Mechanism of sulfur transfer from 1,2,4-dithiazolidine-3,5-diones to triphenylphosphines

Article abstract of DOI:10.1002/poc.3131

The mechanism of sulfurization of substituted triphenylphosphines with 4-(3- and 4-substituted)-1,2,4-dithiazolidine-3,5-diones in acetonitrile, dichloromethane, tetrahydrofuran and toluene at 25°C was studied. The reaction pathway involves rate-limiting initial nucleophilic attack of the phosphorus at sulfur followed by fast decomposition of the phosphonium intermediate to the corresponding phosphine sulfide, phenylisocyanate and carbonylsulfide. From the Hammett correlations and from the solvent dependency, it was concluded that the transition-state structure is very polar and resembles the zwitter-ionic intermediate. The extent of P-S bond formation and S-S bond cleavage is very similar in the solvents series, but the latter gradually decreases with the decreasing polarity of the solvent. Copyright 2013 John Wiley & Sons, Ltd. The mechanism of sulfurization of substituted triphenylphosphines with 4-(3- and 4-substituted)-1,2,4-dithiazolidine-3,5- diones in acetonitrile, dichloromethane, tetrahydrofuran and toluene at 25°C was studied. The reaction pathway involves rate-limiting initial nucleophilic attack of the phosphorus at sulfur followed by fast decomposition of the phosphonium intermediate to the corresponding phosphine sulfide, phenylisocyanate and carbonylsulfide. The transition-state structure is very polar and resembles the zwitter-ionic phosphonium intermediate. Copyright

Full text of DOI:10.1002/poc.3131

Research Article
Received: 18 February 2013,
Revised: 28 March 2013,
Accepted: 4 April 2013,
Published online in Wiley Online Library: 7 May 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3131
Mechanism of sulfur transfer from 1,2,4-
dithiazolidine-3,5-diones to
triphenylphosphines
a
b
a
ꢀ
ꢀ
ꢀ
Oleksandr Ponomarov , Zdenka Padelková and Jirí Hanusek *
The mechanism of sulfurization of substituted triphenylphosphines with 4-(3- and 4-substituted)-1,2,4-
ꢀ
dithiazolidine-3,5-diones in acetonitrile, dichloromethane, tetrahydrofuran and toluene at 25 C was studied. The
reaction pathway involves rate-limiting initial nucleophilic attack of the phosphorus at sulfur followed by fast
decomposition of the phosphonium intermediate to the corresponding phosphine sulfide, phenylisocyanate and
carbonylsulfide. From the Hammett correlations and from the solvent dependency, it was concluded that the
transition-state structure is very polar and resembles the zwitter-ionic intermediate. The extent of P–S bond formation
and S–S bond cleavage is very similar in the solvents series, but the latter gradually decreases with the decreasing
polarity of the solvent. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: 1,2,4-dithiazolidine-3,5-diones; kinetics; mechanism; sulfurization; triphenylphosphines
studied in acetonitrile (ACN), dichloromethane (DCM), tetrahy-
INTRODUCTION
drofuran (THF) and toluene (TOL) at 25 ꢀC. Using 31P NMR, we
have found that the reaction of all 1,2,4-dithiazolidines 1a–g
with triphenylphosphines in CDCl3 and CD3CN proceeds
smoothly and the only products are the corresponding
triphenylphosphinsulphides together with phenylisocyanates
and carbonylsulfide which is consistent with previous[12,13,15–18]
results. Similar experiment with the least reactive triphenyl
phosphite used in our previous study[20] surprisingly showed
that the parallel oxidation of trivalent phosphorus occurs and
4-Substituted 1,2,4-dithiazolidine-3,5-diones are known from
1966 when Zumach and his co-workers prepared[1,2] a modest
series of compounds containing this skeleton. Ten years later,
Barany and Merrifield found the 1,2,4-dithiazolidine-3,5-dione
ring to be a very efficient orthogonal amino protecting group[3]
and since that time it has been extensively used in the synthesis
of peptides,[3–5] aminosugars in glycopeptides[6–10] and peptide
nucleic acids.[11] More recently 1,2,4-dithiazolidine-3,5-diones
were recognized as a nucleophilic isocyanate building block[12–14]
and used in the Mitsunobu reaction[15,16] or in the synthesis
of linear polyurethanes[17] and sulfonyl ureas.[18] The formation
of an isocyanate from 1,2,4-dithiazolidine-3,5-dione ring most
frequently involves sulfur transfer from the disulfide bridge to
triphenylphosphine[12,13,15–18] with simultaneous elimination of
carbonylsulfide. However, not only triphenylphosphine has to be
used for this purpose. Other P(III) compounds can also trigger the
reaction. For example, the sulfurization of phosphites with the par-
ent 1,2,4-dithiazolidine-3,5-dione was used for the efficient synthesis
of phosphorothioate-containing oligodeoxyribonucleotides.[19] The
1,2,4-dithiazolidine-3,5-diones can therefore be considered as a suit-
able sulfurization agent of various P(III) compounds. The sulfurization
efficiency of 4-phenyl-1,2,4-dithiazolidine-3,5-dione towards various
P(III) compounds was recently[20] evaluated and compared with
other known sulfurization agents, but the detailed mechanistic study
of this reaction has not been published until now. Therefore, we
decided to explore the kinetics and detailed mechanism for the
reaction of 4-(3- and 4-subst. phenyl)-1,2,4-dithiazolidine-3,5-diones
(1a–g) with triphenylphosphines (Scheme 1).
the product therefore contains
a mixture of the desired
triphenyl phosphorothioate (d(CDCl3) = 53.7 ppm)[21] and
triphenyl phosphate (d(CDCl3) = ꢁ17.3 ppm)[22] in the ratios 1:3
– 1:5 depending on the water content (based on integration
of peaks at 53.7 ppm and ꢁ17.3 ppm in 31P NMR spectrum).
This result could not be caused only by dissolved oxygen in
deuterated solvent or by the presence of oxygen in the air
above the sample in the NMR tube during measurement
because the triphenyl phosphite itself does not undergo oxida-
tion under these conditions. In order to avoid complications
with undesirable oxidation, we performed kinetic study with
triphenylphosphines only. The position of the transition state
* Correspondence to: J. Hanusek, Institute of Organic Chemistry and Technol-
ogy, Faculty of Chemical Technology, University of Pardubice, Studentská
573, CZ-532 10 Pardubice, The Czech Republic.
E-mail: jiri.hanusek@upce.cz
a O. Ponomarov, J. Hanusek
Institute of Organic Chemistry and Technology, Faculty of Chemical Technol-
ogy, University of Pardubice, Studentská 573, CZ-532 10, Pardubice, The Czech
Republic
RESULTS AND DISCUSSION
ꢀ
b Z. Padelková
Sulfurization reactions of triphenylphosphines with 4-(3- and
4-subst. phenyl)-1,2,4-dithiazolidine-3,5-diones (1a–g) were
Department of General and Inorganic Chemistry, Faculty of Chemical Technology,
University of Pardubice, Studentská 573, CZ-532 10, Pardubice, The Czech Republic
J. Phys. Org. Chem. 2013, 26 560–564
Copyright © 2013 John Wiley & Sons, Ltd.

MECHANISM OF SULFUR TRANSFER FROM 1,2,4-DITHIAZOLIDINE-3,5-DIONES TO TRIPHENYLPHOSPHINES
Scheme 1. Reaction of 4-(3- and 4-subst. phenyl)-1,2,4-dithiazolidine-
3,5-diones (1a–g) with P(III) compounds
on the reaction coordinate and the charge distribution in the
transition state can be determined from studies of the effect
of solvent polarity on reaction rates and from studies of the
transmission of substituent effects. Therefore, the kinetics of
the sulfurization reaction were determined by monitoring the
decrease in concentration of 1a–e spectrophotometrically at
336 nm in the above-mentioned solvents in the presence of
Figure 2. Hammett plot for sulfurization of triphenylphosphine with 1a-e
excess triphenylphosphine. The absorbance decreased exponen-
tially with time from which was obtained a pseudo-first order
rate constant which varied linearly with the concentration of
the substituted triphenylphosphine (as an example see Fig. 1
for ACN) to give the corresponding second-order rate constant
(k in Table 1).
From the data in Table 1, it is obvious that the second-order
rate constants show a significant dependence on solvent polarity
which indicates development of substantial charges in the
transition state. Such observations are completely different[20]
to what was observed in the case of structurally similar 1,2,4-
dithiazole-5-ones and 5-thiones where the influence of solvent
in acetonitrile (●), dichloromethane (■), tetrahydrofuran (○) and toluene
(□) at 25 ꢀC
polarity on the rate of sulfurization was only negligible. From the
data in Table 1, it is also possible to plot corresponding Hammett
correlations (Fig. 2) in each solvent whose positive-slopes reflect
the negative charge developing on the leaving sulfur in the
transition state.
These Hammett r-values are only slightly dependent on a
solvent polarity, but their absolute values (rACN = 2.49 ꢂ 0.17,
r
DCM = 2.33 ꢂ 0.11,
rTHF = 2.16 ꢂ 0.11,
rTOL = 1.90 ꢂ 0.16) are
higher than those found for the reaction of related 3-(3- and 4-
subst. phenyl)-1,2,4-dithiazole-5-ones and 5-thiones with
triphenyl phosphite[20] (r = 1.1–1.6) or triphenylphosphine
(r = 1.7; for source data see Table 10 in Supporting Information).
This confirms the proposal that the transition state structures
existing on the reaction coordinate are more polarized. It
appears that the polarization of S–S bond in the transition state
is enhanced by the increasing polarity of the solvent as seen
from an increase in r-values changing from the lowest value
found in non-polar TOL to the highest value found in strongly
polar ACN. In other words, the S–S bond cleavage in the transi-
tion state is more advanced in polar ACN than in non-polar TOL.
To get a better insight into the reaction mechanism, especially
into the extent of P–S bond formation in the transition state, we
also studied the transmission of polar substituent effects in
reaction of 4-phenyl-1,2,4-dithiazolidine-3,5-dione (1a) with
substituted triphenylphosphines. Again, it was possible to deter-
mine second-order rate constant (k in Table 2) from the slopes of
linear dependences of kobs vs. c(4-X-Ph3P) and plot these
constants against the sum of Hammett sp-constants (Fig. 3).
Figure 1. Dependence of the observed rate constant (kobs) on the
triphenylphosphine concentration (cPh3P) for the reaction of 1a (●), 1b
(■), 1c (▲), 1d (□, inset) and 1e (○, inset) in acetonitrile at 25 ꢀ
C
Table 1. Second-order rate constants (k) for the reactꢀion of 4-(3- and 4-subst. phenyl)-1,2,4-dithiazolidine-3,5-diones (1a–e)a with
triphenylphosphine in ACN, DCM, THF and TOL at 25 C
Compound
k(ACN) (l ꢃ mol–1 ꢃ s–1)
k(DCM) (l ꢃ mol–1 ꢃ s–1)
k(THF) (l ꢃ mol–1 ꢃ s–1)
k(TOL) (l ꢃ mol–1 ꢃ s–1)
1a
1b
1c
1d
1e
33.9 ꢂ 0.6
20.2 ꢂ 0.1
3.51 ꢂ 0.06
6.30 ꢂ 0.22
48.9 ꢂ 0.6
3.62 ꢂ 0.12
0.828 ꢂ 0.018
1.27 ꢂ 0.03
11.35 ꢂ 0.14
18.2 ꢂ 0.2
1.40 ꢂ 0.02
0.31 ꢂ 0.01
0.44 ꢂ 0.01
3.16 ꢂ 0.10
4.72 ꢂ 0.19
11.0 ꢂ 1.4
20.6 ꢂ 0.2
191.0 ꢂ 2.8
434.8 ꢂ 33.9
120.0 ꢂ 1.3
aReaction with derivatives 1g,h was too fast even for stopped-flow measurements.
J. Phys. Org. Chem. 2013, 26 560–564
Copyright © 2013 John Wiley & Sons, Ltd.
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ꢀ
O. PONOMAROV, Z. PADELKOVÁ AND J. HANUSEK
Table 2. Second-order rate constants (k) for the reaction of 4-phenyl-1,2,4-dithiazolidine-3,5-dione (1a) with tris(4-subst.phenyl)
ꢀ
phosphines in ACN, DCM, THF and TOL at 25 C
(4-X-Ph)3P X
k(ACN) (l ꢃ mol–1 ꢃ s–1)
k(DCM) (l ꢃ mol–1 ꢃ s–1)
k(THF) (l ꢃ mol–1 ꢃ s–1)
k(TOL) (l ꢃ mol–1 ꢃ s–1)
OCH3
CH3
H
9488 ꢂ 389
1070 ꢂ 49
33.9 ꢂ 0.6
1.20 ꢂ 0.04
5601 ꢂ 207
2367 ꢂ 99
699 ꢂ 18
550 ꢂ 13
120 ꢂ 2
46.7 ꢂ 1.6
20.2 ꢂ 0.1
3.62 ꢂ 0.12
0.338 ꢂ 0,008
1.40 ꢂ 0,02
Cl
0.619 ꢂ 0.025
0.0925 ꢂ 0.0010
leaving sulfur is close to unit value (+1 and ꢁ1, respectively) and
the transition structure closely resembles the structure of
zwitter-ionic intermediate T in Scheme 2.
The great difference between the structures of these late tran-
sition states found here and the early transition states recently
found[20,23] for the related 1,2,4-dithiazole-5-one and 5-thione
derivatives is probably caused by the poorer leaving ability of
the sulfur. The slower S–S bond cleavage presumably occurs
because there is some cross-conjugation[27] between the leaving
sulfur/nitrogen lone pairs and the carbonyl group (see Scheme 2).
Such competition shifts the position of transition state on the
reaction coordinate close to the highly polarized zwitter-ionic
intermediate
T together with substantial rate retardation
(cf. value k(1a) = 33.9 l ꢃ mol–1 ꢃ s–1 from Tables 1 and 2 with
value k = 1.14 ꢃ 103 l ꢃ mol–1 ꢃ s–1 found[20] for the reaction of 3-
phenyl-1,2,4-dithiazole-3-thione with triphenylphosphine in
ACN). Further evidence for efficient conjugation of the nitrogen
lone pair with carbonyl group(s) comes from X-ray
crystallographic analysis of compound 1f. The ORTEP diagram
(Fig. 4) shows that the 1,2,4-dithiazolidine ring in 1f is
essentially planar (torsion angles C1–S1–S2–C2 and S1–C1–N1–
C2 are less than 0.54(7)ꢀ) and the plane of the benzene ring is
strongly deviated from the plane of the parent heterocycle
(dihedral angle is 63.4ꢀ. This observation supports the idea of
efficient nitrogen lone pair delocalization with both carbonyl
groups and no conjugation with benzene ring. Although a high
degree of nitrogen lone pair conjugation is observed, the C–S
and S–S bonds being attributed to be a single one similarly as
in another four examples of 1,2,4-dithiazolidine-3,5-diones
found in the literature.[13,17,28,29] Typical values of all bond
lengths were taken from Ref..[30]
Figure
3. Hammett plot for sulfurization of 4-substituted
triphenylphosphines with 1a in ACN (●), DCM (■), THF (○) and TOL (□) at 25 ꢀC
From Fig. 3, it is clear that the slopes of Hammett correlations in
individual solvents are virtually the same (rACN = ꢁ2.66 ꢂ 0.17,
rDCM = ꢁ2.69 ꢂ 0.15, rTHF = ꢁ2.55 ꢂ 0.15, rTOL = ꢁ2.63 ꢂ 0.27) but
their absolute values are again much higher than those obtained
for reaction of related 3-(3- and 4-subst. phenyl)-1,2,4-dithiazole-
5-ones and 5-thiones with triphenyl phosphites[20] (r = ꢁ1.07 and
ꢁ1.28) or triphenylphosphines (r = ꢁ1.63). Even smaller
values were found for the reaction of 3-amino-1,2,4-dithiazole-5-
thione and 3-dimethylamino-1,2,4-dithiazole-5-thione with
triphenylphosphines (r = ꢁ0.86 and ꢁ0.92)23. On the other hand,
a high negative Hammett r-value[24,25] was also found for the reac-
tion of m- and p-substituted triphenylphosphines with elemental
sulfur in benzene and TOL (r = ꢁ2.5) for which a highly polarized
transition state was proposed.
The interpretation of the Hammett r-value in terms of
absolute charge distribution in the transition state requires a
reference reaction, ideally a corresponding r-value for an
equilibrium reaction. Unfortunately, the only equilibrium studied
for P(III) compounds concerns protonation of phosphines in
nitromethane which gave[26] Taft r* = ꢁ2.6. The polarity of nitro-
methane in terms of relative permittivity (er = 38.97) is virtually
the same as for ACN (er = 37.50) from which it can be concluded
that the absolute charge on attacking phosphorus as well as on
Figure 4. ORTEP view of compound 1f (thermal ellipsoids at 40% prob-
ability level). Selected interatomic distances [Å]; angles and interplanar
angle [ꢀ]: C1 S1 1.7715(15), S1 S2 2.0621(5), S2 C2 1.7608(13), C2 N1
1.382(2), N1 C1 1.3914(19), O1 C1 1.1994(19), O2 C2 1.206(2); C1 S1 S2
95.85(5), S1 S2 C2 95.76(6), S2 C2 N1 113.84(11), C2 N1 C1 121.50(12),
N1 C1 S1 113.04(11) and interplanar angle of the phenyl ring versus
the heterocyclic ring is 63.4
Scheme 2. Structure of the zwitter-ionic intermediate T formed during
sulfurization of triphenylphosphines with 4-(3- and 4-subst. phenyl)-
1,2,4-dithiazolidine-3,5-diones (1a–g)
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J. Phys. Org. Chem. 2013, 26 560–564

MECHANISM OF SULFUR TRANSFER FROM 1,2,4-DITHIAZOLIDINE-3,5-DIONES TO TRIPHENYLPHOSPHINES
Compound 1f
EXPERIMENTAL
C8H4N2O4S2, M = 256.25, monoclinic, P 21, a = 7.8220(3), b = 7.8601(2),
Syntheses
c = 7.9299(3) Å, b = 92.726(3) ꢀ, Z = 2, V = 486.99(3) Å3, Dc = 1.748 g ꢃ cm–3
,
All the 1,2,4-dithiazolidine-3,5-diones (1a–g) were synthesized and
purified according to published general procedures[31] in two steps from
phenyl isothiocyanate (see Supporting Information).
m = 0.545 mm–1, Tmin/Tmax = 0.887/0.932; –9 ≤ h≤ 10, –10 ≤ k≤ 8, –8 ≤ l ≤ 10;
3535 reflections measured (Ymax = 27.5 ꢀ), 3515 independent (Rint
=
0.0242), 1875 with I > 2s(I), 145 parameters, S = 1.090, R1(obs.
data) = 0.0194, wR2(all data) = 0.0497; max., min. residual electron
density = 0.270, –0.191 eǺ–3
.
In the crystal packing of 1f (Fig. 4), no interesting interatomic
interactions were observed except of non-classical H-bonding
and short contacts between C–H ꢃ ꢃ ꢃ O and N = O ꢃ ꢃ ꢃ S–S atoms or
moieties, respectively.
Kinetic measurements
The kinetic measurements were carried out on a Diode Array Stopped-Flow
SX.18 MV-R (Applied Photophysics) spectrophotometer or on a Hewlett
Packard HP 8453 Diode Array spectrophotometer at 25 ꢀC in 1 cm closable
cells under pseudo-first order conditions (triphenylphosphines in large
excess) in ACN, DCM, THF and TOL. The observed pseudo-first order rate
constants kobs were calculated from the measured time dependence
of the absorbance at 336 nm with the help of an optimization program
(See Tables 3–9 in Supporting Information). In all kinetic runs, the stan-
dard deviation in the fit was always less than 5% of the quoted value and
was more usually between 0.2 and 0.4% of the quoted value. Solvents
used for kinetic measurements were of HPLC quality and were dried
and distilled under argon prior to use. All triphenylphosphines were
from commercial sources and were used as received because the
amount of corresponding oxidation product or another phosphorus-
containing compound was only negligible according to 31P NMR
spectrum. Due to potential oxidation of triphenylphosphines, all the
solutions were freshly prepared just before kinetic measurements.
SUPPORTING INFORMATION
Supporting information may be found in the online version of
this article.
Acknowledgement
The authors thank the Ministry of Education, Youth and Sports of
the Czech Republic for the institutional support.
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Þ
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ꢂ
ꢃ
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2
2
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h ꢀ
ꢁ ꢀ
ꢁi
2
2
2
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Þ
= ΣwðFo
Þ
§for all data:
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J. Phys. Org. Chem. 2013, 26 560–564
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