Photolysis of dialkoxy disulfides: A convenient source of alkoxy radicals for addition to the sphere of fullerene C60

Article abstract of DOI:10.1021/jo952169e

Photolysis of dialkoxy disulfides ROSSOR (R = Me, Et, i-Pr, t-Bu, i-PrCH₂, t-BuCH₂) yields the radicals RO^?, ROS^?, and ROS^?=O that were identified on the basis of product analysis and spin trapping techniques. It has been shown that only the alkoxy radicals RO^?, produced from ROSSOR, add to the sphere of fullerene C₆₀ in steady state conditions to yield the RO-C₆₀^? adducts which can be detected by ESR spectroscopy. The different trend of the hydrogen splitting constants in the RO-C₆₀^? with respect to the corresponding RS-C₆₀^? adducts previously reported has been interpreted as a consequence of the different C-O and C-S bond lengths.

Full text of DOI:10.1021/jo952169e

J . Org. Chem. 1996, 61, 3327-3331
3327
P h otolysis of Dia lk oxy Disu lfid es: A Con ven ien t Sou r ce of Alk oxy
Ra d ica ls for Ad d ition to th e Sp h er e of F u ller en e C60
R. Borghi,1 L. Lunazzi,* and G. Placucci*
Dipartimento di Chimica Organica “A. Mangini”, Universita`, Risorgimento, 4, Bologna 40136, Italy
G. Cerioni
Dipartimento di Scienze Chimiche, Universita`, via Ospedale, 72, Cagliari, Italy
A. Plumitallo
Dipartimento Farmaco Chimico Tecnologico, Universita`, via Ospedale, 72, Cagliari, Italy
Received December 6, 1995X
Photolysis of dialkoxy disulfides ROSSOR (R ) Me, Et, i-Pr, t-Bu, i-PrCH2, t-BuCH2) yields the
radicals RO•, ROS•, and ROS•dO that were identified on the basis of product analysis and spin
trapping techniques. It has been shown that only the alkoxy radicals RO•, produced from ROSSOR,
•
add to the sphere of fullerene C60 in steady state conditions to yield the RO-C60 adducts which
can be detected by ESR spectroscopy. The different trend of the hydrogen splitting constants in
•
•
the RO-C60 with respect to the corresponding RS-C60 adducts previously reported has been
interpreted as a consequence of the different C-O and C-S bond lengths.
It has been recently reported2 that alkoxy radicals
(RO•), photolytically generated from their peroxides
(ROOR), add to the sphere of fullerene C60 yielding the
In addition to product analysis (see the following text)
a convenient approach for this purpose is the spin
trapping technique5 which allows the identification of the
radicals produced in a reaction by means of the ESR
spectra of the corresponding stable nitroxides.
•
RO-C60 radical adducts which can be detected by ESR
spectroscopy. However, the very simple peroxides (R )
Me, Et, i-Pr...) are dangerous to make and to handle (e.g.,
MeOOMe is a gas3 requiring special apparatus to be
conveniently employed) and are thus not well suited for
use as reactants for this purpose. Indeed, only three
Accordingly, photolysis of various ROSSOR derivatives
(R ) Me, Et, i-Pr, t-Bu, i-PrCH2, t-BuCH2) was performed
in the presence of 2-methyl-2-nitrosopropane (t-Bu-NO)
so that the ESR spectra of the corresponding nitroxides
t-BuN(O•)X could be observed, X being the group derived
from the fragmentation of the photolyzed compounds.
By far the most intense ESR signals observed, in all
cases, between -20 and +80 °C, were those of a nitroxide
(see Figure 1 for the case of MeOSSOMe) having a 1:1:1
nitrogen splitting of 12.4 G without any further fine
structure and a g-factor equal to 2.00615 ( 1.5 × 10-4
(Table 1). Clearly, these data indicate that X• radicals
different from the desired alkoxy radicals have been
trapped. For, alkoxy nitroxide of the type t-BuN(O•)OR
(i.e., those expected from the trapping of RO•) are known6
to have much larger aN splittings (their range being 27-
30 G) and to display, quite often, an additional fine
structure due to the hydrogens of the R group. We
identified these intense signals as those of nitroxides
t-BuN(O•)S(O)R, due to the trapping of ROS(O)•. The
latter radical can be thought as derived by the cleavage
of the ROSSOR bond, yielding the ROS• radical which
subsequently undergoes an oxidation.7 An analogous
oxidation process had been observed, albeit in a different
environment, when disulfides RSSR are photolyzed to
•
RO-C60 radical adducts (i.e., those with R ) t-Bu,
PhCMe2, and CF3) could so far be detected with such a
•
method, contrary to the case of the analogous RS-C60
adducts which can be conveniently obtained for any type
of substituent R by means of the photolysis of the
corresponding disulfides RSSR.2 Because of these limita-
tions the characteristical properties of the alkoxyfulleryl
radical adducts could not be thoroughly investigated nor
meaningfully compared to those of the corresponding
(alkylthio)fulleryl analogs.
It occurred to us that a more convenient photolytic
source of alkoxy radicals for addition to fullerene might
possibly be dialkoxy disulfides ROSSOR. These com-
pounds are either liquids or solids and can be easily (and
safely) prepared4 with any kind of R group by reacting
the appropriate alcohols with ClSSCl.
Of course, photolysis of dialkoxy disulfides might not
yield, as we had anticipated, the desired alkoxy radicals,
or even if produced, they might be accompanied by a
number of other unwanted radicals which would make
the spectral appearance much more complex, thus mak-
ing the investigation either extremely difficult or impos-
sible. As a consequence, it was important to identify all
the radicals produced by photolysis of the ROSSOR
derivatives.
(5) (a) J anzen, E. G. Acc. Chem. Res. 1971, 4, 31. (b) Perkins, M. J .
In Advances in Physical Organic Chemistry; Gold,V., Bethelle, D., Eds.;
Academic Press: London, 1980; Vol. 17, p 1. (c) Cowley, D. J .;
Nonehebel, D. C. In Organic Reaction Mechanism 1979; Knipe, A. C.,
Watts,W. E., Eds.; J ohn Wiley and Sons: London, 1981; Chapter 3, p
75.
(6) Forrester, A. R. In Landolt Bo¨rnstein Magnetic Properties of Free
Radicals; Fischer, H., Hellewege, K.-H., Eds.; Springer Verlag: Berlin,
1979; Vol. 9, Part C1, Chapter 6.31, p 914.
X Abstract published in Advance ACS Abstracts, April 1, 1996.
(1) In partial fulfillment of the requirements for the Ph.D. Thesis
in Chemical Sciences, University of Bologna.
(2) Cremonini, M. A.; Lunazzi, L.; Placucci, G.; Krusic, P. J . J . Org.
Chem. 1993, 58, 4735.
(3) Rieche, A. Ber. 1928, 61, 951.
(4) (a) Lengfeld, F. Ber. 1895, 28, 449. (b) Thompson, Q. E.;
Crutchfield, M. M.; Pieron, E. J . Org.Chem. 1965, 30, 2692.
(7) Of course, the result would be the same if oxidation had occurred
first, yielding ROS(O)SOR, which might have subsequently produced
ROS(O)• by cleavage of the S-S(O) bond. However the absence of EtOS-
(O)SOEt amongst the reaction products of photolysis of EtOSSOEt
makes this hypothesis less plausible.
S0022-3263(95)02169-4 CCC: $12.00 © 1996 American Chemical Society

3328 J . Org. Chem., Vol. 61, No. 10, 1996
Borghi et al.
Ta ble 1. Hyp er fin e Sp littin gs (Ga u ss) of th e Va r iou s
Nitr oxid es Obta in ed by Tr a p p in g th e Ra d ica ls
Gen er a ted fr om th e P h otolysis of ROSSOR w ith t-Bu NO
g-factor (Table 1) of this second spectrum, are typical of
an alkoxyalkyl nitroxide [in this example t-BuN(O•)OMe],
produced by the trapping of the RO• radicals. This
interpretation is fully confirmed by the additional fine
structure observed for the analogous nitroxides obtained
by photolysis of ROSSOR having R ) Et, i-PrCH2, and
t-BuCH2, i.e., a 1:2:1 triplet due to the coupling with the
hydrogens of the CH2 moieties (Table 1). These features
agree with those reported for the same nitroxides pro-
duced in a matrix by γ-radiolysis of the corresponding
alcohols.6 Also, the absence of a doublet, due to the CH
hydrogen, might have been expected for the case of
i-PrON(O•)Bu-t (derived from photolysis of i-PrOSSOPr-
RON(O•)Bu-tb
ROS(O)N(O•)Bu-ta
ROSN(O•)Bu-tc
R
aN
aN
aH
aN
Me
Et
i-Pr
t-Bu
i-PrCH2
t-BuCH2
12.4
12.1
12.4
12.4
12.5
12.5
29.6 1.5 (3H)
29.0 1.0 (2H)
28.6
27.0
29.2 1.25 (2H)
26.8 1.6 (2H)
15.5
18.4
18.6
a
b
g-factors ) 2.00615 ( 1.5 × 10-4
.
g-factors ) 2.0055 ( 1 ×
10-4
.
c g-factors ) 2.0070 ( 1.5 × 10-4
.
6
i), is confirmed by the literature report (this splitting
is in fact too small to be resolved, owing to an unfavorable
conformational disposition). Finally, the nitroxide ob-
tained by trapping the t-BuO• radical derived from
t-BuOSSOBu-t does not exhibit, as expected,6 any further
fine structure (Table 1) in that the H-atoms are too far
away from the radical center.
Quite similar results, both with regard to the relative
proportion of the two nitroxide signals and to the values
of the spectral parameters, were obtained when an
analogous spin trap, such as 2,3,5,6-tetramethylni-
trosobenzene, was used under the same conditions (the
only drawback being a lower resolution due to broader
line widths).
Analysis (GC-MS) of the products derived from the
photolysis of EtOSSOEt (in benzene as solvent) showed,
in addition to a certain amount of elemental sulfur (S8),
the presence of only two products, namely the diethyl
sulfite EtOS(O)OEt and diethyl sulfoxylate EtOSOEt,
approximately in a 2:1 relative proportion. These prod-
ucts were identified by comparing their mass spectra with
those of two authentic samples. Whereas in this com-
parison isolated EtOS(O)OEt was employed, the diethyl
sulfoxylate EtOSOEt was obtained in situ by reacting
EtOSSOEt with EtONa, according to the literature.10 The
GC-MS of the products of the latter reaction showed how
the fragmentation pattern of EtOSOEt (M+ ) 122) was
identical (as was the corresponding retention time) to
that of the compound (also having M+ ) 122) we had
observed in the photolysis of EtOSSOEt.
The two mentioned products resulting from the pho-
tolysis of EtOSSOEt are conceivably accounted for by
the coupling of the radical EtO• (unambiguously identi-
fied by the spin trapping experiment) with the radicals
EtOS(O)• and EtOS•, respectively. Such a product analy-
sis, therefore, makes even more plausible the structural
assignment of the nitroxides responsible for the most
intense ESR spectra, i.e., ROS(O)N(O•)Bu-t.
It is also worth mentioning that when the spin trap-
ping is carried out at very low temperature (i.e., below
-100 °C in cyclopropane as solvent) the spectra of the
two nitroxides we had observed at higher temperature
become much less intense, and very weak signals, due
to a third, quite labile nitroxide, are visible. The latter
spectra comprise 1:1:1 triplets (aN ) 15.5-18.6 G,
depending on the substituent R) with g-factors ) 2.0070
( 1.5 × 10-4 (Table 1). It is not unreasonable to suggest
that at such a low temperature the ROS• radical, initially
formed by cleavage of the S-S bond, is more persistent,
owing to a slower oxidation rate. Consequently, it might
survive long enough to be trapped by t-BuNO, yielding
F igu r e 1. ESR spectrum (central starred triplet) of the
nitroxide MeOS(O)N(O•)Bu-t obtained by photolysis of
MeOSSOMe in the presence of t-BuNO at -20 °C. At the edges
appear the wings (N-spin quantum number +1 and -1) of the
spectrum (vertically amplified 100 times) of the nitroxide
MeON(O•)Bu-t, due to the trapping of the MeO• radical as
revealed by the additional 1:3:3:1 hydrogen fine structure.
yield the RS(O)• radicals.8 The reason we feel that the
trapping of the initially formed ROS• radical does not
occur in these conditions, but it is the oxidated species
ROS(O)• which is trapped to yield the most intense ESR
signal, is based on the following experiment. Photolysis
(and thermolysis as well) of diethyl sulfite EtOS(O)OEt,
in the presence of t-BuNO, does yield a nitroxide spec-
trum equal to the more intense triplet (aN ) 12.1 G)
obtained from photolysis of EtOSSOEt. Since it is
expected that cleavage of either of the two EtO-S bonds
of EtOS(O)OEt would generate the EtOS(O)• radical, the
identity of the most intense signals derived from EtOS-
(O)OEt and EtOSSOEt strongly supports the proposed
assignment.9
As shown in Figure 1, in addition to the intense
(starred) spectrum [here assigned to t-BuN(O•)S(O)OMe],
a second spectrum is also observed, in a smaller propor-
tion (about 1:20). This second spectrum comprises a 1:1:1
triplet (aN ) 29.6 G) further split into a 1:3:3:1 quartet
(a3H ) 1.5 G). Owing to the similarity of the g-factors of
the two spectra, the central signal of the weaker spec-
trum is obscured under the central line of the more
intense one. The aN and aH splittings, as well as the
(8) (a) Kawamura, T.; Krusic, P. J .; Kochi, J . K. Tetrahedron Lett.
1972, 4075. (b) Gilbert, B. C.; Kirk, C. M.; Norman, R. O. C.; Laue, H.
A. H. J . Chem. Soc., Perkin Trans. 2 1977, 497.
(9) Further proof of this interpretation is offered by the observation
that photolysis of diethyl sulfite EtOS(O)OEt yields, simultaneously,
a second spectrum due to the trapping of the EtO• radical, which is
the other fragment produced by the cleavage of the EtO-S bond. The
ESR parameters of this second spectrum correspond, 6 in fact, to those
of the t-BuN(O•)OEt radical reported in Table 1.
(10) (a) Meuwsen, A.; Gebhardt, H. Ber. 1936, 69, 937. (b) Thompson,
Q. E. J . Org. Chem. 1965, 30, 2703.

Photolysis of Dialkoxy Disulfides
J . Org. Chem., Vol. 61, No. 10, 1996 3329
the spectrum of ROSN(O•)Bu-t which is sufficiently long
living, at such low temperatures, to be detected. Al-
though independent spectra of such nitroxides are not
available for comparison, the absence of additional proton
fine structure and the values of the aN splittings and
g-factors (Table 1) appear to be reasonable for the
proposed structure. For instance, nitroxides such as
RSN(O•)Bu-t have11 aN ) 18.9 G with g ) 2.0064 when
R ) Me, aN ) 17.4 G with g ) 2.0063 when R ) Et, and
aN ) 17.0 G with g ) 2.0069 when R ) i-Pr. In addition,
this interpretation is in keeping with the presence of
EtOSOEt as one of the two reaction products derived
from photolysis of EtOSSOEt.
The larger intensity of the spectra of the ROS(O)N-
(O•)Bu-t nitroxides with respect to those of nitroxides due
to the trapping of the EtO• and EtOS• radicals is somehow
at variance with the much smaller ratio observed for the
reaction products EtOS(O)OEt and EtOSOEt (2:1, as
mentioned above). Such a large excess of the ROS(O)•
radicals in the spin trapping experiment seems to be the
consequence of a favorable oxidizing environment, which
might be provided by the t-BuNO itself.
Thus, it seems likely to expect that in the absence of
such a spin trap the amount of the ROS(O)• radicals
might be much lower. Furthermore, with many sub-
strates other than t-BuNO, the reaction rate for addition
of RO• might be faster than that of the bulkier ROS(O)•
radicals. If these conditions were actually verified,
photolysis of ROSSOR would then become a convenient
source of RO• radicals.
This prediction was, apparently, confirmed by experi-
ments we carried out with a quite different spin trap,
i.e., 5,5-dimethyl-1-pyrroline N-oxide (DMPO).12 Ther-
molysis (50-70 °C) of ROSSOR (R ) Me, Et, i-Pr,
i-PrCH2) in the presence of DMPO yielded ESR spectra
whose most intense signals correspond to nitroxide
spectra (all having g ) 2.0061) with hyperfine splittings
•
F igu r e 2. ESR spectra of the radical adducts RO-C60 (R )
Me, Et, i-Pr, and t-Bu) obtained by photolysis of the corre-
sponding ROSSOR derivatives in the presence of fullerene C60
(in the case of R ) t-Bu the first and the 10th line have a signal
to noise ratio too low to be detected).
played, the corresponding spectral parameters being
collected in Table 2.
(the values cover the ranges aN ) 12.8-13.0 G, a1H
)
6.5-7.0 G, and a1H ) 2.0 G) very close to those reported12
for nitroxides produced by DMPO trapping RO• radicals
generated by reaction of lead tetraacetate with various
alcohols. Also, room temperature photolysis of t-BuOS-
SOBu-t in the presence of DMPO yields the spectrum of
a nitroxide (g ) 2.0061) with splittings (aN ) 13.1 G, a1H
) 8.0 G, and a1H ) 2.0 G) which are indistinguishable
from those we obtained, in the same experimental
conditions, when using t-BuOOBu-t as a source of t-BuO•
radicals.
At first glance the spectral multiplicity of these spectra
appears quite surprising in that there is no splitting due
to the hydrogens bonded to the carbons in the R-position
to the oxygen atom. On the contrary, noticeable split-
tings are observed for the more distant hydrogens bonded
to carbons in the â-position. Indeed, the spectrum of
MeO-C60• displays a single line (owing to the absence of
splittings due to the three methyl hydrogens) whereas
the hydrogens of the methyl groups in the â-position to
oxygen yield a four-line, a seven-line, and a 10-line
We therefore took advantage of this behavior by
photolyzing t-BuOSSOBu-t in the presence of fullerene
•
multiplet in the case, respectively, of EtO-C60 , i-PrO-
C
60•, and t-BuO-C60• (the appropriate binomial intensity
C
60 to obtain the ESR spectrum of a single radical adduct
distribution is slightly distorted by a small CIDEP effect14
which makes the upfield lines appear somewhat taller
than the corresponding downfield lines). Accordingly, the
which was identical to the one observed13 when reacting
t-BuOOBu-t under the same conditions with C60. There
is, therefore, no doubt that photolysis of dialkoxy disul-
fides can be employed to add RO• radicals to one of the
30 double bonds of fullerene C60, without their ESR
spectra being interfered by the presence of other visible
radical adducts.
•
spectrum of Me2CHCH2OC60 displays a doublet due to
the methine hydrogen (Table 2), without any fine struc-
ture from the methylene hydrogens (a small additional
splitting was also observed for the six hydrogens of the
two methyl groups). For the same reason photolysis of
PhCH2OSSOCH2Ph yields a single line spectrum due to
the PhCH2O-C60• adduct which, not having any hydrogen
bonded to the carbon in the â-position to the oxygen atom,
does not display any fine structure (photolysis at higher
temperature also yields, superimposed to the single line
•
In Figure 2 the ESR spectra of a few RO-C60 radical
adducts (R ) Me, Et, i-Pr, and t-Bu), produced by using
the appropriate ROSSOR dialkoxy disulfides, are dis-
(11) See ref 6, Chapter 6.35, p 986.
(12) J anzen, E. G.; Liu, J . E. P. J . Magn. Reson. 1973, 9, 510.
(13) In both cases photolysis was carried out at low temperature
•
(+10 °C) to make negligible the amount of the MeC60 adduct due to
the addition of the methyl radical2 deriving from the â-scission of
t-BuO•.
(14) Morton, J . R.; Preston, K. F.; Krusic, P. J .; Knight, L. B., J r.
Chem. Phys. Lett. 1993, 204, 481.

3330 J . Org. Chem., Vol. 61, No. 10, 1996
Ta ble 2. Hyp er fin e Sp littin gs (Ga u ss) of th e Ra d ica l Ad d u cts RX-C60 (X ) O, S)a
Borghi et al.
•
X ) Ob
X ) Sc
R
a(CH3)
a(CH2)
a(CH)
a(CH3)
a(CH2)
a(CH)
Me
Et
i-Pr
t-Bu
i-PrCH2
3Hδ ≈ 0
3Hδ ) 0.38
3Hꢀ ) 0.29
6Hꢀ ) 0.21
9Hꢀ ) 0.25
6Hú ) 0
3Hꢀ ) 0.37
6Hꢀ ) 0.275
9Hꢀ ) 0.35
6Hú ) 0.15
2Hδ ≈ 0
2Hδ ≈ 0
2Hδ ) 0.31
2Hδ ) 0.40
1Hδ ≈ 0
1Hδ ) 0.22
1Hꢀ ) 0.21
1Hꢀ ) 0.475
The values for X ) S are from ref 2. The g-factors are in the range 2.002 30-2.002 35. c The g-factors are in the range 2.002 34-
2.002 40.
a
b
(Figure 3, left).19 Since in these systems the spin density
experienced by the H-atoms should mainly originate from
a direct overlap of the 1s H-orbital with the mentioned
carbon pz-orbital, the geometry accounts for the ap-
preciable values of both the Hδ and Hꢀ splittings observed
in EtS-C60• and in the analogous alkylthio adducts. On
the contrary, when X ) O, the shorter lengths of the pair
of the C-O bonds pull the δ-hydrogens of the CH2 group
away from the pz-orbital (Figure 3, right), thus reducing
the extent of the overlap and, hence, the aH values of the
δ-hydrogens which eventually become too small to be
detected. At the same time, the ꢀ-hydrogens of the CH3
group are placed in a position allowing an even better
overlap with the pz-orbital, thus explaining the larger aH
splittings observed for Hꢀ in the RO-C60• with respect to
F igu r e 3. Representation of the top portion of the computed
structure of the radical adducts EtS-C60• (left) and EtO-C60
(right), showing how both the CH3 and CH2 groups are in a
position allowing the interaction with the spin of the unpaired
electron in the former case, whereas only the CH3 group can
experience such an interaction in the latter case (the MM
calculations18 were carried out for the whole sphere, although
only the part of interest is reported for convenience).
•
•
the RS-C60 radical adducts. Analogous geometrical
considerations, based on this model, also account for the
unnegligible splitting (0.15 G) of the methyl hydrogens
•
in the ú-position in i-PrCH2O-C60 to be compared with
the corresponding null value2 in i-PrCH2S-C60 (Table
•
spectrum, a second spectrum with the same splittings
reported15 for the PhCH2-C60• adduct: the benzyl radical
is obviously a byproduct of the high-temperature pho-
tolysis of PhCH2OSSOCH2Ph).16
To comply with the usual labeling of radicals, where
the atomic positions are counted with respect to the
radical center indicated as R, the hydrogens bonded to
the carbon directly attached to the oxygen atom will be
called δ in Table 2, those one carbon further away ꢀ, and
so on.2
2).20
Exp er im en ta l Section
Ma ter ia l. (EtO)2SO, t-BuOOBu-t, t-BuNO, ArNO (Ar )
2,3,5,6-tetramethylphenyl), and DMPO are commercially avail-
able. The dialkoxy disulfides ROSSOR were prepared accord-
ing to the general method described in ref 4b. Derivatives with
(19) Although the unpaired electron density is, in part, also delo-
calized upon other carbons of the sphere, all the calculations (carried
Whereas we have seen how the Hδ splittings of the
•
RO-C60 radical adducts are too small to be resolved,
•
out for the H-C60 radical) agree in indicating that the density upon
•
the carbon marked as black in Figure 3 is the largest one. See, for
instance: Matsuzawa, N.; Dixon, D. A.; Krusic, P. J . J . Phys. Chem.
1992, 96, 8317. Percival, P. W.; Wlodeck, S. Chem. Phys. Lett. 1993,
207, 31. Morton, J . R.; Negri, F.; Preston, K. F. Can. J . Chem. 1994,
72, 776. Reid, I. D.; Roduner, E. Hyperfine Interact. 1994, 86, 809.
Borghi, R.; Lunazzi, L.; Placucci, G.; Krusic, P. J .; Dixon, D. A.;
Matsuzawa, N.; Ala, M. Submitted for publication.
those of the corresponding RS-C60 were found to be
equal to or larger than their Hꢀ splittings.2 Also, the Hꢀ
•
splittings of all the RO-C60 radical adducts we made
available with these experiments (Table 2) are definitely
larger than those of the corresponding RS-C60• analogs.2
(20) An alternative explanation, for which we are indebted to an
anonymous reviewer, might be based on the assumption that confor-
mations having the RX group (X ) O, S) pointing away from the
direction of the pz orbital (as shown below for EtXC60•) are also
significantly populated:
A possible explanation for this behavior can be found
in the different geometry engendered by the longer C-S
with respect to the C-O bonds. As shown in the
molecular mechanics calculated18 structures displayed,
•
as an example, in Figure 3 for the EtX-C60 radical
adducts (X ) O, S), the CH3 and CH2 moieties occupy a
position, when X ) S, where both groups can similarly
overlap with the pz orbital bearing the unpaired electron
Within this framework one might further assume that conformation
A should be preferred when X ) O, whereas conformation B should be
preferred when X ) S in that the longer C-S bond lengths would allow
the methyl group not to “bump” against the C60 surface. The possibility
of a through-bond transmission of the hyperfine interactions when “zig-
zag” (or W) pathways21 are available would thus make the CH2 splitting
larger for X ) S (structure B) than for X ) O (structure A). The same
assumptions would predict an opposite trend for the splitting of the
CH3 group, which lies along a “zig-zag” path when X ) O (structure
A), but not when X ) S (structure B). This explanation, however, does
not exclude the one we proposed, and both might occur simultaneously.
(21) (a) Krusic, P. J .; Rettig, T. A. J . Am. Chem. Soc. 1970, 92, 722.
(b) Perkins, C. W.; Martin, J . C.; Arduengo, A. J .; Lau, W.; Angria, A.;
Kochi, J . K. J . Am. Chem. Soc. 1980, 102, 7753. (c) Chatgilialoglu, C.;
Lunazzi, L.; Macciantelli, D.; Placucci, G. J . Am. Chem. Soc. 1984, 106,
5252.
(15) Morton, J . R.; Preston, K. F.; Krusic. P. J .; Wasserman, E. J .
Chem. Soc., Perkin Trans. 2 1992, 1425.
(16) In ref 15 the splitting due to the pair of aromatic protons of
the PhCH2-C60• adduct was not assigned. By repeating the experiment
•
using 3,5-dimethylbenzyl bromide as a source of ArCH2 radicals (Ar
) 3,5-dimethylphenyl), we did not observe any change with respect to
•
the spectral patterns of the PhCH2-C60 adduct. As a consequence,
the mentioned splitting must originate from the pair of ortho protons,
contrary to what was observed17 in the Ph-C60 adduct, where the
•
aromatic splitting originates from the meta protons.
(17) Borghi, B.; Lunazzi, L.; Placucci, G.; Krusic, P. J .; Dixon, D.
A.; Knight, L. B., J r. J . Phys. Chem. 1994, 98, 5395.
(18) MMX forcefield as in PCMODEL, Serena Software, Blooming-
ton, IN.

Photolysis of Dialkoxy Disulfides
J . Org. Chem., Vol. 61, No. 10, 1996 3331
R ) Me, Et, and i-Pr had been previously reported.4 The
dialkoxy disulfides employed in the present investigation were
identified as follow.
ESR Sp ectr oscop y. The spin trapping experiments were
carried out by mixing the various precursors with the nitroso
compound in benzene as solvent. The samples were degassed
by connecting to a vacuum line the Suprasil quartz tubes,
containing the solutions, which were subsequently sealed in
vacuo. Photolysis was carried out in the cavity of an ESR
spectrometer (Varian E 104) using a 500 W high-pressure Hg
lamp. For the low-temperature experiments cyclopropane,
condensed in the tubes by means of liquid nitrogen, was added
to the solutions.
Dim eth oxy Disu lfid e, MeOSSOMe. 1H NMR (CDCl3) δ:
3.66 (s). 13C NMR (CDCl3) δ: 61.54 (CH3). IR: ν 975, 675,
650.
Dieth oxy Disu lfid e, EtOSSOEt. 1H NMR (CDCl3) δ: 1.21
(m, 6H), 3.78 (m, 2H), 3.90 (m, 2H). 13C NMR (CDCl3) δ: 15.48
(CH3), 70.89 (CH2). IR: ν 1000, 870, 695, 650.
Diisopr opoxy Disu lfide, i-P r OSSOP r -i. 1H NMR (CDCl3)
δ: 1.30 (d, 6H), 1.45 (d, 6H), 4.12 (m, 2H). 13C NMR (CDCl3)
δ: 22.19 (CH3), 22.84 (CH3), 78.25 (CH). IR: ν 1100, 905, 825,
700.
Di-ter t-bu toxy Disu lfid e, t-Bu OSSOBu -t. Bp (1 mm): 49
°C. 1H NMR (CDCl3) δ: 1.20 (s). 13C NMR (CDCl3) δ: 28.40
(CH3), 81.17 (C(CH3)3). IR: ν 1160, 835, 765, 670. Anal. Calcd
for C8H18O2S2: C, 45.68; H, 8.62; S, 30.48. Found: C, 45.88;
H, 8.79; S, 30.55.
Diisob u t oxy Disu lfid e, i-P r CH2OSSOCH2P r -i. Bp (1
mm): 58 °C. 1H NMR (CDCl3) δ: 0.90 (d, 12H), 1.90 (m, 2H),
3.50 (dd, 2H), 3.62 (dd, 2H). 13C NMR (CDCl3) δ: 18.98 (CH3),
19.03 (CH3), 28.75 (CH), 82.15 (CH2). IR: ν 980, 720, 675.
Anal. Calcd for C8H18O2S2: C, 45.69; H, 8.62; S, 30.48.
Found: C, 45.75; H, 8.73; S, 30.67.
Din eop en toxy Disu lfid e, t-Bu CH2OSSOCH2Bu -t. Bp
(0.8 mm): 72 °C. 1H NMR (CDCl3) δ: 0.89 (s, 18 H), 3.41 (d,
2H), 3.57 (d, 2H). 13C NMR (CDCl3) δ: 26.43 (CH3), 32.53
(C(CH3)3), 86.00 (CH2). IR: ν 970, 770, 715. Anal. Calcd for
C10H22O2S2: C, 50.38; H, 9.30; S, 26.90. Found: C, 50.29; H,
9.44; S, 27.14.
The ESR spectra of the radical adducts of fullerene C60 were
obtained by mixing benzene solutions of C60 with dialkoxy
disulfides ROSSOR (the equivalent ratios covering the range
1:1-1:5). The samples were degassed and photolyzed as
previously described2,17 at about 60 °C and 80 °C (only
t-BuOSSOBu-t having been photolyzed at 10 °C). The g-factors
of the nitroxide radicals were determined with respect to that
of DPPH (contained in an internal capillary sample) and those
of the RO-C60• radical adducts with respect to that of the line
due to the C60 excited triplet (g ) 2.001 35).22
Ack n ow led gm en t. Thanks are due to Dr. P. J .
Krusic, Dupont, Wilmington, DE, for reading the manu-
script and Miss B. Guidi, University of Bologna, for
skillful technical assistance. The work was carried
out with the financial support of the Italian Nation-
al Research Council (CNR) and of the Ministry for
the University and the Scientific Research (MURST),
Rome.
Bis(ben zyloxy)d isu lfid e, P h CH2OSSOCH2P h . Mp: 58-
59 °C. 1H NMR (CDCl3) δ: 4.93 (d, 2H), 4.82 (d, 2H), 7.37 (s,
10 H). 13C NMR (CDCl3) δ: 76.58 (CH2), 128.16 (CH), 128.4
(CH), 128.59 (CH), 136.18 (C-quat). IR: ν 945, 910, 655. Anal.
Calcd for C14H14O2S2: C, 60.40; H, 5.07; S, 23.04. Found: C,
60.33; H, 5.02; S, 23.13.
J O952169E
(22) Closs, G. L.; Gautam, P.; Zhang, D.; Krusic, P. J .; Hill, S.;
Wasserman, E. J . Phys. Chem. 1992, 96, 5228.