylaryl intermediates (benzyl and benzhydryl radicals) and alkyl halide intermediates (chloromethyl, dichloromethyl, and trifluoromethyl radicals) is performed by methods of laser photoemission. Techniques, aimed at the determination of thermodynamic and kinetic properties of intermediates (standard potentials E ⁰ of redox pairs R/R^-, standard adsorption free energies -G _a(R) ⁰ , values of rate constants W ⁰ at an equilibrium potential, as well as lifetimes (times of death in the bulk) _R of radicals R and _X of products of their reduction R^-) from a comparison of Tafel plots for quasi-reversible reduction of intermediates with calculated ones and standard potentials E ⁰—from Tafel plots for irreversible electroreduction of intermediates, are presented. The transition from irreversible to quasi-reversible reduction in aprotic solvents at EE ⁰ is observed only in the case of benzyl, benzhydryl, and trifluoromethyl radicals, for which this particular collection of thermodynamic and kinetic properties is obtained, and is not observed for the chloromethyl and dichloromethyl radicals. In this case redox characteristics of intermediates (E ⁰, W ⁰) are estimated from absolute values of rates of their electroreduction. Possible reasons for the differences in the probability of a reversible electron transfer are discussed for the systems studied.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 157–174.Original Russian Text Copyright © 2005 by Krivenko, Kotkin, Kurmaz.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

KINETICS STUDY ON MELT GRAFTING ACRYLIC ACID ONTO LLDPE USING REACTIVE EXTRUSION

The kinetics of melt grafting acrylic acid(AA)onto linear low density polyethylene(LLDPE)by using reactive extrusion was investigated.The polymeric peroxides(POOP and POOH)generated by electron beam irradiation were used to initiate the graft reaction.The samples taken out from the barrel at five ports along screw axis were analyzed by FTIR.The spectra show that both the graft copolymerization and homopolymerization proceed in two stages:the graft degree(or mass of homopolymer)increases linearly with the reaction time in the initial stage,and then gradually in the second stage.The rate of graft copolymerization R_g is always faster than that of homopolymerization R_h in the present system and the activation energy is 131 kJ·mol~(-1) for graft copolymerization and 127 kJ·mol~(-1) for homopolymeirzation.These results were interpreted in terms of solubility and diffusion of monomer,as well as the reactivity and the concentration of reactive species. The relationships between reaction rate and monomer concentration and peroxide concentration were found to he:R_g ∝ [M]~(1.46)[POOP+POOH]~(0.53) and R_h ∝[M]~(1.08)[POOH]~(0.51),which indicate that the addition of monomer to polymeric radicals is a slow step for the graft copolymerization.     

Ruthenium catalysts bearing N-heterocyclic carbene ligands in atom transfer radical reactions

The catalytic activity of ruthenium-p-cymene complexes bearing N-heterocyclic carbene ligands in atom transfer radical addition (ATRA) or polymerisation (ATRP) strongly depends on the substituents of the carbene ligand, thereby providing a nice illustration of the importance of organometallic engineering and ligand fine tuning in homogeneous catalysis.