Chiral properties of tetrathiatriarylmethyl spin probes

Article abstract of DOI:10.1039/c1cc10988j

We report that tetrathiatriarylmethyl (trityl) EPR probes are chiral molecules at room temperature, the two stereoisomers that differ in their helicity being configurationally stable enough to be separated and stored independently. The Royal Society of Ch

Full text of DOI:10.1039/c1cc10988j

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COMMUNICATION
Chiral properties of tetrathiatriarylmethyl spin probesw
a
a
b
Benoıt Driesschaert,ab Raphael Robiette, Fabio Lucaccioni, Bernard Gallez and
¨
Jacqueline Marchand-Brynaert*a
Received 19th February 2011, Accepted 25th February 2011
DOI: 10.1039/c1cc10988j
We report that tetrathiatriarylmethyl (trityl) EPR probes are
chiral molecules at room temperature, the two stereoisomers
that differ in their helicity being configurationally stable enough
to be separated and stored independently.
which feature an enantiomeric relationship (Fig. 2). If the
conformational equilibration between these two helices is
rapid, then the molecules will not display chiral properties.
However, due to the large steric bulk of the three aryl moieties,
one can expect the rotation around the three carbon–carbon
bonds connected to the central carbon to be restricted, thus
limiting the stereoisomerisation of the two helices. If this is the
case and the interconversion between the two enantiomeric
helices is slow, then the two enantiomeric helices could
potentially be separately isolated. It is therefore crucial to
study the dynamics of this conformational equilibrium, since
the chirality could have important consequences for the
applications, especially as different enantiomers can display
different biological activities. In addition, the connection of
chiral substituents to the trityl core would result in the
formation of diastereoisomeric mixtures11 possibly with
different chemical, biological and physical properties (e.g.
different EPR spectra). Finally, the chirality associated with
an open-shell system would confer interesting chiro-optical
properties to the tetrathiatriarylmethyl radicals , such as
nonlinear optical (NLO) responses.13
As trityl alcohol is also a propeller-shaped molecule,14
tetrathiatriarylmethyl alcohols, the direct precursors of the
radicals (see Fig. 1), might similarly be chiral if the rotations
around the three carbon–carbon bonds connected to the
central carbon are restricted. Indeed, when we added Pirkle’s
alcohol, a known chiral solvating agent (CSA), to a solution of
2d15 in CDCl3, we observed splitting of the 1H and 13C NMR
signals on the spectra measured at room temperature (see
ESIw), indicating the formation of two diastereoisomeric
solvation complexes. This observation shows that the inter-
conversion of the helicity does not occur rapidly on the NMR
Stable tetrathiatriarylmethyl radicals, 1a (Fig. 1), belong to a
family of trityl radicals extensively used for electron para-
magnetic resonance imaging (EPRI),1 oximetry,2 dynamic
nuclear polarization (DNP)3 and the detection of superoxide
radical anions.4 These water soluble, paramagnetic species
have unique properties, namely an exceptional biostability, a
narrow EPR linewidth (the anoxic linewidth is o100 mG),
and they display a low concentration-dependent broadening of
the EPR line.5
Accordingly, this class of carbon-centered radicals has been
selected by several laboratories for chemical modifications
with the aim of diversifying their applications and improving
their performances. In this context, a series of new tetra-
thiatriarylmethyl radicals have recently been synthesized for
various applications: dendritic trityl radicals that possess a
higher stability,6 ester-derivatized forms used for intracellular
oxygen measurement,7 amino-derivatized forms allowing dual
pH/oxygen assessment,8 fluorinated variants used to quantify
tissue oxygenation with a high sensitivity in perfluorocarbon
formulations,9 aldehyde derivatives with enhanced sensitivity
to oxygen10 and trityl–nitroxide biradicals dedicated to the
simultaneous measurement of redox status and oxygenation.11
Surprisingly, during these numerous developments of trityl
radicals for biomedical uses, the fundamental question of the
potential chirality associated with their helicoidal conformation
has never been addressed. However, it has been shown that
tetrathiatriarylmethyl radicals adopt a propeller shape in
which all of the rings twist in the same direction.12 This
geometrical arrangement allows two conformations to occur,
namely the right-handed (P) and left-handed (M) helices,
a Institute of Condensed Matter and Nanosciences (IMCN),
´
Molecules, Solids and Reactivity (MOST), Universite Catholique de
Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium.
E-mail: jacqueline.marchand@uclouvain.be;
Fax: +32 (0) 10 47 41 68; Tel: +32 (0) 10 47 27 40
b Louvain Drug Research Institute (LDRI), Biomedical Magnetic
´
Resonance Section, Universite Catholique de Louvain,
Avenue Mounier 73.40, B-1200 Bruxelles, Belgium
w Electronic supplementary information (ESI) available: Details of
experimental procedures, characterizations and computational details.
See DOI: 10.1039/c1cc10988j
Fig. 1 Representative tetrathiatriarylmethyl radicals with biomedical
uses (1a) and their precursors.
c
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It is interesting to note that our calculation suggests that
solvation should not have an important effect on the activation
free energy.
According to the calculated activation free energy in
solution (29.5 kcal molꢀ1), the two enantiomers of 1d should
be configurationally stable enough to be isolated and stored
separately. We thus undertook the resolution of a tetra-
thiatriarylmethyl radical on a preparative scale.
We first aimed to synthesize the diastereoisomers by
derivatization of the trityl core with an enantiomerically pure
compound. Our strategy consisted of derivatizing the corres-
ponding trityl alcohol instead of the radical because of its
higher chemical stability; the radical could easily be generated
from the alcohol17 after separation of the diastereoisomers.
Accordingly, trityl alcohol 3a has been synthesized by
aminolysis of 2b with (S)-a-methylbenzylamine as the chiral
Fig. 2 The two conformational enantiomers of tetrathiatriarylmethyl
radical 1d.
time scale at 25 1C. Further experimental evidence of the
chirality of tetrathiatriarylmethyl alcohols was obtained for
alcohol 2b15 by analytical HPLC carried out on a chiral
stationary phase (CSP HPLC) at room temperature. Indeed,
the resolution of the peaks corresponding to the two enantiomers
of 2bz shows that there is no rapid isomerisation process
on the HPLC time scale either (Fig. 3a). Under the same
conditions, resolution of the tetrathiatriarylmethyl radical 1b
was also very visible (Fig. 3b). As expected for enantiomers,
the CD spectra recorded by stopped-flow spectrometry at the
maximum of CD intensity for the two peaks of 1b are mirror
images (see ESIw). This is the first direct experimental proof of
the chirality of a tetrathiatriarylmethyl radical at room
temperature.
1
derivatizing agent (Scheme 1). The H and 13C NMR spectra
of 3a clearly show the presence of two diastereoisomers
contrasting with the benzylamine analogue 3b that does not
possess an additional chiral center (see ESIw). Unfortunately,
all of our attempts to separate diastereoisomers 3a by crystall-
ization or chromatography failed. Notably, no induction of
asymmetric carbons on the helicity has been observed as the
ratio of diastereoisomers of 3a was 50 : 50 (determined
by NMR).
We then investigated the possibility of direct resolution of
the enantiomers using semi-preparative CSP HPLC. Again,
the resolution of trityl alcohol 2b instead of radical 1b has been
considered, due to the better resolution obtained for alcohol
2b by analytical CSP HPLC (see Fig. 3). Transposition of the
analytical CSP HPLC method to a semi-preparative scale
allowed us to obtain the two enantiomers of 2b in milligram
amounts with an enantiomeric excess of 100% (see ESIw).
Further scaling-up is limited due to the low solubility of 2b in
the eluent. The two optically pure enantiomers of 2b were then
separately converted into the corresponding enantiomers of
radical 1b by treatment with neat trifluoroacetic acid, a known
procedure for the quantitative conversion of tetrathiatriaryl-
methyl alcohols into radicals.17 Analytical CSP HPLC of the
two enantiomers confirmed their enantiomeric purity (see ESIw).
Having these two enantiomers 1b in hand, we investigated
their conformational stability in solution by following the
kinetics of their racemization (see ESIw). In the event, we
found that, at room temperature, 1b isomerizes slowly in
EtOAc (t1/2 enantio E 1 month), thus confirming that the two
enantiomers of 1b can be separately stored for months in the
freezer. The free activation energy calculated from the
obtained kinetic constant, using the Eyring equation, is
25.98 ꢁ 0.02 kcal molꢀ1 at 25 1C. This experimental value is
in good agreement with our calculations and supports the
two-ring flip as the enantiomerization mechanism. A complete
study of the enantiomerization mechanism and the determination
of the activation parameters for other substituted tetra-
thiatriarylmethyl radicals, tetrathiatriarylmethanols and tetra-
thiatriarylmethanes are currently under study and will be
reported elsewhere.
In order to study the kinetic of isomerization, we first
investigated the conformational pathway using computational
(DFT) methods.y There are four potential pathways for
interconversion: the n-ring flip mechanisms (n = 0, 1, 2, 3)
in which zero, one, two or three aromatic ring(s) rotate(s) in a
conrotatory fashion across the plane perpendicular to the one
formed by the three aromatic carbons linked to the central
carbon, while the other ring(s) rotate(s) in the opposite
direction through the reference plane (Fig. 4, for the complete
pathways see ESIw).16 We have investigated all these four
possible mechanisms for radical 1d (the model compound).
Transition state structures could be found for the two- and the
three-ring flip mechanisms, TS-two and TS-three, respectively.
For the other mechanisms, our calculations revealed that
bringing two of the rings into the same plane would imply
such a spatial proximity between the thio substituents of these
aryl groups (o0.5 A between two S atoms) that these structures
are simply not realistic. Calculating the transition state energies
showed that the lower lying pathway for conformational
equilibrium is the one involving a two-ring flip (Table 1).
In conclusion, we have shown that tetrathiatriarylmethyl
radicals, which are very attractive spin probes, are chiral
molecules at room temperature. The two enantiomers that
Fig. 3 CSP HPLC chromatograms with UV and CD detection of
trityl alcohol 2b at 274 nm (a) and radical 1b at 267 nm (b).
c
4794 Chem. Commun., 2011, 47, 4793–4795
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These observations are of primary importance and should be
taken into account in the further development and derivatization
of these species for biomedical uses.
This work was supported by the Universite Catholique de
Louvain (UCL) and the Fonds de la Recherche Scientifique-
FNRS (F.R.S.-FNRS). R.R. and J.M.-B. are Chercheurs
qualifies F.R.S.-FNRS. We are grateful to L. Collard for
technical assistance with HPLC and Dr P. Leveque for
recording the EPR spectra of radical 1b.
Notes and references
z The right- and left-handed helices showing opposing helicity in the
circular dichroism (CD) chromatogram and identical UV spectra (see
ESIw).
y Geometry optimization was carried out using the UB3LYP hybrid
density functional, as implemented in Jaguar 7.5, with the standard
split valence polarized 6-31G* basis. Single point energy calculations
were carried out at the UB3LYP-D level of theory (including an
approximation correction for dispersion) with the larger 6-311+G**
basis, using the ORCA package. Solvation effects were estimated at
the UB3LYP-D/6-31G* level using the conductor-like screening
model (COSMO) as implemented in ORCA, using the parameters
appropriate for ethyl acetate. See ESIw for full computational details
and related references.
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Fig. 4 The four potential transition states for the isomerization of
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a
a
a
TS-zero
TS-one
TS-two
TS-three
—
—
—
—
—
—
a
a
a
25.6(27.8)
74.8
28.3
77.9
29.5
79.6
a
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Scheme 1
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differ in their helicity are configurationally stable enough to be
separated and independently stored for months in the freezer
but slow racemization occurs in solution at room temperature.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4793–4795 4795