Biradicals, Radicals in Excited States, Carbenes and Related Species PDF
Preview Biradicals, Radicals in Excited States, Carbenes and Related Species
General introduction IX General introduction A Definition and coverage In the following tables rate constants for reactions of free radicals in liquids are collected. The species covered are mostly paramagnetic molecules deriving their paramagnetism from a single unpaired valence electron. They are neutral molecular fragments or ions with positive of negative charges. Emphasis is on polyatomic organic free radicals. Excluded are some small species like the hydrated electron, the hydro- gen and other atoms and a variety of polyatomic inorganic radicals. For reaction rates of these in aqueous solutions recent other compilations are available [88Bux and earlier tables]. A table on organic biradicals is included since their reactions are similar to those of monoradicals, and of carbenes, nitrenes and related compounds which also have similar reactivities. The volume is divided grossly into sections dealing with individual types of free radicals such as carbon-centered radicals, nitrogen-centered radicals, nitroxyl radicals, oxygen-centered radicals and radicals centered on other heteroatoms. These sections deal mainly with irreversible reactions. In addi- tion, there are sections on proton transfer processes and their equilibria and a chapter on radicals reacting in excited states. An index of radicals formulae will facilitate data retrieval. The reactions covered involve bimolecular processes between like and unlike radicals and between radicals and molecules via atom, group or electron transfer, or addition and unimolecular processes like fragmentations or rearrangements. Within the chapters dealing with special radical types a subdivision according to the types of reaction is applied. In such subdivisions the entries are organized according to the molecular formula of the radical involved, and formulae are in the order of increasing number of C, H atoms and then all other elements (except D, listed with H) in alphabetical order. The main subject of the volume is the compilation of absolute rate constants for established re- actions. In part they were deduced from published relative rate data if the absolute rate constants of the reference reactions were known. Relative rate constants of qualitative data as reaction half-lifes are included occasionally, in particular for important classes of radicals or radical reactions for which absolute data are not yet available. Thus, the tables do not represent a comprehensive compilation of all reactions known to date, yet of all those with sufficiently characterized reaction kinetics. For details on subdivision into subvolumes, chapters and ordering within chapters, see Table of contents and the introductory sections of individual chapters. The literature is generally covered from the date of appearance of the precursor volumes Landolt-Börnstein New Series II/13a-e and 1993. B Arrangement and contents of tables As indicated by the general table headings there is one separate entry for each specific reaction or each set of competing reactions. Besides specifying the reaction the entry contains information on the technique of radical generation, the method of rate determination, and experimental conditions such as solvent and temperature. It lists the rate constants, the equilibrium constants and other rate data, such as activation parameters of the reactions, and gives the pertinent reference plus additional references. Further relevant information is given in footnotes. The following explanations apply to the individual parts of the entries. REACTION: The reaction or the competing reactions are written in stoichiometric form starting with the reacting radical. For reactions between different radicals the radical appearing first in the adopted ordering of substances (see above) is written first and specifies the location of that entry. A cross reference to this reaction is entered at that later position of the tables which corresponds to the order of the second radical. The same principle is obeyed in the ordering of the substrates in competing radical molecule reactions. Where deviations from this scheme occur the reader is referred to the introduction of the individual chapters. Where necessary, structural formulae of radicals, reactants and products are written out in full detail. Repeatedly occuring structures are abbreviated by capital bold letters and an entry R = group may specify a substituent within the general structure. Self-evident structures of X General introduction products are also abbreviated occasionally as OO- or NN-dimers of otherwise. Product structures are not given if they could not be identified from the original literature. RADICAL GENERATION: The technique of radical production is outlined in short using abbreviations given in the List of symbols and abbreviations. METHOD: The methods in use for the determination of reaction rate data are manifold, and a variety of abbreviations had to be introduced (see also List of symbols and abbreviations). Whereas earlier literature mostly applied the rather indirect techniques of measurements of product ratios (PR), the consumption of inhibitors (inh. cons.), rates of oxygen absorption (ROA) or consumption of other reactants (RRC) the progress of time resolved spectroscopy is evident more recently, and the most accurate rate data for irreversible processes are now obtained from kinetic absorption spectroscopy (KAS), kinetic electron spin resonance spectroscopy (KESR) or conductometry (cond.) in conjugation with pulsed radical generation. For reversible processes steady-state techniques of absorption spectroscopy (SAS) or electron spin resonance (SESR) or line-shape analyses in magnetic resonance (ESRLA, NMRLA) are common. For details of methods, the reader is referred to the original literature. SOLVENT: Where possible the solvent is given by its molecular formula or name. Special conditions such as pH or solvent composition are indicated. TEMPERATURE T [K]: The temperature of the sample during the rate measurement is given in K. RT stands for an unspecified room temperature. Where activation parameters of rate constants were measured, the column temperature indicates the temperature range of measurement. RATE DATA: Rate constants of uni- and bimolecular processes are given in their usual dimensions s-1 and M-1s-1, equilibrium constants in their corresponding appropriate dimensions. The same applies to ratios of rate constants. All rate constants k are defined for product appearance. Consequently, 2k governs the rate of radical disappearance in bimolecular self-reactions of radicals. Since the rate of radical disappearance is often measured in these cases, the value of 2k is displayed. If available the Arrhenius activation parameters, i.e. the parameters of the equation k = A x exp(-E /RT) are also listed a with A given in logarithmic form and E in kJ mol-1. The column rate data may also give enthalpies a (D H*), entropies (D S*), and volumes (D V*) of activation in SI-units. For acid-base equilibria pK-values are listed. Errors are given in units of the last digit displayed for the data. REFERENCE/ADDITIONAL REFERENCE: The first entry specifies the reference from which the data were extracted with the first two numbers for the year of appearance (92 = 1992), the following three letters for the family name of the first author and the last number ordering the publications in the year of publication. Additional references contain earlier less reliable work on the same subject, theoretical treatments of rate data or other relevant information. The following list of symbols and abbreviations is common for all chapters. Additional symbols and abbreviations may appear as necessary in individual chapters. For these and additional information on contents and coverage, on arrangements and ordering and on special data display the reader is referrred to the introductory sections of the individual chapters. C Important monographs, series, compilations 75Ash Ashmore, P.G. (ed.): Specialist Periodical Reports, Reaction Kinetics, Vol. 1ff. London: The Chemical Society 1975 ff 81Ker Kerr, J.A., Moss, J.S. (eds.): CRC Handbook of Bimolecular and Termolecular Gas Reactions, Vols. I, II. Boca Raton: CRC Press 1981 86Gie Giese, B.: Radicals in Organic Synthesis. Oxford: Pergamon 1986 86Vie Viehe, H.G., Janousek, Z., Merényi, R. (eds.): Substituent Effects in Radical Chemistry. Dordrecht: Reidel 1986 88Alf Alfassi, Z.B. (ed.): Chemical Kinetics of Small Organic Free Radicals, Vols. I-IV. Boca Raton: CRC Press 1988 General introduction XI 88Bux Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B.: Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 17 (1988) 513 88Fis Fischer, H., Heimgartner, H. (eds.): Organic Free Radicals. Berlin: Springer 1988 89Min Minisci, F. (ed.): Free Radicals in Synthesis and Biology. Dordrecht: Kluwer 1989 89War Wardman, P.: Potentials of One-Electron Couples Involving Free Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 18 (1989) 1637 90Tan Tanner, D.D. (ed.): Adv. Free Radical Chem. Greewich: JAI Press 1990 ff 93Ben Bensasson, R.V., Land, E.J., Truscott, T.G.: Excited States and Free Radicals in Biology and Medicine. Oxford: Oxford University Press 1993 93Lef Leffler, J.E.: An Introduction to Free Radicals. New York: Wiley 1993 93Moz Mózcik, G., Emerit, I., Fehér, J., Malkovics, B., Vincze, A.: Oxygen Free Radicals and Scavengers in the Natural Sciences. Budapest: Akadémiai Kiadó 1993 93Ste Steiner, U., Wicke, E. (eds.): Magnetic Field and Spin Effects in Chemistry. München: Oldenbourg 1993 94Per Perkins, M.J.: Radical Chemistry. Hemel Henstad: Ellis Horwood 1994. 95Fos Fossey, J., Lefort, D., Sorba, J.: Free Radicals in Organic Chemistry. New York: Wiley 1995. D List of symbols and abbreviations Symbols D(R- X) bond dissociation energy E0,E0´ standard red uction potential G radiation chemical yield H Hammett acidity function 0 k [s-1, M-1s-1] rate constant K equilibrium constant 2k rate constant of self-termination t D G free enthalpy of activation D H enthalpy of activation D S entropy of activation D V volume of activation D H° [kJ mol-1] enthalpy of dissociation D S° [J K-1mol-1] entropy of dissociation l monitoring wavelength mon h [cP] viscosity el molar decadic absorption coefficient at wavelength l r (s ), r (s +), r (s -) Hammett´s rho based on s , s + or s - scales t [s, min, day] half-life ½ t [ns] biradical lifetime T [K] temperature V/V volume by volume mixture 1:1 m equimolar mixture XII General introduction Abbreviations a) General aq aqueous ox. oxidation absorpt. absorption r reverse Ac acyl rad. radiolysis add. addition reduct. reduction Ar aryl RT room temperature c cyclo s secondary cath. cathodic satd. saturated conc. concentrated, concentration soln. solution cons. consumption spectr. spectroscopy corresp. corresponding t tertiary decomp. decomposition temp. temperature e electron temp.dep. temperature dependence f foreward therm. thermolysis i iso TR time-resolved irr. irradiation var. various mixt. mixture vis. visible n normal b) Methods AS absorption spectroscopy NMRLA nuclear magnetic resonance line- chemil. chemiluminescence shape analysis ch. r. chain reaction ox. oxidation CIDEP chemically induced dynamic PAC photoacoustic calorimetry electron polarization phot. photolysis CIDNP chemically induced dynamic Pol. polarography nuclear polarization Potent. titr. potentiometric titration Cond. conductometry PR product ratio Co-ox. cooxidation pulse rad. pulse radiolysis CV cyclic voltammetry ROA rate of oxygen consumption DPSC double potential step RRC rate of reactant consumption chronoamperiometry RS rotating sector EDA electron donor-acceptor SAS steady-state absorption ESR electron spin resonance spectroscopy ESRLA electron spin resonance lineshape SESR steady-state electron spin analysis resonance FSCV fast scan cyclic voltammetry spin trap. spin trapping glc gas liquid chromatography SSCV slow scan cyclic voltammetry inh. cons. inhibitor consumption Stern-Volmer competitive studies based on ISC intersystem crossing yields; same type of dependence HPLC high performance liquid as Stern-Volmer plots in chromatography photochemistry KAS kinetic absorption spectroscopy therm. coup. thermocouple method KESR kinetic electron spin resonance TRAS time-resolved absorption KRRS kinetic resonance Raman spectroscopy spectroscopy TRFS time-resolved fluorescence LFP laser flash photolysis spectroscopy LIF laser induced fluorescence General introduction XIII c) Substances or parts of substances acac acetylacetone OX oxalate ACHN a ,a ´-azo-bis- PBN phenyl-t-butyl nitrone cyclohexanecarbonitrile PC dicyclohexylperoxydicarbonate AIBN a ,a ´-azo-bis-isobutyronitrile PHEN phenantroline An anisyl PNAB 4-nitroacetophenone BIPY bipyridinium PNBPA pentaamine(4-nitrobenzoato)- BIP bipyridine cobalt(III)2+ BMP 2,6-di-t-butyl-4-methylphenol PVA polyvinylacetate CTAB cetyltrimethylammonium bromide PY pyridine CTAC cetyltrimethylammonium chloride SDS sodium dodecyl sulfate cyp cyclopentadienyl SEP 1,3,6,8,10,13,16,19-octaaza- DBPO dibenzoyl peroxide bicyclo[6.6.6]eicosane DCP di-a -cumyl peroxide ssDNA single strand DNA diNOsat 1,8-dinitro-1,3,6,10,13,16,19-hexa- TBAB tetra-n-butyl ammonium azabicyclo[6.6.6]eicosane bromide DLPC dilinoleoylphosphatidylcholine TBO t-butoxyl DME dimethoxyethane TERPY terpyridine DMF dimethylfuran thd 2,2,6,6,-tetramethyl-3,5-heptane- DMPO 5,5-dimethyl-1-pyrroline-1-oxide dionato chelate DNA deoxyrobonucleic acid THF tetrahydrofuran DOPA 3,4-dihydroxyphenylaniline THP tetrahydropyran DPA diphenylamine TMPD N,N,N,N-tetramethyl-p-phenylene- DPE diphenyl ether diamine DPM diphenylmethanol TQ triquat DPPH a ,a -diphenyl-b -picryl hydrazyl V viologen DPPH-H a ,a -diphenyl-b -picrylhydrazine DQ diquat H2O water DTBH di-t-butyl hyponitrite CH3OH methanol DTBK di-t-butyl ketone C H ethylene 2 4 DTBP di-t-butyl peroxide C H OH ethanol 2 5 DTBPO di-t-butyl peroxalate C H ethane 2 6 DTB di-isopropyldithiophosphate c-C H cyclopropane 3 6 EDTA ethylene diamine tetraacetic acid C H propyl EN ethylene diamine 3 7 C H OH propanol EPA diethylether:isopentane:ethanol 3 7 C H propane (5:5:2 by volume) 3 8 FAD flavin adenine dinucleotide i-C4H10 isobutane FMN flavin mononucleotide c-C5H10 cyclopentane fod 1,1,1,2,2,3,3,3-heptofluoro-7,7- n-C H n-pentane 5 10 dimethyl-4,6-octanedionato chelate C H benzene 6 6 HMPA hexamethylphosphoramide c-C H cyclohexane 6 12 LTA lead tetraacetate n-C H n-hexane 6 14 MTBP methyl-t-butyl peroxide c-C H cyclooctane MTHF methyl tetrahydrofuran 8 16 i-C H isooctane MV methyl viologen 8 18 n-C H n-octane NAD nicotinamide adenine dinucleotide 8 18 NBS N-bromosuccinimide NTA nitrilo triacetate Ref. p. 80] 12 Biradicals 1 12 Biradicals (J.C. NETTO-FERREIRA, J.C. SCAIANO) 12.0 General introduction This chapter on biradicals is a follow-up on that published in 1985 in Volume 13, subvolume ‘e’of this series [85Sca6]. We have tried to cover all the available literature on absolute rate constants and lifetimes through 1995 and it should be essentially complete for 1996, at least for those articles included in Chemical Abstracts by the end of 1996. During the last decade, considerable progress has been made in our understanding of biradical kinetics in solution. Notably, several articles have dealt with singlet biradicals, for which little was known a decade ago. In addition, many ‘remote’biradicals, such as those covered in Sections 12.1.5, are now well established. The subdivision and coverage of this chapter is very similar to that published in 1985, except for the need to subdivide some sections where such subdivision was not justified before given the limited data available in 1985. Considerably less emphasis has been placed in the coverage of competitive studies, given the availability of absolute data. The exception is the singlet biradicals derived from furan, pyrrol and thiophene structures covered in Section 12.3.3; these are systems where absolute data are more scarce. Two other approaches have been used to determine biradical lifetimes from competitive studies. Adam and coworkers [90Ada1] have carried out extensive work using biradical trapping with oxygen as a relative pro- cess against which other rate constants are calibrated. It is generally assumed (on the basis of spin statistics) that the rate constant for the interaction of triplet biradicals with molecular oxygen is given by 4/9 k , diff where k is the rate constant for diffusion. The assumption is consistent with experimental absolute values diff of biradical scavenging by oxygen; many of these values have been included in this compilation or in its predecessor. Chart 1 summarizes a large number of biradical lifetimes determined by this technique. Chart 1. Biradical Lifetimes determined by the oxygen trapping technique · · · · · · · · · · 93 ±11 ns 42 ±7 ns < 0.1 ns < 0.1 ns < 0.1 ns [87Ada2] [87Ada2] [85Eng1] [85Eng1] [85Eng1] · C H 6 5 · · · · · · · C H 6 5 C H 6 5 > 1 ns 52 ±20 ns ≈390 ns 280 ±40 ns [85Eng1] [84Ada1] [89Ada1] [87Ada1, 88Ada1] Landolt-Börnstein New Series II/18E2 2 12.0 General introduction [Ref. p. 80 · · · · · · · · 10–20 ns 10–20 ns 94 ±15 ns ≈0.1 ns [90Ada1] [90Ada1] [90Ada1] [84Ada2] 56 ±18 ns 52 ±21 ns 10–20 ns [87Ada2] [87Ada2] [87Ada2] · · · · · · · · 0.3 > t> 3.3 ns < 0.1 ns < 0.1 ns < 0.1 ns [87Ada4] [85Ada1] [86Ada1] [90Ada1] · O · · · · · < 0.1 ns ≈1 ns < 0.1 ns [90Ada1] [87Ada1, 88Ada1] [90Ada1] A similar competitive approach has also been used to determine biradical lifetimes by using calibrated free radical clocks attached to one of the radical termini. The concept was reviewed by Griller and Ingold [80Gri1] and is widely employed in free radical chemistry. The most common “clocks” have been the cyclopropylcarbinyl rearrangement and the cyclization of substituted 5-hexenyl radical centers. A repre- sentative example, reported by Hastings and Weedon [91Has1] is shown in Scheme 1. The biradicals are produced by photoreaction of N-benzoylindole with vinylcyclopropane. Product studies yield the ratio k /k , where k is the reciprocal of the biradical lifetime. The rate constant k can be estimated from A B A B established free radical systems, with the critical assumption that radical centers in biradicals behave in much the same way as in monoradicals. While the overall mechanism is somewhat more complex than shown in Scheme 1, it serves to illustrate the way in which radical clocks are employed in the deter- mination of biradical lifetimes. Chart 2 summarizes data obtained in this manner. Scheme 1.Application of a free radical clock (from [91Has1] · · f · · N r N C H C H O 6 5 O 6 5 k k A B N N C H O 6 5 C H O 6 5 (A) (B) Landolt-Börnstein New Series II/18E2 Ref. p. 80] 12.0 General introduction 3 Chart 2.Biradical lifetimes based on radical clocks · O O · · · · · · · 50 ns 50 ns 28 ns 0.3 < t < 3.3 ns [90Rud1] (further ref.: [90Rud1] (further ref.: [85Eng1] [88Ada2] [91And1]) [91And1]) O · · · · · · · · R 59 ns 0.1 < t < 1 ns 4–14 ns 50 ns [91Eng1] [87Ada4] [94Eng1] [89Bec1] O · OH O · OH · ·· O · · · N CH 3 O C H O 6 5 ≈1.0 ns ≈1.0 ns 35 ns 100 ns [93Wag1] [93Wag1] [94Che1] [91Has1] (further ref.: [92And1]) · · N C H O 6 5 100 ns [91Has1] A section on biradical quenchingof excited statesincluded in the earlier compilation has been excluded in this revision. This involved the interaction of excited states with persistent biradicals, largely dinitroxides. Little progress has been made in this area and the information reviewed earlier has been of limited usefulness. A number of reviews containing kinetic data on biradicals have been reported during the last few years. [93Joh1; 95For1; 94Bor1; 94Gri1; 82Sca3; 82Tur2; 92And1; 90Ada1; 89Joh1]. Landolt-Börnstein New Series II/18E2 4 12.1 Unimolecular biradical processes [Ref. p. 80 12.1 Unimolecular biradical processes 12.1.1 Reactions of 1,3-biradicals to yield molecular products Radical’s gross formula Reaction Radical generation Ref./ Method Solvent T[K] Rate data add. ref. [CH] 5 8 · · decay Benzophenone sensitized LFP (248 nm, 25 ns) of the azoalkane precursor TR-PAC acetonitrile RT t = 258(14) ns 93Ada2 [CH] 5 8 · · Phot. of diazene precursor TR-PAC benzene RT t = 316(80) ns 1) 88Her1 t = 75(21) ns 2) [CH] 6 8 · · CH CH 2 2 Benzophenone sensitized LFP (308 nm, 25 ns) of the azoalkane precursor TR-PAC acetonitrile RT t = 281(36) ns 93Ada2 [CH ] 8 12 · · Phot. of diazene precursor TR-PAC benzene RT t = 913(196) ns 1) 88Her1 t = 122(25) ns 2) [C H ] 11 12 C H · · 6 5 decay Benzophenone sensitized LFP of diazene precursor KAS benzene 298 t = 390(50) ns 89Ada1/ acetonitrile 298 t = 380(30) ns 87Ada1 1) Benzophenone sensitized irr. (365 nm), argon purged. 2) Benzophenone sensitized irr. (365 nm), oxygen saturated. Landolt-Börnstein New Series II/18E2 Ref. p. 80] 12.1 Unimolecular biradical processes 5 Radical’s gross formula Reaction Radical generation Ref./ Method Solvent T[K] Rate data add. ref. [C H ] 12 12 C H 6 5 · · C H 6 5 CH CH 2 2 Benzophenone sensitized LFP (308 nm, 25 ns) of the azoalkane precursor TR-PAC acetonitrile RT t = 334(8) ns 93Ada2 [C HBr] 13 9 · · Br Br UV irr. of the corresp. cyclopropane UV, IR, ESR 3) solid poly- 134 t = 1.7 · 106ns 87Fis1 ethylene 120… 160 E = 23.0(20) kJmol–1 a log [A/s–1] = 6.2(5) [C H ] 13 10 · · + UV irr. of the corresp. cyclopropane UV, IR, ESR 3) solid poly- 134 t = 29 · 106ns 87Fis1 ethylene 120… 160 E = 22.2(40) kJmol–1 a log [A/s–1] = 5.1(10) [C H ] 13 10 · · decay Benzophenone sensitized LFP (351 nm, 75 mJ, 20 ns) of diazo precursor KAS acetonitrile 293 t > 20 · 106ns 90Ada2 120… 140 E = 22.2(42) kJmol–1 a log [A/s–1] = 5.1(10) 3) Based on disappearance of UV, IR, ESR signals of the biradicals. Landolt-Börnstein New Series II/18E2
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