Synthesis, enantiomeric conformations, and stereodynamics of aromatic ortho-substituted disulfones

Article abstract of DOI:10.1002/hlca.200390020

Aromatic ortho-disulfone derivatives are readily accessible from diiodide precursors by Cu^I-mediated reaction with sodium sulfinate salts (DMF, 110°). The sulfonyl substituents adopt in solution and in the solid state two enantiomeric conformations (λ and δ) as evidenced by ³¹P- and ^IH-NMR data of the chiral D₃-symmetric tris{4,5-bis[(4-methylphenyl)sulfonyl]benzene-1,2-diolato(2-)- κO,κO′}phosphate(v) anion (3a) and 1,2-bis(camphor-10-sulfonyl)-4,5-dimethoxybenzene((=1, 2-bis{{[(1S,4R)-7,7-dimethyl-2-oxobicyclo[2.2.1]- hept-1-y1]methyl}sulfonyl}-4,5-dimethoxybenzene; 6c). X-Ray structure analysis of 1,2-dimethoxy-4,5-bis(methylsulfonyl)benzene (6a) and 1,2-dimethoxy-4,5-bis(4-methylphenyl)sulfonyl]benzene (6b) confirmed in the solid state the preferred chiral orientation of the sulfonyl groups. Dynamic conformational isomerism was detected for 6c in its ^IH-NMR in the temperature range of 110°, the corresponding free energy being 19.8 kcal mol^-1.

Full text of DOI:10.1002/hlca.200390020

Helvetica Chimica Acta Vol. 86 (2003)
65
Synthesis, Enantiomeric Conformations, and Stereodynamics of
Aromatic ortho-Substituted Disulfones
by Je¬ro√me Lacour*a), David Monchauda), Jiri Maredaa), France Favargera), and Ge¬rald Bernardinellib)
a
¬
¬
¡
) Departement de Chimie Organique, Universite de Geneve, quai Ernest-Ansermet 30, CH-1211 Geneva 4
b
¬
¡
) Laboratoire de Cristallographie, Universite de Geneve, quai Ernest-Ansermet 24, CH-1211 Geneva 4
Aromatic ortho-disulfone derivatives are readily accessible from diiodide precursors by CuI-mediated
reaction with sodium sulfinate salts (DMF, 1108). The sulfonyl substituents adopt in solution and in the solid
state two enantiomeric conformations (l and d) as evidenced by 31P- and 1H-NMR data of the chiral D3-
symmetric tris{4,5-bis[(4-methylphenyl)sulfonyl]benzene-1,2-diolato(2À)-kO,kO'}phosphate(v) anion (3a)
and 1,2-bis(camphor-10-sulfonyl)-4,5-dimethoxybenzene ((ˆ1,2-bis{{[(1S,4R)-7,7-dimethyl-2-oxobicyclo[2.2.1]-
hept-1-yl]methyl}sulfonyl}-4,5-dimethoxybenzene; 6c). X-Ray structure analysis of 1,2-dimethoxy-4,5-bis(me-
thylsulfonyl)benzene (6a) and 1,2-dimethoxy-4,5-bis(4-methylphenyl)sulfonyl]benzene (6b) confirmed in the
solid state the preferred chiral orientation of the sulfonyl groups. Dynamic conformational isomerism was
detected for 6c in its 1H-NMR in the temperature range of 1108, the corresponding free energy being 19.8 kcal ¥
molÀ1
.
Introduction. The octahedral geometry of a pentavalent hexacoordinated P-atom
allows the formation of chiral phosphate anions D and L enantiomers by
complexing the P-atom with three identical symmetrical bidentate ligands [1].
Hexacoordinated tris(benzenediolato)phosphate anion 1, of particular interest for its
easy preparation from pyrocatechol (ˆ benzene-1,2-diol), PCl5, and an amine, is
unfortunately configurationally labile as an ammonium salt in solution due to an acid-
induced racemization mechanism [2]. Recently, we reported that the introduction of
electron-withdrawing groups (Cl-atoms) at the aromatic rings increases the configura-
tional stability of the resulting tris(tetrachlorobenzenediolato)phosphate(v)
(ˆ TRISPHAT) derivative 2 (Fig. 1). This anion can be resolved associated with a
chiral ammonium cation. It is an efficient NMR chiral shift reagent, a powerful
resolving agent for ruthenium(II) complexes, and a chiral inducer onto iron(II)
tris(diimine) compounds [3].
To extend the pool of chiral anions to choose from for possible asymmetric
applications, we decided to prepare novel hexacoordinated phosphate anions derived
from new electron-deficient pyrocatechols. We decided to focus on the synthesis of
pyrocatechol derivatives substituted with strong electron-withdrawing groups, such as
the sulfonyl groups MeSO2 and p-TolSO2 [4]. The new pyrocatechols should be at least
as electron-poor as tetrachloropyrocatechol, and literature precedents convinced us of
the need to introduce at least two sulfonyl substituents [4]. The pattern of distribution
of the two electron-withdrawing groups on the pyrocatechols was also essential.
Asymmetrically substituted pyrocatechols would lead in octahedral geometry to the
synthesis of meridional and facial isomers of the phosphate anions. The two

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Helvetica Chimica Acta Vol. 86 (2003)
Cl
SO2R
Cl
Cl
Cl
SO2R
RO2S
O
O
P
Cl
Cl
O
O
P
SO2R
SO2R
R
R
SO2
1
5
O
O
O
O
Cl
Cl
O
O
O
O
RO
O
O
O
O
2S
O
O
O
P
P
RO2S
O
4
O
Cl
Cl
O
O
SO2
O
Cl
SO2R
RO2S
Cl
SO2R
1
4
3a R = p-Tol
R = Me
2 ∆-TRISPHAT
b
Fig. 1. Hexacoordinated phosphate anions: 1, 2 (TRISPHAT), 3, and 4
symmetrical 4,5- and 3,6-disubstitutions were then compatible with the making of D3-
symmetric anions of the general structures 3 and 4 (Fig. 1).
Herein, we report that the introduction of sulfonyl substituents at C(4) and C(5) of
the pyrocatechol ligand led to mixtures of diastereoisomeric hexacoordinated
phosphate anions 3. This is due to the adoption of enantiomeric conformations (d
and l) by each pair of the ortho-positioned sulfonyl groups. To demonstrate further this
effect and characterize the slow interconversion between the d and l forms,
enantiomerically pure 1,2-bis(camphor-10-sulfonyl)-4,5-dimethoxybenzene (6c) was
prepared.
2. Results and Discussion. 2.1. Syntheses of 4,5-Bis(sulfonyl)pyrocatechols. Of the
two symmetrical modes of disubstitution in agreement with the making of D3-
symmetric anions, a simple retrosynthetic analysis led us to tackle first the synthesis of
4,5-disubstituted derivatives (Scheme 1). Recently, Suzuki et al. reported a simple and
high-yielding route to aromatic sulfones by a CuI-mediated reaction of aryl iodides with
aromatic sodium sulfinate salts [5]. No ortho-diiodobenzene derivatives had been
tested. However, the generality of this reaction made us confident that we could use it
to introduce the two ortho-positioned sulfonyl groups. Furthermore, literature
precedents indicated that 4,5-diiodoveratrol (ˆ1,2-diiodo-4,5-dimethoxybenzene; 5)
could be prepared in a single step from veratrol with a combination of iodine and
periodic acid, compound 5 being an ideal starting material [6].
We decided to attempt the sulfonylation reaction under reported conditions with
commercially available sodium p-toluenesulfinate (ˆsodium 4-methylbenzenesulfi-
nate). We also extended the coupling methodology to the readily prepared
methanesulfinate derivative [7]. As reported, diiodination of veratrol with I2/H5IO6
proceeded smoothly to give 5 in 83% yield (Scheme 2). Bis-sulfonylation of 5 resulted,
after optimization, in the syntheses of 6a (R ˆ p-Tol; 88%) and 6b (R ˆ Me; 95%) with
high yields. The CuI-mediated coupling can thus be applied with either aromatic and
aliphatic sulfinate salts. Longer reaction times (>64 h) were required for the double
sulfonylation. Prolonged heating at higher temperatures (1258) led to lower amounts of
1
bis-sulfonylated derivatives. H- and 13C-NMR analyses of 6a,b did not reveal any
particularity, except for a signal broadening that could only be understood in the course
of this study. Simple deprotection of the methylated derivatives 6a,b under classical

Helvetica Chimica Acta Vol. 86 (2003)
67
Scheme 1. Retrosynthetic Scheme for the Synthesis of 4,5-Bis(sulfonyl)pyrocatechol Derivatives
4
2
R'O
R'O
SO2R
SO2R
R'O
R'O
I
I
R'O
R'O
5
1
R' = H, Me
conditions (BBr3, CH2Cl2) led to the 4,5-bis(p-tolylsulfonyl)pyrocatechol (7a) and 4,5-
bis(methylsulfonyl)pyrocatechol (7b) in 93 and 100% yield, respectively (Scheme 2)
[8]. The reaction of 7a required more elevated temperature (458 instead of 248) to
proceed with high yield.
Scheme 2
OMe
OMe
I
I
OMe
OMe
RO2S
RO2S
OMe
OMe
RO2S
RO2S
OH
OH
a)
c)
b)
5
7a R = p-Tol
b R = Me
6a R = p-Tol
b R = Me
a) I2, H5IO6, MeOH, 708; 5 (83%). b) CuI, RSO2Na (3.0equiv.), DMF, 110 8; 6a (R ˆ p-Tol, 88%) and 6b
(R ˆ Me, 95%). c) BBr3 (5.0equiv.), CH 2Cl2; 7a (R ˆ p-Tol; 93%) and 7b (R ˆ Me, 100%).
2.2. Syntheses of Tris[4,5-bis(sulfonyl)benzenediolato]phosphate Anions 3a and 3b.
The syntheses of the corresponding hexacoordinated phosphate anions were first
attempted under conditions developed in our laboratory for the synthesis of TRI-
SPHAT 2. However, reactions of 7a or 7b and PCl5 in toluene and addition of Bu3N in
CH2Cl2 did not yield the desired (Bu3NH) ¥ 3a and (Bu3NH) ¥ 3b salts. 31P-NMR
Analyses of the crude reaction mixtures revealed signals between 0and 5 ppm
corresponding to classical tetracoordinated phosphate derivatives. Assuming that this
failure was the result of poor solubility in toluene of pyrocatechols 7a,b and/or the
resulting P-derivatives, we circumvented the problem by choosing more-polar solvent
conditions. Addition of PCl5 to solutions of the pyrocatechols in CH2Cl2 (7a) or MeCN
(7b), then reaction, and addition of Bu3N in DMF afforded the desired (Bu3NH) ¥ 3a
and (Bu3NH) ¥ 3b salts along with minor amounts of degradation products (Scheme 3).
Purification of the crude mixtures could be effected by metathesis reactions with
malachite green oxalate and chromatography (SiO2) to afford the bis [4-(dimethyla-
mino)phenyl]phenylmethylium (8) salts, i.e., 8 ¥ 3a and 8 ¥ 3b, in 63 and 82% yield,
respectively (over two steps and nonoptimized).
2.3. 31P-NMR Analysis: Multiple Isomers for Anion 3a. Characterization of salt 8 ¥
3b by 31P-NMR revealed one signal corresponding to the hexacoordinated phosphate
anion 3b ((D)6acetone, d À76.2, Fig. 2). However, 31P-NMR analysis of 8 ¥ 3a was, at
first glance, somewhat puzzling. Four signals were observed while only one was
expected in the À80ppm region characteristic of the tris(benzenediolato)phosphate
anions ((D)6DMSO; d À76.7, À77.3, À77.5, and À78.2, Fig. 3). This result was
furthermore surprising as only one set of signals corresponding to anion 3a could be
1
observed in the H-NMR spectrum. Further characterization of salt 8 ¥ 3a by mass
spectrometry (ES-MS) confirmed its structural integrity. The four signals in the 31P-

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Helvetica Chimica Acta Vol. 86 (2003)
Scheme 3. Synthesis and Isolation of Hexacoordinated Phosphate Anions 3a and 3b
1. PCl5
RO2S
RO2S
O
O
RO2S
RO2S
OH
OH
2. DMF, r.t.
P
[Bu3NH]
3. Bu3N, r.t.
3
3a R = p-Tol
b R = Me
7a,b
N
N
RO2S
RO2S
O
O
P
malachite green
chromatography
3
3a R = p-Tol
b R = Me
8
Fig. 2. Partial 31P-NMR spectrum ((D6)acetone, 162 MHz) of 8 ¥ 3b
Fig. 3. Partial 31P-NMR spectrum ((D6)DMSO; 162 MHz) of 8 ¥ 3a
NMR spectrum could not be explained by the presence of four different chemical
compounds. The easiest means to explain the four signals was to consider that anion 3a
exists in four diastereoisomeric forms.
2.4. Hypothetical Enantiomeric Conformations for 4,5-Bis(sulfonyl)pyrocatechols.
The observation of four stereoisomers for 3a was somewhat reminiscent of what is

Helvetica Chimica Acta Vol. 86 (2003)
69
usually noticed for tris(bidentate) octahedral complexes made of nonplanar chelate
rings, such as [Co(en)3]3‡ (en ˆ ethane-1,2-diamine). As first noted by Corey and
Bailar, when chelate rings adopt chiral conformations (d and l), four diastereoisomers
always appearing in enantiomer pairs are generated due to the inherent chirality of
the octahedral complexes (D and L): D(ddd)/L(lll), D(ddl)/L(lld), D(dll)/L(ldd),
D(lll)/L(ddd) [9][1]. However, the planarity of the chelating pyrocatechol rings in 3a
was not compatible with such an explanation. If chiral conformations were to be the
source of the four signals observed in the 31P-NMR spectrum, they would have to result
from the spatial arrangement of the sulfonyl substituents.
We turned our attention to the work of Collard and co-workers [10]. In 1995, they
reported the synthesis of highly twisted substituted arenes of the general 1,4-dialkyl-
2,3,5,6-tetra(alkylsulfonyl)benzene skeleton (e.g., 9). In these compounds, X-ray
structural analysis showed that the ortho-positioned sulfonyl groups adopt an
alternating orientation, and the rings are highly twisted with external torsion
angles of 468. Collard and co-workers also observed dynamic NMR behavior
corresponding to a slow isomerism of the sulfonyl side chains (barrier of interconver-
sion of ca. 17 kcal ¥ molÀ1).
C4H9
C10H21O2S
SO2C10H21
SO2C10H21
C10H21O2S
C4H9
9
2.5. Solid-State Analysis of 4,5-Bis(p-tolylsulfonyl)veratrol 6a and 4,5-Bis(methyl-
sulfonyl)veratrol 6b. X-Ray structural analyses of crystals of 6a1) and 6b were
undertaken and confirmed the existence of an −up-and-down× arrangement of the ortho-
positioned sulfonyl groups in the solid state (Figs. 4 and 5). In both compounds, the
sulfonyl groups are twisted around a pseudo-C2 axis passing through the midpoint of
the C(1)ÀC(2) and C(4)ÀC(5) bonds. The SÀCÀCÀS torsional angles are close to 08
(3.4(6)8 and À2.0(6)8 and 6a and 6b, resp.) meaning a quasi-planar pyrocatechol ring
contrary to what was observed by Collard and co-workers [10]. The crystal structures of
6a and 6b being centrosymmetric, both d and l configurations were observed in the
solid state. The molecular packing of 6b shows stacking interactions of the
pyrocatechol rings (Fig. 6) on both sides of inversion centres and running along the
[100] direction (interplane distances: 3.481(9) and 3.527(9) ä).
2.6. Synthesis, Diastereoisomeric Conformations, and Stereodynamics of the 4,5-
Bis(camphor-10-sulfonyl)veratrol (6c). To demonstrate in solution the presence of
these enantiomeric d and l conformations, we decided to study a model compound
which would be easily accessible following the synthetic route already established and
whose conformational isomerism could be determined by 1H-NMR. The observation of
1
)
The present report follows a previous communication of our group, which described the structure of 6a in
detail (CCD refcode: qoycuc) [11].

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Fig. 4. Views of the conformations adopted by ortho-positioned sulfonyl groups and configurational assignment
(d and l). a ¥¥¥¥ a constitutes the plane of the phenyl ring, and b ¥¥¥¥ b represents the line passing through the R
substituents.
Fig. 5. ORTEP View of 6b (d configuration) with the atom numbering. Ellipsoids are represented with 40%
probability level. Selected torsional angles [8]: C(3)ÀC(4)ÀO(1)ÀC(7) ˆ À4.7(6), C(6)ÀC(5)ÀO(2)ÀC(8) ˆ
À9.3(6), C(2)ÀC(1)ÀS(1)ÀC(9) ˆ 74.2(4), and C(1)ÀC(2)ÀS(2)ÀC(10) ˆ 79.8(4).
the enantiomeric d and l conformations necessitates the presence of another
stereogenic element, and the introduction of a chiral probe was thus mandatory. The
rapid access to the enantiomerically pure sodium camphor-10-sulfinate salt 10 from the
(‡)-camphor-10-sulfonyl chloride led us to consider the synthesis of the derived 4,5-
bis(camphor-10-sulfonyl)veratrol (6c). Chiral C2-symmetric compound 6c would fit
perfectly the above-mentioned criteria.
The synthesis of 6c from diiodoveratrol 5 turned out to be more challenging than
those of 6a and 6b (Scheme 4). Sulfinate salt 10 was unstable and decomposed (IR
monitoring) slowly at room temperature even under inert conditions (N2). A large
excess of 10 (25 equiv.) and longer reaction times were required for the CuI-mediated
bis(sulfonylation) reaction to proceed with good yield (85%). 1H-NMR Analysis of 6c
at room temperature revealed as expected two sets of signals corresponding to the
diastereoisomeric d and l conformations. The diastereotopic protons CH2(10) (Fig. 7)

Helvetica Chimica Acta Vol. 86 (2003)
71
Fig. 6. Perspective view of the crystal packing of 6b along the [100] direction showing the alternate stacking
interactions of l and d configurations
and the geminal Me groups of the camphorsulfonyl units gave rise to split signals, as
well did the aromatic protons HÀC(3') and the MeO groups of the veratrol ring
(Table). The two diastereoisomeric d and l conformations in 6c are not identically
populated as the integration of the respective signals revealed a 2.25 :1 diastereomer
ratio. Thus, the chiral camphorsulfonyl units lead not only to the magnetic non-
equivalency of diastereoisomeric conformations but also to an asymmetric induction in
favor of one of the configurations2).
Dynamic conformational isomerism was detected for 6c in the 1H-NMR spectrum
in a temperature range of 1108 (Fig. 8), demonstrating without ambiguity the slow
interconversion at room temperature between diastereoisomeric d and l conformations
Scheme 4
O
H
H
O
25 equiv.
O2S
O2S
OMe
I
I
OMe
OMe
10
SO2Na
CuI, DMF, 110 °
85 %
OMe
5
H
H
6c
O
2
)
No attempts were made to determine which of the two diastereoisomeric conformations is preferred.

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1
Fig. 7. Partial H-NMR spectrum (400 MHz, CDCl3) of 6c. Methylene protons CH2(10) of the major (M) and
minor (m) diastereoisomeric conformations d or l2).
Table. 1H-NMR Chemical Shifts (400 MHz, CDCl3) for Selected Protons of the Major (M) and Minor (m)
Diastereoisomeric Conformations of 6c2)
m
M
HÀC(3')
HÀC(10)
MeO
8.29
5.19, 4.64
3.16
8.20
4.98, 4.63
3.28
Me
1.04, 0.91
1.05, 0.90
Fig. 8. Variable-temperature 1H-NMR measurements (400 MHz, (D6)DMSO) of 6c. HÀC(3') signal of the
major (M) and minor (m) diastereoisomeric conformations d and l2).

Helvetica Chimica Acta Vol. 86 (2003)
73
of aromatic ortho-disulfones. The corresponding free energy of interconversion is ca.
19.8 kcal ¥ molÀ13).
2.7. Diastereoisomeric Conformations of Tris(4,5-bis(sulfonyl)benzenediolato)phos-
phate Anions. Taking thus into account the existence in the solid state and in solution
of enantiomeric d and l conformations of the ortho-disulfones of each of the three
chelating pyrocatechols, four diastereoisomers always appearing in enantiomer pairs
can be considered for anions 3a and 3b: D(ddd)/L(lll), D(ddl)/L(lld), D(dll)/
L(ldd), and D(lll)/L(ddd). Two of the four diastereoisomers of anion 3b are depicted
in Fig. 9, wherein the orientation of the sulfonyl groups determined by the X-ray
structural analysis is used4). For anion 3a, diastereoisomers D(ddd) and D(lll) are
represented in Fig. 10, wherein a geometry calculated by molecular mechanics (MM‡ )
was used for the sulfonyl groups5).
Fig. 9. Views (Chem3D) of two diastereoisomers of 3b (D(ddd) and D(lll))
For anion 3a, each of the signals observed in the 31P-NMR spectrum can thus be
assigned to one of the diastereoisomers considering that the rate of interconversion of
the p-tolylsulfonyl groups is slow compared to the NMR time scale. A stereodynamic
study was attempted by variable-temperature 31P-NMR measurements. Within the
range of temperature studied (25 1478), no coalescence of the four signals was
observed, showing an even slower equilibration of the d and l conformations than for
6c6). Unfortunately, degradation of anion 3a occurred at high temperature most
probably through an acid-catalyzed decomposition pathway limiting the scope of this
investigation (Fig. 11)7).
3
)
The relationship DG=ˆ RTc (22.96 ‡ ln (Tc/Dn)) was used to determine the activation energy DG= from
the coalescence temperature Tc [K] and the frequency separation of the peaks Dn [Hz].
No calculations have been attempted on those structures.
4
5
6
)
)
)
Hyperchem 5.0, HyperCube Inc., 1996.
The activation energy DG= is higher than 21.4 kcal ¥ molÀ1, considering a Tc ꢀ 1478 and a minimum Dn
value of 29 Hz (Dd ˆ À77.35 to À77.53).
7
)
Dynamic conformational isomerism was not detected for 3a in the 31P-NMR spectrum at 1478, the
corresponding free energy is thus higher than 19.5 kcal ¥ molÀ1
.

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Fig. 10. Views (Chem3D) of two diastereoisomers of 3a (D(ddd) and D(lll))
Fig. 11. Partial 31P-NMR spectrum (162 MHz, DMSO) of salt 8 ¥ 3a a) before and b) after heating at 1478
For anion 3b, two explanations can be proposed to rationalize the observation of a
single signal in the 31P-NMR spectrum. The four diastereoisomers are either
isochronous or the rate of interconversion between the d and l conformations is rapid
compared to the NMR time scale, the first explanation being more likely than the
second. The smaller size of the methylsulfonyl groups compared to the p-tolylsulfonyl
induce probably less strain between the adjacent chelating pyrocatechol rings of the
four diastereoisomers, and identical 31P-NMR chemical shifts for the four derivatives is
understandable.
2.8. Computational Studies of the 4,3-Bis(sulfonyl)arenes. To gain insight into the
rate of conversion for 4,5-bis(sulfonyl)veratrols, we have performed quantum-chemical
computations for the barrier to internal rotation of sulfonyl groups in the model
compound: 4,5-bis(methylsulfonyl)veratrol (6b). The computations at the B3LYP level
[12] and the 3-21G basis set [13] as implemented within the Gaussian 98 package [14]
were used for the geometry optimizations of local minima. The structures of 4,5-
bis(methylsulfonyl)veratrol (6b) in different conformations were optimized by
minimization of the energy with respect to all the geometrical parameters, thus leading
to the global minimum (l-6b, Fig. 12). In this particular conformation, the two ortho-
positioned sulfonyl groups adopted the characteristic −up-and-down× arrangement

Helvetica Chimica Acta Vol. 86 (2003)
75
Fig. 12. Schematic B3LYP/3-21G potential-energy surface for the rotational barrier of the sulfonyl groups of 6b
and optimized molecular structures for key stationary points
observed also in the X-ray structure. Extended investigations of the potential-energy
surface for the internal rotation of the sulfonyl groups were also undertaken. In the
initial step, the potential-energy-surface scan for the internal rotation about the CÀS
bonds was performed by imposing the C(3)ÀC(2)ÀS(2)ÀC(10) and
C(6)ÀC(1)ÀS(1)ÀC(9) dihedral angles (for atom numbering, see Fig. 5) at the
regular increments. The reaction coordinate started from the initial value of 110.88
corresponding to the equilibrium geometry of conformer l-6b, and was incrementally
decreased to 08. As shown in Scheme 5, the same dihedral-angle deformation imposed
to both sulfonyl groups results in their conrotatory movement. At each of the fixed
dihedral angles, the full geometry optimization was performed. The minimum-energy
reaction path thus established led eventually to an additional local minimum where the
C(3)ÀC(2)ÀS(2)ÀC(10) and C(6)ÀC(1)ÀS(1)ÀC(9) dihedral angles decreased to
34.78. The optimized geometry projection of this new rotamer 11 is shown in Fig. 12,
and its relative energy is 32.7 kcal ¥ molÀ1 higher when compared to the global minimum
of l-6b.
The rotamer corresponding to the highest point of the minimum energy profile was
used to locate the opposed conformer of the transition state TS1 for the veratrol 6b
racemization (Scheme 5 and Fig. 12). The vibrational-frequency analysis of TS1
confirmed the presence of a unique imaginary frequency: a necessary condition for the
characterization of transition states. We also undertook the investigation of the

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Scheme 5. Conrotatory and Disrotatory Mode for the Conformational Isomerism in 4,5-Bis(sulfonyl)veratrols
potential-energy surface corresponding to the disrotatory mode of isomerization via
the geared transition state TS2 (Scheme 5). However, for this pathway, we were not
able to locate any stationary point at this level of theory8).
The intrinsic-reaction-coordinate calculations (IRC) [15] were initiated from the
TS1 to ensure that the rotamer 11 is connected to this saddle point. The main geometry
deformation along this intrinsic reaction coordinate appears to follow the conrotatory
movement of the sulfonyl groups. It is noteworthy that, during the IRC procedure, we
did not restrict any geometry parameters explicitly. The IRC eventually reached the
local minimum 11, and the investigation was not pursued beyond this point. Indeed, it
was obvious from the previously established minimum-energy reaction path that the
local minimum 11 and the global minimum l-6b are connected on the potential-energy
surface via the internal rotation of the sulfonyl groups. The schematic coordinate
diagram for the barrier to internal rotation of veratrol l-6b is summarized in Fig. 12.
This barrier amounts to 41.4 kcal ¥ molÀ1 as computed by the B3LYP/3-21G method.
Clearly, this barrier is much higher than that observed by NMR for 6c. While the 3-21G
basis set is convenient for exploration of the potential-energy surfaces for molecules
having the size of veratrol 6b, such a modest basis set is not, however, reliable enough
for the energy calculations. Therefore, we resorted to use the B3LYP with the 6-31G*
basis set [16] for the energy calculations and geometry optimizations of the key
stationary points located previously on the B3LYP/3-21G surface. The geometry of
8
)
In addition to the conrotatory and disrotatory modes of isomerization, it is possible that step-by-step
processes can take place. In this case, only one sulfonyl group would rotate at a time. Our preliminary
modeling calculations at the semiempirical level indicated, however, that such a reaction proceeds via
energy barriers higher than those determined for the concerted rotation of the two functional groups.

Helvetica Chimica Acta Vol. 86 (2003)
77
each stationary point was re-optimized without any restriction, and the vibrational
analysis was performed for the transition states. With the larger basis set, the key
dihedral angle C(1)ÀC(2)ÀS(2)ÀC(10), which controls the orientation of the
methylsulfonyl group, is optimized to 73.58 and 179.98 for the global minimum l-6b'
and the transition state TS1', respectively. The optimized geometry parameters of
stationary points were generally quite similar for both theory levels. However, with the
larger set, the structure optimization of the local minimum 11 resulted in its
rearrangement into the global minimum l-6b'. Therefore the energy profile of the
veratrol racemization is simplified, as shown in Fig. 13. At the B3LYP/6-31G* level of
theory, the barrier to internal rotation of sulfonyl groups amounts to 22.3 kcal ¥ molÀ1.
This result is in good agreement with the value observed for 6c. However, it should be
repeated that computations were done on the model compound 6b with methylsulfonyl
groups, while the experimental value was observed for the camphorsulfonyl-substituted
veratrol 6c. It should be noted though, that in the conrotatory mode of isomerization
(Scheme 5), the steric bulk of the alkyl substituent of the sulfonyl group is not the
critical factor for the height of the energy barrier for the process.
Fig. 13. Schematic B3LYP/6-31G* potential-energy surface for the conformational isomerism of compound 6b
and optimized molecular structures for key stationary points
3. Conclusions. Aromatic ortho-disulfone derivatives (ortho-(RSO2)2Ar) adopt
two precise enantiomeric conformations in solution that can be detected by the
formation of chiral hexacoordinated tris[4,5-bis(sulfonyl)benzene-1,2-diolato]phos-
phate anions or camphorsulfonyl derivatives. The rate of conformer interconversion is

78
Helvetica Chimica Acta Vol. 86 (2003)
slow on the NMR time scale and seems to depend upon the nature of the sulfonyl side
chains RSO2 (R ˆ Bu, DG=ˆ 17 kcal ¥ molÀ1 [10]; R ˆ camphor-10-yl, DG=ˆ 19.8 kcal ¥
molÀ1; R ˆ p-tolyl, DG=> 20kcal ¥ mol À1).
We thank Damien Jeannerat and Scott Smith (NHMFL) for their attempts to simulate the NMR
stereodynamic, as well as the Swiss National Science Foundation, the Federal Office for Education and Science
¬ ¬
¬
¡
(COST D11), the Societe Academique de Geneve for financial support, and the Fondation de Famille Sandoz
(J. L. ).
Experimental Part
General. Solvents and chemicals are used without purification unless indicated. Toluene was distilled from
Na metal, and MeOH, hexane, and CH2Cl2 from calcium hydride. Dimethylformamide (DMF) was distilled
from MgSO4 under vacuum and stored over 4 ä molecular sieves under N2. CHCl3 (Fluka) and CDCl3 were
filtered through a plug of basic alumina prior to use. Flash chromatography (FC): pressure 0.1 0.2 bar; J. T.
Baker silica gel (30 60 mm, 230 400mesh) or Fluka neutral alumina type 507C (100 125 mesh). Anal. gas
chromatography (GC): Hewlett-Packard-HP6890 chromatograph; 30m  0.32 mm nonpolar column (95%
dimethylpolysiloxane/5% diphenylpolysiloxane, 0.25 mm) from Alltech (HP5); classical conditions: 808 for
2 min, then 58/min, 1808 for 5 min; tR in min. M.p.: in open capillary tubes; Stuart-Scientific SMP3 melting-point
apparatus; uncorrected. IR Spectra: Perkin-Elmer 1600 FT-IR spectrophotometer, with KBr. NMR Spectra:
Bruker AMX-400 and Varian XL-200 spectrometers; at r.t., unless otherwise specified; chemical shift d in ppm
from internal standard (SiMe4 for d(H) and d(C), H3PO4 for d(P)), coupling constants J in Hz. MS: for
electrospray (ES), Finnigan SSQ-7000 spectrometer, for electron ionization (EI) Varian CH4 or SM1
spectrometer; in m/z (rel. %).
1. 4,5-Diiodoveratrol (ˆ1,2-Diiodo-4,5-dimethoxybenzene; 5). A soln. of H5IO6 (2.9 g, 12.8 mmol,
1.0equiv.) in MeOH (19.0ml) was stirred for 15 min at 20 8. I2 (6.4 g, 25.1 mmol, 2.0equiv.) was then added
and the mixture stirred vigorously for 10min. Veratrol ( ˆ1,2-dimethoxybenzene; 4.0ml, 31.4 mmol, 2.5 equiv.)
was added, and the mixture was stirred for 4 h at 708 ( ! important white precipitate). The hot mixture was then
poured into a diluted sodium pyrosulfite soln., and the white solid formed was collected by filtration over a
B¸chner funnel and washed with MeOH. The resulting product was then dissolved in AcOEt and the soln. dried
(Na2SO4) and evaporated: 11.5 g (29.4 mmol, 94%) of 5. No further purification was required. GC: tR 8.19. M.p.
1338 (AcOEt/cyclohexane). 1H-NMR (CDCl3, 200 MHz): 7.21 (s, 2 H); 3.81 (s, 6 H). 13C-NMR (CDCl3,
‡
50MHz): 149.6; 121.6; 96.0; 56.1. EI-MS (70eV): 390 (8,
(C8H8I2O2 ; calc. 389.86139).
M ), 264 (100), 249 (23). HR-MS: 389.86205
‡
2. Sodium Methanesulfinate. A soln. of sodium sulfite (5.0g, 39.6 mmol, 1.0equiv.) in H 2O (19.0ml) was
vigorously stirred for 10min at 20 8. NaHCO3 (6.8 g, 81.4 mmol, 2.0equiv.) was added, and the mixture was
stirred for 1 h at 508. Methanesulfonyl chloride (3.0ml, 39.2 mmol, 1.0equiv.) was carefully added, and after the
addition, the mixture was vigorously stirred at 508 for 4 h. After cooling to 208, the H2O was evaporated for
several h. MeOH (5.0ml) was added to the white residue to separate the sodium methanesulfinate from MeOH-
insoluble salts, and the mixture was stirred overnight. Filtration and evaporation gave 3.7 g (94%) of the title
product. IR (KBr): 1040 960s (SO2À). 1H-NMR (CD3OD, 200 MHz): 2.71. 13C-NMR (CD3OD, 50MHz):
49.63.
3. Sodium Camphor-10-sulfinate (ˆ Sodium (1S,4R)-7,7-Dimethyl-2-oxobicyclo[2.2.1]heptane-1-methane-
sulfinate; 10). As described for sodium methanesulfinate, from sodium sulfite (750mg, 6.0mmol, 1.0equiv.) in
H2O (9.0ml), NaHCO 3 (1.0g, 12.0mmol, 2.0equiv.), and ( ‡)-camphor-10-sulfonyl chloride (ˆ(1S,4R)-7,7-
dimethyl-2-oxobicyclo[2.2.1]heptane-1-methanesulfonyl chloride; 1.4 g, 5.7 mmol, 0.9 equiv.): 1.4 g (100%) of
10. IR (KBr): 1040 960s (SO2À). 1H-NMR (CD3OD, 400 MHz): 2.40 2.26 (m, 2 H); 2.16 (dd, J ˆ 191, 13,
2 H); 2.10 1.99 ( m, 2 H); 1.87 (d, J ˆ 18, 1 H); 1.60 1.53 ( m, 1 H); 1.46 1.40( m, 1 H); 1.0 6 (s, 3 H); 0.90
(s, 3 H). 13C-NMR (CD3OD, 100 MHz): 218.3; 60.2; 58.4; 42.7; 42.4; 26.5; 25.5; 18.8; 18.7.
4. 4,5-Bis(p-tolylsulfonyl)veratrol (ˆ1,2-Dimethoxy-4,5-bis[(4-methylphenyl)sulfonyl]benzene; 6a). A
soln. of 5 (3.3 g, 8.5 mmol, 1.0equiv.) in freshly distilled DMF (from MgSO 4; 67.0ml) was vigorously stirred
under Ar for 10min at 20 8. CuI (5.0g, 26.2 mmol, 3.0equiv.) was added, and the mixture was stirred for 10min.
Sodium 4-methylbenzenesulfinate (5.0g, 28.1 mmol, 3.3 equiv.) was added and the mixture was vigorously
stirred under Ar for 72 h at 1108. The mixture was allowed to cool to 208 and diluted with H2O (100.0 ml) and
AcOEt (100.0 ml). The precipitate formed was removed by filtration over a B¸chner funnel. The org. layer was

Helvetica Chimica Acta Vol. 86 (2003)
79
washed with H2O (2 Â 70.0 ml) and sat. NaHCO3 soln. (2 Â 50.0 ml), dried (Na2SO4), and evaporated: 3.0g
(88%) of 6a. Recrystallization from AcOEt/hexane gave 6a. White needles. GC: tR 9.66. M.p.: 193 1948
(AcOEt/cyclohexane). 1H-NMR (CDCl3, 200 MHz): 7.91 (s, 2 H); 7.81 (d, J ˆ 4, 4 H); 7.27 (d, J ˆ 4, 4 H); 4.03
(s, 6 H); 2.39 (s, 6 H). 13C-NMR (CDCl3, 50MHz): 151.9; 143.9; 139.2; 133.0; 129.3; 127.8; 115.3; 56.8; 21.6. EI-
‡
MS (70 eV): 446 (100, M ), 414 (31); 382 (70), 243 (60), 212 (59), 139 (73), 91 (89). HR-MS: 446.08817
‡
(C22H22O6S2 ; calc. 446.08578).
5. 4,5-Bis(methylsulfonyl)veratrol (ˆ1,2-Dimethoxy-4,5-bis(methylsulfonyl)benzene; 6b). As described for
6a (Exper. 4), from 5 (1.0g, 2.6 mmol, 1.0equiv.), DMF (21.0ml), CuI (1.5 g, 7.7 mmol, 3.0equiv.), and sodium
methanesulfinate (786 mg, 7.7 mmol, 3.0 equiv.): 750 mg (2.6 mmol, 100%) of 6b. Recrystallization from
AcOEt/cyclohexane gave 6b. Colorless rectangular crystals. GC: tR 9.71. M.p. 222 2248 (AcOEt/cyclohexane).
1H-NMR (CDCl3, 200 MHz): 7.76 (s, 2 H); 4.0 2 (s, 6 H); 3.42 (s, 6 H). 13C-NMR (CDCl3, 50MHz): 114.6; 56.8;
‡
‡
45.6. EI-MS (70 eV): 294 (100, M ), 169 (61), 140 (27), 79 (7), 63 (10). HR-MS: 294.02109 (C10H14O6S2 ; calc.
294.02319).
X-Ray Crystal Structure of 6b. C10H14O6S2, Mr 294.3; m ˆ 3.830mm À1, dx ˆ 1.477 g ¥ cmÀ3, monoclinic, P 21/c,
Z ˆ 4, a ˆ 7.5870(3), b ˆ 8.5460(7), c ˆ 20.417(1) ä, b ˆ 90.923(3)8, Vˆ 1323.6(3) ä3; colourless prism 0.15 Â
0.18 Â 0.29 mm. Cell dimensions and intensities were measured at r.t. on a Stoe Stadi4 diffractometer with
graphite-monochromated Cu[Ka] radiation (l ˆ 1.5418 ä), w-2q scans, scan width 1.058 ‡ 0.35 tg q, and scan
speed 0.068/s. Two reference reflections measured every 45 min showed no variation. À7 < h < 7; 0 < k < 8; 0 <
l < 21. The structure was solved by direct methods [17]. Measured reflections, 1658; unique reflections, 1605, of
which 1277 were observables (j Fo j> 4s (Fo)); Rint ˆ 0.030. Data were corrected for Lorentz and polarization
effects and for absorption [18] (Tmin,max ˆ 0.43306, 62375). All calculations were performed with the XTAL
system [19]. Full-matrix least-squares refinement based on F with a weight of 1/(s2(Fo) ‡ 0.00012(Fo2)) gave the
final values R ˆ 0.044, wR ˆ 0.042, and S ˆ 1.97(4) for 206 variables and 1277 contributing reflections. The
maximum shift/error on the last cycle was 0.56 ¥ 10À2. The final difference electron-density map showed a
maximum of ‡ 0.27 and a minimum of À 0.30 eäÀ3. H-Atoms of the Me substituents were refined with
restraints on bond lengths and bond angles and both aromatic H-atoms were refined without restraint.
Crystallographic data for 6b (excluding structure factors) have been deposited with the Cambridge
Crystallographic Data Center (deposition N8 CCDC 178337). Copies of the data can be obtained, free of charge,
on application to the CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: ‡ 44(1223)336-033; e-mail:
deposit@ccdc.cam.ac.uk).
6. 4,5-Bis(camphor-10-sulfonyl)veratrol (ˆ1,2-Bis{{[(1S,4R)-7,7-dimethyl-2-oxobicyclo[2.2.1]hept-1-yl]-
methyl}sulfonyl}-4,5-dimethoxybenzene; 6c). As described for 6a (Exper. 4), from 5 (299 mg, 0.7 mmol,
1.0equiv.), DMF (31.0ml), CuI (432 mg, 2.3 mmol, 3.0equiv.), and 10 (4.5 g, 19.1 mmol, 25.0equiv.): 370mg
(0.7 mmol, 85%) of 6c as a brown oil, which was purified by FC (SiO2, AcOEt/cyclohexane 1:1): pale yellow oil.
GC: tR 2.95. 1H-NMR (CDCl3, 400 MHz; (m) ˆ minor; (M) ˆ major): 8.27 (s, 2 H (m)); 8.1827 (s, 2 H (M));
5.19 (d, J ˆ 14.3, 2 H (m)); 4.98 (d, J ˆ 14.2, 2 H (M)); 4.64 (d, J ˆ 14.3, 2 H (m)); 4.63 (d, J ˆ 14.2, 2 H (M));
3.27 (s, 6 H (M)); 3.56 (d, J ˆ 14.9, 2 H (M)); 3.36 (d, J ˆ 15.4, 2 H (m)); 2.90( d, J ˆ 14.9, 2 H (M)); 2.79 (d, J ˆ
15.4, 2 H (m)); 3.15 (s, 6 H (m)); 2.43 1.43 (m, 10 H); 1.04 (s, 6 H (M)); 1.03 (s, 6 H (m)); 0.90 (s, 6 H (m));
0.89 (s, 6 H (M)). 13C-NMR (CDCl3, 100 MHz): 216.4 (m); 214.3 (M); 163.6; 162.9; 162.4; 70.8 (m); 65.6 (M);
60.3; 59.4 (m); 58.9 (M); 51.4 (M); 50.1 (m); 49.1 (m); 48.5 (M); 42.5; 36.6 (m); 35.9 (M); 32.3 (M); 31.5 (m);
‡
27.0( M); 26.8 (m); 26.1 (m); 25.5 (M); 19.7; 19.5 (M); 19.3 (m). EI-MS (70eV): 566 (2, M ), 534 (100), 502
(21), 216 (45), 151 (53).
7. 4,5-Bis(p-tolylsulfonyl)pyrocatechol (ˆ4,5-Bis[(4-methylphenyl)sulfonyl]benzene-1,2-diol; 7a). A soln.
of 6a (1.8 g, 3.9 mmol, 1.0equiv.) in CH 2Cl2 (80.0 ml) was stirred under Ar for 10 min at 208. BBr3 (2.4 ml,
19.7 mmol, 5.0equiv.) was then slowly and carefully added, and the mixture was vigorously stirred at 45 8 during
16 h. The mixture was allowed to cool to 08, and H2O was carefully added. The org. layer was washed with 10%
HCl soln. (2 Â 50ml), dried (Na 2SO4), and evaporated: 1.5 g (93%) of 7a. GC: tR 9.43. 1H-NMR (CDCl3,
200 MHz): 7.98 (s, 2 H); 7.78 7.74 (d, J ˆ 4, 4 H); 7.49 (s, 2 H); 7.24 7.20( d, J ˆ 4, 4 H); 2.35 (s, 6 H). 13C-NMR
‡
(CDCl3, 50MHz): 147.2; 144.2; 138.7; 131.9; 129.4; 127.7; 120.1; 21.6. EI-MS (70eV): 418 (15, M ), 354 (11), 264
‡
(53), 156 (17), 155 (30), 108 (37), 91 (100), 65 (37). HR-MS: 418.05277 (C20H18O6S2 ; calc. 418.05447).
8. 4,5-Bis(methylsulfonyl)pyrocatechol (ˆ4,5-Bis(methylsulfonyl)benzene-1,2-diol; 7b). As described for
7a (Exper. 7), from 6b (475 mg, 1.6 mmol, 1.0equiv.), CH 2Cl2 (26.5 ml), and BBr3 (1.0ml, 8.2 mmol, 5 equiv.) at
208 for 16 h: 430 mg (1.6 mmol, 100%) of 7b. Recrystallization from acetone/hexane gave 7b. White solid. GC:
1
tR 9.66. M.p.: 2658 (dec.; acetone/hexane). H-NMR ((D)6acetone, 200 MHz): 9.60 (s, 2 H); 7.72 (s, 2 H); 3.36
‡
(s, 6 H). 13C-NMR ((D)6acetone, 100 MHz): 149.8; 132.7; 120.6; 45.5. EI-MS (70 eV): 266 (100, M ), 203 (46),
‡
187 (23), 141 (39), 125 (46), 79 (49), 63 (55). HR-MS: 265.99112 (C8H10O6S2 ; calc. 265.99188).

80
Helvetica Chimica Acta Vol. 86 (2003)
9. Bis[4-(dimethylamino)phenyl]phenylmethylium Tris{4,5-bis[(4-methylphenyl)sulfonyl]benzenedi-
olato(2À)-kO,kO'}phosphate(1À) (8 ¥ 3a). In a flame-dried flask under N2 (addition funnel for solid and reflux
condenser topped with a gas outlet connected to a trap of conc. NaOH soln.), a soln. of 7a (35 mg, 83 mmol,
3.0equiv.) in CH 2Cl2 (1.0ml) was stirred for 10min at r.t. PCl (6 mg, 27 mmol, 1.0equiv.) was added and the
5
mixture stirred for 10min at r.t. The CH 2Cl2 was then evaporated, the DMF (1.0ml) added and the mixture
stirred for 12 h at r.t. Then Bu3N (6.5 ml, 27 mmol, 1.0equiv.) was added. After 8 h of additional stirring at 25 8,
the solvent was evaporated and the resulting brown oil purified by cation exchange with malachite green in
CH2Cl2 followed by FC (SiO2, CH2Cl2): 8 ¥ 3a (27.5 mg, 63%). Deep green oil. 1H-NMR ((D)6DMSO,
400 MHz): 8.07 (br. s, 2 H); 8.00 (s, 6 H); 7.83 (br. d, J ˆ 8.3, 16 H); 7.75 (br. d, J ˆ 1.8, 4 H); 7.34 (br. d, J ˆ 9.1,
4 H); 7.30 7.26 ( m, 17 H); 6.83 (d, J ˆ 9.1, 4 H); 3.21 (s, 12 H); 2.42 (s, 18 H). 13C-NMR ((D)6DMSO,
100 MHz): 155.5; 148.3; 148.2; 148.0; 143.7; 143.4; 139.7; 139.4; 139.2; 132.7; 131.9; 129.2; 129.1; 127.9; 127.7;
126.6; 120.2; 114.6; 114.4; 112.3; 77.2; 40.3; 21.5. 31P-NMR ((D)6DMSO, 162 MHz); À76.72; À77.34; À77.53;
À78.23. ES-MS: pos. mode: 329 (100); neg. mode: 1279 (36), 1115 (100); 141 (39), 125 (46), 79 (49), 63 (55).
10. Bis[4-(dimethylamino)phenyl]phenylmethylium Tris[4,5-bis(methylsulfonyl)benzenediolato(2À)-
kO,kO']phosphate](1À) (8 ¥ 3b). As described for 8 ¥ 3a (Exper. 9), from 7b (100 mg, 375 mmol, 3.0equiv.) in
MeCN (8.0ml), PCl 5 (26 mg, 125 mmol, 1.0equiv.), then in DMF (8.0ml) with Bu 3N (30 ml, 124 mmol,
1.0equiv.). Of the crude pale yellow oil (188 mg), an aliquot (24 mg, 24 mmol, 1.0equiv.) was purified by cation
exchange with malachite green (24.5 mg, 26 mmol, 1.1 equiv.) in CH2Cl2/acetone 1:2 (3 ml), followed by FC
(SiO2, CH2Cl2/AcOEt 8 :2): 8 ¥ 3b (15 mg, 55%). This corresponds to an overall yield of 82%. Deep green oil.
1H-NMR ((D)6acetone, 400 MHz): 7.77 7.74 (m, 1 H); 7.67 (br. s, 1 H); 7.64 7.60( m, 2 H); 7.53 7.52
(m, 5 H); 7.44 7.38 (m, 6 H); 7.10( d, J ˆ 9.6, 4 H); 3.38 (br. s, 12 H); 3.36 (br. s, 18 H). 13C-NMR ((D)6acetone,
100 MHz): 157.0; 148.5 (d, 2J(C,P) ˆ 5.0); 140.6; 134.4; 133.0; 132.9; 128.5; 127.0; 119.6; 113.6; 113.2
(d, 3J(C,P) ˆ 17.4); 59.5; 44.6; 40.1. 31P-NMR ((D)6acetone, 162 MHz): À76.16. ES-MS: pos. mode: 329
(100); neg. mode: 823 (36), 539 (42), 265 (36), 173 (39), 113 (88), 95 (100).
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Received May 29, 2002