Preparation and characterization of alkenyl aryl tetrafluoro- λ6-sulfanes

Article abstract of DOI:10.1002/anie.201306507

Substituted alkenyl aryl tetrafluoro-λ⁶-sulfanes have been prepared by the direct addition of readily accessible chlorotetrafluorosulfanyl arenes to primary alkynes. Substitution of an apical fluorine of the pentafluorosulfanyl group enables mo

Full text of DOI:10.1002/anie.201306507

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Angewandte
Communications
DOI: 10.1002/anie.201306507
Structure Elucidation
Preparation and Characterization of Alkenyl Aryl Tetrafluoro-l6-
sulfanes**
Linbin Zhong, Paul R. Savoie, Alexander S. Filatov, and John T. Welch*
Abstract: Substituted alkenyl aryl tetrafluoro-l6-sulfanes have
been prepared by the direct addition of readily accessible
chlorotetrafluorosulfanyl arenes to primary alkynes. Substitu-
tion of an apical fluorine of the pentafluorosulfanyl group
enables modulation of the reactivity of this little explored
functional group while at the same time facilitating the direct
investigation of aryl substituent effects on the aryl tetrafluoro-
sulfanyl-substituted products.
reactivity, structure, and bonding of the product alkenyl aryl
tetrafluorosulfanes remained an elusive goal.
The addition of triethylboron to an ethereal solution of
a the chlorotetrafluorosulfanyl arene 3 and alkynes 2
promotes formation of 1 in 15 minutes. The required chlor-
otetrafluorosulfanyl arenes 3 are easily and inexpensively
prepared by a general process which is applicable to a variety
of substituted chlorotetrafluorosulfanyl arenes.[1] Yields of
isolated and recrystallized 1 are reported in Scheme 1. The
yield (crude reaction mixture) of 1a was excellent, but the
recovery of the lower melting crystalline product was more
challenging than with higher melting solids 1b–e.
S
ubstituted alkenyl aryl tetrafluoro-l6-sulfanes (1; for struc-
ture see Scheme 1) were prepared by the direct addition of
chlorotetrafluorosulfanyl arenes[1] to primary alkynes. Sub-
stitution of the apical fluorine of the pentafluorosulfanyl
group enables modulation of the reactivity of this functional
group while facilitating the direct investigation of the aryl
substituent effects on the aryl tetrafluorosulfanyl-substituted
products.
In spite of the intriguing properties of hypervalent
fluorinated sulfur compounds,[2] the preparation of these
substances has been limited to the synthesis of molecules
bearing pentafluorosulfanyl arenes.[3] However, preparations
of pentafluorosulfanylated aliphatic compounds are increas-
ingly common.[4] Investigations of tetrafluoro-l6-sulfanes,
where the electronic effects of hypervalent fluorinated
sulfur may be modulated by selective substitution, are rarer.
Selective substitution can be a tool for the potential control of
reactivity, but synthetic access to the substituted compounds
is challenging. Diaryl tetrafluorosulfanes were prepared by
fluorination of the corresponding diaryl sulfides with
trifluoromethylhypofluorite[5] or fluorine.[6] Aryl trifluoro-
methyl tetrafluorosulfanes were prepared by direct fluorina-
tion of the corresponding aryl trifluoromethyl sulfides.[7]
Trifluoromethyl tetrafluorosulfanyl chloride, prepared by
oxidative fluorination of trifluoromethylsulfenyl chloride
with chlorine fluoride, was used to prepare a variety of alkyl
trifluoromethyl tetrafluorosulfanes.[8] Photolytic initiation
was necessary to promote the reaction of the trifluoromethyl
tetrafluorosulfanyl chloride with alkenes.[9] Unfortunately,
systematic investigation of the influence of the carbon
substituent, which is the apical substituent on sulfur, on the
Scheme 1. Triethylboron-catalyzed addition of chlorotetrafluorosulfanyl
arenes to alkynes.
The facile formation of the desired tetrafluoro-l6-sulfanes
in good yields is supported by computations suggesting that
À
homolytic cleavage of the S Cl bond of 3a requires approx-
imately 6.7 kcalmolÀ1 less energy than homolytic cleavage of
the S Cl bond of SF5Cl (B3LYP/6-31 + G(d,p)).[10] In contrast
À
to the triethylboron-facilitated addition of SF5Cl to alkenes
and alkynes where a nonpolar solvent such as pentane n-
pentane or trichlorofluoromethane is required to suppress
ionic halofluorination,[11] the ease of homolytic S Cl bond
À
cleavage in 3, presumably a consequence of the reduced
electronegativity of aryl tetrafluoro-l6-sulfane, enables the
use of ethereal solvent.
To functionalize the alkenyl aryl tetrafluorosulfanes,
Suzuki coupling of the boronic acids 4 with 1d formed 1e–
g, in good yields, and is consistent with coupling of penta-
fluorosulfany-substituted arenes (Scheme 2).[12] In contrast to
the reaction of pentafluorosulfanyl aryl bromide, addition to
1 required a significantly higher reaction temperature of
1208C, as opposed to room temperature. A variety of solvent
systems and temperatures were examined to determine the
optimum reaction conditions for coupling reactions of 1.
Reactions in 1,4-dioxane, over a range of temperatures from
room temperature to 1008C with either potassium carbonate
or potassium acetate are slower than those in THF. Use of
acetone or DMF leads to decomposition. Even on heating at
1208C for 96 hours with potassium acetate, the yields of the
[*] L. Zhong, P. R. Savoie, A. S. Filatov, Prof. J. T. Welch
Department of Chemistry, University at Albany, SUNY
1400 Washington Ave, Albany, NY 12222 (USA)
E-mail: Jwelch@albany.edu
[**] We gratefully acknowledge the National Science Foundation
(CHE0957544) for financial support of this work and UBE, America
Inc., for generous provision of intermediates.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306507.
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Table 1: Selected distances and angles, from single-crystal X-ray struc-
tural studies, and 13C chemical shifts and s values for 1a–e.
1a
1b
1c
1d
1e
Distances[a]
À
C4 S
À
S C5
1.81(2)
1.811(2)
0.0148
1.8196(17) 1.814 (4) 1.822(4) 1.818(2)
1.8057(18) 1.813 (4) 1.798(4) 1.805(2)
Scheme 2. Stability of aryl tetrafluoro-l6-sulfanes under Suzuki cou-
pling conditions.
À
S F differ-
0.012
0.0237
0.028
0.003
ence[b]
Angles[c]
C4-S-C5
176.91(10) 175.75(8) 175.33(17) 175.4(2) 178.1(1)
coupling reactions of 1d demonstrated the robustness of the
tetrafluoro-l6-sulfane nuclei of 1d–g.
13C NMR shifts[d]
The influence of arene substituents correlates with the
13C NMR chemical shifts of 1a–e.[13] In 1a–e, the 13C chemical
shift of the resonance attributed to C4 (see Figure 1 for
C4
C5
s[e]
160.2
144.67
0.0
158.3
144.32
0.23
163.7
142.55
0.78
158.9
143.6
0.23
159.4
144.7
À0.01
À
[a] In ꢀ. [b] Maximum variation in S F bond lengths. [c] In degrees.
[d] d [ppm]. [e] See Ref. [15].
This substituent effect on sulfur geometry is consistent with
the increased significance of an ionic resonance form to the
C4-S-C5 bonding and the consequent distortion of geometry
around sulfur. The ionic resonance form would also be
À
predicted to result in the S F1 bonds syn to the alkene being
[19]
À
shortened relative to the anti S F2 bonds, as was found.
In addition to elucidating structural trends in aryl tetra-
fluorosulfanyl compounds, X-ray data revealed the manifold
potential of this novel class of materials. Especially interesting
was the crystallization of 1d and 1c in a polar, non-
centrosymmetric space group, as shown in Figures 2 and
Figure 1. Molecular structure of 1c. Thermal ellipsoids are shown at
50% probability.
numbering; d = 158.3–163.7 ppm) correlates weakly with
increasing s values,[14] however a better correlation is found
with the computed NMR shielding tensors (B3LYP, 6-31G
(d), r2 = 0.9545). Consistent with the Spiesecke–Schneider
relationship,[15] these shifts may be a barometer of charge
density, with an increase in s leading to lower-field chemical
shifts for C4. However, the 13C chemical shift of C5, located at
the opposite apical position of sulfur, decreases from d =
144.7 to d 142.5 with increasing s (r2 = 0.9826). Correlation
of C5 chemical shifts with the shielding tensors is worse (r2 =
0.8056). The decrease in C5 chemical shift with strongly
electron-withdrawing p-substituents was unanticipated. For
comparison, in styrene, the corresponding Ca chemical shifts
decrease, but only slightly, from d = 135.8 to d 135.0. The
decrease in chemical shift at C5 on introduction of electro-
negative substituents suggests an increase in charge density,
and is consistent with the contribution of ionization of the
Figure 2. Packing of 1d illustrates alignment of molecular dipoles with
infinite columns of molecules apparently organized by a favorable
bromine–arene interaction.
Figure 3, respectively. Not only do crystals of 1d form
assemblages with oriented dipoles, but the columnar assem-
blies interact to form two-dimensional sheets. The columns
are associated by interaction of the bromine of the bromoaryl
tetrafluorosulfanyl group with the arene of a neighboring
column. (Figure 2)
The structure of 1c differs from that of 1d in that
substitution of the nitro group for bromine disrupts the
interactions between pairs of columns with the resultant
elements of the columnar pair having classic edge–p inter-
actions, yet no obvious interaction with a neighboring assem-
bly (Figure 3). The compounds 1c [space group Ima2) and 1d
(space group Pna2(1)] both crystallize in the mm2 point
group, and therefore crystals of both 1c and 1d could be
C5 S bond to the Rundle–Pimentel–Coulson[16]/charge-
À
shift[17] description of bonding at sulfur. Four-bond carbon–
fluorine coupling constants (4JC,F), typically 1.8 Hz, are
observed in the spectra of 1. In contrast, with benzotrifluor-
4
5
ide, JC,F couplings are not detected,[18] but JC,F values
(1.11 Hz) are reported.[18b]
The aryl substituent effect can also be assessed by scrutiny
of the data from single-crystal X-ray diffraction studies of 1a–
e (Table 1). The C4-S-C5 bond angle shows a systematic
distortion from linearity with compression of the C4-S-C5
bond angle from 178.18 to 175.338 and increasing s values.
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Table 2: Selected distances and angles, from single-crystal X-ray struc-
tural studies, and 13C chemical shifts for 6a–c
6a
6b
6c
Distances[a]
À
C4 S
À
S C5
1.809 (3)
1.731 (4)
0.0043
1.8137 (15)
1.7445 (16)
0.0087
1.820 (3)
1.736 (3)
0.0091
[b]
À
S F difference
Angles[c]
C4-S-C5
179.40 (16)
178.81 (7)
179.69 (14)
13C NMR shifts[d]
C4
C5
Figure 3. Packing of 1c illustrates alignment of molecular dipoles with
discretely separate infinite columns of molecular pairs enforced by the
nitro group.
159.6
95.5
157.6
95.1
163.1
94.3
À
[a] In ꢀ. [b] Maximum variation in S F bond lengths. [c] In degrees.
[d] d [ppm].
anticipated to exhibit piezoelectric behavior as well as be
polar and possess pyroelectric properties. Such pyroelectric
properties are intriguing in the design of new ferroelectric
materials.[20] In the parent structure 1a, as well as in 1b and
1e, conventional p–p stacking leads to alternate dipolar
orientations of the columnar assemblies in centrosymmetric
crystals.
influence the structural and spectroscopic properties, which
are predictors of the reactivity of both the reactants and the
addition products, with strongly electron-donating groups not
leading to isolable products. The effects are consistent with
contemporary understanding of the bonding of fluorinated
hypervalent sulfur. Manipulation of these structures can lead
not only to the formation of intriguing polar crystals with
potential utility in sensors, but also to control of the reactivity
of the adducts in further synthetic transformations. Future
experiments will be devoted to expanding the scope of
reactions of arene tetrafluorosulfanyl chlorides with substi-
tuted alkynes and alkenes.
The alkenyl aryl tetrafluoro-l6-sulfanes 1 discussed above
can also undergo additional transformations without decom-
position of the tetrafluorosulfanyl group. Dehydrochlorina-
tion with lithium hydroxide formed the alkynyl aryl tetra-
fluoro-l6-sulfanes 6 in excellent yields (Scheme 3).
Received: July 25, 2013
Published online: November 28, 2013
Keywords: fluorine · radicals · structure elucidation ·
.
substituent effects · sulfur
Scheme 3. Dehydrochlorination of 1 to form the acetylenes 6.
DMSO=dimethylsulfoxide
In contrast to the carbon–fluorine coupling observed for 1,
the 5JC,F couplings in 6 were observed to aryl carbon atoms of
the aryl acetylide portion of 6 where JC,F = 1.7 Hz. This is in
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À
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