The X-ray structures of sulfoxides

Article abstract of DOI:10.1007/s10870-008-9493-9

We have structurally characterized and investigated a range of sulfoxide compounds containing aryl and alkyl substituents. Compounds 1 and 3-6 all crystallize in an orthorhombic space group, where compounds 2 and 7 crystallize in a monoclinic space group.

Full text of DOI:10.1007/s10870-008-9493-9

J Chem Crystallogr (2009) 39:407–415
DOI 10.1007/s10870-008-9493-9
ORIGINAL PAPER
The X-Ray Structures of Sulfoxides
Amy L. Fuller Æ R. Alan Aitken Æ Bruce M. Ryan Æ
Alexandra M. Z. Slawin Æ J. Derek Woollins
Received: 23 May 2008 / Accepted: 10 October 2008 / Published online: 4 November 2008
Ó Springer Science+Business Media, LLC 2008
Abstract We have structurally characterized and investi-
gated a range of sulfoxide compounds containing aryl and
alkyl substituents. Compounds 1 and 3–6 all crystallize in an
orthorhombic space group, where compounds 2 and 7 crys-
tallize in a monoclinic space group. The unit cell parameters
aromatic ring with the S–O bond or both. Furthermore, if the
sulfur is flanked by an alkyl group, a CH proton of S–CH –R
2
2
can be properly oriented to participate in an intermolecular
hydrogen bond with the sulfoxide oxygen of another
molecule.
˚
of the compounds are as follows: 1 (Fdd2), a = 17.653(5) A,
˚
b = 53.153(14) A, and c = 10.071(3) A; 2 (P2 /c), a =
˚
Keywords Sulfoxide ꢀ Interaction ꢀ Conjugation ꢀ
Intramolecular ꢀ Intermolecular
1
˚
˚
˚
7
.894(13) A, b = 5.653(10) A, and c = 27.02(5) A, b =
˚
˚
9
7.347(15)°; 3 (P2 2 2 ), a = 5.7569(6) A, b = 12.2139(12) A,
1
1 1
˚
and c = 17.5974(18) A; 4 (Pca2 ), a = 8.256(4) A, b =
˚
1
˚
˚
.470(3) A, and c = 23.995(13) A; 5 (P2 2 2 ), a = 5.848(4) A,
˚
5
Introduction
1
1 1
˚
b = 7.568(5) A, and c = 27.650(17) A; 6 (Pbca), a =
˚
˚
0.0569(15) A, b = 9.9403(16) A, and c = 17.843(3) A;
˚
˚
1
A sulfoxide is a molecule with the general formula R–
0
˚
and 7 (Pc), a = 13.217(4) A, b = 5.3766(14) A, and
˚
S(=O)–R , where R is an organic group. Structurally, these
˚
c = 8.370(2) A, b = 90.673(6)°. The S=O bond distances
molecules display some interesting characteristics. There
has been some debate over the nature of the S=O bond and
a comparison with other well known molecules possessing
˚
in these compounds range from 1.489(7) to 1.515(8) A. In all
seven structures, the O(1)–S–C bond angles vary from
0
1
05.1(4) to 111.7(30)° and the C(1)–S(1)–C(11) bond angles
the R(X=O)R motif (where X=C or P) illustrates why the
range from 94.1(4) to 100.56(12)°. The compounds contain
unique intra- and intermolecular interactions depending on
the groups attached to the sulfoxide moiety. The polarity of
the sulfoxide bond in these compounds allows for intramo-
lecular SꢀꢀꢀO interactions to occur. When the sulfur is bound
to alkyl groups, there tends to be a shorter SꢀꢀꢀO intermo-
lecular distance than when the sulfur is bound to aromatic
substituents. Additionally, if the sulfur is flanked by an aryl
group, the S–C bond distance is slightly shorter than if
flanked by an alkyl group, suggesting a possible weak
OꢀꢀꢀHaryl intramolecular interaction, weak conjugation of the
S=O bond in sulfoxides is debated [1]. In the carbon analog
0
R(C=O)R , the carbon atom forms a typical p–p p bond
with oxygen. In the sulfoxide or phosphine oxide (O=PR3)
molecules, however, it has been suggested that the oxygen
contributes electrons from its unshared lone pairs from the
2p orbital to an empty 3d orbital of the central sulfur or
phosphorus atom, i.e., d–p p bonding [1, 2]. However,
there is some debate over the compatibility of the energy
level overlap of the 3d orbital with the oxygen 2p orbital.
The sulfoxide bond is probably best represented as being
somewhere in between a double and a single bond, with
significant ionic character [3]. This is represented by the
two resonances structures in Fig. 1.
A. L. Fuller ꢀ R. A. Aitken ꢀ B. M. Ryan ꢀ
A. M. Z. Slawin ꢀ J. D. Woollins (&)
School of Chemistry, University of St Andrews, St Andrews,
Fife KY16 9ST, UK
Another important characteristic of sulfoxide molecules
is their ability to be chiral. Chiral centers are mainly
associated with tetrahedral carbon centered compounds,
0
e-mail: jdw3@st-and.ac.uk
however, the carbon analog of sulfoxide, R(C=O)R , is
123

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J Chem Crystallogr (2009) 39:407–415
Compounds 2–4, 6, and 7 were prepared by oxidation of
the relevant sulfides with NaIO in aq. MeOH [9–13]. All
O
S
O
S
4
compounds were recrystallized from a CH Cl /pentane
2
2
solution.
R
R'
R
R'
Fig. 1 Two resonance structures of the sulfoxide bond
X-ray Crystallography
Table 1 lists details of data collections and refinements for
–7. Data for 1, 2, 3, 5, and 7 were collected using a
Rigaku SCX-Mini diffractometer (Mercury2 CCD) and 4
O
C
O
S
O
1
e
R''
R'
P
was collected using the St Andrews Robotic diffractometer
R
R'
R
R'
R
(
Saturn724 CCD) at either 125 or 293 K with graphite-
˚
Fig. 2 Geometric comparison of a carbonyl carbon, a sulfoxide and a
tertiary phosphine oxide
monochromated Mo-Ka radiation (k = 0.71073 A) whilst
was collected using a Rigaku MM007 RA/confocal optics
6
and Mercury CCD at 93 K [14–16]. Intensity data were
collected using x (and / for 7) steps accumulating area
detector images spanning at least a hemisphere of reci-
procal space. All data were corrected for Lorentz
polarization and long-term intensity fluctuations. Absorp-
tion effects were corrected on the basis of multiple
equivalent reflections or by semi-empirical methods.
Structures were solved by direct methods and refined by
planar. When sulfur is bound to three substituents, such as
in a sulfoxide, the lone pair of electrons on the sulfur atom
forces the substituents into a pyramidal geometry. Since the
sulfur now has four unique arms, it is most similar to chiral
0
00
tertiary phosphine oxides (O=PRR R ) (Fig. 2). The Cahn-
Ingold-Prelog priority rules are used when deciding the
stereochemistry of chiral sulfoxide compounds, and the
unpaired electrons are assigned as the lowest priority group
2
full-matrix least-squares against F (SHELXL) [17].
Hydrogen atoms were assigned riding isotropic displace-
ment parameters and constrained to idealized geometries.
Details are available from the Cambridge Crystallographic
Data Centre CCDC 689268–689274.
[
3]. Many reaction pathways have been investigated in
attempts to synthesize a sulfoxide with specific chirality,
including the use of inorganic, organic, and enzymatic
catalysts [4–6]. Specific chirality enables sulfoxides to be
used as catalysts to transfer their chirality to carbon com-
pounds [5]. Sulfoxide chirality is also becoming
increasingly important in pharmaceutical synthesis [3].
Three classes of sulfoxide compounds have been pre-
viously described as having very specific hydrogen
bonding interactions that enforce particular conformations
in the molecule and influence crystal packing [7]. Here, we
have investigated the structural properties of three types of
sulfoxides where the R groups are alkyl–alkyl, alkyl–aryl,
and aryl–aryl arms using X-ray crystallography. We dis-
cuss both intra- and intermolecular interactions that
influence the packing of these compounds.
Results and Discussion
Structural Analysis Around Sulfur
Structures from the single X-ray analysis of the sulfoxides
1–7 are shown in Fig. 3. Compounds 1 and 6 have two
alkyl arms attached to the sulfoxide moiety, 5 has two aryl
groups attached, and 2, 3, 4, and 7 have one alkyl (benzyl)
and one aryl arm.
The bond lengths around the sulfur atom in 1–7 are
shown in Table 2. These compounds have similar S–O
˚
bond distances ranging from 1.489(7) to 1.515(8) A and
Experimental
are consistent with the reported average sulfoxide distance
˚
of 1.497(13) A [18]. However, the S–C bond distances
General
seem more sensitive to the substituents and range from
˚
1
.746(12) to 1.865(10) A. There is a very slight difference
Crystals of dibenzyl sulfoxide 1, benzyl 4-chlorophenyl
sulfoxide 2, benzyl 4-methylphenyl sulfoxide 3, benzyl
phenyl sulfoxide 4, di(p-tolyl) sulfoxide 5, benzyl ethyl
sulfoxide 6, and 4-nitrobenzyl phenyl sulfoxide 7 were
analyzed by X-ray crystallography. Crystallographic data
in bond length depending on the organic group attached to
the sulfur, though the range is larger for the alkyl case. If
the group is aromatic, the S–C distances have a tendency to
˚
be slightly shorter (ranging from 1.798(2) to 1.811(2) A)
than if the group is alkyl (ranging from 1.746(12) to
˚
1.839(2) A). This could be due to a very weak conjugation
for diphenyl sulfoxide (SOPh ) was already determined [8].
2
Compounds 1 and 5 were purchased from Aldrich.
of the p-system in the aromatic ring with the S=O double
1
23

J Chem Crystallogr (2009) 39:407–415
409
Fig. 3 Thermal ellipsoid plots (30% probability ellipsoids) of 1–7
bond. The average reported S–C bond distance is
˚
impact of the lone pair of electrons on the sulfur atom. In 6,
the disorder in the oxygen atoms may be responsible for the
larger O(2)–S–C bond angles of 109.2(3) and 111.7(3)°.
The O(1)–S(1)–O(2) bond angle in 6 is 119.1(3)°.
1
.818(1) A [18].
Selected bond angles around the sulfur atom are shown
in Table 2. Due to the lone pair of electrons on the sulfur, it
adopts a pyramidal structure. The O(1)–S–C bond angle for
all seven structures vary from 105.1(4) to 108.5(4)°, with
the two extremes being present in 7. The C(1)–S(1)–C(11)
bond angles are smaller, ranging from 94.1(4) to
OꢀꢀꢀHaryl Intramolecular Interactions
When an aryl group is attached to the sulfur, not only is
there the possibility of a weak conjugation of the double
1
00.56(12)°. This difference reflects the stereochemical
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J Chem Crystallogr (2009) 39:407–415
411
bonds, but it has been suggested that an intramolecular
interaction can exist between the sulfoxide oxygen and the
ortho-hydrogen (Haryl) on a neighboring aromatic ring [7].
similar to the phenyl arms in SOPh , they display different
2
structural characteristics. Only one OꢀꢀꢀH
distance
aryl
˚
(2.53(1) A) is similar to SOPh , while the other one is
2
˚
significantly longer (2.75(1) A). The O–S–C–C torsion
The strength of the OꢀꢀꢀH interaction influences how the
aryl
aromatic ring is oriented in the molecule and will ulti-
mately influence crystal packing. Three measures can be
angles for the two arms are also much larger in 5 than in
SOPh , twisting to 23.48(1) and 32.37(1)° (Fig. 5).
2
used to determine the strength of the OꢀꢀꢀH
intramo-
Across the entire series, as the OꢀꢀꢀH distance increa-
aryl
aryl
lecular interaction; (1) the distance between the oxygen and
hydrogen atom, (2) the O–S–C–C torsion angle, and (3) the
O atom deviation from the S-aryl ring plane. For example,
ses, so does the torsion angle; 2 has the smallest OꢀꢀꢀH
aryl
distance and the smallest torsion angle and 5 has the largest
distance and the largest torsion angle. There are
OꢀꢀꢀH
aryl
a stronger interaction will result in a shorter OꢀꢀꢀH
dis-
only two exceptionsto this trend: one arm of compound 5 and
one arm of compound 3. Interestingly enough, in both
exceptions, it seems to be a p-tolyl substituent causing the
deviations. However, a crystallographic example of (-)-(S)-
4-aminophenyl p-tolyl sulfoxide is known [19] (Fig. 6). The
aryl
tance, a smaller torsion angle, and a smaller deviation of
the oxygen atom from the S-aryl plane. These three values
can be found in Table 2. Figure 4 illustrates the possible
OꢀꢀꢀH interaction and the O–S–C–C torsion angle being
aryl
measured.
OꢀꢀꢀH
distance of the p-tolyl group in this compound is
aryl
˚
Compounds 2–4 and 7 have one aryl group adjacent to
the sulfoxide moiety enabling an intramolecular interaction
2.31(1) A, which is much closer than the equivalent distance
in 3 or 5. The p-tolyl group also has a O–S–C–C torsion angle
of -10.8(3)°, which is less than that of 5 and only slightly
larger than that of 3.
to exist between the O and the Haryl. The OꢀꢀꢀH distance
aryl
˚
in these compounds range from 2.50(1) to 2.61(1) A. For
the most part, as the OꢀꢀꢀH distance increases, the torsion
aryl
angle also increases, reflecting the displacement of the O
atom from the plane of the aromatic group. 2 has the
˚
shortest OꢀꢀꢀH interaction (2.50(1) A), the smallest torsion
angle (3.27(1)°), and the smallest oxygen deviation from
˚
the plane (0.142(3) A). The OꢀꢀꢀH
distance increases
aryl
2
\ 4 \ 3 * 7 whilst the O–S–C–C torsion angle and the
mean deviation of the oxygen atom increases
\ 3 \ 4 \ 7. Compound 3 displays some interesting
behavior, as it has the second longest OꢀꢀꢀH distance of
2
aryl
˚
the series (2.61(1) A), but a very tight torsion angle
(
8.16(1)°).
Compound 5 does not fit into the previous structural
group because it has two aryl arms attached to the sulf-
oxide. However, this compound can be compared to the
well known compound diphenyl sulfoxide (SOPh ). Both
2
Fig. 5 The X-ray structure of 5 showing the large O–S–C–C torsion
angles
phenyl arms in SOPh have similar OꢀꢀꢀH bond distances
2
aryl
˚
of 2.51(1) and 2.57(1) A and similar O–S–C–C torsion
angles of 11.38(1) and 11.70(1)° [8]. Compound 5 has two
p-tolyl substituents. While these arms are structurally
O
S
H
C
R
C
Fig. 4 Possible intramolecular interactions between the OꢀꢀꢀHaryl can
be described by the OꢀꢀꢀHaryl distance, the O–S–C–C torsion angle,
and the oxygen deviation from the S-aryl plane
Fig. 6 ORTEP drawing of (-)-(S)-4-aminophenyl p-tolyl sulfoxide:
˚
O1ꢀꢀꢀHaryl interaction is 2.31 A and O1–S1–C7–C8 torsion angle is
1
9
-10.8(3)°
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J Chem Crystallogr (2009) 39:407–415
413
Fig. 7 Left, X-ray structure of
6
showing the disorder in the
oxygen atom (O(1), 80% and
O(2), 20% occupancy). Right, 6
is rotated bringing the ethyl
group forward to show the
alignment of O(2) and Hmethyl
OꢀꢀꢀHmethyl Intramolecular Interactions
–CH –R group [7]. An interaction between the oxygen
2
atom of one sulfoxide molecule and the hydrogen from the
Compound 6 has two alkyl groups attached to the sulfoxide
intramolecular
–CH –R group can be difficult to demonstrate because it is
2
moiety, and it appears that an OꢀꢀꢀH
so weak. The OꢀꢀꢀH
distance, the OꢀꢀꢀC
distance,
methyl
alkyl
alkyl
interaction could be present. In 6 there is a disordered
oxygen atom, with 80% O(1) and 20% O(2) occupancy.
Structurally there is a close contact between O(2) and
C(12) (Fig. 7). In addition, O(2) and one Hmethyl are per-
and the OꢀꢀꢀH–C angle can be used as evidence to help
support or contradict this theory.
Significant intermolecular hydrogen bonding interac-
tions are considered in these compounds if the OꢀꢀꢀH
˚
fectly eclipsed with a distance of 2.93(1) A. This
˚
distances are\2.70 A and the OꢀꢀꢀH–C angle is[120° [7].
orientation does not exist when looking at O(1). Even
Table 2 shows the OꢀꢀꢀH
distance, the OꢀꢀꢀC distance,
alkyl
though the O(1)ꢀꢀꢀH
distance is slightly shorter
and the OꢀꢀꢀH–C angle for compounds 1–7. The weaker
OꢀꢀꢀH–C interactions are longer than classical OꢀꢀꢀH–O
methyl
˚
(
2.80(1) A), the atoms are *12° degrees out of alignment.
˚
hydrogen bond distances (*2.30 A), and they are less
S, O Intermolecular Interactions
sensitive to deviations from ideal geometries than stronger
H-bonds [20]. Therefore, a larger OꢀꢀꢀH–C angle can
deviation from linearity, but that doesn’t necessarily mean
the hydrogen bond is weaker [7].
The sulfoxide bond has been described as a single bond
with ionic character, with the sulfur bearing a formal
positive charge and the oxygen bearing a formal negative
charge [1, 3]. The large dipole moments in these bonds
allow unique intermolecular interactions in the packing of
these molecules. The intermolecular SꢀꢀꢀO distances range
In this study, 1 and 6 have two alkyl arms adjacent to the
sulfur. 1 has two benzyl groups (crystallizing with two
independent molecules in the asymmetric unit cell, 1a and
1b). The values in Table 2 show the average interaction
distances associated with each molecule. The average
values for 1a and 1b are very close and are in most cases
within experimental error of each other. Of all of the
reported compounds, 1b has the shortest SꢀꢀꢀO distance
˚
from 3.57(1) to 4.37(1) A. The shortest distance is in 1 and
the longest is in 5 (Table 2). In 1–7 sulfur bound to alkyl
groups tends to have a shorter intermolecular SꢀꢀꢀO distance
than when sulfur is bound to aromatic substituents. This
could be due to one or a combination of several reasons.
When aryl substituents are present, conjugation of the
entire pi system in the molecule could reduce the positive
dipole on the sulfur atom, lengthening the intermolecular
SꢀꢀꢀO distance. Alternatively, alkyl groups could simply
give more space for close approach of the oxygen atom.
Thirdly, when alkyl substituents are present, any intermo-
˚
˚
distance (2.77(1) A),
(3.57(1) A), but the longest OꢀꢀꢀH
alkyl
and the OꢀꢀꢀH–C atoms form a 132.72(1)° angle (Fig. 8).
In 6, the sulfur is flanked by one benzyl group and one
˚
ethyl group. The SꢀꢀꢀO distance is 3.99(1) A, which is
longer than in 1. Intermolecular OꢀꢀꢀH
interactions with
alkyl
the OꢀꢀꢀH–C of the benzyl arm are shorter and more linear
˚
distance in 6 is 2.53(1) A and
than that of 1. The OꢀꢀꢀH
alkyl
lecular OꢀꢀꢀH
interactions could help pull the S and O
an OꢀꢀꢀC–H angle is 145.91(0)°. The ethyl arm shows an
˚
alkyl
closer together.
even shorter OꢀꢀꢀH
distance of 2.48(1) A, with a more
alkyl
linear OꢀꢀꢀC–H angle of 149.04(1)°.
OꢀꢀꢀHalkyl Intermolecular Interactions
Given the benchmarks discussed earlier, if there is an
OꢀꢀꢀH
interaction, it would have to be considered very
alkyl
Sulfoxide compounds display some unique intermolecular
interactions in their crystal packing. It has been suggested
weak. However, two weak OꢀꢀꢀH–C intermolecular inter-
actions could pull the molecules closer together and
decrease the OꢀꢀꢀS distance and decrease the OꢀꢀꢀH–C angle
(Fig. 9).
that a type of intermolecular OꢀꢀꢀH
interaction can
alkyl
occur in sulfoxides when the sulfur atom is flanked by a
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J Chem Crystallogr (2009) 39:407–415
Fig. 8 Possible intermolecular interactions in 1
Fig. 10 Possible intermolecular interactions in 2
H
H
Conclusions
H
H
H
H
H
H
H
Ar
Ar
S
H
H
δ+
We have structurally characterized and investigated sulf-
oxide compounds 1–7. The S=O bond distances in these
compounds are all very similar ranging from 1.489(7) to
S
δ+
R
R
δ−
R
R
R
O
δ−
˚
1.515(8) A. In all seven structures, the O(1)–S–C bond
O
angles vary from 105.1(4) to 108.5(4)° and the C(1)–S(1)–
C(11) bond angles range from 94.1(4) to 100.56(12)°.
We find that compounds 1–7 contain unique intra- and
intermolecular interactions depending on the groups
attached to the sulfoxide moiety. The polarity of the sulf-
oxide bond in these compounds allows for an
intramolecular SꢀꢀꢀO interaction to occur. When the sulfur
is bound to alkyl groups, there tends to be a shorter SꢀꢀꢀO
intermolecular distance than when the sulfur is bound to
aromatic substituents. Additionally, if the sulfur is flanked
by an aryl group, the S–C bond distance is slightly shorter
than if flanked by an alkyl group. These distances suggest a
possible interaction, which could be weak conjugation,
θ
H
θ
S
S
δ+
δ+
R
O δ−
O δ−
Fig. 9 Simple depiction of intermolecular hydrogen bonding inter-
actions in sulfoxide compounds with two alkyl arms (left) and an
alkyl and aryl arms (right)
Compounds 2–4 and 7 have one alkyl and one aryl
group. The SꢀꢀꢀO interactions in these molecules are
˚
slightly longer (range from 3.95(1) to 4.35(1) A) than the
OꢀꢀꢀH
intramolecular interaction, or both. The strength
aryl
compounds with two alkyl groups (1: 3.57(1) and 3.58(1)
˚
of these combined interactions would also determine the
amount of twisting the aryl group can undergo and would
also influence the molecular packing. Furthermore, if the
and 6: 3.99(1) A). 3 has the longest SꢀꢀꢀO distance of
˚
4
.35(1) A. The OꢀꢀꢀH
distances range from 2.29(1) in 2
alkyl
˚
to 2.50(1) A in 3. The OꢀꢀꢀHalkyl–C angles are more linear
in these compounds than in the alkyl/alkyl compounds
ranging from 1.47.90(1) in 7 to 170.84(1)° in 2. Possible
intermolecular interactions for 2 are shown in Fig. 10. This
sulfur is flanked by an alkyl group, a CH proton of S–
2
CH –R can be properly oriented to participate in an
2
intermolecular hydrogen bond with the sulfoxide oxygen of
another molecule.
could be due to having only one –CH -R arm available for
2
OꢀꢀꢀH
intramolecular interaction, which would allow
alkyl
the molecules to align in a more linear fashion. It could
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