Synthesis and identification of solution-stable sulfenic acids: Perfluoroalkanesulfenic acids

Article abstract of DOI:10.1002/ejoc.201301563

Taking advantage of the strong electron-withdrawing effect of perfluoroalkyl groups, solution-stable perfluoroalkanesulfenic acids were synthesized for the first time by Cope-type elimination of the corresponding imines; the acids were identified by ¹H NMR, ¹⁹F NMR, and IR spectroscopy and mass spectrometry. Trapping reagents were utilized to capture the in situ generated sulfenic acids, which provided further experimental evidence for the formation of these fluorinated sulfenic acids. Owing to the strong electron-withdrawing effect of perfluoroalkyl groups (R _f), solution-stable perfluoroalkanesulfenic acids are synthesized by Cope-type elimination of the corresponding imines at 80 °C; the acids are identified by ¹H NMR,¹⁹F NMR, and IR spectroscopy and mass spectrometry. Trapping agents are used to capture the in situ generated sulfenic acids, which provides the experimental evidence. Copyright

Full text of DOI:10.1002/ejoc.201301563

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301563
Synthesis and Identification of Solution-Stable Sulfenic Acids:
Perfluoroalkanesulfenic Acids
Xiao-Bo Li,[a] Ze-Feng Xu,[a] Li-Juan Liu,[a] and Jin-Tao Liu*[a]
Keywords: Sulfenic acids / Fluorine / Reactive intermediates / Elimination / Rearrangement
Taking advantage of the strong electron-withdrawing effect
spectrometry. Trapping reagents were utilized to capture the
of perfluoroalkyl groups, solution-stable perfluoroalkanesulf- in situ generated sulfenic acids, which provided further ex-
enic acids were synthesized for the first time by Cope-type
elimination of the corresponding imines; the acids were iden-
tified by 1H NMR, 19F NMR, and IR spectroscopy and mass
perimental evidence for the formation of these fluorinated
sulfenic acids.
steric hindrance[4a–4f] (Figure 1, a) and intramolecular hy-
drogen bonding[5] (Figure 1, b), or a combination of both
of these factors[4g] (Figure 1, c), results in the great stability
of sulfenic acids (Scheme 1). Besides, strong electron-with-
drawing groups also give rise to stable sulfenic acids (Fig-
ure 1, d),[6] because these groups can sharply reduce the nu-
cleophilicity of the sulfur atom. However, the design and
synthesis of these kinds of sulfenic acids are rare. The spe-
cies in Figure 1d containing a tropylium cation[7] is the first
example of a stable sulfenic acid that does not bear a bulky
residue and that does not form stabilizing hydrogen bonds.
The authors concluded that the stability of the tropylium
cation supported the earlier proposal that the attenuated
nucleophilicity of the sulfenic acid moiety by electron-with-
drawing groups could increase the stability.
It is well known that perfluoroalkyl groups have a con-
siderable electron-withdrawing effect, which may stabilize
sulfenic acids. In fact, trifluoromethylsulfenic acid[8a] (Fig-
ure 1, e) was previously postulated as an intermediate in the
hydrolysis of ditrifluoromethyl disulfide, and pentafluo-
rophenylsulfenic acid[8b] (Figure 1, e) was detected as the
product of the elimination reaction of pentafluorophenyl
butyl sulfoxide. However, both of them were unstable under
their formation conditions and further studies of their
chemistry were not reported.
Introduction
Sulfenic acids (RSOH) have been implicated as key inter-
mediates in a wide variety of reactions, including biological
transformations, and they still attract great attention.[1] The
possibility that sulfenic acids may regulate the catalytic ac-
tivity of certain enzyme systems provides more incentive for
understanding their chemistry. The difficulty in elucidating
the fundamental chemistry of these species results not only
from their high reactivity but also from the scarcity of
methods for their preparation.[2] In the absence of suitable
trapping agents, sulfenic acids generally undergo self-con-
densation to give thiosulfinates and water. The reaction is
promoted by dimerization through intermolecular hydrogen
bonding. Then, the thiosulfinates are further converted into
thiosulfonates and disulfides through redox conversion
(Scheme 1).[3] Therefore, the chemistry of sulfenic acids and
the mechanism of their reactions is derived indirectly from
rationalization of the final products.[1f]
Scheme 1. Self-condensation of sulfenic acids.
Even though several stable sulfenic acids have been syn-
thesized, few of them are suitable for directly studying the
chemistry of the sulfenic group (–SOH) in traditional ways
because of troubles in preparation, multireactive sites in the
molecules, and huge steric hindrance. As a result, the explo-
ration of the chemistry of sulfenic acids is still fragmentary.
In this context, we report a new type of solution-stable
alkanesulfenic acids that only bear an electron-withdrawing
perfluoroalkyl group without bulky residues. These kinds
of sulfenic acids are stable in solution and can be charac-
Many efforts have been made to synthesize and isolate
stable sulfenic acids. Representative examples from pervious
research are illustrated in Figure 1. It can be concluded that
[a] Key Laboratory of Organofluorine Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Science
345 Lingling Road, 200032 Shanghai, China
E-mail: jtliu@sioc.ac.cn
http://liujintao.sioc.ac.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301563.
1
terized by H NMR, 19F NMR, and IR spectroscopy and
mass spectrometry (MS).
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Synthesis and Identification of Solution-Stable Sulfenic Acids
Figure 1. Representative sulfenic acids; Phth = phthalyl.
Results and Discussion
In our preliminary experiments, perfluoroalkanesulfin-
imine 1a (0.1 m in toluene) was treated with 1,3-cyclopenta-
diene (2, 10.0 equiv.) at 80 °C to study its Diels–Alder reac-
tion in a sealed tube. However, rather than the predicted
Diels–Alder products, isomers 3a and 3b were obtained in
a total yield of 82%. Interestingly, if 1b or 1c derived from
a different aldehyde was used instead of 1a, the same prod-
ucts were obtained (Scheme 2); this indicates that the same
intermediate was formed from 1a, 1b, and 1c in these reac-
tions and that it was trapped by the dimer of 2 to form 3.
The structures of 3a and 3b were determined by spec-
troscopy, MS, and elemental analysis (see the Supporting
Information). Furthermore, the X-ray crystal structure of
3b was obtained (Scheme 2).[9]
Scheme 2. Reactions of imines 1 with 2.
Davis and co-workers[10] previously reported that N-alk- 8.04 ppm disappeared, and a broad peak appeared at δ =
1
ylidenearylsulfinamides underwent Cope-type elimination 3.91 ppm in the H NMR spectrum of 5a; this indicates
to yield arylsulfenic acids that could be trapped by activated that an active proton was produced after heating at 80 °C
olefins and alkynes through a reversible six-electron sigma- (Scheme 3, a). The 19F NMR spectra illustrated the disap-
tropic rearrangement. On the basis of literature pre- pearance of chirality (AB signals disappeared) on the sulfur
cedent,[1a,10] if imine 1 is heated in toluene at 80 °C, it may atom in 1d and the appearance of a new triplet signal (δ =
first undergo the same procedure to give nonafluoro-n- –102.45 ppm, t, J = 9.5 Hz, 2 F; Scheme 3, b). There were
butanesulfenic acid 5a, which can react with the dimer of characteristic bands of the –SOH group at 3236 (O–H) and
1,3-cyclopentadiene to produce 3 through the six-electron 813 cm–1 (S–O) in the IR spectrum of 5a. Furthermore,
sigmatropic rearrangement, as depicted in Scheme 2.
characteristic signals for the molecular ions of 5a and corre-
Comparison of the 1H NMR spectra of imine 1d and 5a, sponding thiosulfonate 6 as well as fragment 7 were found
which was prepared by heating imine 1d in C6D6 overnight, in the ESI-MS of 5a at m/z = 267.0 [M5a – H], 534.9 [M6
revealed that the sharp single resonance of Ha in 1d at δ = + H], and 299.0 (fragment 7), respectively (Scheme 3, c). It
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SHORT COMMUNICATION
1
Scheme 3. H NMR, 19F NMR, and mass spectra analysis.
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Synthesis and Identification of Solution-Stable Sulfenic Acids
Scheme 4. Experimental evidence for the formation of 5a.
is clear that 5a underwent a self-condensation process,
Under the same conditions, 2-chlorotetrafluoroeth-
which is the typical reaction in sulfenic acid chemistry. Ac- anesulfenic acid (5b) and tridecylfluoro-n-hexanesulfenic
cordingly, 5a can be determined as nonafluoro-n-butane- acid (5c) were prepared in high yields upon heating the cor-
sulfenic acid.
responding imines in toluene at 80 °C overnight (Table 1).
As additional evidence, imine 1e was synthesized and Both of them were stable in toluene if the concentration did
heated in toluene for 3 days, and nitrile 4d was isolated in not exceed 0.1 m.
54% yield. Moreover, compounds 9a and 9b, the addition
Table 1. Synthesis of perfluoroalkanesulfenic acids 5.
products of 5a and 4d, were isolated in 8 and 4% yield,
respectively, which also indicated the formation of sulfenic
acid 5a in this transformation (Scheme 4, a). To further
confirm the formation of 5a, several sulfenic acid trapping
reagents were then used to capture it, as illustrated in
Scheme 4 (b–e).
A mixture of imine 1d and ethyl propiolate was heated
in toluene at 80 °C overnight to give sulfoxides 10a and 10b
in 93% yield with a ratio of 1:0.32, and upon adding ethyl
propiolate to a solution of 5a, preprepared by heating imine
1d in toluene overnight, the same products were obtained
in 83% yield with a ratio of 1:0.46 (Scheme 4, b). However,
if phenylacetylene was used instead of ethyl propiolate, only
product 11 was obtained selectively in 88% yield (Scheme 4,
c). Another typical sulfenic acid trapping agent, that is,
Entry
1
R
Rf
5
Yield [%][a]
1
2
3
4
1a
1d
1f
iBu
tBu n-C4F9
tBu CClF2CF2
iPr
n-C4F9
5a
5a
5b
5c
94
94
92
91
1g
n-C6F13
[a] Determined by 19F NMR spectroscopy by using p-BrC6H4CF3
as an internal standard.
We found that as a result of the presence of perfluoro-
benzenethiol, also captured sulfenic acid 5a to afford di- alkyl groups, 5 showed some special properties that were
sulfide 12 in 88% yield (Scheme 4, d). In addition, upon different from those of nonfluorinated sulfenic acids
treatment with sulfuryl chloride and hexamethyldisilazane (Scheme 5). For example, 5a can be stored in toluene for
(HMDS) in sequence, 5a was converted into nonafluoro-n- more than four months at 0 °C or for days at 80 °C without
butanesulfinamide (Scheme 4, e). All of the above experi- any transformation. In contrast, tert-butanesulfenic acid
mental results indicate that sulfenic acid 5a was formed has a half-life of 3.6 min in benzene at 35 °C at a concentra-
upon heating imines 1 at 80 °C in toluene.
tion of 0.13 m.[11] It was also found that 5a is very sensitive
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SHORT COMMUNICATION
Scheme 5. Reactions of 5.
to water. Adding water to a solution of 5a in toluene ac- (Scheme 5, c). If dl-menthol was utilized, desired sulfenate
companied by severe stirring resulted in the formation product 18 was obtained along with defluorination prod-
of 2,2,3,3,4,4,4-heptafluorobutanoic acid (13) in 2 h ucts 19 and 20 (Scheme 5, d). Unfortunately, 5b did not
(Scheme 5, a). This is also inconsistent with tert-butanesulf- react with ordinary electrophiles such as MeI, BnBr, and
enic acid, which is stable in water.[11] It was reported that BnOTf (Tf = trifluoromethanesulfonyl, Scheme 5, e).
both acids and bases can accelerate the self-condensation
All of the above results indicate that perfluoroalkanesulf-
reaction of tert-butanesulfenic acid. However, acids (such enic acids 5 have some properties in common with ordinary
as CH3COOH, n-C5H11COOH, CF3COOH, dilute HNO3, sulfenic acids, such as the electrophilicity and reactivities to
H2SO4, and H3PO4) have no clear effect on the stability of C=C and CϵC bonds. Moreover, because of the presence
5a, although some bases (5% NaOH aq., 5% NaHCO3 aq., of a perfluoroalkyl group, acids 5 also bear some special
Et3N) can accelerate the decomposition of 5a. Hydrogen properties. For example, they are thermally stable in aro-
halides HX (X = Cl, Br) can convert 5a into corresponding matic solvents and are easy to defluorinate, probably
sulfenyl halides 14 easily (Scheme 5, b), which is in accord through fluorinated oxanthiirane intermediate 21,[12] as
with nonfluorinated sulfenic acids. Under both basic and proposed in Scheme 6.
acidic conditions, no self-condensation products of 5a were
observed.
The influence of solvents on the stability of 5a was also
investigated. If another solvent was added to a solution of
5a in toluene (Vsolvent/Vtoluene = 1:1), a different phenome-
non was observed. DMSO and acetonitrile made 5a decom-
pose to give 13 at room temperature. Nothing happened in
the case of CH2Cl2, CHCl3, acetone, THF, or Et2O; how-
Scheme 6. Proposed defluorination process.
ever, if the resulting mixture was heated to 80 °C for 6 h,
disulfide 8, which might be formed through the route indi-
cated in Scheme 3 (c), and 13 were obtained. No change
Because hydrogen bonding is crucial for the stability of
was observed upon adding aromatic solvents such as benz- sulfenic acids and because
a characteristic band at
ene and dimethylbenzene. It is clear that aromatic solvents 3236 cm–1 was observed in the IR spectrum of 5a in tolu-
such as toluene can stabilize 5a efficiently. In contrast, tert- ene, which corresponds to the O–H vibration, we performed
butanesulfenic acid is more stable in polar solvents such as further investigations to see if intra- or intermolecular hy-
DMSO and water.[11]
drogen bonding existed. IR spectroscopy is an efficient tool
As a preliminary investigation into the chemistry of 5, 5b to solve this kind of problem. However, the corresponding
was treated with diphenylmethanamine, and sulfenamide 15 signal in the IR spectrum of a solution of 5 with a concen-
was obtained, as expected, in 16% yield. However, amides tration of about 0.1 m was very weak, and great error was
16 and 17 were also formed in 5 and 40% yield, respectively introduced upon diluting the solution further. Therefore,
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Synthesis and Identification of Solution-Stable Sulfenic Acids
19F NMR spectroscopy was used to study this hydrogen-
bonding interaction.
Taking a solution of 5b in toluene, prepared by heating
imine 1f in toluene, as the object, a series of 19F NMR
spectra were recorded at different concentrations (0.092–
0.0023 m, Figure 2). It was found that the signal of the
ClCF2 group shifted from δ = –69.03 to –69.23 ppm and
the signal of the CF2SOH group shifted from δ = –102.66
to –102.75 ppm if the concentration of 5b was changed
from 0.092 to 0.0023 m. The tiny but clear upfield chemical
shift of both resonances indicated that intramolecular hy-
drogen bonding between the sulfenic acid group and an ad-
jacent fluorine atom does not exist and that there is a weak
intermolecular hydrogen-bonding interaction between two
sulfenic acid molecules.[13]
Figure 3. 19F NMR spectra of 5b with gradual addition of aceto-
nitrile.
philicity of the sulfur atom by the strong electron-with-
drawing perfluoroalkyl group cuts off the self-condensation
process. Both aspects result in good stability of sulfenic acid
5.
Conclusions
In summary, perfluoroalkanesulfenic acids with good
stability in solution, which results from the strong electron-
withdrawing nature of the perfluoroalkyl group, were pre-
pared and characterized for the first time; this provides the
second example of a class of stable sulfenic acids stabilized
by electron-withdrawing groups. With the advantages of
considerable stability, synthetic simplicity, and functional
group unicity, these novel fluorinated sulfenic acids will
provide a way for researchers to study the chemistry of sulf-
enic acids and their reaction mechanisms in a normal fash-
ion.
Figure 2. 19F NMR spectra of 5b at different concentrations in tol-
uene.
19F NMR spectra were also recorded for a 0.092 m solu-
tion of 5b in toluene with gradual addition of acetonitrile
(1.0–300 equiv., Figure 3). Upon the addition of CH3CN,
the signal of ClCF2 shifted downfield by 0.17 ppm (from δ
= –69.03 to –68.86 ppm), whereas the signal of CF2SOH
shifted downfield by only 0.05 ppm (from δ = –102.66 to
–102.61 ppm). These results indicate that the intermolecular
hydrogen bond formed by 5b was quite weak and that the
addition of an excess amount of acetonitrile might lead to
the formation of new intermolecular MeCϵN···HO hydro-
gen-bonding interactions.
Considering the perfluoroalkyl groups in 5a–c are not as
bulky as those in the compounds outlined in Figure 1 (a,b),
the steric effect is negligible in these sulfenic acids. There-
fore, the strong electron-withdrawing effect of the per-
fluoroalkyl groups plays a key role in stabilizing sulfenic
acid 5: (1) A decrease in the electron density of the –SOH
group makes it a poor hydrogen-bond acceptor and weak-
ens or even blocks the intermolecular hydrogen-bonding in-
teraction between two sulfenic acid molecules. As a result,
the dimerization of 5 is very difficult. (2) Although there is
Experimental Section
Typical Procedure: Under
a N2 atmosphere, the sulfinimine
(18.00 mmol) and freshly distilled toluene (180.0 mL) were added
to a 250 mL, two-necked, round bottle charged with a condensing
tube. The mixture was heated at 80 °C. Upon completion of the
reaction, as indicted by 19F NMR spectroscopy, the mixture was
cooled to room temperature and stored in the refrigerator. The
yield of the sulfenic acid was determined by 19F NMR spectroscopy
by using 4-trifluoromethylbromobenzene as an internal standard.
Supporting Information (see footnote on the first page of this arti-
cle): Procedures for the synthesis of perfluoroalkanesulfinamide, 1,
1
3, 5, 9–20, copies of H, 13C and 19F NMR spectra for new com-
pounds, and mass and IR spectra of 5a.
Acknowledgments
Financial support from the National Natural Science Foundation
of China (NSFC) (grant number 21172243) is gratefully acknowl-
an impact of the α-effect,[14,15] the decrease in the nucleo- edged.
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SHORT COMMUNICATION
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Published Online: January 23, 2014
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