Russian Chemical Reviews 70 (10) 885 ¡À 895 (2001) Recent trends in the chemistry of sulfur-containing reducing agents S V Makarov Contents I. Introduction II. The structure of molecules of sulfur-containing reducing agents III. Classification of sulfur-containing reducing agents and the fields of their application IV. Methods of synthesis of sulfur-containing reducing agents V. The kinetics and mechanisms of reactions involving sulfur-containing reducing agents VI. Conclusion Abstract. reactivity and stability synthesis, structure, the on Data Data on the structure, synthesis, stability and reactivity of D bonds r with agents reducing sulfur-containing of sulfur-containing reducing agents with C7S o S or S7S bondsD sodium and hydroxymethanesulfinate sodium dithionite, sodium dithionite, sodium hydroxymethanesulfinate and thiourea thiourea oxides and anaerobic of Reactions surveyed.are D oxides D are surveyed. Reactions of anaerobic and aerobic aerobic decomposition are agents reducing sulfur-containing of decomposition of sulfur-containing reducing agents are discussed. discussed. The of studies the in compounds these of applications The applications of these compounds in the studies of non-linear non-linear phenomena in chemical kinetics and in guanidine syntheses are phenomena in chemical kinetics and in guanidine syntheses are considered. references 165 includes bibliography The considered. The bibliography includes 165 references. I. Introduction Compounds containing an S7S or C7S bond, namely, sodium dithionite, sodium hydroxymethanesulfinate (HMS, commercial name rongalite) and thiourea dioxide (TUDO), have long been used in chemistry and chemical technology as reducing agents.The traditional fields of application including printing and dyeing of textiles,1 production of synthetic rubber,2 manufacture of uranium and transuranium element compounds,3 preparative organic 4 and inorganic 5 chemistry have been covered fairly comprehensively in monographs.6, 7 In recent years, some new fields of application appeared and the above-mentioned fields (mainly, organic synthesis 8, 9) have been further developed. In particular, increasing numbers of publications are devoted to the use of these compounds in biochemistry,10 ¡À 12 in organofluorine chemistry,13 and in investigations of non-linear phenomena in chemical kinetics.14, 15Apromising field of application of thiourea dioxides and the products of their oxidation, trioxides, is the synthesis of guanidines.16 ¡À 20 The researchers' interest in the guanidine properties, which has increased in recent years,21, 22 is due to the discovery of an important biological function of nitrogen oxide, the precursor of which in an organism is L-arginine (2-amino-5-guanidinovaleric acid), and to the search for new medical drugs.18, 20 Thus, there is a need for a review that would reflect the state-of-the-art of the chemistry of sulfur-containing reducing agents.S V Makarov Ivanovo State University of Chemistry and Technology, prosp.F Engelsa 7, 153460 Ivanovo, Russian Federation. Fax (7-093) 241 79 95. Tel. (7-093) 232 73 97. E-mail: makarov@icti.ivanovo.su Received 13 February 2001 Uspekhi Khimii 70 (10) 996 ¡À 1007 (2001); translated by Z P Bobkova #2001 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2001v070n10ABEH000659 885 885 886 888 888 893 II. The structure of molecules of sulfur-containing reducing agents 4 6 5 6 44 4 4 5 6 The molecular structures of sodium dithionite, HMS and TUDO have been studied in detail by IR and Raman spectroscopy and by X-ray diffraction analysis. The structure of sodium dithionite has aroused the greatest interest. Seemingly, the structure of the S2O2¡¦ ion should resemble the structures of well-known sulfur-and- oxygen anions with the S7S bonds, e.g., S2O2¡¦ and S2O2¡¦ (see Ref.23). One might expect that S2O2¡¦ and S2O2¡¦ ions would correspond to D3d and C2h point groups of symmetry, respec- tively. This prediction came true only for the latter anion, while for sodium dithionite, unexpected results were obtained. According to X-ray diffraction data,24, 25 the S2O2¡¦ ion belongs to the C2u point group and contains an abnormally long S7S bond (2.39 A). Later, it was shown by Raman spectroscopy 26, 27 that in aqueous solutions of sodium dithionite, the structure of the S2O2¡¦ ion changes, namely, it becomes centrosymmetrical and corresponds to the C2h point group. The influence of the nature of the salt cation on the structure of the dithionite ion has been studied.23 It was shown that the `abnormal' eclipsed conformation (C2u) of S2O2¡¦ can be found in salts with small cations, for example, with sodium. In the salts with large cations, in particular, in tetraethyl- ammonium dithionite, this anion is centrosymmetrical both in the solid state and in solution.Thus, the abnormal structure of sodium dithionite in the solid phase is due to the influence of the cation and to the associated packing effects 23 rather than to the properties of the S2O2¡¦ 4 anion itself. These experimental data were substantiated theoretically by the results of LCAO-Xa density functional calculations.28 It was shown that the curve for the total energy vs. S7S bond length for dithionite ion, unlike these curves for S2O2¡¦ or S2O2¡¦ ions, has a very broad minimum.This indicates that the S7S bond length in S2O2¡¦ 4 depends appreciably on external factors, in particular, on the packing effects in the crystal. It was also found that the barrier to rotation in the dithionite ion is very low. This accounts for the easy dehydration of Na2S2O4 . 2H2O and the concomitant change in the dithionite structure (for example, the S7S bond length increases from 2.298 A in the dihydrate to 2.393 A in the anhydrous salt). Therefore, it can be expected that the properties of solid dithion- ites should depend on the nature of solvents from which they have been crystallised. Indeed, it has been demonstrated in relation to lithium dithionite 29 that the products isolated from aqueous and tetrahydrofuran solutions have different properties; in particular, the latter is extremely susceptible to oxidation and to self-ignition in air.When lithium dithionite is stored in a sealed capillary, its886 2 2 2 crystal lattice is transformed giving rise to defects, and this accelerates solid-phase processes. A similar transformation but taking place at elevated temperatures was observed for the sodium salt. Thus, conditions used to prepare dithionites affect appreci- ably their properties. A substantial dependence of the stability on the nature of the solvent used for recrystallisation is also typical of solid sodium hydroxymethanesulfinate.30 Due to the long and rather weak S7S bond, dithionites tend to undergo homolytic decomposition giving rise to the SO¡ radical anions.The structural parameters of the SO¡ radical anion have been determined;31 the (r) S7O distance is 1.5230.02 A, the O7S7O angle (aO7S7O) is 115.62.0 8. The structures of the SO¡ radical anion and the NaSO2 charge transfer complex were also determined by ab initio calculations.32 According to X-ray diffraction studies,33 HMS is the sodium salt of hydroxymethanesulfinic acid, HOCH2SO2Na. The crystal lattice parameters of HMS and the interatomic distances in the HOCH2SO¡2 anion were determined; the C7S bond was found to be the longest one (1.838 A). X-Ray diffraction analysis of TUDO was first performed by Sullivan and Hargreaves;34 later, the data were refined.35, 36 In the solid phase, TUDO exists as (NH2)2CSO2.The CSO2 group is pyramidal, the S7O and C7N bond lengths are 1.496 and 1.296 A, respectively, and the C7S bond (1.867 A) is much longer than this bond in the thiourea (1.716 A) molecule. The bond lengths in theTUDOmolecule found from X-ray diffraction data are in good agreement with the results of ab initio calcu- lations.36 It was suggested that TUDO is a combination of two zwitter-ion forms O O7 H2N + and C S . H2N +C +S O7 O7 H2N H2N In these ions, the negative charge is concentrated on the oxygen and nitrogen atoms and the positive charge is on the hydrogen, carbon and sulfur atoms, mainly, on sulfur. The results of calculations indicate that nucleophilic attack is directed at the carbon and sulfur atoms.The structure of thiourea dioxide depends substantially on the solvent. It was found 37 that after dissolution of TUDO in water, the acidity increases, a constant pH value being established very slowly. It was found in special experiments that these changes are not related to decomposition of TUDO or to the presence of oxygen. The 1H NMR spectra of aqueous 38 and dimethyl sulfoxide 39 solutions of TUDO were found to vary with time. The peak observed initially in the TUDO spectrum is rapidly split into a doublet, these changes taking place only in the first minutes after dissolution of thiourea dioxide. Quantum-chemical calculations by the AM1 method for TUDO in the (NH2)2CSO2 and NH2C(=NH)SO2H (aminoiminometha- nesulfinic acid) forms 40 showed that solvation with water or dimethyl sulfoxide decreases substantially the total energy of the system.Note that unlike the gas phase, in aqueous solutions, aminoiminomethanesulfinic acid is the thermodynamically more stable form. Thus, quantum-chemical calculations imply the possibility of TUDO tautomerisation in aqueous solutions. The tautomeric transformations of TUDO in aqueous solutions have been studied theoretically using the method of critical points on the potential energy surface.41 The most probable mechanism of tautomerisation includes stages of successive intermolecular pro- ton transfer in TUDO oligomers and decomposition of the oligomers to give solvated monomers of aminoiminomethanesul- finic acid.The structure of thiourea trioxide (TUTO) has also been studied.42 It was found that the C7N bonds in TUTO are virtually equivalent (1.298 and 1.297 A) and are much shorter than the typical C7Nsingle bond (1.470 A), while the C7S bond length (1.815 A) is slightly greater than the sum of the covalent radii of the S and C atoms (1.790 A). The tetrahedral S atom forms three nearly equivalent S7O bonds (1.439, 1.431, and S V Makarov 2 1.446 A). The structures of thiourea di- and trioxides are stabilised by intermolecular hydrogen bonds.36, 42 The unusually high den- sity calculated 42 for TUTO, 1.948 g cm73 (the density of thiourea dioxide is 36 1.70 g cm73) points to a highly efficient packing of (NH2)2CSO3 molecules in the crystal lattice.Apparently, the structure of TUTO is similar to that of TUDO. The research into the structures of sodium dithionite, HMS and TUDO showed that each of these molecules contains an abnormally long bond, which is, hence, highly prone to rupture (S7S bond in the dithionite and C7S bonds in HMS and TUDO). It is this structural feature that accounts for the high reducing activities of these compounds. As will be shown below, the chemical reactivities of dithionites, HMS, TUDO and their analogues are governed by transformations of the same inter- mediates, namely, sulfoxylic acid H2SO2 or its anions and the SO¡ radical anion. This fact, together with the similarity of methods for the use of these compounds explain why they have been combined in one group.III. Classification of sulfur-containing reducing agents and the fields of their application The sulfur-containing reducing agents considered here (Table 1) can be classified into dithionites, a-hydroxyalkanesulfinates, and thiourea dioxides. Thiourea trioxides are also included in Table 1. Data on a-aminoalkanesulfinates not reflected in the Table can be found in a monograph.7 Note that the properties of most of compounds listed in Table 1 have been little studied. The only exceptions are sodium dithionite, HMS and TUDO, which have found wide practical use. Sodium dithionite and hydroxymethanesulfinate are used most often for the reduction of vat dyes in textile industry.1 The processes involving these reducing agents form the basis of all industrial printing and vat dyeing techniques.The textile industry also makes use of zinc hydroxymethanesulfinates, which possess a lower reducing activity than HMS.6 Conversely, a-hydroxyetha- nesulfinate is a stronger reducing agent than HMS.7 An important application of HMS and TUDO is the manu- facture of synthetic rubber.2 For instance, a mixture of HMS with isopropylbenzene hydroperoxide and ethylenediaminetetraaceta- toferrate(III) initiates copolymerisation of butadiene with styrene. The Fe2+ ±TUDO±H2O2 system is used to initiate polymer- isation of various vinyl monomers.59 Other effective initiating systems are TUDO±KBrO3 60 and TUDO±KMnO4.61 In addi- tion, TUDO is used for waste paper processing 62 and for wool bleaching.63 Numerous publications have been devoted to the use of the reducing agents in question, mainly sodium dithionite, in biochemistry.10 ± 12, 64 ± 66 Sodium dithionite is used for the reduc- tion of many natural and synthetic electron transfer agents including nicotinic acid derivatives, flavins and cytochromes;64, 65 TUDO is used for reduction of ferredoxin, cytochrome C and other substrates 66 and for inactivation and modification of cytidine triphosphate synthase.11, 12 N,N0-Dimethylthiourea is a highly efficient antioxidant, which has found wide use in bio- chemical and medical research.67 Thus the reaction of hydrogen peroxide with N,N0-dimethylthiourea, resulting in the dioxide, can be used to determine the content of H2O2 in biological systems.55 Sulfur-containing reducing agents are widely used in organic synthesis.Thus sodium dithionite serves as the reagent for the reduction of nitro, nitroso, azido and azo compounds, aldehydes, ketones, keto esters, benzyls, quinones, heterocycles, and com- pounds with double bonds as well as for the synthesis of sulfones.4 Hydroxymethanesulfinate and thiourea dioxide are used in sim- ilar processes, in particular, in the preparation of a light-stabiliser for plastics, Benazol P,68 a stabiliser for motor oils called Diafen FP 69 and sulfones.70 ± 74 In recent years, the possibility of using these reducing agents in the chemistry of organofluorine com- pounds has been studied intensively.13, 75 Thiourea dioxide is also employed to reduce organosulfur compounds (sulfylimines, sulf-Recent trends in the chemistry of sulfur-containing reducing agents Table 1.Sulfur-containing reducing agents. Name of compound Dithionites Sodium dithionite Potassium dithionite Lithium dithionite Zinc dithionite Tin dithionite Tetraethylammonium dithionite a-Hydroxyalkanesulfinates Sodium hydroxymethanesulfinate (rongalite) Disodium hydroxy- methanesulfinate Zinc hydroxymethanesulfinate Zinc hydroxyl hydroxy- methanesulfinate Zinc hydroxymethanesulfinate, disubstituted Calcium hydroxymethanesulfinate, disubstituted Titanium hydroxymethane- sulfinate Barium hydroxymethanesulfinate, disubstituted Lead(II) hydroxymethanesulfinate, disubstituted Mercury(II) hydroxy- methanesulfinate, disubstituted Sodium a-hydroxyethanesulfinate Sodium a-hydroxytrifluoro- ethanesulfinate Potassium a-hydroxyethanesulfinate MeCH(OH)SO2K 6 Sodium a-hydroxypropanesulfinate EtCH(OH)SO2Na Potassium a-hydroxy-2-propane- sulfinate Sodium a-hydroxy-n-butane- sulfinate Thiourea dioxides Thiourea dioxide N-Methylthiourea dioxide N-Propylthiourea dioxide N-Isopropylthiourea dioxide N-Butylthiourea dioxide N-But-2-ylthiourea dioxide N-tert-Butylthiourea dioxide N-Neopentylthiourea dioxide N-Hydroxymethylthiourea dioxide N-Hexylthiourea dioxide N-Dodecylthiourea dioxide N-Phenylthiourea dioxide N-p-methylphenylthiourea dioxide N-o-hydroxyphenylthiourea dioxide N-o-methoxyphenylthiourea dioxide N-2,6-Dimethylphenylthiourea dioxide Name of compound Ref.Formula Thiourea dioxides N-2,6-Diethylphenylthiourea dioxide N-2,6-Diisopropylphenyl- thiourea dioxide N-Guanylthiourea dioxide 7 43 29 44 25 45 Na2S2O4 K2S2O4 Li2S2O4 ZnS2O4 Sn(S2O4)2 (Et4N)2S2O4 7 HOCH2SO2Na 6 NaOCH2SO2Na 46 47 (HOCH2SO2)2Zn HOCH2SO2Zn(OH) OCH2SO2Zn OCH2SO2Ca (HOCH2SO2)4Ti N-Diphenylmethylthiourea dioxide N-Triphenylmethylthiourea dioxide N,N-Dimethylthiourea dioxide N,N-Dipropylthiourea dioxide N,N-Dibutylthiourea dioxide N,N-Di(2-methylpropyl)thiourea dioxide N,N0-Dimethylthiourea dioxide N,N0-Dibut-2-ylthiourea dioxide N,N0-Diphenylthiourea dioxide N,N0-Di-o-methylphenylthiourea dioxide N,N0-Dicyclohexylthiourea dioxide (cyclo-C6H11NH)2CSO2 OCH2SO2Ba Thiourea trioxides OCH2SO2Pb 666666 OCH2SO2Hg 48 49 MeCH(OH)SO2Na CF3CH(OH)SO2Na Thiourea trioxide N-Methylthiourea trioxide N-Propylthiourea trioxide N-But-2-ylthiourea trioxide N-tert-Butylthiourea trioxide N-Allylthiourea trioxide 76 PrnCH(OH)SO2Na N-Phenylthiourea trioxide Me2C(OH)SO2K 6 N-Benzylthiourea trioxide N-o-Methylphenylthiourea trioxide N-o-Chlorophenylthiourea trioxide N-p-Fluorophenylthiourea trioxide N-2-Isopropylphenylthiourea trioxide 36 50 50 50 N-2,6-Dimethylphenylthiourea trioxide N-2,6-Diethylphenylthiourea trioxide (NH2)2CSO2 MeNHC(NH2)SO2 PrnNHC(NH2)SO2 Me2CHNHCSO2 NH2 BunNHC(NH2)SO2 Et(Me)CHNHC(NH2)SO2 Me3CNHC(NH2)SO2 Me3CCH2NHC(NH2)SO2 HOCH2NHC(NH2)SO2 50 50 50 50 51 N-2,6-Diisopropylphenylthiourea trioxide N-2,6-Dichlorophenylthiourea trioxide N,N-Dimethylthiourea trioxide 52 52 51 53 C6H13NHC(NH2)SO2 C12H25NHC(NH2)SO2 PhNHC(NH2)SO2 p-MeC6H4NHC(NH2)SO2 51 o-HOC6H4NHC(NH2)SO2 51 50 o-MeOC6H4NHCSO2 NH2 2,6-Me2C6H3NHCSO2 NH2 2,6-Cl2C6H3NHCSO3 NH2 Me2NC(NH2)SO3 N-Ethyl-N-phenylthiourea trioxide Ph(Et)NC(NH2)SO3 N,N0-Diethylthiourea trioxide N,N0-Diisopropylthiourea trioxide (Me2CHNH)2CSO3 N,N0-Dibut-2-ylthiourea trioxide N,N0-Di-tert-butylthiourea trioxide (Me3CNH)2CSO3 N,N0-Dineopentylthiourea trioxide (Me3CCH2NH)2CSO3 N,N0-Dicyclohexylthiourea trioxide (cyclo-C6H11NH)2CSO3 N,N0-Diphenylthiourea trioxide N,N0-Dibenzylthiourea trioxide 887 Ref.Formula 50 2,6-Et2C6H3NHCSO2 NH2 50 2,6-Pri2C6H3NHCSO2 NH2 NH2CNC(SO2H)NH2 54 NH Ph2CHNHC(NH2)SO2 50 50 Ph3CNHC(NH2)SO2 Me2NC(NH2)SO2 Prn2 NC(NH2)SO2 Bun2 NC(NH2)SO2 (Me2CHCH2)2NCSO2 50 50 50 50 NH2 (MeNH)2CSO2 [Et(Me)CHNH]2CSO2 (PhNH)2CSO2 (o-MeC6H4NH)2CSO2 55 50 51 53 50 17 56 56 56 56 56 (NH2)2CSO3 MeNHC(NH2)SO3 PrnNHC(NH2)SO3 Et(Me)CHNHC(NH2)SO3 Me3CNHC(NH2)SO3 NHCSO3 NH2 PhNHC(NH2)SO3 PhCH2NHC(NH2)SO3 o-MeC6H4NHC(NH2)SO3 56 56 17 17 o-ClC6H4NHC(NH2)SO3 16 p-FC6H4NHC(NH2)SO3 56 56 17 2-Me2CHC6H4NHCSO3 NH2 2,6-Me2C6H3NHCSO3 NH2 2,6-Et2C6H3NHCSO3 NH2 56 2,6-Pri2C6H3NHCSO3 NH2 17 (EtNH)2CSO3 [Et(Me)CHNH]2CSO3 (PhNH)2CSO3 (PhCH2NH)2CSO3 56 57 57 56 56 56 56 56 57 58888 oxides, disulfides),76, 77 for the preparation of a number of com- pounds of selenium and tellurium 78, 79 and deoxygenation of various heteroaromatic N-oxides;8 in addition, it serves as a convenient reductant in phase transfer catalysis.76, 80 Sodium hydroxymethanesulfinate is also utilised in the syntheses of various compounds of selenium and tellurium 81, 82 and for the preparation of sultins.9 An important application of thiourea oxides is the synthesis of guanidines and their derivatives,16 ± 20, 83 ± 85 which consists in the reaction of thiourea oxides with primary 83 or heterocyclic secon- dary amines, for example, morpholine,85 or with amino acids 84 and other amino compounds. The reactions of thiourea oxides with amines are carried out in organic solvents (methanol,83 acetonitrile 85), while the reactions with amino acids take place in aqueous solutions.84 The mechanism proposed for this reaction includes the addition of the nucleophile (i.e., amine) to thiourea oxide and decomposition of the intermediate giving rise to guanidine.85 The use of trioxides is more convenient because it permits the preparation of guanidines 85 and guanidine acids 84 in high yields.Currently, thiourea trioxide is among the most important guanidylating agents used in the syntheses of guani- dines with various structures.18 ± 20, 86 ± 98 Unfortunately, the kinetics of the reactions of thiourea oxides with amino compounds has not yet been studied, despite the fact that the interest in guanidines is very high due to the discovery of the crucial role of nitrogen oxide in functioning of the vascular system.99 ± 101 It was found that it is the guanidine fragment of L-arginine that is oxidised in vivo to give citrulline and nitrogen oxide (in a recent publication,87 the possibility of using thiourea trioxide in the syntheses of L-arginine derivatives is demonstrated).Guanidines have proved efficient in the treatment of cardiovascular dis- eases.100 Thus chemists of the Du Pont company have developed the DuP 714 preparation,18 which prevents thrombus formation; this synthesis was a success only when thiourea dioxide was used to introduce the guanidine group.The attempts to synthesise this preparation using other guanidylating agents failed. Thiourea trioxide 86 ± 98 and N-phenylthiourea trioxide 16 have also been employed for the preparation of other medicinal drugs. Thiourea trioxides are the initial compounds in the syntheses of amino- iminoethanenitriles, 5-aminotetrazoles, N-cyanoguanidines and N-hydroxyguanidines.57 The reaction of (NH2)2CSO3 with methyl anthranylate affords 5H-quinazolino[3,2-a]quinazoline-5,12(6H)- dione.102 Numerous studies have been devoted to the use of reduction reactions involving HMS and TUDO to produce metals � silver,103 cadmium,104 technetium,105 nickel,106 and metallic coat- ings on fibres.107 IV.Methods of synthesis of sulfur-containing reducing agents Metal dithionites are prepared by the reduction of sulfur dioxide with zinc, sodium formate 44 or borohydrides.43 A method for the synthesis of extra pure sodium dithionite containing 99% of the required substance has been developed.108 A procedure for the preparation of tetraethylammonium dithionite using ion exchange resins is documented.45 Several methods for the synthesis of sodium hydroxymetha- nesulfinate are known.6 The preparation ofHMSby the reduction of sodium hydroxymethanesulfonate with disperse zinc is used most often. Thiourea mono-, di- and trioxides are produced upon the oxidation of thiourea with hydrogen peroxide in a neutral medium.7 The synthesis of TUDO is performed at 0 ± 58C.Unlike the dioxide, thiourea monoxide is quite unstable. More stable monoxides are formed from thiourea derivatives containing bulky substituents at the nitrogen atoms. Thus N-phenyl- and ethyl- enethiourea monoxides have been prepared.109 The formation of these compounds in solution was proved by the test reaction with FeCl3, typical of mono-S-oxygenated thioamides and thiocarba- S V Makarov mates. Neutral aqueous solutions of the monoxides synthesised are stable over a period of 24 h at 0 8C; however, these compounds are exceptionally unstable in the solid state. N-Phenyl-, N,N0- diphenyl-, N-o-hydroxyphenyl- and N-o-methoxyphenylthiourea dioxides have been prepared by using sodium molybdate 51 to catalyse the reaction between the corresponding thiourea deriva- tive and H2O2.Dioxides of other N-aryl-substituted thioureas have also been prepared.53 Syntheses of N-aryl- and N-alkyl- substituted thiourea dioxides in 80%± 90% yields without cata- lysts have been reported.50, 110 In order to prepare N-phenyl- thiourea dioxide containing *100% of the required substance, the synthesis should be carried out at 77 to 12 8C.17 An attempt to prepare N,N0-diphenylthiourea dioxide by a procedure pro- posed previously 53 failed.17 Thiourea trioxides can be synthesised by treatment of thioureas or thiourea dioxides with three or one equivalent of peracetic acid, respectively.56 An electrochemical procedure for the synthesis of thiourea trioxides has been pro- posed.111 The major products formed in the reaction of thiourea with hydrogen peroxide in strongly acidic media and in the presence of metal ions are formamidine disulfide salts.112 There is no con- sensus of opinion on the mechanism of the reaction between thiourea and H2O2.The kinetics and the mechanism of the reactions of thiourea and N,N0-dialkylthiourea with hydrogen peroxide in acid media have been studied.113 Presumably, in this case, these thioureas act as nucleophiles by replacing the peroxide oxygen, i.e., the reaction does not involve free radicals. However, the use of spin traps, namely, N-benzylidene-tert-N-butylamine N-oxide and 5,5imethyl-1-pyrroline N-oxide (DMPO) allowed detection of the carbamidinothiyl radical NH2(=NH)CS upon oxidation of thiourea with hydrogen peroxide.114 The stability of this radical was found to depend substantially on the medium pH, the most intense EPR signal being observed at 2.5 ± 3.0.V. The kinetics and mechanisms of reactions involving sulfur-containing reducing agents Decomposition of sulfur-containing reductants and their reac- tions with oxidants are closely interrelated because they proceed via the formation of the same intermediates. Therefore, we shall first consider the results of research into decomposition of sulfur- containing reducing agents. The stability of these compounds in aqueous solutions depends appreciably on the pH. Sodium dithionite and HMS are less stable in acid media.Oxygen-free decomposition of sodium dithionite in acid solutions has been studied in detail.115, 116 Polarography was used to measure the time variation of the concentrations of dithionite and the products of its decomposition, which include sulfide, sulfite, thiosulfate, and active sulfur (Sa); the term `active sulfur' implies atomic sulfur, the S .H2O hydrate, the HSOH thioperoxide, the polymeric biradical and sulfur in a polysulfide chain. Decomposition was found to be autocatalytic, active sulfur and sulfide functioning as the catalysts. The influence of sulfide, sulfite, thiosulfate and sulfur dichloride additives (and also of mixtures of these compounds) on the rate of dithionite decom- position has been studied.115 On the basis of the results obtained, it was concluded that the following reactions are responsible for the dithionite decomposition: �non-catalysed reaction proceeding during the induction period, (1) Sa+3SO2+2H2O; 2H2S2O4 � autocatalytic reaction mainly proceeding at the fast decom- position stage, (2) H2S+5SO2+2H2O.3H2S2O4 Simultaneously, a series of side reactions take place. On the basis of experimental data,115 a mathematical model of the dithionite decomposition was proposed.116 Calculations per- formed in terms of this model made it possible to estimate theRecent trends in the chemistry of sulfur-containing reducing agents contribution of individual homogeneous reactions and to demon- strate that the proposed two-stage scheme is adequate to describe the real mechanism of decomposition.Non-catalysed and catalysed decomposition of sodium dithionite in weakly acidic solutions in the presence of sulfur- containing admixtures has been investigated.117 Particular atten- tion is devoted to the influence of sulfite. In the opinion of the researchers cited,117 in the case of a high sulfite concentration in the solution, decomposition of S2O2¡ 4 can follow two mechanisms, a heterolytic and a homolytic one. The former includes the following stages: (3) [O2S(O2)S7SO2OH]37, S2O2¡ 4 +HSO¡3 [O2S(O2)S7SO2OH]37+H+ S3O2¡ 6 +H2O. (4) In this case, reaction (3) is the rate-determining stage. The 2 trithionate formed in reaction (4) participates, together with the SO¡ radical ion, in the decomposition by the homolytic mecha- nism (5) 2 SO¡ S2O2¡ 2 , 4 (6) S2O5H2¡ , HSO¡3 +SO¡2 (7) S2O5H2¡ +S3O2¡ 6 SO2+HSO¡3 +SO23 ¡ +S2O¡3 , (8) S2O2¡ +SO2.3 S2O¡3 +SO¡2 The reaction between two dianions [reaction (7)] determines the rate of the whole homolytic process. When the sulfite concentration in the solution is low, the process follows a third pathway (9) [O2S(O2)S7OSOOH]37 , (10) S2O2¡ +HSO¡2 , 5 (11) 2 (12) S2O2¡ 4 +HSO¡3 [O2S(O2)S7OSOOH]37 2HSO¡ S2O2¡ 3 +H2O , S2O2¡ 2HSO¡3 . 5 +H2O Unlike the heterolytic mechanism mentioned above, in this pathway, the initial reaction is the transfer of an oxygen atom from the bisulfite ion to the dithionite sulfur atom.A serious drawback of the mechanisms proposed by Holman and Bennett 117 [see reactions (3) ± (12)] is that nothing is said about the role of sulfur or sulfides in the dithionite decomposition. This is explainable for a large excess of sulfite present because in acid media, HSO¡3 rapidly reacts with sulfur and sulfides, which virtually cancels out their effect on the dithionite decomposition. However, in the case of low initial concentrations of sulfite in the solution, the influence of sulfur or sulfide, which are the main products of dithionite decomposition in acid media, cannot be neglected. Decomposition of sodium hydroxymethanesulfinate in acid media has been studied.118, 119 This process is autocatalytic, being accelerated by active sulfur.The catalytic effect of Sa is attribut- able to its influence on the decomposition of sodium dithionite, which is one of the main intermediate compounds formed upon decomposition of HMS. The following sequence of reactions was proposed as taking place during oxygen-free decomposition of HMS in acidic aqueous solutions: 2 HSO¡2 +CH2O, 2 S2O2¡ 3 +H2O, Sa+SO2¡ 3 , 3 HSO¡3 , (13) (11) (14) (15) (16) S2O2¡ 4 +H2O, (5) HOCH2SO¡ 2HSO¡ S2O2¡ SO2¡ 3 +H+ HSO¡2 +HSO¡3 S2O2¡ 2SO¡2 . 4 An important role in the mechanisms of decomposition of sodium dithionite as well as HMS and TUDO is played by reaction (5), i.e., the homolytic cleavage of S2O2¡ 4 . 889 The equilibrium constant of reaction (5) at 298 K is equal to 1.461079 mol litre71 (see Ref.120). Studies of the temperature dependence of this equilibrium constant gave quite contradictory results. Thus it has been found 121 that an increase in the temper- ature from 298 to 353 K results in a 10-fold increase in the concentration of the SO¡2 radical anion, which entails an increase in the equilibrium constant of reaction (5) by a factor of 10. A similar pattern of dependence of the constant on the temperature was also indicated by other researchers.122 However, according to some other studies,120, 123, 124 the increase in the equilibrium constant upon an increase in the temperature is much more gradual. In view of the inconsistency of the data on the temperature dependence of the equilibrium constant, the thermodynamic characteristics of reaction (5) were determined using other methods. x The standard enthalpies of formation (DfH8) of the SO¡ (x=2 ± 4) radical anions were calculated.125 The DfH8(SO¡x , 298) values in an aqueous solution were determined on the basis of thermodynamics data for liquid-phase decomposition of ions of the [R7R]27 type including the dithionite ion: (17) R¡ +R¡ .[R7R]27 The standard enthalpy of formation of the product of this reaction can be estimated using the following relation: DfH8(R¡ , 298) = = 0.5 {DrG8(298)+TDrS8(298)+DfH8([R7R]2¡, 298)}, (18) 4 where DrG8(298) and DrS8(298) are the change in the standard Gibbs energy and the standard entropy in reaction (17) at 298 K, respectively, DfH8([R7R]2¡, 298) is the standard enthalpy of formation of the [R7R]2¡ ion at 298 K.The DrG8(298) (52.31.3 kJ mol71) and DfH8(S2O2¡ 4 , 298) (7753.7 kJ mol71) values for the decomposition of S2O2¡ are known. The uncertainty in TDrS8(298) is due to the absence of data on the standard entropy of formation of the SO¡x anion at 298 K [S f (SO¡x , 298)]. These values were estimated using the empirical dependence of the standard entropy of the [R7R]2¡ oxy anion on its mass (M), charge (Z), the distance between the central atom and the peripheral oxygen atoms (r), and the empirical factor ( f) that takes into account the ion geometry (19) S f (298)76.3 R(lnM) = 2767338.6 (Z/r) , 2 have not been determined yet. 2 4 where R is the universal gas constant, r is an empirical variable equal to r/f .For SO¡2 , the r value was found proceeding from the values r=0.151 nm, f=0.870.11. The S f (SO¡2 ) value at 298 K found on the basis of relation (19) amounted to 13425 J mol71 K71 [for the dithionite ion, S f (S2O24 ¡)= 104.5 J mol71 K71]. The DrS 8(298) value for reaction (5) corre- sponding to these parameters is equal to 163.550 J mol71 K71. The DfH8(Sxcl;2 , 298) value determined from Eqn (18) amounts to 7326.58 kJ mol71. The researchers cited 125 did not compare the resulting DrH8(298) (100.717 kJ mol71) with the values found from the temperature dependence of the equilibrium con- stant of reaction (5). It is noteworthy that this value is close to DrH8(298)=89.1 kJ mol71, presented in a publication.122 Unfortunately, the standard enthalpies and entropies of forma- tion of the sulfoxylate ion SO2¡ Equilibrium (5) in aprotic media has been studied using tetraethylammonium dithionite.45 It was shown by EPR and UV spectroscopy that the S2O2¡ dimer dissociates to give the SO¡ radical anions in DMF, DMSO and acetonitrile, the equilibrium constants in non-aqueous solvents being 107 times as high as that in water.Thus, the equilibrium constant for reaction (5) depends appreciably on the nature of the solvent. Unlike HMS or sodium dithionite, TUDO is much less stable in alkaline solutions than in neutral or acidic solutions. Comparative investigation of the composition of the products of decomposition of thio- and N-phenylthiourea di- and trioxides890 and N,N0-diphenylthiourea trioxide in neutral and alkaline sol- utions has been carried out.17 Ureas are the major products of thiourea dioxide decomposition in alkaline media.Conversely, decomposition of thio- and N-phenylthiourea trioxides affords mainly cyanamides (pH 13 ± 14) (N,N0-diphenylthiourea trioxide is mainly converted into N,N0-diphenylurea). In less alkaline solutions (pH 10), the percentage of ureas in the products of dioxide decomposition decreases, while the content of cyanamides increases. Under similar conditions, thiourea trioxide gives mela- mine in 52% yield; the formation of cyanoguanidine (dicyanodia- mide) was also detected (the formation of substituted triazine related to melamine has been observed previously in the decom- position of N-propylaminoiminomethanesulfonic acid in the presence of tert-butylamine 16).Presumably, a product of self- condensation of TUTO is formed intermediately in the reaction of melamine formation. The essentially different decomposition mechanisms of thio- urea dioxides and trioxides and, as a consequence, different compositions of the reaction products have been attributed,17 first of all, to the different stabilities of NH2NHRCSO2 and NH2NHRCSO3 in alkaline media (R=H, Ph). The trioxide is assumed to be much more stable in solution than the dioxide; however no experimental evidence supporting this assumption is available. No data on the sulfur-containing products of decom- position of thiourea oxides are available either. 2 It has been assumed 126 that an important role in the decom- position of TUDO is played by dithionite ions and the SO¡ radical anions.The appearance of these species in alkaline solutions ofTUDOwas detected by polarography and interpreted as being due to the reaction (20) (NH2)2CSO2+OH7 HSO¡2 +(NH2)2CO, as well as reactions (5) and (16) presented above. Presumably,126 decomposition of TUDO gives sulfoxylate as the primary product [however, it is not indicated what process is responsible for the formation of sulfite needed for the formation of dithionite by reaction (16)]. Yet another mechanism of TUDO decomposition has also been proposed;104 according to this mechanism, the strong reducing properties of TUDO in alkaline media are due to the homolytic decomposition of the H2N(=NH)CSO¡2 anion to give reactive species, SO¡2 and (NH2)2C OH.It has also been suggested 127 that the SO¡2 radical anion (in the aqueous alkali ± ethanol system) is formed as the primary product of TUDO decomposition. Unfortunately, in most of the above-mentioned studies, no integrated approach is used to investigate the processes of decom- position of sulfur-containing reducing agents. This is especially true for thiourea oxides. The transformations of their `sulfur' and `nitrogen' components have been studied independently by chem- ists working in different fields. The former, `sulfur' part has been studied by inorganic chemists and specialists in chemical kinetics, while the latter, `nitrogen' part has been an object of investigations of organic chemists and biochemists. These investigations have not been connected with one another and, moreover, the results have been published in journals of different disciplines. The fewness of kinetic data concerning the processes of decomposition of sulfur-containing reducing agents also appears surprising.Data on the decomposition mechanisms are fragmentary and often contradictory. Unfortunately, in many publications, no attention is paid to the role of oxygen in the decomposition of the compounds in question. Often, it is even impossible to grasp whether the reaction was carried out in an inert atmosphere or in air (this is especially typical of papers in organic chemistry).It will be shown below that it is the effect of oxygen on the composition of the intermediate and final products in the decomposition of sulfur-containing reducing agents that is the main reason for the contradictions mentioned above. Let us consider the reactions of sulfur-containing reductants with oxygen. Creutz and Sutin,128 who confirmed the previously proposed 120 mechanism of the dithionite reaction with oxygen, S V Makarov (5) 2SO¡ S2O2¡ 2 , 4 (21) products, SO¡2 +O2 showed also that the process rate is determined by reaction (5), the rate constant for the forward reaction being virtually independent of the pH: k=2.5 (pH 6.5) and 1.8 s71 (pH 13).The researchers also determined the lower limit for the rate constant for reaction (21): k21516108 litre mol71 s71. Later, the value k21=2.46109 litre mol71 s71 was found.129 The kinetics of the reaction of sodium dithionite with hydro- gen peroxide has been studied.128 As in the case of oxygen, the SO¡2 radical anion rather than the S2O24 ¡ dithionite ion enters into the reaction with the oxidant. The influence of oxygen on the decomposition ofHMSandTUDOhas been studied.119, 130 It was found that in the HMS decomposition under aerobic conditions, the appearance of dithionite is preceded by an induction period, which is missing when the experiment is carried out under an inert atmosphere.119 Study of decomposition ofHMSin the presence of superoxide dismutase, catalase and sodium formate showed that active forms of oxygen (superoxide, peroxide, hydroxyl radicals) have little influence on the rate of dithionite formation. Sulfite additives exert a much more pronounced influence under both anaerobic and aerobic conditions.Decomposition of air-saturated alkaline solutions of thiourea dioxides is also accompanied by the formation of dithionite, preceded by an induction period.130 However, unlike HMS, sulfite additives do not affect decomposition of TUDO. In addition, decomposition of thiourea dioxides under anaerobic conditions does not give dithionite. Thus, the S2O2¡ 4 ion and the SO¡2 radical anion occurring in equilibrium arise upon the reaction of a sulfur- containing product of thiourea dioxide decomposition with oxy- gen.Apparently,130 this sulfur-containing product is the sulfox- ylate ion SO2¡ 2 . This species reacts with oxygen to give the SO¡2radical anion, which in turn reacts with oxygen [reaction (23)] and dimerises [back reaction (5)]: (22) SO¡2 +O¡2 , SO2¡ 2 +O2 (23) SO2+O¡2 , SO¡2 +O2 (5) 2SO¡ S2O2¡ 2 4 These reactions occur during the induction period. Since dithionite ion is accumulated after the completion of the induction period, i.e., after oxygen has disappeared, the sulfur-containing species react with superoxide or with the product of its dismuta- tion, peroxide. However, it is known that superoxide is quite stable in strongly alkaline media 131 (most experiments were carried out in a solution of NaOH with a concentration of 0.5 mol litre71) and is unlikely to undergo dismutation over the period of TUDO decomposition.Thus, the peroxide can be formed only upon the reduction of superoxide. This was proved by an independent research of the reaction of TUDO with potassium superoxide in the absence of oxygen. The shape of the resulting kinetic curves was similar to that typical of the curves of TUDO decomposition and dithionite accumulation in the pres- ence of oxygen. The same regularities were also observed in the reaction of TUDO with peroxide under anaerobic conditions; this is due to reactions of the sulfoxylate (arising upon decomposition of TUDO deprotonated in a strongly alkaline medium) and the SO¡2 radical anion with peroxide, giving rise to a hydroxyl radical [represented in Eqns (25) and (26) as the deprotonated form O¡ ]: SO2¡ 2 +NH=C=NH, SO¡2 +O¡ +OH7, (24) (25) (26) SO2+O¡ +OH7, (5) 2 , 4 (27) (NH2)2CO.(NH)2CSO2 SO2¡ 2 +HO¡2 SO¡2 +HO¡2 S2O2¡ 2SO¡ NH=C=NH+H2O Thus, study of the kinetics of reactions of thiourea dioxide with oxygen and its reduced forms made it possible to prove thatRecent trends in the chemistry of sulfur-containing reducing agents 2 2 the sulfoxylate ion SO2¢§ rather than the SO¢§ radical anion, as had been proposed previously,104 is the primary sulfur-containing product of TUDO decomposition in strongly alkaline media. It is the formation of sulfoxylate that accounts for the strong reducing properties of TUDO in alkaline media.The aerobic decomposition of N-methyl- and N,N0-dime- thylthiourea dioxides follows regularities similar to those consid- ered above for TUDO.130 N,N0-Dimethylthiourea dioxide is the most reactive reductant, apparently due to the much higher stability of the intermediate formed primarily in its decomposi- tion, namely, N,N0-dimethylcarbodiimide, compared to the stabilities of carbodiimide and N-methylcarbodiimide.132 Con- versely, the rates of the reactions of oxygen and its reduced forms withN-methylthiourea dioxide (MTUDO) are lower than those in the case of TUDO. Among the compounds considered, MTUDO is the most stable. N-Phenylthiourea dioxide (PTUDO) also exhibits a higher stability in solutions than TUDO.133 Data on the reactivity of thiourea dioxides can be important for interpret- ing the toxicity of thioureas.Many thioureas, especially N-sub- stituted ones, are known to be highly toxic. Thus lethal doses of a-naphthyl- and N-phenylthioureas amount to 0.5 and 5 mg kg71, respectively.17 The toxicity of thioureas is supposed to be due to their oxidation 134 ¡¾ 137 and the tendency of the resulting oxides to undergo desulfurisation;17 the higher this tendency, the more toxic the compound. However, data of some publications 130, 133 are at variance with this conclusion:17 the rates of desulfurisation of N-methyl- and N-phenylthiourea dioxides are lower than that of thiourea dioxide; nevertheless, N-methyl- and N-phenylthioureas are much more toxic than the unsubsti- tuted thiourea.Apparently, there are also other reasons for the high toxicity of N-substituted thioureas. Therefore, the formation and the properties of adducts produced by thiourea oxides and proteins are being vigorously studied.137 2 In weakly alkaline solutions of thiourea dioxides, dithionite ions are accumulated much more slowly. Among other reasons, this is due to the sharp increase in the reactivity of hydrogen peroxide upon a decrease in the pH, which results in a change in the mechanism of interaction of H2O2 with TUDO. It was found that at pH<13, TUDO reacts directly with the peroxide to give thiourea dioxide radicals and hydroxyl radicals.138 The latter have been detected by EPR in the presence of DMPO.Yet another possible reason for the appreciable decrease in the rate of dithionite formation in solutions of thiourea dioxides is a change in the mechanism of their decomposition. To verify this, a comparative kinetic study 138 of the processes of TUDO decom- position and its reactions with oxygen at various pH has been carried out. The rate of the reaction of thiourea dioxide with oxygen at [TUDO]0 44 [O2]0 was found to be described by the equation u=k [TUDO]. In nearly neutral and in strongly alkaline media, the rate constants for both processes virtually coincide. However, in the pH range of 9 ¡¾ 12, substantial differences are observed: the rate constant for TUDO decomposition increases much more rapidly and reaches a constant level at pH 10, whereas the rate constant for the reaction of thiourea dioxide with O2 continues to grow even in the pH 10 ¡¾ 13.7 range.The non- coincidence of the mechanisms of TUDO decomposition at pH 10 and 13 has been also discovered by other authors. Thus it has been found 139 that the reaction of glycine with TUDO in concentrated ammonium hydroxide solutions affords guanidino- acetic acid in 36% yield. When the reaction is carried out in a solution of K2CO3 with a concentration of 1 mol litre71 (pH 10), guanidineacetic acid is not formed.17 In addition, ammonia was detected upon decomposition ofTUDOin this solution 17 (ammo- nia is also formed when thiourea trioxide decomposes in weakly acidic media 42). These facts led to the conclusion 138 that decom- position of thiourea dioxide at pH 9 ¡¾ 12 follows two pathways.According to one of them, ammonia is eliminated (as this takes place, TUDO loses the guanidylating capacity towards glycine) but the C7S bond is not cleaved, the SO2¢§ anion is not formed, and the reaction with oxygen does not occur. Conversely, the 891 2 second pathway includes a stage of formation of the SO2¢§ anion and the C7N bond remains intact; therefore, TUDO actively reacts with oxygen and glycine. It was found138 that both path- ways have a stage of TUDO ionisation. Since the dependence of the rate constant for the reaction of thiourea dioxide on pH is shifted to the region of high pH, the pathway involving the formation of sulfoxylate is assumed to result from decomposition of TUDO dianions.In the TUDO monoanion, the C7N bond is apparently much weaker than the C7S bond; hence, the primary decomposition stage yields ammonia. In acid media, thiourea dioxide is very stable and barely decomposes at room temperature.7 On heating in glacial acetic acid, TUDO and some its analogues decompose to give formami- dine acetate and sulfur dioxide.110 The data presented here indicate that oxygen and active forms of oxygen (AFO) play an important role in the reactions with sulfur-containing reductants. Let us consider published data dealing with the reaction of the TUDO precursor, thiourea, with AFO (oxidation with peroxide has been mentioned above, in the discussion of the methods of synthesis of thiourea oxides).The strongest oxidant among the AFO is the hydroxyl radical.140 The reaction of thiourea with the OH radicals proceeds at a very high rate 141 (k=1.261010 litre mol71 s71). The hydroxyl radicals also oxidise very effectively N,N0-dimethylthiourea 55 and tetra- methylthiourea.141 The reactivity of the O¢§2 superoxide is substantially lower than that of OH radicals. In aqueous solvents, superoxide reacts with thiourea to give cyanamide.142 It is assumed that TUDO is formed as an intermediate of this process. The reaction of super- oxide with diarylthiourea follows a different pathway 143 giving rise to a substituted guanidine. In nonaqueous solutions, super- oxide reduces sulfur dioxide (28) SO2+O¢§2 SO¢§2 +O2 .2 2 2 4 2 The equilibrium constant for this reaction, calculated on the basis of redox potentials (E8) of the O2/O¢§ and SO2/SO¢§ couples, equals 4.8;144 however, experimental data indicate that reaction (28) proceeds virtually to completion. This is due to the subsequent transformations of the SO¢§ radical anion, namely, dimerisation to give S2O2¢§ and complexation with SO2. In the aqueous medium, superoxide does not reduce sulfur dioxide: the redox potential of the O2/O¢§ couple at pH 7 and pH 14 equals 70.16 V; that at pH 0 is +0.12 V;145 E8 of the SO2/SO¢§2 pair is 70.26 V.146 In some studies,147 ¡¾ 149 electron transfer reactions in the O2/O¢§2 ¡¾ metal complex systems are considered in terms of the Marcus theory.The equations derived (29) k12 = k11k22K12f12 , (30) pAAAAAAAAAAAAAAAAAAAAAAAAA OlogK12U2 log f12 a 4 logOk11k22=z2U , where z is a factor taking into account the frequency of collisions of the reacting species in solution, were used to determine the rate constant for the electron exchange in the O2/O¢§2 system (k22) on the basis of known constants for electron exchange in metal complexes (k11) and the experimental rate constants for the reduction of metal complexes by superoxide (k12), for example(31) Co(NH3)2a +O2 Co(NH3)3a +O¢§26 6 [in Eqns (29), (30), K12 is the equilibrium constant for reaction of type (31)]. However, the k22 values found in these calculations,147 based on the data on the kinetics of reactions of O¢§2 with various metal complexes, varied in the range of 1078 ¡¾ 105.7 litre mol71 s71.In the opinion of the researchers cited,145 the unsuitability of the Marcus theory for the calculation of k22 was due to the unequal solvations of O2 and O¢§2 . Later,148 a different approach was used to determine the k22 values, namely, the kinetics of the reaction of metal complexes with oxygen892 (32) CrL3a +O¢§ CrL2a +O2 2 , 3 3 rather than with superoxide was studied (L is 2,20-bipyridine, 1,10- phenanthroline and their derivatives). The use of this approach provided the k22 values in the range of 1 ¡¾ 10 litre mol71 s71 (in a later review,150 a value of 10 litre mol71 s71 was recommended). The reasons for the so great difference between the k22 values calculated from the data on the kinetics of reduction of complexes with superoxide remain unknown.No corresponding data concerning the superoxide reduction to peroxide can be found from the literature. 2 2 2 Comparison of the rate constants for the reactions of the O¢§ and SO¢§2 radical anions with identical oxidants, metal complexes, published in a study is of interest.149 The ratio of the rate constants for the reactions involving SO¢§ and O¢§ was found to be approximately 103; it does not depend on the nature of the oxidant, which is consistent with the Marcus theory and the equation derived from it (33) sAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA kOSO2=SO¢§2 U kOO2=O2¢§ U 1016:9 DE, kOSO¢§ kOO2¢§ U a 2 U 2 2 where kOSO¢§2 U and kOO¢§2 U are the rate constants for the reaction of SO¢§ and O¢§ with the same substrate; DE is the difference between the reduction potentials ofO2 (70.16 V) and SO2 (70.26 V), and kOSO2=SO¢§2 U and kOO2=O¢§2 U are the self-exchange rate constants of the SO2/SO¢§ If we assume that kOSO¢§ 2 and O2/O¢§2 systems, respectively. 2 U/kOO¢§2 U=103, the kOSO2=SO¢§2 U/ 2 3 2 kOO2=O¢§2 U ratio should be equal to*104.In most studies devoted to the electron exchange in the SO2/ SO¢§ couple, dithionite served as the source of SO¢§2 .149 The kOSO2=SO¢§2 U values calculated for the outer-sphere oxidation of CrL2a (L is bipyridine, phenanthroline, and their substituted derivatives) with sulfur dioxide were compared with the values determined previously in experiments using SO¢§ reagent.151 It was found that, unlike the k(O2/O¢§ and SO¢§2 as the 2 ) values, the kOSO2=SO¢§2 U constants calculated from the kinetic data for reactions involving SO2 are rather close: kOSO2=SO¢§2 U=(1 ¡¾ 18)6104 litre mol71 s71 at 298 K.These values are approximately 104 times as high as the value kOO2=O¢§2 U=10 litre mol71 s71 reported in a review,150 which is consistent with the results cited above.149 2 2 . Thus, the published data indicate that the Marcus theory is applicable to the SO2/SO¢§2 and O2/O¢§2 couples. It is noteworthy that the SO¢§2 /SO22 ¢§ couple in which SO¢§2 is the oxidised form has not been studied. Apparently, this is due to the difficulty of reduction of the SO¢§ radical anion.Oxidation of sulfoxylate would be the preferred variant. The above data indicate that thiourea dioxides can be used as sources of SO2¢§ Let us consider data on the reactions of thioureas with yet another active form of oxygen, namely, singlet oxygen 1O2. Thus oxidation of thiourea with photochemically generated singlet oxygen has been studied.152 It was shown previously that solutions of thiourea do not undergo photolysis either in the absence or in the presence of oxygen even on prolonged exposure to UV light. Photooxidation of thioureas was observed when their alcohol solutions containing oxygen and a sensitiser (dye) were exposed to the visible light. The composition of photooxidised products depends on the nature of thiourea and, to a lesser extent, the dye.The major products formed in the reaction of thiourea with 1O2 include SO2, sulfur andTUDO (the sensitisers used are Methylene Blue or chlorophyll). In the presence of Rose Bengal, the for- mation of dicyanediamide was also observed. Photochemical destruction of thiourea was discovered when titanium dioxide was used as the sensitiser.153 The reaction yields cyanamide and dicyanamide. The acid HOCl is also an active form of oxygen.55 The kinetics of reactions of thiourea with oxohalogen compounds draws special attention.14, 15, 154, 155 Until recently, studies of non-linear phenomena in chemical kinetics have been mainly related to the S V Makarov chemistry of halogens.However, at present, oscillators based on sulfur compounds are becoming more and more important. The combination of halogen and sulfur compounds in one reaction is rather attractive. However, data on the kinetics of such reactions are scarce. One of the first chemical oscillators based on sulfur-contain- ing compounds 156 is a mixture of sulfide, sulfite, oxygen and the Methylene Blue dye. It has been reported that the reaction between chlorite and thiourea is accompanied by oligo-oscilla- tion.154 Diverse non-linear effects have been discovered in the reactions of thiourea with iodate, bromate, chlorine dioxide, iodine and bromine.155, 157, 158 However, the greatest attention of researchers is still attracted by the reaction with chlorite.Never- theless, there is no consensus of opinion concerning the mecha- nism of the reaction between thiourea and chlorite. Thus it has been assumed 155 that the protonated form of the chlorite HClO2 is much more reactive than the deprotonated form. However, other researchers 159 hold to the opposite opinion. They proposed a mechanism for the reaction of ClO2 and (NH2)2CS, according to which two ClO2 molecules successively transfer the oxygen atoms to HOSCNHNH2 to give HO3SCNHNH2. However, other researchers 155 believe that the reaction between ClO2 and the thiourea monoxide is accompanied by electron transfer giving rise to ClO¢§2 and H2NNHCSO (it has been assumed 60 that the reaction of thiourea dioxide with bromate also follows a radical mechanism).It was shown experimentally 155 that the rates of reactions of formamidine disulfide and thiourea dioxide with ClO2 depend substantially on the time elapsed after the prepara- tion of solutions of the sulfur-containing compounds. This was attributed 155 to accumulation of reactive aminoiminomethane- sulfenic acid (thiourea monoxide) in the solution. However, this explanation can hardly be considered to be convincing because thiourea monoxide is exceptionally unstable at 298 K and can hardly be expected to accumulate in solution. It was shown 158 that, among other reasons, non-coincidence of the kinetic data obtained by different researchers may be due to the influence of impurities of metal compounds, especially copper and iron compounds (Cu and Fe compounds are known to catalyse efficiently the processes of reduction by thiourea).How- ever, the main reason for the inconsistency mentioned above is that no kinetic data concerning individual stages of the multistage reaction between thiourea and oxohalogen compounds have been available until recently. Only in 1993 ¡¾ 1995, were reactions between an intermediate of (NH2)2CS oxidation, i.e. thiourea dioxide, with iodate,160 bromate 15 and chlorite 14 studied. The kinetic parameters thus found made it possible to correct (in some cases, the corrections were quite significant) the rate constants for the individual stages of thiourea oxidation published previously. However, the reported 14 ratios of the rate constants for reactions of thiourea di- and trioxides with the same oxidants raise doubts.Thus the rate constants for the reactions of TUDO and TUTO with HOCl are nearly equal (9.56103 and 6.56103 litre mol71 s71) but those for the reaction with ClO2 differ by a factor of 60 (66102 and 10 litre mol71 s71, respectively). The reasons for this discrepancy have not been elucidated. The interpretation 161, 162 of the results of kinetic studies of the reactions of hydroxymethanesulfinate with bromate and chlorite in acid media is even more objectionable. Presumably, the reactions of HMS with any oxohalogen compounds either present initially or formed at intermediate stages (BrO3, HBrO2, HOBr, ClO¢§2 , HOCl, ClO2, Cl2 O2) afford hydroxymethanesulfo- nate.161, 162 As a result of oxidation, this product gives sulfate, i.e., as in the case of reactions involving thiourea, the C7S bond rupture takes place after the appearance of sulfonate.Meanwhile, it is known that hydroxymethanesulfonate is very stable in acid media and does not tend to participate in redox reactions.7 Conversely, HMS rapidly decomposes in acid solutions, the products of its decomposition exhibiting strong reducing proper- ties. Unfortunately, in the studies cited above,161, 162 no mention is even made of theHMSdecomposition or the role of its products�¢Recent trends in the chemistry of sulfur-containing reducing agents sulfoxylate and formaldehyde (later, the same researchers demonstrated 163 that formaldehyde reacts with chlorite at a rather high rate).The results of study of Makarov et al.164 made it possible to introduce substantial changes to the schemes of reactions of halogens and oxohalogen compounds with thiourea, thiourea dioxide and hydroxymethanesulfinate. It was shown 164 that the rates of oxidation of sulfonates (thiourea trioxide and hydroxymethanesulfonate) are much lower than the rates of oxidation of sulfinates (thiourea dioxide and hydroxymethane- sulfinate) and even thiourea. Thus, it appears that thiourea trioxide and hydroxymethanesulfonate are not intermediates of the reactions of TUDO, thiourea and HMS with halogens and oxohalogen compounds. During the oxidation of sulfinates, rupture of the C7S bond takes place before the formation of sulfonates.164 The active reducing agent thus formed, sulfoxylate, is then oxidised to sulfite and sulfate.VI. Conclusion The data on the properties of sodium dithionite, sodium hydrox- ymethanesulfinate and thiourea dioxides considered in the review provide a number of conclusions. An important feature common to these compounds is decomposition in solutions to give strong reducing agents. However, the opposite patterns of dependence of the stability of TUDO and HMS on the pH entail differences between the processes of their decomposition. Indeed, in solutions of HMS, the dithionite ion is formed predominantly according to an `anaerobic' mechanism, whereas in solutions of TUDO, this occurs by an `aerobic' mechanism.Sulfur-and-oxygen ions (sul- fite, thiosulfate) barely affect the TUDO decomposition, whereas the mechanism of decomposition of hydroxymethanesulfinate largely depends on admixtures. Thus, in the case of HMS decom- position, the subsequent reactions of sulfur-containing com- pounds are much more significant than in the decomposition of TUDO. Decomposition of HMS is affected most appreciably by dithionite ions. The very low stability of these ions in acid media and the capability of decomposing in aqueous solutions by an autocatalytic mechanism account for the autocatalytic mode of HMS decomposition as a whole. The unique character of dithion- ite is due to the presence of the very long S7S bond. Therefore, insertion of monoatomic sulfur into the S7S bond becomes possible and, as a consequence, decomposition of dithionite and hydroxymethanesulfinate is accelerated. Conversely, in the thio- urea dioxide molecule, direct insertion of sulfur into the shorter C7S bond is difficult.No conjugate pathway for dithionite destruction exists in the decomposition of TUDO (in alkaline solutions of thiourea dioxide under aerobic conditions, dithionite formation does take place but it is much more stable in these media than TUDO). The following important circumstance should also be men- tioned. Thiourea dioxides are the most convenient precursors of the little studied sulfoxylate ion (SO2¡ 2 ). 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