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Free Radicals and Free-Radical Reactions of Monovalent and Divalent Sulfur.

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Free Radicals and Free-Radical Reactions of Monovalent and Divalent Sulfur [*J
Organic free radicals of monovalent sulfur have not been hitherto observed in solution or
in melts. Steric hindragce and resonance stabilization, which are responsible ,for the stability
of the triarylrnethyl, diphenylnitrogen, and phenoxyl radicals, are apparently insufficient
to stabilize the organic free radicals of monovalent sulfur in such concentrations that they
can be detected by current physical methods. - I t was only in I963 that arninopolysulfur
radicals (R2N-S,, -S.) were detected in solution, and arylsulfur radicals (Ar-S.) and
phenylselenium radicals were isolated at ca. -180 "C. - Organically bound sulfur can be
stabilized in the free-radical state if association of the radicals is prevented by fixing in a
crystal lattice ("cystine radical"), by repulsion between radical ions (sulfinium salts),
or by freezing-in (arylsulfrir radicals).
I. Introduction
The discovery of aryl-substituted carbon, nitrogen, and
oxygen radicals had a decisive influence on our views
concerning the chemical bond, and, together with the
discovery of short-lived alkyl radicals, provided a basis
for discussion of free-radical reaction mechanisms.
Although numerous reactions are known in the organic
chemistry of silicon, phosphorus, and sulfur [**I which
certainly proceed by a free-radical mechanism, no free
radicals of trivalent silicon and germanium, divalent
phosphorus and arsenic, or monovalent sulfur and
selenium have as yet been observed. Trivalent radicals
of the higher elements of Group IV have been sought
in vain by Krause, Schlenk, Gilman, and Selwood [l].
Earlier evidence for the existence of trivalent tin and
lead radicals, obtained by molecular-weight measurements, requires confirmation by ESR spectroscopy [2].
Kuchen [3] was unable to detect the formation of diphenylphosphorus radicals from tetraphenyldiphosphine,
in analogy with the decomposition of tetraphenylhydrazine into diphenylnitrogen radicals. Despite many investigations [4-121, in particular by Lecher, Schunberg,
[*] Free-radical reactions of tetra- and hexavalent sulfur are not
considered in this work. For a survey of these reactions, see [16],
p. 326.
[**I For free-radical additions of Si and P compounds to olefins,
see F. W. Sfacey and J . F. Harris in : Organic Reactions. Wiley,
New York 1963, Vol. XIII, pp. 209 et seq.
[la] Review: W. Huckel: Theoretische Grundlagen der Organischen Chemie. 7th Edit., Akademische Verlagsgesellschaft, Leipzig 1952, Vol. I, p. 153.
[Ib] Review: H . A. Staab: Einfiihrung in die Theoretische Organische Chemie. 1st Edit., Verlag Chemie, Weinheim/Bergstr.
1959, p. 452.
[2] In hexamesityldiplumbane - which M . Lesbre, J. Satge, and
D. Voigt [C. R. hebd. SBances Acad. Sci. 246,594 (1958)] believed
to dissociate into free radicals, on the basis of molecular-weight
determinations - no dissociation into free radicals could be detected by ESR spectroscopy [E. Miiller, F. Gunter, K. Scheffler,
and H. Fettel, Chem. Ber. 91,2888 (1958)l. It has been also shown
by magnetic measurements that the hexaryl compounds of digermane, diplumbane, and distannane, whose dissociation into
free radicals had been assumed from cryoscopic measurements,
are diamagnetic [P. W. Selwood et al., J . Amer. chem. SOC.61,
3168 (1939); 62, 2765 (1940); 63, 2509 (1941); 64, 1727 (1942)].
W. V . Farrar has concluded from light-absorption measurements
Selwood, Pearson, Dimroth, and Rundel, the formation of
monovalent sulfur radicals from disulfides was shown,
prior to 1963, only in the case of cystine on irradiation
with X-rays [ 131.
T h e factors stabilizing free radicals such as triphenylniethyl,
diphenylnitrogen, a n d aroxyl, i.e. steric hindrance of dimerization a n d resonance of t h e radical, ar e apparently n o longer
sufficient in t h e case of t h e elements of t h e second a n d third
periods. Owing t o the greater covalent radii of these elements,
t h e accumulation of phenyl groups or other bulky substituents
does n o t prevent dimerization. Furthermore, resonance stabilization of t h e radical is more difficult, owing t o t h e greater
distance between t h e hetero-atom a n d t h e phenyl group [I b].
We succeeded recently in isolating several arylsulfur
radicals by photolysis of aromatic disulfides or mercaptans in the gaseous state, followed by quenching at
77 "K [13a-l3c]. By the same method the diphenylphosphorus [14], diphenylarsenic [ 151, phenylselenium
that diphenyl ditelluride dissociates completely in 1 '% solution
to form phenyltellurium radicals [Research 4 , 177 (195111. Our
own ESR measurements on diphenyl ditelluride solutions at room
temperature gave no evidence of free radicals.
[3] W. Kuchen and H . Buchwald, Chem. Ber. 91, 2871 (1958).
[4] Review of previous publications: H. Z. Lecher, Science
(Washington) 120, 220 (1954).
[5] A. Schonberg, E. Rupp, and W. Gumlich, Ber. dtsch. chem.
Ges. 66, 1932 (1933).
[6] H. G. Cuforth and P. W. Selwood, J . Amer. chem. SOC.70,
278 (1948).
[7] H. P. Koch, J. chem. SOC.(London) 1949, 401.
[8] A. Schonberg, A. Mustafa, and W. Askar, Science (New York)
109, 522 (1949).
[9]A. Mustafaand M.Kamel, Science(Washington)ll8,41l(1953).
[lo] K. Dimroth and G. Oosterloo, Angew. Chem. 70, 165 (1958).
[I I] D. E. Pearson, D. Caine, and L. Field, J. org. Chemistry 25,
867 (1960).
[I21 W . Rundel, Z. Naturforsch. ISb, 546 (1960); Angew. Chem.
73, 437 (1961).
[13] Y. Kurita and W. Gordy, J. chem. Physics 34, 282 (1961).
[13a] U.Schmidt and A. Miiller, Angew. Chem. 75, 299 (1963);
Angew. Chem. internat. Edit. 2, 216 (1963).
[13b] U.Schmidt, A. Miiller, and K . Markau, Tetrahedron Letters
17, 1091 (1963).
[13c] U. Schmidt, A. Miiller, and K . Markau, Chem. Ber. 97, 405
[I41 U . Schmidt, F. Geiger, A. Muller, and K. Markau, Angew.
Chem. 75, 640 (1963); Angew. Chem. internat. Edit. 2,400 (1963).
[IS] U. Schmidt, A. Miiller, and K. Markau, unpublished work.
Diphenylarsenic radical: g ( (= 2,034; g
2,004; diphenyltel2,002.
lurium radical: gll= 2,0342; g
Angew. Chem. internat. Edit. 1 Vol. 3 (1964)
/ No. 9
[13a, I3b], and phenyltellurium radicals [ 151 were isolated by photolysis of tetraphenyldiphosphine, diphenylarsine. diphenyl diselenide, and diphenyl ditelluride, and were characterized by ESR spectroscopy.
The thermal stability can be determined from the rate
of recombination on heating. which is measured optically by the fading of the free-radical color. It is greater
for the diphenylarsenic and phenylselenium radicals
than for the diphenylphosphorus and phenylsulfur
Since in these radicals steric hindrance of dimerization probably n o longer plays a n important part, their stability must
be d u e t o t h e weak bonding between t h e hetero-atoms in t h e
dimer. T h e bonding between elements of t h e fourth period
(As a n d Se) is weaker t h a n between elements of t h e third
period (P a n d S ) , owing t o t h e greater distance between t h e
a t o m s a n d the consequently smaller overlap of t h e electron
of mercaptans and organic disulfides are known which
occur on heating, irradiating, or in the presence of freeradical formers, r.g. :
a) The dispropxtionation of a mixture of symmetrical
disulfides in the molten state to form unsymmetrical
disulfides, and the reverse prccess, can be plausibly interpreted by means of a homolytic dissociation (c) into
organic sulfur radicals which react with the disulfide
in a displacement reaction (d) [20].
$) The regulating action of many disulfides on vinyl
polymerization depends on a free-radical displacement
on the sulfur of the disulfide by the macroradical, according to Equation (a) [17,18]. The liberated organic
sulfur radical adds onto the monomer M, with formation of a carbon radical [Equation (b)], which starts a
new chain.
+ R-S-S-R
n-s. + M
-+ (M),-M-S-R
+ R-S*
+ R-S-M.
y) The primary process in the denaturation of proteins
by ionizing radiation is probably free-radical formation
on the protein sulfur [19]. Numerous investigations on
the action of ionizing or ultraviolet radiation on proteins
containing sulfur have demonstrated the formation of
sulfur radicals which are stable in the crystalline state.
Although there is no compelling evidence for the formation of monovalent sulfur radicals (sulfenyl or thiyl
radicals) RS . in solution [*I, many free-radical reactions
[I61 C. W a l h g : Free Radicals in Solution. J. Wiley, New York
1957, p. 335.
[I71 See [16], pp. 156, 319.
[I81 Review: W . A. Pryor: Mechanism of Sulfur Reactions.
McGraw-Hill Book Company, New York 1962, p. 46.
[I91 W . Gordy and H . Shields, Radiation Research 9, 61 1 (1958);
W. Gordy in: Symposium on Information Theory in Biology.
Pergamon Press, New York 1958, p. 241.
[*I W . Rundel and K . Scheffler have observed weak ESR signals
without hyperfine structure during ultraviolet irradiation of solutions of 2,4,6-tri-t-butylthiophenol[Z.Naturforsch. 18b, 984
Chem. internat. Edit. / VoI. 3 (1964) No. 9
+ P(OR)~
f R - S.
+ R-S-~OR)~
+ S=P(OR’)3
R* + RSH + R H + RS-
+ RS.
+ R-S-S-R
+ R-S-R
c) In the addition of mercaptans to double bonds, which
is induced by free-radical formers or by light [22] and
is often used in preparative work, an organic sulfur
radical adds onto the C-C bond. The resulting carbon
radical dehydrogenates the mercaptan to form a sulfur
radical which propagates the chain according to (i)
and (k) :
+ R-SH
+ RHC(SR)-;HR,
+ R--S*
d) Dehydrogenation or oxidation of mercaptans in the
presence of styrene or dienes leads, after addition of an
organic sulfur radical to a double bond, to co-oxidation, telomerization, or polymerization [23].
e) When a mixture of a disulfide and a hydrogen donor
is heated, considerable quantities of mercaptan are
formed [24] until equilibrium is attained [Equations (1)
and (m)].
2 R--S.
11. Free-Radical Reactions of Organic Sulfur
b) The reactions of disulfides or mercaptans with phosphites and free-radical formers to form hydrocarbons
and thioethers [20a,b], have been explained by Walling
by the radical chains (e), (f), (g) and (e), (0, (h) [21]:
Among the radicals mentioned, organic sulfur radicals
are of particular interest, since they are assumed to be
intermediates in important technical and physiologicalchemical processes, e.g. :
or) The vulcanization of rubber has been often interpreted as proceeding via organic sulfur radicals [16].
Attempts have been made to detect the benzothiazolylsulfur radical, probably because of the accelerating
action of benzothiazolyl mercaptan.
+ R-S-S-R
+ H?-Don
2R-Si’- 2 R-SH
+ Don
[20] See [l8], p. 51.
[20a] F. W . Hoffman, R . J. Ess, T. C . Simmons, and R . S . Hanzel,
J . Amer. chem. SOC.78, 6414 (1956).
[20b] H . I . Jakobson, R . G. Harvey, and E. V . Jensen, J. Amer.
chern. SOC.77, 6064 (1955).
[21] C. Walling and R. Rabinowitz, J. Amer. chem. SOC.79, 5326
(1957). For the further reaction of the alkyl radicals formed according to Equation (f), in the presence of C O and disulfides to
form thione esters, see C. Walling, 0 . H. Basedow, and E. S.
Savas, J. Amer. chem. SOC.82, 2181 (1960). - Recently, the reaction of trialkyl phosphites with disulfides has been plausibly
interpreted as occurring by a polar mechanism: R . G. Harvey,
H . I. Jacobson, and E. V. Jensen, J. Amer. chem. SOC.85, 1618
[22] Reviews: [16], p. 314 and [18], p. 75; F. W. Sracey and J . F.
Harris in Organic Reactions. Wiley, New York 1963, Vol. XIII,
p. 164 et seq.
[23] E . L. Jenner and R . V. Lindsey j r . , J. Amer. chem. SOC.83,
1911 (1961); ibid. 85, 1969 (1963), where further references are
given; A. 0. Oswald, J. org. Chemistry 26,842 (1961); A. 0.Oswald, K . Criesbaum, and B. E. Hudson j r . , ibid. 28, 2355 (1963);
G. P . Scotf and J . C. Wang, ibid. 28, 1314 (1963); Review: F. W .
Sracey and J. F. Harris in Organic Reactions. Wiley, New York
1963, Vol. XIII, p. 185 et seq.
Characteristic reactions proceeding via organic radicals of monovalent sulfur may be summarized as
a ) Initiation of free-radical reactions of mercaptans and
disulfides by light or free-radical formers (peroxides,
azo-compounds) [e.g. initiation according to reactions
( 4 , ( 4 , (e), (i), or (111.
p) Free-radical displacement reactions on the sulfur of
mercaptans or disulfides [e.g. according to (a), (d), (g),
y) Recombination of the short-lived, and so far not
physically detected, organic sulfur radicals formed
according to a ) or p), to yield disulfides [Equations (c)
or (01.
6) Addition of sulfur radicals to C=C bonds [e.g. (b),
E) Dehydrogenation of aliphatic and hydroaromatic
compounds by thermally or photolytically formed organic sulfur radicals [e.g. Equation (m)]. Analogous
behavior is shown by the polysulfur radicals formed by
fission of S8 rings: they react with aliphatic and hydroaromatic compounds with dehydrogenation [25], substitution [26], or trithione formation [27a,b], and induce the Willgerodt reaction possibly by a-substitution
of ketones via N-S radicals [27b,28].
The stabilization of arylsulfur radicals by steric hindrance of dimerization appears to be possible only with
very large substituents in the ortho-positions. Experiments with diary1 disulfides which were hexasubstituted
by phenyl [lo], iso-propyl [Ill, or t-butyl groups [I21
gave no indication of dissociation into radicals. Since
the phenoxyl radical has been recently shown by ESR
spectroscopy [32] to occur during dehydrogenation of
phenol, similar experiments with mercaptans in solution
seem promising [33].
III. Attempts to Detect Organic Sulfur Radicals
Oxidation of aromatic thioethers in concentrated sulfuric acid yields deeply colored solutions of divalent
radical ions (sulfinium salts) (2), some of which are
stable for several days. The state of oxidation of the
radical ion is between those of the thioether and the
sulfoxide. The same radical ion is formed from aromatic
sulfoxide on dissolution in sulfuric or polyphosphoric
acid, probably by the removal of an OH radical from
the cation (3) of the sulfoxide salt. Diarylsulhium
salts are stable only if substituted in the p-positions with
electron-donating groups. Sulfur radical ions of the
type of Wurster’s salt ( 4 ) are especially stable, and
are formed even in the oxidation of nitro-substituted
p-phenylene-dithiodiaryl ethers.
Earlier attempts have been already reported [4]. The
evidence for the dissociation of disulfides into monovalent sulfur radicals, based on determinations of molecular weight, light absorption, and magnetism, has
been either refuted by ESR spectroscopy [29] or is not
convincing 1301. The methods are much too inaccurate
to detect the expected small amounts of free radicals,
but nevertheless are often quoted, even in recent
textbooks and monographs. The strong thermochromism of many disulfides, which is often quoted as an
argument in favor of dissociation, can be plausibly
explained by thermal broadening of the absorption
bands [31].
[24] Review: [16], p. 322; A . Schonberg and A . Mustafa, J. Amer.
chem. SOC.73,2401 (1951) und J. chem. SOC.(London) 1949,889;
C. Walling and R . Rabinowitz, J. Amer. chem. SOC.81, 1137
(1959); Y . Schaafsma, A. F. Bickel, and A. C. Kooyman, Tetrahedron 10, 76 (1960).
[25] R. Wegler, E. Kiihle, and W . Schafer in W . Foerst: Neuere
Methoden der praparativen organischen Chemie. Verlag Chemie,
Weinheim/Bergstr. 1961, Vol. 111, p. 1 ; see [18], p. 117.
[26] See [16], p. 336.
[27a] B. Bottcher and A . Liittringhaus, Liebigs Ann. Chem. 557,
89 (1947).
[27b] A . Liittringhaus, H . B. Konig, and B. Bottcher, Liebigs Ann.
Chem. 560, 201 (1948).
[28] F. Asinger et al., Angew. Chem. 75, 1050 (1963); Angew.
Chem. internat. Edit. 2, 220 (1963).
[29] R . E. Davis and C . Perrin, J. Amer. chem. SOC.82, 1590
(1960), footnote [15].
[30] For example, G. Leandri and A. Tundo explain an average
deviation of 10 % in the Rast molecular-weight determination
for numerous disulfides by a dissociation into free radicals [Ann.
Chimica 44, 63 (1954)l.
[31] J. C. D . Brand and J. R. Davidson, J. chem. SOC.(London)
1956, 15.
1. The Wystine Radical” [13]
Irradiation of cystine crystals with X-rays produces
radicals which remain stable for years in the solid state.
Govdy has established the structure of the radical ( I )
by ESR hyperfine structural analysis of irradiated single
crystals of cystine dihydrochloride. After homolytic
cleavage of the S-S bond, rotation about the C2-C3
bond gives rise to an arrangement with widely separated
radical groups, which is stable in the crystal. The freeradical state disappears immediately when the crystal
is dissolved.
2. Diarylsulfinium Salts [34,35]
A r - S -Ar
2 Ar-5-Ar
[32] T. J. Stone and W . A. Waters, Proc. Roy. SOC.(London)
Ser. A 1962, 253.
[33] In the oxidation of thiophenolates by nitroso-compounds
in a flow apparatus, no S radicals could be detected by ESR
spectroscopy, but anion radicals of the nitroso compound were
found [F. J. Smentowski, J. Amer. chem. SOC.85, 3036 (1963)l.
[34] U. Schmidt, K. H . Kabitzke, K . Markau, and A. Miiller, Angew. Chem. 72, 708 (1960).
[35] U. Schmidt, K . H . Kabitzke, and K . Markau, Liebigs Ann.
Chem. 672, 78 (1964). Presented at the GDCh-Meeting, Heidelberg (Germany), Sept. 1963.
Angew. Chem. internat. Edit. 1 Vol. 3 (1964)
I No. 9
The tetraarylthiophenium salts (7) may be classified
as sulfur-containing members of the series of stable
five-membered cyclic radicals, such as pentaphenylcyclopentadienyl [36] (5) and pentaphenylpyrrolium
salts [37] (6). They are formed from tetraarylthiophenes
in concentrated sulfuric acid.
The sulfinium hexachlorostannates, which are stable
in the solid state but are very sensitive to moisture,
react rapidly in suspension in benzene with phenols or
phenolethers to form sulfonium salts:
+ C ~ H S - O C H-+~
2 Ar-i-Ar
+ Ar-S-Ar
+ He
The reaction between sulfoxides and phenols or phenol ethers
in concentrated acid to form sulfonium salts, which was
discovered by Smiles [38] and which Liittringhaus [39] has
formulated, in the case of phenol/HCl, as passing through a
sulfoxide salt, probably proceeds via a sulfur radical ion in
many cases, and certainly in sulfuric acid. The radical ion is
formed by removal of an OH radical from the initially produced sulfoxide salt.
Table 1. Data from the ESR spectra and colors of some sulfinium
Radical cation
H . 0 1
of lines
2.009 1 12.8
nium salt: 13 lines) to the corresponding hydroxyl compounds, the number of lines in the spectra decreases [4,4‘dihydroxydiphenylsulfinium salt : five lines, due to coupling
of the four o-protons; 4,4’-dihydroxy-2,2’,5,5’-tetrachlorodiphenylsulfiniurn salt: three lines, due to coupling of the
two o-protons; tetra-(4-hydroxyphenyl)thiophenium salt:
five lines, due to coupling of a group of four protons, probably the four o-protons of the phenyl groups a t positions 2
and 5.
The transfer of electrons between N-and S-compounds
is of interest with regard to the denaturation of proteins
by radiation. For example, diarylsulfinium salts and
triarylamines momentarily yield the nitrogen radical
ion, which is strongly stabilized by resonance owing to
its three aryl groups :
+ Ar3N
-t Ar3Nx
3. Thianthrene and Phenoxathiin
(Phenoxthin) Radical Ions
High concentrations of free radicals (8) hake been
detected in the deep-blue solutions of thianthrene in
concentrated sulfuric acid, by Hirshon, Cardner, and
Fraenkel [40], using ESR measurements. Wertz and
Vivo [41] recorded a five-line spectrum. Shine, DaviJ,
and Small [42] as well as Rundel and Schefler [43] deduced from the fine structure of substituted thianthrene
radical ions that, in addition to the sulfur atoms, carbon
atoms 2, 3, 6, and 7 possess high spin densities.
High free-radical concentrations have been also observed by
ESR spectroscopy in the deep-blue solutions of chlorinated
phenoxathiin S-oxides in sulfuric acid [44].Lamotte, Rassat,
and Servoz-Gavin [45]later recorded the ESR spectrum of
the blue-violet solution o f unsubstituted phenoxathiin in
sulfuric acid [radical ( 9 ) ] . However, the number of lines in
the hyperfine structure cannot be established with certainty.
Moreover, the spectrum is unsymmetrical and hence cannot
be explained unambigously. Systematic investigation of the
solutions of substituted phenoxathiin S-oxides yielded simpler symmetrical spectra with fine structure, which allowed
conclusions to be drawn regarding the position probability
of the single electron 1461 ( e . g . 1,3,4,5,6,8-hexachlorophenoxathiin radical ion: three lines, due to coupling of the two
protons para to the free-radical sulfur).
(8): X = S
x= 0
4. Sulfur-Containing Oxygen and Nitrogen Radicals
with High Spin Density on the Sulfur
Di-(0-nitrophenylsulfeny1)hydrazine (10) dissociates reversibly in solution above SO “C to form o-nitrophenylsulfenylimine radicals (11) [47]. The triplet structure o f the ESR
Strong ESR signals were recorded with solid and dissolved
sulfinium salts (cf. Table 1). The repeatedly observed fine
structure of the spectra could be interpreted, e.g. the seven
lines of the ditolylsulfinium salt by coupling of the six methyl
protons. On passing from the methoxyl-substituted compounds (dianisylsulfinium salt: 9 lines; tetraanisylthiophe[36]K. Ziegler and B. Schnell, Liebigs Ann. Chem. 445,266(1925).
[37] R. Kuhn and H . Kainer, Chem. Ber. 85, 498 (1952).
[38] S. Smiks and R. LeRossignol, J. chem. SOC.(London) 89,
696 (1906); J. Goerdler in Houben- Weyl: Methoden der Organischen Chemie. 4th Edit., Georg Thieme, Stuttgart 1955,Vol. IX,
p. 184.
[39] A . Liittringhaus, Ber. dtsch. chem. Ges. 72, 890 (1939).
Angew. Chem. internat. Edit. 1 Vol. 3 (1964)
I No. 9
[40] J. M . Hirshon, D . M . Gardner, and G . K . Fraenkel, J. Amer.
chem. SOC.75, 41 15 (1 953).
1411 J. E. Wertz and J . L. Vivo, J. chem. Physics 23, 2193 (1955).
[42] H . J. Shine, C . F. Davis, and R . J. Small, J. chem. Physics 38,
569 (1963).
[43]W . Rundel and K . Scheffler,Tetrahedron Letters 17,993(1963).
[44] K . H. Kabitzke, Diploma Thesis, Universitat Freiburg, 1960.
[45] B. Lamotte, A . Rassat, and P . Servoz-Gavin, C . R. hebd.
Seances Acad. Sci. 225, 1508 (1962).
[46] U . Schmidt, I(. H . Kabitzke, and I(. Markau, Chem. Ber. 97,
498 (1964).
[47] U.Schmidt, K . H . Kabitzke, and K . Markau, Angew. Chem.
76, 376 (1964); Angew. Chem. internat. Edit. 3, 373 (1964).
spectrum shows a certain spin density on the nitrogen. However, since the equal distances between lines of equal intensity
are only half as great (1 1 gauss) as for a comparable nitrogen
6. Arylsulfur and Aryldisulfur Radicals
(2.0076), which is within the range of the values found for
radicals by mercaptan formation, and to determine their
lifetimes by varying the distance between the zone of
irradiation and the point of entry of the hydrogen donor,
were successful only with aromatic disulfides [ 13aI.
When a condenser, covered with cumol as the hydrogen
donor and cooled with liquid nitrogen, was fitted past
the irradiation zone, a red condensate precipitated i n
the photolysis of diphenyl disulfide; the red color faded
on thawing. The red substance separates out on the
condenser in a higher concentration when the disulfide
is vaporized with accurate temperature control, and
sulfinium salts [CH3O-ChHd-S-C6H4-OCHj :g = 2.00791
and outside that for pure N-radicals [diphenylpicrylhydrazyl :
g = 2.00361. Above about 8OoC, considerable evolution of
Nz occurs in toluene solution. The sulfenylimine radical (ZI)
then decomposes according to Equation (r), presumably
forming the arylsulfur radical - which recombines t o give
the disulfide (12) - and nitrene, which reduces the N-S
radical t o the sulfenamide ( 1 3 ) ; compounds (12) and (13)
have been detected as products. Only (IZ), N2, and NH3 are
formed in glacial acetic acid and in the dry decomposition
of (10); the N2 and NH3 probably result from the disproportionation of NH. When (10) decomposes in acrylonitrile
solution, the solvent is rapidly polymerized.
+ 2ArS.+ 2 N H
(1 I )
2 ArS.
(1 2 )
2 ArS-<H
+ 2 NH
2 ArS-NH2
+ Nz
The red product of the photolysis of diphenyl disulfide
was identified as the phenylsulfur radical by recombination to form the colorless diphenyl disulfide, by
reaction with cumol to form thiophenol, and by paramagnetic resonance spectroscopy [13a-l3c]. The red
radical is stable at the temperature of liquid nitrogen
but recombines within a few minutes even at 165°K.
The ESR signal is very strong (Fig. 1); the strong
anisotropy and the high g-value indicate a phenylsulfur
radical with the single electron largely localized on the
sulfur atom [50].
Aroxyls with thioether groups in the p-position have been
studied by Miiller, Stegmann, and Sclieffler 1481. A n ESR
spectrum with nine lines was recorded for p-methylthio-o,o'di-t-butylphenoxyl, and was explained by participation of
sulfur free-radical structures. No coupling of the protons of
thep-phenylthio group was however observed in the spectrum
of the p-phenylthio compound, in which even stronger participation of sulfur free-radical structures should be expected
(cf. the stability of the diarylsulfinium salts [34,35]).
5. Aminopolysulfur Radicals
By means of ESR measurements Hodgson, Buckler, and
felers [49] have found recently that solutions of sulfur
in amines contain stable polysulfur radicals (14) at
room temperature. These
probably result from
the dissociation of the diaminopolysulfane ( / 5 ) , and
are possibly responsible for the catalytic action of
amines in numerous reactions of sulfur [*I.
[48] E. Miiller, H . B. Stegmann, and K . Scheffler, Liebigs Ann.
Chem. 645, 79 (1961).
[49] W. G . Hodgson, S. A. Buckler, and G . Peters, J. Amer. chem.
SOC.85, 543 (1963).
[*I See, for example, [ZS] and C. G . Moore and R. W. Saville, J.
chem. SOC.(London) 1954,2082; H . Krebs, Silicon, Sulfur, Phosphat. Colloq. Sec. Inorg. Chem. Intern. Union of pure and App.
Chem. Miinster (Germany), 1954, p. 107; Chem. Abstr. 52, 5015
2 0031
Fig. I . ESR spectra of aromatic sulfur radicals. The measurements were
recorded at 77 "K with a Varian V 4500 spectrometer with 100 kclsec
modulation, The g values were determmed by measurement of the
microwave frequency and of the magnetic field with a nuclear resonance
probe. Frequency 9500 Mclsec.
Phenylsulfur (CaH5-S.); __ p-xenylsulfur
- - - - - photolysis product of dixenyl disulfide.
Abscissa: Field strength.
Ordinate: d(absorption) d(fie1d strength).
[50] J. D . Michaelsen [Ph. D. Thesis, Catholic University, Washington, 19551 has observed colored coatings on a very low-temperature condenser in the pyrolysis of aromatic disulfides. However, the free-radical nature of the colored condensate was not
proved, and the results have not been published. Three weeks
after the publication (March Zlst, 1963) of our first communication [13a] on the isolation of the phenylsulfur radical and the
detection of its free-radical nature by ESR spectroscopy, P . J.
Zandstra and J. D . Michaelsen published [J. chem. Physics 39,
933 (1963); received on April 1 Ith, 19631 a paper describing ESR
measurements on the pyrolysis products of diphenyl disulfide and
p,p-dinaphthyl disulfide; the products were obtained by methods
described by Michaelsen in his thesis. Besides a g value of 2.000,
these authors found g = 2.1246 for the pyrolysis product of the
disulfide. They constructed the spectrum of the phenylsulfur
Angew. Chem. internat. Edit.
1 Vol. 3 (1964)
No. 9
A few substituted arylsulfur radicals were isolated by
the same method, and the effect of the substituent on the
recombination temperature was investigated [13b, 13~1.
Table 2 shows that electron-donating groups, or a
phenyl group in the p-position, stabilize the radicals and
displace their light absorption towards longer wavelengths. Similarities can be recognized between the
stability of the sulfur radicals and the free-radical
cleavage of the corresponding disulfides by cyangisopropyl radicals or the macroradicals in the vinyl polymerization (Table 3) : with unstrained disulfides, the
ease of the free-radical cleavage increases with increasing
stability of the corresponding sulfur radical.
as cleavage of the S-S bond, giving rise to the polysulfur diradical ( .S-S,-S .) or the p-xenyldisulfur
radical (CGH~-C&-S-S .). This view is supported by
the position of the anisotropic ESR signal at higher g
values (which correspond to the g value of 2.030 for the
aminopolysulfane radical R2N-Sn-S. [49]), by the
approximately equal dissociation energies of the S-S
and C-S bonds [51], and by the appearance of hydrogen sulfide [*] on reduction of the recombination product with LiAlH4 [13c].
IV. Free-Radical Displacement Reactions on the
Sulfur of Organic Disulfides
Table 2. Color a n d stability of monovalent organic sulfur radicals.
Color of
Bright yellow
Bright yellow
[a] The color of the radical fades in a few minutes at 165 "K.
[bl The color of the radical is still weakly present after 10 minutes at
190 "K.
Arylsulfur radicals are also formed in the photolysis
of thiophenols [13c], which is not surprising in view of
the large chain-transfer constants [*I [17,18] and the
numerous free-radical addition reactions [22] of the
mercaptans [51]. The ESR spectrum of the prussianblue p-xenylsulfur radical produced by photolysis of pphenylthiophenol (p-xenyl mercaptan) is very similar
to that of the phenylsulfur radical (Fig. 1). On photolysis of dixenyl disulfide, on the other hand, the spectrum
of the xenylsulfur radical shows a second free radical
signal with a high g value. It is believed that in this case
homolysis of the S-C bond has also occurred, as well
The regulating action of disulfides on the vinyl polymerization has been already mentioned in the introduction (Section 1.p). The chain-transfer constants have
been measured for many disulfides [17,18]. A simple
displacement reaction on the disulfide sulfur is the
reaction of triarylmethyl radicals with disulfides to form
thioethers [52].
To examine the effect of substituents on the homolysis
of the S-S bond, reaction (s) of the cyanoisopropyl
Angew. Chem. internat. Edit. 1 VoI. 3 (1964)
I No.
+ RS.
radical with various disulfides was studied under standardized conditions [53] (see Table 3). With aromatic
disulfides, free-radical displacement on the sulfur of the
S--S bond is facilitated by the presence of electron
donors in the p-position. This agrees with the fact that
Table 3. Homolytic cleavage of disulfides by cyanoisopropyl radicals
or by macroradicals durin; styrene polymerization.
Cleavage 1%I by
radicals [531 [a]
K ( "C) [bl of the
styrene polymerization [17, IS]
radical after mathematical elimination of a foreign radical signal,
and carried out a n MO calculation of the electron distribution
in the phenylsulfur radical. Our own .ESR measurements,
repeated several times, on the phenylsulfur radical prepared by
our method fromdiphenyldisulfide or thiophenol, have only given
traces of a free-radical signal a t the high g value of 2.1246, the
maximum strength of this signal being 3 yo of that of the main
signal. The high g value of 2.1246 published by the American
authors is not, therefore, attributable to the phenylsulfur radical,
but to a foreign radical which is formed in high concentrations
together with the phenylsulfur radical under the drastic conditions used by Michaelsen. The agreement between the result of
the MO calculation described by Zandstra and Michaelsen and
their experimental results is probably fortuitous.
[ * ] The chain-transfer constant of the mercaptan o r disulfide is
defined as K = VK/VM, where VK = rate of reaction of the
macroradical with the chain-transfer agent (disulfide o r mercaptan); V M = rate of addition of the macroradical to the monomeric olefin. The greater the chain-transfer constant of a mercaptan or disulfide, the more readily will its S-H o r S-S bond
be cleaved homolytically by a carbon radical. For the determination of chain-transfer constants, see the standard works on
polymerization kinetics.
[51] The dissociation energy of the S-H bond was
at least in
the aliphatic series - found to be rather greater than that of the
S-S bond: CH3S-H = 88.8 kcal/mole; CH3S-SCH3 = 73.2 kcl/a
mole; CH3-SCH3 = 73.2 kcal/mole [J. L. Franklin and H. E.
Lumpkin, J. Amer. chem. SOC.74, 1023 (19S2)J.
0.33(99) [cl
0.1 l(50)
0.005(99) [dl
lipoic acid
R- S
[a] The disulfides were allowed to react under standardized conditions
with a tenfold excess of azo-isobutyronitrile in dioxan until the azocompound was completely decomposed. The degree of conversion was
then determined from the concentration oi the unreacted disulfide.
[b! For the definition and determination of the chain-transfer constant
K, see footnote [*] in opposite column.
[c] Transfer constant of di-(p-ethoxyphenyl) disulfide.
[dl Transfer constant of diethyl disulfide.
[ * ] Hydrogen sulfide can arise only from the sulfur or the
polysulfides which result on heating the p-xenyldisulfur radical.
[S2] H. Lecher, Ber. dtsch. chem. Ges. 48, 524 (1915); A . Schonberg, A . Stephenson, H . Kaltschmitt, E. Petersen, and H. Schulten,
ibid. 66, 237 (1933).
[S3] U.Schmidt andA. Muller,LiebigsAnn. Chem. 672, 9 0 (1964).
the chain-transfer constant of dianisyl disulfide is greater
than that of diphenyl disulfide. Among the aliphatic
disulfides, only those with strained five- and six-mem[S4] W. A. Pryor and P. K. Plait [J. Amer. chem. SOC.85, 1496
(1963)J have recently investigated the reaction between phenyl
radicals and aliphatic disulfides. As the steric hindrance of the
disulfide group increases, the free-radical displacement on the
sulfur is suppressed in favor of a dehydrogenation by the phenyl
radical. The effect of steric hindrance of aliphatic disulfides in a
free-radical displacement reaction on the sulfur also manifests
itself in the smaller chain-transfer constants of branched disulfides.
bered rings react with the inert cyanoisopropyl radical
[54]; the greater strain of the five-membered ring manifests itself clearly in the higher rate of reaction on freeradical attack. It should be mentioned that dibenzyl
disulfide undergoes free-radical cleavage much more
easily than the purely aliphatic disulfides; the chaintransfer constant is also greater than for aliphatic disulfides.
Received, January 21st, 1964
[A 366/176 IE]
German version: Angew. Chem. 76, 629 (1964)
Translated by Express Translation Service London
Chromatographic Separations on Porous Gels [*I
Over the past f e w years, fractionation of mixtures of substances on the basis of differences
in their molecular weights has become an important method in chemistry and biochemistry.
A survey is given of the theory arid practice of separations applying porous gels, and the
multifarious uses of such gels are indicated.
1. Introduction
Modern methods of separation, e. g. partition chromatography, countercurrent distribution, adsorption or ionexchange chromatography, and electrophoresis, generally make use of differences in solubility or molecular
charge. It is only comparatively recently that advantage
has been taken of differences in molecular sizes [I, la],
although zeolites [2],which have been known for some
time, are in fact molecular sieves consisting of threedimensional networks of SiO4 and A104 tetrahedra
enclosing cavities (about 50 % of the volume) connected
by pores. Zeolites are now produced synthetically and
are used, for example, for drying gases or for separating
olefins from gas mixtures ; even at extreme temperatures
they have specific affinities for polar substances.
The first generally applicable method for separating
large from small molecules, namely dialysis through
porous membranes, was derived directly from macroscopic filtration. In the meantime, dialysis techniques
have been refined by the development of membranes
with graduated, well-defined pore sizes. “Ultrafilters”
[*I Extended version of lectures given at Farbwerke Hoechst,
Frankfurt/Main-Hoechst (Germany), December 1962; Osterreichische Gesellschaft fur Physiologische Chemie, Vienna (Austria),
February 1963 ; Kolloid-Gesellschaft, Bad Oeynhausen (Germany), October 1963.
[ I ] Reviews of certain aspects of gel filtration: P. Flodin, Ph.D.
Thesis, University of Uppsala (Sweden), 1962; obtainable from
A. B. Pharmacia, Uppsala (Sweden); J . Porarh, Advances Protein
Chem. 17, 209 (1962); B. Gelottet New Biochemical Separations.
D. van Nostrand, London 1963; A. Tiselius, J. Porath, and P. A.
Albertsson, Science (Washington) 141, 13 (1963).
[la] It is assumed in this paper that molecular weight is proportional to molecular size.
[2] Review: E. A. Scheuermann, Chemiker-Ztg. 20, 767 (1961).
or “membrane filters” [2a] consist of foams of cellulose
derivatives or regenerates, and are used predominantly
in biochemistry for the concentration and desalting of
protein solutions, or in electrodialysis. In these preparations, in contrast to the zeolites, pores and cavities
are identical and account for about 80 % of the volume
of the filter.
The pore sizes of agar membranes have recently been determined by diffusion of proteins and viruses of various particle weights [3]. Craig used pretreated cellulose membranes
t o obtain information about the shapes of peptide and protein
molecules in solution from their rates of diffusion [4]. Synge
reported attempts t o separate cytochrome c, insulin, gramicidin S, and valine by multistage electrodialysis IS]; in the
course of their migration in a n electric field, the ions were
made t o pass through membranes of increasing density, being
more or less retained by these according to size. This technique may be compared with the multistage dialysis processes
due t o Signer [6] and Craig [7], in which the dialysate of the
first unit becomes the liquid to be dialysed in the second
stage, and so on. The output from the separation is poor, as
regards both quality and quantity, in relation t o the effort
Much better separations can be achieved by the simple
c h r o m a t o g r a p h i c process of gel filtration. This in[2a] Manufacturer and literature: Membranfilter-Gesellschaft
GmbH., Gottingen (Germany).
[3] G. K. Ackers and R. L. Steere, Biochim. biophysica Acta 59,
137 (1962).
[4] L. C. Craig and E. Harfenist, Report at the 6th European
Peptide Symposium, Athens (Greece) 1963; cf. Angew. Chem.
75, 63 (1964).
[S] R. L. M . Synge and M . A . Youngson, Biochem. J. 78, 3 I P
[6] R. Signer, H . Hanni, W. Koestler, W. Rottenberg, and P. v.
Tavel, Helv. chim. Acta 29, 1894 (1946); P. v. Tavel, ibid. 30, 334
[7] L. C. Craig and T . P. King, J. Amer. chem. SOC.77, 6620
(1955); 78, 4170 (1956).
Angew. Chem. internat. Edit. 1 Vol. 3 (1964) 1 No. 9
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