Thermochemical properties of dibenzyloxy disulfides

Article abstract of DOI:10.1016/j.tca.2012.10.028

The in situ thermal behavior of dialkoxy disulfides (OSSO) has been previously explored, however their neat thermal stability has yet to be examined. We synthesized a library of ten dibenzyloxy disulfide derivatives with various para-substituents. Each derivative was analyzed by TGA and DSC to discern molecular fragmentation. A correlation of the pattern of fragmentation to Swain and Lupton's R-value was observed. Also, DSC analysis revealed that when the para-substituent was a phenyl (i.e. bis(p-phenylbenzyloxy) disulfide), and successive runs were performed, the thermogram showed the presence of the fragmentation, and upon ¹H NMR analysis its corresponding alcohol and aldehyde were observed.

Full text of DOI:10.1016/j.tca.2012.10.028

Thermochimica Acta 551 (2013) 99–103
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Thermochemical properties of dibenzyloxy disulfides
Eric G. Stoutenburgb, Anne E. Palermob, Ronny Priefera,b,∗
a College of Pharmacy, Western New England University, Springfield, MA 01119, USA
b Department of Chemistry, Biochemistry, and Physics, Niagara University, NY 14109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 August 2012
Received in revised form 25 October 2012
Accepted 29 October 2012
Available online 16 November 2012
The in situ thermal behavior of dialkoxy disulfides ( OSSO ) has been previously explored, however
their neat thermal stability has yet to be examined. We synthesized a library of ten dibenzyloxy disul-
fide derivatives with various para-substituents. Each derivative was analyzed by TGA and DSC to discern
molecular fragmentation. A correlation of the pattern of fragmentation to Swain and Lupton’s R-value
was observed. Also, DSC analysis revealed that when the para-substituent was a phenyl (i.e. bis(p-
phenylbenzyloxy) disulfide), and successive runs were performed, the thermogram showed the presence
of the fragmentation, and upon 1H NMR analysis its corresponding alcohol and aldehyde were observed.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Dialkoxy disulfides
TGA
DSC
Fragmentation
Substituent constants
1. Introduction
autocatalyzed decomposition while in solution and irradiated
under UV-light in a manner depicted in Scheme 1 [14].
Although the dialkoxy disulfide was first synthesized by
Lengfeld in 1895 [1], very little has been reported on this func-
tionality until recently [2–11]. Whether the moiety existed as the
linear arrangement ( OSSO ), or as its isomeric form, thionosul-
fite ( OS( S)O ), was not confirmed until 1997 [3]; however, it
has been reported to exist as the latter thionosulfite when in cyclic
structures [4]. Due to the ability of a lone pair of electrons from one
of the sulfurs to donate into the ꢀ* orbital of the adjacent S O bond,
Thermally, benzylic [7] or cubylcarbinolic [8] dialkoxy disulfides
have been heated in solution in the presence of dienes to afford
cyclic di- and tetra-sulfides, in a pseudo-Diels-Alder reaction pre-
sumably via the liberation of S2. Harpp had proposed [11], and
we have further validated [15], a cage-based mechanism for this
process (Scheme 2).
Although a number of studies have been conducted on the
thermolytic behavior of dibenzyloxy disulfides in solution, no
work has been reported on their thermal profile while neat.
Herein, we report the synthesis of ten dibenzyloxy disulfides and
their degradative properties when examined by thermal gravi-
metric analysis (TGA) as well as differential scanning calorimetry
(DSC).
˚
the S S bond is unusually short (ca. 1.95 A) [5] and has an enhanced
˚
barrier of rotation of ca. 18 kcal/mol [6], compared to ∼2.05 A and
∼8 kcal/mol for simple disulfides [12]. In addition, although thio-
sulphurous acid (HOSSOH) has been successfully synthesized at
−70 ◦C, it readily decomposes at 20 ◦C to afford SO2 [13], and no
salts of this functionality have been reported.
The uniqueness of this functional group has been explored
both photo- and thermolytically. Lunazzi and Placucci reported
that under photolytic conditions, the alkoxy radicals could be
formed from the dialkoxy disulfides, which were subsequently
reacted with fullerenes [9]. In addition, Harpp and coworkers
observed the conversion of bis-cubylmethyl-dialkoxy disulfide to
bis-cubylmethyl sulfite with ambient light while open to the air
[8]. Dibenzyloxy disulfides have also been shown to go through
2. Experimental
2.1. Materials, preparation and characterization of compounds
All chemicals were reagent grade with the exception of
4-phenoxybenzyl alcohol which was synthesized according to pub-
lished procedure [16]. 1H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were recorded on a Varian instrument with solvent indi-
cated with tetramethylsilane (TMS) as an internal standard and
compared to authentic samples. Chemical shifts (ı) are given in
ppm, coupling constants (J) in Hz, and spin multiplicities as s (sin-
glet), d (doublet), ABq (AB quartet) and m (multiplet). Dibenzyloxy
disulfides 8–10 are new chemical entities and full characterization
∗
Corresponding author at: College of Pharmacy, Western New England Univer-
sity, Springfield, MA 01119, USA. Tel.: +1 413 796 2438; fax: +1 413 796 2266.
E-mail address: ronny.priefer@wne.edu (R. Priefer).
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tca.2012.10.028

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E.G. Stoutenburg et al. / Thermochimica Acta 551 (2013) 99–103
Scheme 1. Priefer’s proposed mechanism of the photolytic, autocatalyzed decomposition of dibenzyloxy disulfides [13].
including HRMS recorded on a ThermoFinnigan MAT 95XL with ESI
II source for electrospray were performed.
Bis(4-trifluorobenzyloxy) disulfide (8, 0.17 g, 68%) as clear liquid.
1H NMR (400 MHz, CDCl3): ı 4.85, 4.96 (ABq, J = 12.0 Hz, 4H), 7.44
(d, J = 7.2 Hz, 4H) 7.62 (d, J = 7.2 Hz, 4H). 13C NMR (100 MHz, CDCl3)
ı 75.7, 124.0 (J = 272.1 Hz), 125.5 (J = 3.8 Hz), 128.4, 130.6 (J = 32.2)
140.4. Exact mass calculated for C16H12O2F6S2 414.0177, found
414.01779.
2.2. Representative synthesis of a dibenzyloxy disulfide
Bis(4-nitrobenzyloxy) disulfide (1): p-Nitrobenzyl alcohol (0.25 g,
1.63 mmol) was dissolved in anhydrous CH2Cl2 under N2. Triethyl-
amine (0.227 mL, 1.63 mmol) was added and the resulting solution
was cooled to 0 ◦C. S2Cl2 (65.3 ␮L, 0.82 mmol) was added dropwise
over 20 min. The solution was stirred at 0 ◦C for 2 h before being
allowed to equilibrate to room temperature for 3 h. The reaction
was quenched with dH20, washed with 2 × 20 mL aliquots of brine.
The aqueous phase was extracted with CH2Cl2 (2 × 10 mL), and the
combined organic was dried over MgSO4, filtered, and concentrated
under reduced pressure. Column chromatography with a 2.5:1 ratio
of hexanes:ethyl acetate afforded:
Bis(4-cyanobenzyloxy) disulfide (9, 0.12 g, 67%) as white-yellow
solid mp. 27–29 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.84, 4.96 (ABq,
J = 12.6 Hz, 4H), 7.44 (d, J = 7.6 Hz, 4H) 7.44 (d, J = 7.2 Hz, 4H).
13C NMR (100 MHz, CDCl3) ı 75.4, 112.0, 118.6, 128.6, 132.4,
141.6. Exact mass calculated for C16H12O2N2S2 328.0335, found
328.03448.
Bis(4-phenoxybenzyloxy) disulfide (10, 0.65 g, 88%) as off white
solid mp. 24–26 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.77, 4.88 (ABq,
J = 11.4 Hz, 4H), 6.98 (d, J = 8.4 Hz, 4H), 7.01 (d, J = 7.6 Hz, 4H), 7.13
(t, J = 7.2 Hz, 2H), 7.33 (m, 8H). 13C NMR (100 MHz, CDCl3) ı 76.3,
118.6, 119.1, 123.5, 129.8, 130.5, 131.3, 156.8, 157.7. Exact mass
calculated for C26H22O4S2 462.0954, found 462.09551.
Bis(p-nitrobenzyloxy) disulfide (1, 0.23 g, 93%) as an off white
solid mp. 102–104 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.89, 5.01 (ABq,
J = 12.6 Hz, 4H), 7.49 (d J = 8.8 Hz, 4H), 8.22 (d, J = 8.8 Hz, 4H). 13C
NMR (100 MHz, CDCl3) ı 75.1, 123.8, 128.7, 143.6, 147.9.
2.3. Measurements
Bis(4-phenylbenzyloxy) disulfide (2, 0.23 g, 90%) as white solid.
mp.105–107 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.85, 4.96 (ABq,
J = 11.4 Hz, 4H), 7.35 (t, J = 7.6 Hz, 2H), 7.43 (d, J = 8.0 Hz, 4H), 7.59
(m, 12H). 13C NMR (100 MHz, CDCl3) ı 76.5, 127.1, 127.3, 127.5,
128.8, 129.2, 135.5, 140.6, 141.4.
Bis(4-chlorobenzyloxy) disulfide (3, 0.23 g, 92%) as off white solid
mp. 102–104 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.75, 4.86 (ABq,
J = 11.4 Hz, 4H), 7.30 (d, J = 7.2 Hz, 4H), 7.33 (d, J = 7.2 Hz, 4H). 13C
NMR (100 MHz, CDCl3) ı 75.8, 128.8, 130.0, 134.4, 134.9.
Bis(4-methoxybenzyloxy) disulfide (4, 0.19 g, 76%) as white solid.
mp. 20–22 ◦C. 1H NMR (400 MHz, CDCl3): ı 3.81 (s, 6H), 4.73, 4.84
(ABq, J = 11.0 Hz, 4H), 6.88 (d, J = 8.4 Hz, 4H), 7.24 (d, J = 8.4 Hz, 4H).
13C NMR (100 MHz, CDCl3) ı 55.3, 76.5, 113.9, 128.8, 130.5, 159.8.
Bis(4-methylbenzyloxy) disulfide (5, 0.21 g, 84%) as off white solid
mp. 25–27 ◦C. 1H NMR (400 MHz, CDCl3): ı 2.35 (s, 6H), 4.76, 4.87
(ABq, J = 11.2 Hz, 4H), 7.17 (d, J = 8.0 Hz, 4H), 7.24 (d, J = 8.0 Hz, 4H).
13C NMR (100 MHz, CDCl3) ı 21.3, 77.2, 128.8, 129.2, 133.6, 138.2.
Bis(4-hydrogenbenzyloxy) disulfide (6, 0.20 g, 80%) as off white
solid mp. 43–45 ◦C. 1H NMR (400 MHz, CDCl3): ı 4.80, 4.92 (ABq,
J = 11.4 Hz, 4H), 7.17 (m, 10H). 13C NMR (100 MHz, CDCl3) ı 76.8,
128.4, 128.5, 128.6, 136.6.
Thermal properties were examined by thermogravimetric anal-
ysis, TGA, with TA instruments TGA Q50 System in a N2 atmosphere
and heated at a rate of 10 K/min. The differential scanning calorime-
try, DSC, was run with a Perkin-Elmer Pyris 6 calorimeter in a N2
atmosphere. The solid sample was placed in an aluminum capsule,
heated and cooled at the rate of 5 K/min. All results were taken
from the multiple heating and cooling runs, detecting the level of
enthalpy change that is associated with a respective phase transi-
tion. The transition enthalpy, ꢁH (kJ/mol), was determined from
the peak area of the DSC thermogram. The transition entropy, ꢁS
(J/mol K), was calculated with the equation ꢁS = ꢁH/T, where T was
the transition temperature corresponding to the DSC maximum.
The thermodynamic data were mean values of several independent
measurements carried out on different samples.
3. Results and discussion
3.1. Synthesis
Previous work on dibenzyloxy disulfide focused on seven para-
substituted derivatives [14,15]. For this study we expanded this
to ten total dibenzyloxy disulfides. Synthesis of the library was
performed as previously described [14,15]. In short, two equiva-
lents of a desired commercially available benzyl alcohol (with the
Bis(4-tertbutylbenzyloxy) disulfide (7, 0.19 g, 72%) as clear liq-
uid. 1H NMR (400 MHz, CDCl3): ı 1.32 (s, 18H), 4.77, 4.89 (ABq,
J = 11.2 Hz, 4H), 7.29 (d, J = 8.0 Hz, 4H), 7.38 (d, J = 8.0 Hz, 4H). 13C
NMR (100 MHz, CDCl3) ı 31.3, 34.6, 76.7, 125.5, 128.6, 133.6, 151.6.
Scheme 2. Harpp’s proposed cage mechanism of thermolytic decomposition of dialkoxy disulfides [11].

E.G. Stoutenburg et al. / Thermochimica Acta 551 (2013) 99–103
101
Scheme 3. Synthesis of dibenzyloxy disulfides.
exception of 4-phenoxybenzyl alcohol which was synthesized from
its corresponding acid following published procedure [15]) with
two equivalents of Et3N and one equivalent of S2Cl2 were reacted
in CH2Cl2 for 2 h at 0 ◦C than 3 h at room temp (Scheme 3). Com-
pounds were purified via column chromatography and stored in a
freezer until use. Even when stored cold, slight decomposition was
occasionally observed, and these particular dibenzyloxy disulfide
were re-purified prior to any thermal studies.
3.2. Thermogravimetric analysis
In an effort to understand the thermolytic character of diben-
zyloxy disulfides with respect to the electron donating and
withdrawing groups, we initially examined them using TGA. Sev-
eral milligrams of each derivative were individually heated at
10 K/min to 1000 K or until a plateau in weight was reached and no
further degradation was observed (Table 1). Of the ten dibenzyloxy
disulfides that were evaluated, five (i.e. (1) NO2, (2) Ph, (6) H, (8) CN,
(9) CF3) had a single slope degradation profile with some residual
black charred material remaining on the pan. The remaining five
had either a two-step (i.e. (3) Cl, (5) Me, (7) tBu) or three-step (i.e.
(4) OMe, (10) OPh) degradation profile.
In an attempt to understand these observed degradation pat-
terns we initially examined Swain and Lupton’s F-value, which had
been used to explain rates of decomposition in situ [15]. Unfortu-
nately, no correlation was observed. However, utilization of Swain
and Lupton’s R-value [17] did prove successful (Table 2). It does
appear that the more negative the Swain and Lupton’s R-value is
for a given substituent, the more likely it is to undergo a stepwise
degradation. Between an R-value of −0.138 and −0.161 there is a
two-step degradation process. Above −0.500, the degradation is
three steps. On the other end, substituents that possess an R-value
greater than −0.088 have a single step degradation profile.
Fig. 1. Representative TGA curves for (1) NO2, (4) MeO, and (7) tBu.
1, 3 and 6 each had endotherms at 367, 318, and 320 K, ꢁH of
29.8, 38.1, and 19.5 kJ/mol, and ꢁS of 81.2, 119.8, and 60.9 J/mol K,
respectively. However, these three dialkoxy disulfides did not pro-
duce an exotherm upon cooling (Table 3). This is not surprising as
these temperatures were near the onset temperature for degrada-
tion from the TGA results (Table 1). When the aluminum capsules
were opened and NMR spectra were run on the material, no start-
ing dialkoxy disulfides were detected. The temperature, enthalpy,
and entropy of fusion of the dibenzyl disulfide analog of 6 has been
previously studied [18]. The melting temperature of dibenzyl disul-
fide was 342 K, compared to 320 K for 6. In addition, the ꢁH and
ꢁS was reported to be 44.7 kJ/mol and 130.8 J/mol K for dibenzyl
disulfide, which is significant elevated compared to compound 6’s
19.5 kJ/mol and 60.9 J/mol K, respectively. These data suggests that
the dialkoxy disulfide does not possess as strong intermolecular
forces, when in solid state, as its corresponding disulfide.
It appears that substituents that have the ability to donate elec-
tron density into the system have a stabilizing effect. Since both
the bis(methoxybenzyloxy) (4) and bis(phenoxybenzyloxy) (10)
derivatives have a low onset degradation temperature of 383 and
398 K, respectively, the substituents are most likely stabilizing the
daughter fragments rather than the starting dibenzyloxy disulfides.
In addition, regardless of whether the degradation was 2- or 3-step,
all five of these dibenzyloxy disulfides lost mass corresponding to
sulfur between 475 and 525 K (Fig. 1). As a comparison, diben-
zyl disulfide has previously been evaluated via TGA [18]. As with
dibenzyloxy disulfides 6, a one-step thermal decomposition was
observed, with an onset, inflection point, and end point at 509, 544,
and 570 K, respectively. The onset temperature is 111 K higher for
the dibenzyl disulfide than compound 6. However, the end point
is only 80 K higher. Unfortunately, these two compounds cannot
be perfectly compared since the reported dibenzyl disulfide was
heated under air, whereas ours were under a nitrogen atmosphere.
Of the compounds that were run via DSC, only dialkoxy disul-
fide 2 gave both an endotherm upon heating and an exotherm
upon cooling (Fig. 2). On the initial DSC cycle an endotherm at
3.3. DSC analysis
Only compounds 1, 2, 3, and 6 were able to be run via DSC
because from the synthesized library, they were the only solid com-
pounds with melting points above room temperature. Compounds
Fig. 2. DSC thermogram of bis(p-phenylbenzyloxy) disulfide (2).

102
E.G. Stoutenburg et al. / Thermochimica Acta 551 (2013) 99–103
Table 1
Temperature at onset, inflection and end for dibenzyloxy disulfides 1–10.
Compound
Extrapolated onset temperature
for degradation (K)
Temperature of inflection
point (K)
Extrapolated end temperature
for degradation (K)
Nitro (1)
Biphenyl (2)
Chloro (3)
410
403
418
469
383
485
633
403
462
398
423
486
380
393
398
653
755
448
438
447
488
408
514
675
426
497
437
457
507
441
441
479
725
823
533
467
464
513
425
569
789
454
536
490
482
540
532
484
570
743
883
Methoxy (4)
Methyl (5)
Hydrogen (6)
tert-butyl (7)
Cyano (8)
TriFluoro (9)
Phenoxy (10)
Table 2
Swain and Lupton’s R-values as comparison to # of degradation steps.
Compound #
10
Substituent
Swain and Lupton’s R-value
# of degradation steps
OPh
OMe
Cl
Me
tBu
Ph
−0.740
−0.500
−0.161
−0.141
−0.138
−0.088
0
0.155
0.184
0.186
3
3
2
2
2
1
1
1
1
1
4
3
5
7
2
6
1
8
9
H
NO2
CN
CF3
Table 3
Transition temperatures, T, enthalpies, ꢁH, and entropies, ꢁS for dibenzyloxy disulfides 1, 2, 3 and 6.
Compound
Heating
Cooling
T (K)
ꢁH (kJ/mol)
ꢁS (J/mol K)
T (K)
ꢁH (kJ/mol)
ꢁS (J/mol K)
Nitro (1)
Biphenyl (2)
367
381
377
373
369
365
318
320
29.8
43.2
31.6
25.2
19.6
18.4
38.1
19.5
81.2
113.4
83.8
67.6
53.1
50.4
119.8
60.9
365
360
355
349
342
−29.7
−24.1
−19.0
−17.7
−17.4
−81.4
−66.9
−53.5
−50.7
−50.7
Chloro (3)
Hydrogen (6)
381 K (ꢁH = 43.2 kJ/mol; ꢁS = 113.4 J/mol K) and an exotherm at
365 K (ꢁH = −29.7 kJ/mol; ꢁS = −81.4 J/mol K) were observed. This
heating/cooling was repeated four more times, and with each run
there was a decrease in the temperature of the endotherm, ꢁH,
and ꢁS upon heating. On the cooling runs the temperature of the
exotherm decreased, while the ꢁH and ꢁS increased. The changes
from run 1 to 5 are proposed to be due to the formation of a new
product; hence introducing an impurity into the mix, which should
lower and broaden the melting point. We have indeed observed
this phenomenon before, albeit with a cubane based molecule [19].
However, as indicated above, this particular structural moiety is
thermally sensitive and should fragment upon heating. Indeed,
what was observed upon post-DSC NMR was starting alcohol and
its corresponding aldehyde in an approximate 1:1 ratio.
was observed, whereby strongly electron-donating substituents
stabilize daughter fragments. In addition, the DSC profile of bis(p-
phenylbenzyloxy) disulfide (2) presented an unusual thermogram,
where with each successive run the decomposition introduced an
alcohol and aldehyde impurity at an approximate 1:1 ratio which
caused a shift and broadening of both the endo and exotherms.
Acknowledgments
ES, AEP, and RP would like to thank the Niagara University
Academic Center for Integrated Sciences, as well as the Rochester
Academy of Science for financial support. RP would also like to
thank Western New England University, College of Pharmacy for
generous financial support.
4. Conclusion
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