Atropisomerism in a 10-Membered Ring with Multiple Chirality Axes: (3 Z,9 Z)-1,2,5,8-Dithiadiazecine-6,7(5 H,8 H)-dione Series

Article abstract of DOI:10.1021/acs.joc.8b01009

For the first time, chirality in (3Z,9Z)-1,2,5,8-dithiadiazecine-6,7(5H,8H)-dione series was recognized. Enantiomers of the 4,9-dimethyl-5,8-diphenyl analogue were isolated at room temperature, and their thermal stability was determined. X-ray crystallogr

Full text of DOI:10.1021/acs.joc.8b01009

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Featured Article
Atropisomerism in a ten-membered ring with multiple chirality
axes: (3Z,9Z)-1,2,5,8-dithiadiazecine-6,7(5H,8H)-dione series
Vesna Risso, Daniel FARRAN, Guilhem Javierre, Jean-Valère Naubron, Michel Giorgi, Patrick
Piras, Marion Jean, Nicolas Vanthuyne, Roberta Fruttero, Dominique Lorcy, and Christian Roussel
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01009 • Publication Date (Web): 08 Jun 2018
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is usually neutralized with a NaHCO3 aqueous solution.3 We treated first with aqueous NaOH solution
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to eliminate the formed mꢀCPBA salt and in the present case the expected sulfate. H NMR of the
crude in CDCl3 showed the presence of unreacted thiazolinethione 1 and thiazolinone 2 thanks to the
typical chemical shift of the coupled proton in position 5 at 6.33 and 5.85 ppm respectively. In
addition, the analysis of the crude organic reaction medium was performed on a (S,S)ꢀWhelk O1 chiral
column which was installed in a ten chiral column screening unit equipped with on line CD and
polarimeter detectors dedicated to routine enantiomer separation.4 (S,S)ꢀWhelk O1 column is well
known to separate enantiomers of atropisomeric thiazolinethiones and thiazolinones but also to
discriminate thiazolinone from thiazolinethione on the basis of the different hydrogen bonding abilities
of thiocarbonyl versus carbonyl groups in these heterocyclic systems.5,6 The UV trace showed a very
clean crude reaction medium with two large peaks corresponding to the unreacted thiazolinethione 1
and the targeted thiazolinone 2 which were known in the laboratory.2 In addition, two very minor extra
peaks (ca 2% each) which were respectively eluted before and after the two main heterocycles
attracted our attention (see SI). The CD (254 nm) detector trace was as expected silent for the two
main compounds 1 and 2 but quite intriguingly two intense CD signals of opposite sign and similar
intensity were associated to the two very minor peaks hardly detected in UV. These observations
militated in favor of the formation of a very minor chiral byꢀproduct during the oxidation of achiral 1
into achiral 2, a byꢀproduct (3) which enantiomers were nicely separated at room temperature and
presented an intense CD response at 254 nm in the mobile phase. Column chromatography on silica
and amylene stabilized CHCl3 eluent allowed a clean isolation of 3 as the less retained compound. In
this article, we disclose our full studies of the unusual stereochemical features of (3Z,9Z)ꢀ1,2,5,8ꢀ
dithiadiazecineꢀ6,7(5H,8H)ꢀdiones.7
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O
S
S
O
N
Ph
N
S
S
O
N
N
Ph
a/ mCPBA
b/ TEA, Air
+
S
1
2
3 (4%)
Scheme 1. Oxidation of thiazolineꢀ2ꢀthione 1 yielding thiazolinone 2 and unknown 3 as a very minor
compound
Results and Discussion
Chromatography of the isolated 3 on (S,S)ꢀWhelk O1 column (Figure 1) confirmed that 3 was a
racemate. The first eluted enantiomer was dextrorotatory according to a Jasco ORꢀ4090 on line
polarimeter, it also showed a positive CD sign at 300 nm. Several other commercially available chiral
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supports were additionally screened, the UV and CD traces as well as chromatographic parameters are
given in the supporting information, the best separation was achieved on (S,S)ꢀWhelk O1 column ( α
= 2.46 and Rs = 9.75).
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UV 254nm
Jasco polarimeter
CD 300nm
min
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0
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Figure 1. Chromatograms of racemic 3 on (S,S)ꢀWhelk O1 column, 50:50 Heptane / EtOH, 25°C, 1
mL/min: Rt(+) = 7.14, Rt(ꢀ) = 13.28, k(+) = 1.42, k(ꢀ) = 3.50, α =2.46 and Rs = 9.75.
Single crystal Xꢀray analysis definitively identified 3 as (3Z,9Z)ꢀ4,9ꢀdimethylꢀ5,8ꢀdiphenylꢀ1,2,5,8ꢀ
dithiadiazecineꢀ6,7(5H,8H)ꢀdione (Scheme 2), in agreement with the HRMS data which proposed
C20H18N2O2S2 for element composition. The C2 symmetry of the structure accounted for the
deceptively simple 1H and 13C NMR spectra: a single methyl peak (proton and carbon), a single CH=
(proton and carbon), a single carbonyl group and a single monoꢀsubstituted phenyl group (see SI).
O
O
1
4
Ph
Ph
N
N
3
2
P1,M2,M3,M4
= PMMM
S S
Scheme 2. Definition of the four chirality axes. 1: oxamide axis, 2: left Nꢀalkenyl axis, 3: right Nꢀ
alkenyl axis, 4: disulfide axis. For instance the conformer having a P configuration for axis 1, M for
axis 2, M for axis 3 and M for axis 4 (P1,M2,M3,M4) will be abbreviated in Figure 6 as PMMM.
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The crystal structure (C2/c) showed four pairs of enantiomers in the cell for the same conformation
(Figure 2).
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Figure 2. Xꢀray crystal structure of compound 3. The cell contains four pairs of enantiomers (C2/c)
For sake of clarity, two enantiomers were selected among the eight available structures, they are
displayed in Figure 3 with the M or P configuration labelling of each relevant axis. One can go from
one enantiomer to the other by inverting the configuration around four axes: the oxamide bond, the
disulfide bond and the two Nꢀalkenyl (C=CꢀN) bonds. The amides are planar and thus the C(O)ꢀN
axes apparently do not contribute to the local chirality descriptors of the ground state structure.
P
M
M
M
P
P
P
M
Figure 3. Xꢀray crystal structure of 3: for sake of clarity only two enantiomers among the four pairs of
enantiomers found in the cell are displayed. The M or P configuration of each axis is given.
Semiꢀpreparative chiral chromatography on (S,S)ꢀWhelk O1 column nicely afforded the two
enantiomers of 3 (> 94% ee after careful evaporation of the collected fractions).
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Inspection of the literature reveals that eleven derivatives having a (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ
6,7(5H,8H)ꢀdione framework have been characterized in ten articles since the seminal work of
Baldwin and Walker in 1974 (Figure 4).8
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O
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N
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O
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Ph
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Ph
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OH
S
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HO
Figure 4. Display of the eleven (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀdione derivatives
reported in literature. 48ꢀ12, 513 ,69,11, 79,11, 89,11, 910,11, 109,11, 1114, 1215, 1316 and 14.17
They mostly appeared in relation with the synthesis of diazadithiafulvalene derivatives resulting
from the dimerization of the carbene ensuing from the basic treatment of 2ꢀunsubstituted thiazolium or
benzothiazolium salts. According to Itoh et coll. the bridging double bond in diazadithiafulvalene is
then oxidized with molecular oxygen and the resulting addition peroxide rearranges to yield the
corresponding (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀdione derivative.11 Itoh’s mechanism was
recently adapted to account for the formation of a (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀdione
derivative 14 from a vitamin B1 analogue.17 Thiazolineꢀ2ꢀthiones are known to produce 2ꢀ
unsubstituted thiazolium salt under oxidation with H2O2 or mꢀCPBA,18 linking our experimental
protocol (oxidation followed by TEA or NaOH treatment) to the previous formations of a ten
membered ring.
It is quite remarkable that the chirality issues of the (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀ
dione derivatives have been totally ignored in the handful of previous works. If truth be told, some
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clear evidences of the chirality in these series were already available from the reported single crystal
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Xꢀray structure determination or in some case from H NMR spectra. Single crystal Xꢀray structure
determination has been the preferred method to characterize the unfamiliar (3Z,9Z)ꢀ1,2,5,8ꢀ
dithiadiazecineꢀ6,7(5H,8H)ꢀdione skeleton.8,9,11,13,14 Inspection of Cif files from the Cambridge
database indicates that in all cases except one, two enantiomers could be clearly recognized in the cell
while asymmetric conformations were mentioned for two compounds.10 The exception, which is even
more appealing, is for compound 11 which crystallized in a P212121 chiral group indicative of the
probable occurrence of a conglomerate.14 1H NMR provided another obvious proof of the chirality of
some of the literature samples which presented diastereotopic methylenic protons in ethyl or benzyl
groups at room temperature. For instance, the 1H NMR of compound 14 which was published in 2017
presented a nice AB spectra for the methylenic protons of the Nꢀbenzyl groups and a clear ABX2
motif for the methylene groups attached to the ethylenic bonds.17 These clear evidences of the chirality
of the tenꢀmembered ring conformer were completely unexploited. In these ten articles, there is
definitively no mention of VTꢀNMR studies or chiral HPLC attempts in order to address the stability
of the chiral conformers. The low barriers associated to oxamide inversion (see below) and disulfide
inversion, one can find in the literature, might be at the origin of the lack of interest for chiral HPLC
attempts to isolate stable enantiomers in 3 analogues.
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Back to the first isolation of enantiomers in (3Z,9Z)ꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀdione series,
a key step was to establish the thermal stability of the enantiomers of 3 through the determination of
the barrier to enantiomerization in dichloromethane. A barrier of 101.0 kJ/mol was obtained at 25°C,
corresponding to a halfꢀlife time of 7.8 hours; such a barrier is high enough for a safe room
temperature recording of chiroptical data of these unprecedented enantiomers. Table 1 shows the
enantiomerization barriers and halfꢀlife times determined experimentally in four solvents at 30°C.
Solvent
acetonitrile
ethanol
ꢁG≠enantiomerization of 3 (kJ/mol, 30°C)
t1/2 (at 30°C, in hours)
99.2
100.6
101.0
103.3
1.9
3.2
3.8
9.5
dichloromethane
chloroform
Table 1. Enantiomerization barriers and halfꢀlife times for 3 at 30°C in four solvents.
The barriers in chloroform were determined at 30°C and 61°C (See SI), they were not significantly
different (103.3 and 103.5 kJ/mol respectively) pointing out a very small entropy contribution.
Noteworthy, the barrier slightly but significantly decreases as solvent polarity increases: 103.3 kJ/mol
in chloroform versus 99.2 kJ/mol in acetonitrile (Table 1).
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Optical rotations were measured in chloroform: [α]D25= +1250 ±50 (c 0.257, CHCl3); [α]436
=
+3300 ±100 (c 0.257, CHCl3) for (+)3, first eluted enantiomer on (S,S)ꢀWhelk O1 and [α]D25= ꢀ1250
±50 (c 0.051, CHCl3); [α]43625= ꢀ3300 ±100 (c 0.051, CHCl3) for (ꢀ)3, second eluted enantiomer.
Molar rotations [Ф]D25= + or ꢀ 4775 (c 0.257, CHCl3) were in some aspects reminiscent of large
rotations in helicene series.19
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Circular dichroism spectra were recorded in order to determine the absolute configuration of (+)3 and
(ꢀ)3 by comparing the calculated dichroism spectra for a chosen enantiomer with the experimental
ones. Acetonitrile was the solvent for electronic circular dichroism (ECD) measurements while
dichloromethane was used for vibrational circular dichroism (VCD). Calculations were performed for
the optimized structure of the enantiomer having a (M) configuration around the oxamide bond, a (P)
configuration around the disulfide bond and (M) configurations around both of the Nꢀalkenyl bonds.
The optimized structure for the single conformer was similar in terms of local configurations to the
enantiomer originating from Xꢀray displayed on the right side of Figure 3. ECD calculations were
performed using time dependent density functional theory (TDꢀDFT) with the SMD(CH3CN)/CAMꢀ
B3LYP/6ꢀ31++G(d,p) // SMD(CH3CN)/B3LYP/6ꢀ311G(d,p) level while VCD calculations were
performed using density functional theory (DFT) with the SMD(CH2Cl2)/B3LYP/6ꢀ311+G(d,p) level
(see supporting information for details). Experimental ECD spectra for (+)3 and (ꢀ)3 are reported in
Figure 5. Nice mirror images were obtained. It is worth noting that the CD is positive for all the
recorded wavelengths (185 to 350 nm) for (+)3 and negative for (ꢀ)3 in agreement with the same peak
sign observed using polarimeter or CD detection during chromatography on chiral support. Calculated
CD spectra for the (M,P,P,P) enantiomer is strictly negative for all the calculated range. A reasonable
agreement is thus obtained between the calculated ECD spectra of the (M,P,P,P) enantiomer and the
experimental ECD spectra of the second eluted enantiomer (ꢀ)3.
Figure 5. Experimental ECD spectra of (+)3 (green curve, first eluted) and (ꢀ)3 (red curve, second
eluted) in acetonitrile (0.38 mmol/L) and calculated (blue curve) ECD for the optimized (M,P,P,P)
enantiomer.
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The experimental VCD spectra of (+)3 and (ꢀ)3 and calculated VCD spectra of the optimized
(M,P,P,P) enantiomer are reported in Figure 6. The calculated VCD spectra is in good agreement with
the experimental VCD spectra of (ꢀ)3.
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In summary the two circular dichroism methods, ECD and VCD converged to safely assign the
(M,P,P,P) absolute configuration to (ꢀ)3, the second eluted enantiomer on (S,S)ꢀWhelk O1 column.20
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Figure 6. VCD spectra of (+)3 (green curve, first eluted), (ꢀ)3 (red curve, second eluted) and
calculated VCD spectra of (M,P,P,P) enantiomer (blue curve).
DFT calculations (SMD(CH2Cl2)/B3LYP/6ꢀ311G(d,p)) were performed to visualize the inversion
route of the four axes (M,P,P,P) into (P,M,M,M) paving the way of the enantiomerization process.21
Two main experimental features should be corroborated by calculations: the occurrence of a single
pair of enantiomers and a barrier of 101.0 kJ/mol for the enantiomerization process in
dichloromethane. Furthermore, the knowledge of the detailed process could be indicative of the
required structural modifications which might, in future work, greatly affect the barrier and (or)
promote the occurrence of other detectable conformational states. The nature (ground state (GS) or
transition state (TS)) of each stationary point found along the reaction paths have been checked by a
frequency calculation.
A distorted fourꢀdimensional hypercube (distorted tesseract) which accounts for the inversion of 4
axes in a structure like 3 without a C2 symmetry is given in Figure 7.22
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Figure 7. Display of the calculated conformer energies and barriers for compound 3 according to a
distorted tesseract (distorted fourꢀdimensional hypercube). Each stereoisomer is linked to four
neighbors through inversion of configuration about a single axis. The values into brackets ()*
correspond to the calculated Gibbs free energy at 298 K of the pass between two stereoisomers
(kJ/mol), energetic references are MPPP and PMMM conformers as 0 kJ/mol. The configuration and
the energy of the stereoisomers are given in the cartouches. All calculations were performed using
SMD(CH2Cl2)/B3LYP/6ꢀ311G(d,p) theoretical level.
Each conformation is linked to four neighboring conformations differing by the chirality about a
single axis. Eight pairs of enantiomers are thus defined and the interꢀconversion of one enantiomer
into its antipode requires at least four rotations. The front panel of the tesseract displays eight
diastereoisomeric forms while the rear panel is composed of the corresponding enantiomers. The eight
diastereoisomers associated to a (M) absolute configuration about the O=CꢀC=O axis are located on
the West side of the figure and the eight enantiomers with a (P) configuration are located on the East
side. The general scheme could have been simplified taking into account the C2 symmetry in
compound 3: for instance PMPM and PPMM are identical and the two spots could have been merged
in Figure 7; the same hold true for PPMP versus PMPP, MPMM versus MMPM and MMPP versus
MPMP. That simplification is quite useful to save calculation power for both conformer energy and
interꢀconversion barrier but we prefer to display all the conformers to visualize all the interꢀconversion
pathways. Merging the identical conformers would lead to a superimposition of different routes and
would hide the fact that there is no direct connection between twin conformers. The count of the
different routes is important for the statistical correction of the calculated barrier when compared with
the experimental one.
For sake of clarity, the calculated structures of individual conformers and transition states are
displayed in the supporting information. All the displayed calculated energies in Figure 7 are given in
kJ/mol at 298 K in CH2Cl2.
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In Figure 7, the conformers are colored according to their energy: dark blue for the more stable and
red for the less stable, the conformers with intermediate energies are colored in light blue, green,
bright and dark yellow, respectively. Enantiomers display of course the same color, conformers which
are superimposable due to the C2 symmetry have the same color but they are differentiated by the
relief of the cartouche. The MPPP and PMMM enantiomers in dark blue are the most stable (reference
value 0 kJ/mol). They indeed correspond to the ones observed by Xꢀray and used for calculation of
chiroptical properties. The light blue MPPM and PMMP enantiomers (16 kJ/mol) differ from the more
stable by a single inversion along the disulfide bond. The energy gap is sufficient to explain that they
were not detected during routine analyses and that a single conformer was included during VCD and
ECD calculations. The calculated barrier is equal to 86 kJ/mol for the exchange major conformer
(MPPP) to minor conformer (MPPM) via a direct inversion of the disulfide bond; as seen on the
distorted tesseract, the exchange major conformer (MPPP) may be performed according to a less
energetic three step route going through MPMP and MPMM conformers. These routes would have
been difficult to trace without the display of the results on a distorted tesseract.
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It is well documented that VTꢀNMR experiments can be used to detect two conformers in exchange
when one of them is too low in concentration and thus escape detection. If the difference in chemical
shifts is large enough and the barrier of exchange falls in the range of dynamic NMR, a broadening of
the major signal occurs for a temperature range around coalescence.23 Sixteen NMR spectra (500
MHz) of 3 were recorded in CDCl3 between 235 and 315K and the height of the methyl peak was
compared with that of the CHCl3 peak taken as a reference. A very tiny deflection was indeed noticed
around 295ꢀ285 K before a continuous change in relation with a T2 broadening. Such a tiny deflection
cannot be taken as a firm evidence of the exchange process but in the absence of exchange a strictly
unbroken curve would have been obtained (See SI). These VTꢀNMR experiments militate in favor of
the occurrence of very minor undetected conformer in exchange with a major conformer through a
barrier (majorꢀminor) producing a coalescence around 290K. These experiments fit quite well with the
calculations.
The other conformers are of higher energy with a maximum value (87 kJ/mol) for the MMMP and
PPPM pair.
The calculations also reveal a zone of relatively easy exchange between several diastereoisomeric
forms having the same absolute configuration at the O=CꢀC=O axis with of course its counterpart in
the enantiomeric space. These diastereoisomers are separated by calculated barriers ranging between
73 and 86 kJ/mol. These relatively low barriers are labeled in green in SW and NE parts of Figure 7.
The calculations show that depending on the starting conformer from the West part of the scheme,
diastereoisomerization involving the rotation about the O=CꢀC=O axis calls for calculated barriers
equal to 108, 116 or 126 kJ/mol to reach the East part of the scheme. If one excludes the four routes
involving the oxamide axis inversion with a calculated barrier equal to 126 kJ/mol, two routes
involving the inversion of the oxamide axis are calling for barriers equal to 116 kJ/mol and two
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possibly preferred routes are calling for barriers equal to 108 kJ/mol. All these four routes bridging the
West part to the East part of the scheme are all preceded or followed by Nꢀalkenyl barriers of similar
energy (110 kJ/mol). Thus the complete enantiomerization process between the observed MPPP and
PMMM stable enantiomers proceeds through several quasiꢀisoenergetic routes in competition, most of
them having a combination of inversion of the O=CꢀC=O axis and inversion of the Nꢀalkenyl axes as a
probable determining step. The slight but significant solvent effect on the barrier (Table 1) militates in
favor of a sizeable contribution of the oxamide axis inversion as a determining step.
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The literature is very poor in examples of cyclic oxamides forced to adopt a twisted s-cis
conformation in the ground state and in which the inversion of the ring goes through a planar or close
to planar cis conformation. To the best of our knowledge, the only example was reported by Isaksson
and Liljefors who determined the barrier to enantiomerization in a series of seven membered cyclic
oxamides 15 by dynamic NMR (Figure 8).24 The barriers were in the range of 40 to 50 kJ/mol and thus
ca 60 kJ/mol, lower than the one we determined in compound 3.
Figure 8. Selected model compounds from literature providing examples of barrier to inversion in
oxamide 1524, disulfide linkage 1625 and Nꢀalkenyl axis 17.26
Compound 16 provides a relevant example of disulfide linkage in a rather flexible ten membered
ring.25 Five conformers of similar energies were in equilibrium as seen in the NMR spectra. The
barriers corresponding to the ring flip process, amide rotation and disulfide inversion were attained by
dynamic NMR. The disulfide inversion had the lowest barrier: 72 kJ/mol. In compound 3, the
calculated disulfide inversions are in the 79ꢀ99 kJ/mol range depending on the backbone arrangement.
As mentioned before, for some backbone arrangements, inversion around one of the Nꢀalkenyl
((C=C)ꢀN) axis produces calculated barriers in the same range as for the inversion of the O=CꢀC=O
group providing additional determining steps in competition (Figure 6). Clark, Curran et al. recently
reported a large series of barriers to rotation around the Nꢀalkenyl bond in enamide framework 17
(Figure 8).26 In the absence of large substituents in the vicinity of the Nꢀalkenyl bond, the
experimental barriers were in the 45ꢀ80 kJ/mol range.
In addition to compound 16, other tenꢀmembered rings presenting intracyclic amide group
(heterocyclic analogues of dibenzodiazepines)27 or exocyclic amide bond (tenꢀmembered benzazecine
derivative)28
and additional insaturations showed enantiomeric conformations but the
enantiomerization barriers obtained by VTꢀNMR were too low to imagine a room temperature
isolation of the corresponding enantiomers.
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The stepwise calculations reported in Figure 7 clearly show that the enantiomerization process in 3
may proceed through many routes of similar energy. The occurrence of several routes is quite
rewarding when one compares the experimental value (101 kJ/mol in dichloromethane) with the
calculated ones which are all higher in energy. It is worth recalling that the experimental energy of the
barrier is lowered by RTln(n) kJ/mol when n routes are in competition; the barrier calculations are thus
in pretty good agreement with the experimental one. We are aware that some partially or totally
concerted rotations around the different axes may also account for the observed barrier but the
stepwise calculations reported in Figure 7 are a vivid source of inspiration for future work.
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In conclusion we have isolated for the first time two enantiomers (atropisomers) in (3Z,9Z)ꢀ1,2,5,8ꢀ
dithiadiazecineꢀ6,7(5H,8H)ꢀdione series and determined their thermal stability. The enantiomerization
involves the inversion of configuration of four chirality axes. The chiroptical properties and absolute
configuration have been determined. Calculations confirm that a single pair of enantiomers among the
eight possible pairs is populated and that several routes are in competition for the enantiomerization
process, most of them having the inversion about the O=CꢀC=O bond in combination with the Nꢀ
alkenyl inversion for determining steps.
A completely unexplored field of investigation of these unusual chiral objects in which the chirality is
arising from several linked chiral axes to form a ten membered ring is now open. The effect of
chemical modifications of the reactive disulfide and oxamide groups on conformations and barriers to
enantiomerization are underway and will be reported in due time.
Experimental Section
Synthesis of (3Z,9Z)ꢀ4,9ꢀdimethylꢀ5,8ꢀdiphenylꢀ1,2,5,8ꢀdithiadiazecineꢀ6,7(5H,8H)ꢀdione 3:
a) oxidation with m-CPBA: to a solution of 4ꢀmethylꢀ3ꢀphenylꢀ1,3ꢀthiazoleꢀ2(3H)ꢀthione 1 (400 mg,
1.9 mmol) in 3 mL CH2Cl2 (anhydrous), was added dropwise a solution of mꢀCPBA (1064 mg, 3.2 eq)
in 30 mL CH2Cl2 (anhydrous) under stirring and nitrogen at 0°C. The reaction medium turned yellow
and a solid was formed. 60 mL CH2Cl2 (anhydrous) were added to the flask for dissolution and the
resulting mixture was treated with 40 mL (1M NaOH) solution at room temperature. The biphasic
medium was stirred during 12 hrs at ambient. The recovered organic phase was washed three times
with water (3 x 20 mL), dried on MgSO4, filtered and evaporated. The resulting crude medium was
analyzed by NMR and chiral chromatography (see text). Column chromatography on silica (eluent
CHCl3 amylene stabilized) afforded ca 15 mg of pure 3 as the first eluted compound.
b) oxidation with H2O2: to a solution of 4ꢀmethylꢀ3ꢀphenylꢀ1,3ꢀthiazoleꢀ2(3H)ꢀthione 1 (518 mg,
2.50 mmol) in acetone (10 mL) was added H2O2 (30% in water, 1 mL, 2.50 mmol) and HPF6 (1 mL)
under nitrogen atmosphere and at 0°C. The reaction was then left in the same conditions for 30 min.
The solvent was then partially removed by evaporation under reduced pressure and the crude was
cooled at 0°C providing a solid which was collected by filtration (yellow solid) and immediately used
in the next step without further characterization. To a solution of that solid in acetonitrile (50 mL) was
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added dropwise a solution of triethylamine (0.70 mL, 5 mmol) in acetonitrile (25 mL) under nitrogen
atmosphere. The reaction was then left under stirring overnight. The solvent was removed by
evaporation under reduced pressure and dichloromethane (10 mL) was added. The organic layer was
washed with water (20 mL), dried on MgSO4 and evaporated under reduced pressure. The crude was
purified by column chromatography (silica, eluent CHCl3 ꢀamylene stabilizedꢀ), yielding the desired
product as a white solid (39 mg, 4% yield from 4ꢀmethylꢀ3ꢀphenylꢀ1,3ꢀthiazoleꢀ2(3H)ꢀthione).
Note: compound 3 was first eluted with CHCl3 (amylene stabilized) on silica. In all attempts, the
chromatography was stopped after total elution of 3 and the composition of the remaining mixture was
not further analyzed.
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1
Mp = 212°C. Rf 0.38 (Silica plate, amylene stabilized CHCl3). H NMR (400 MHz, CDCl3): δ 2.04
(s, 6H), 6.61 (s, 2H), 7.29ꢀ7.34 (m, overlapping 2H+4H), 7.41ꢀ7.47 (m, 4H). 13C NMR (100 MHz,
CDCl3): δ 21.8, 123.5, 126.0, 127.7, 129.5, 137.5, 152.7, 164.2. HMRS m/z calcd for [M+H]+
+
C20H19N2O2S2 : 383.0882; found 383.0885.
Computational Details for the calculations of the IR/VCD and UVꢀvis/ECD spectra:
Spectra calculations have been performed using the single conformation of the MPPP enantiomer of
3. The vibrational frequencies IR absorption and VCD intensities were calculated using the same
theoretical level as for geometry optimisation SMD(CH2Cl2)/B3LYP/6ꢀ311+G(d,p). Computed
harmonic frequencies have been calibrated using a scaling factor of 0.98 since they are generally
larger than those experimentally observed. IR absorption and VCD averaged spectra were constructed
from calculated dipole and rotational strengths assuming Lorentzian band shape with a halfꢀwidth at
half maximum of 10 cmꢀ1. Based on the SMD(CH3CN)/B3LYP/6ꢀ311G(d,p) optimized geometries
the ECD and UV spectra were calculated using time dependent density functional theory (TDꢀDFT)
with CAMꢀB3LYP functional and 6ꢀ31++G(df,p) basis set. Calculations were performed for vertical
1A singlet excitation using 50 states. For a comparison between theoretical results and the
experimental values the calculated UV and ECD spectra have been modeled with a Gaussian function
using a halfꢀwidth of 0.37 eV. Due to the approximations of the theoretical model used an offset
almost constant was observed between measured and calculated frequencies. Using UV spectra all
frequencies were calibrated by a factor of 0.97. All calculations were performed using Gaussian 16
package.29
Computational Details for the calculations of the inversion mechanisms:
All the mechanisms have been calculated using DFT with B3LYP functional with 6ꢀ311G(d,p) basis
set or ccꢀpvtz basis set, and PBE0 functional with 6ꢀ311G(d,p) basis set. The mean effects of the
solvent (CH2Cl2) have been introduced using the SMD solvation model. The nature (ground state
(GS) or transition state (TS)) of each stationary point found along the reaction paths have been
checked by a frequency calculation. GS and TS have respectively none and a single imaginary
frequency. Intrinsic reaction coordinate (IRC) calculations have been performed on each TS on both
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direction in order to validate the reaction path. All calculations were performed using Gaussian 16
package.29
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Supporting information available: Copies of NMR spectra and chiral HPLC chromatograms. Kinetic
of enantiomerization. XꢀRay data. Optical rotations. ECD and VCD details. Computational details for
the calculations of the IR/VCD and UVꢀvis/ECD spectra. Molecular modeling coordinates of
conformers and transition states. Distorted tesseracts resulting from DFT calculations performed at the
(SMD(CH2Cl2)/B3LYP/ccꢀpvtz and (SMD(CH2Cl2)/PBE0/6ꢀ311G(d,p) levels. Gibbs free energy,
enthalpy and entropy values for every modeled GS and TS.
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ACKNOWLEDGMENTS
VR thanks ERASMUS exchange program for support. CNRS and AMU fundings are acknowledged.
The authors deeply thank Dr Valerie Monnier and Dr Gaetan Herbette from Spectropole AixꢀMarseille
Univ. for HMRS and NMR analytical support.
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