Biradicals, Radicals in Excited States, Carbenes and Related Species

445 Pages·1998·4.269 MB·English

by  H. Fischer (auth.), H. Fischer (eds.)

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