oxidation of halide ions by hydrogen peroxide.1
9-21
The
spectra of the four stereoisomers, M-(R,R,R)-1a/P-(S,S,S)-
1
a and M-(S,S,S)-1b/P-(R,R,R)-1b, are depicted in Figure
complexation properties of enantiomerically pure 1a were
studied by way of their vanadium(V) complexes. Indeed,
hemicryptophane 1a reacted with 1 equiv of vanadium(V)
1
4
3. As shown previously by Collet and co-workers, the CD
3
oxytriisopropoxide to give rise to the C -symmetric com-
pound 11, which was characterized by high-resolution (ESI-
TOF) mass spectrometry peaks at m/z 1066.3440 and by 500
1
22
MHz H NMR spectroscopy shown in Figure 2b. This
preliminary result demonstrates clearly the feasibility of the
metal insertion. The second step to be considered is the
association of the hemicryptophane complex with a substrate
of interest. Using molecular mechanics, we performed the
modeling of the encapsulation of the methyl p-tolyl sulfide
23
inside the cavity of 11 (see Supporting Information). Methyl
p-tolyl sulfide, which is a model substrate for the catalytic
oxidation of sulfide to sulfoxide, is indeed encapsulated
inside the molecular cavity of the vanadyl host, whose
Figure 3. CD spectra in CH
of 1a and 1b (see Supporting Information for concentrations).
2 2
Cl at 20 °C of the enantiomeric pairs
3
volume was approximated to 110 Å . This preliminary work
is encouraging, and we are currently investigating the
encapsulation process, which will be of prime importance
to investigate the potential catalytic activity of the hemic-
ryptophane complexes.
spectra of chiral C
be analyzed in terms of exciton coupling between the
transition moments of the three aryl chromophores. The
3
derivatives of cyclotriveratrylene can
1
5
In conclusion, a novel series of chiral metalloreceptors has
been prepared. Considering the already existing hemicryp-
tophanes displaying complexation properties, these are the
first enantiomerically pure ones. They display CD spectra,
which were used to determine their absolute configuration.
Host 1a efficiently forms an oxovanadium complex and can
be considered as an artificial model of an enzyme.19 This
new family of hemicryptophanes is promising, and supramo-
lecular asymmetric catalytic properties are expected. These
studies are in progress.
CD spectra of 1a and 1b consist of two exciton patterns,
1
roughly centered on the isotropic absorptions of the L
B
(290
1
nm) and L
A
(239 nm) transitions.
1
The L
A
transition pattern was used to determine the
absolute configuration of 1a and 1b by comparison with other
chiral CTVs bearing two different alkoxy groups (OR and
OR′, with R and R′ * H),14 for which the so-called
spectroscopic moment of the bulkier group is greater than
that of the smaller one, which implies that the P stereoisomer
1
(
respectively, M) displays in the L
A
region two oppositely
signed bands with a negative-positive sequence (respec-
Acknowledgment. We warmly thank J e´ r oˆ me Joubert for
tively, positive-negative), from high to lower energies.
his skillful assistance in modeling the complexes.
1
Indeed, in the L
A
region, 1a shows two oppositely signed
Supporting Information Available: Experimental pro-
cedures and characterization for 2, 4-10, 1a, 1b, and
CD bands between ca. 230 and 260 nm; from high to low
energy, the sequence of signs is negative-positive for the
P-(S,S,S) enantiomer and the opposite (positive-negative)
1
complex 11; H NMR spectra of Figure 2; HR ESI-TOF MS
of 11; and structures of free 11 and thioether containing 11
obtained from molecular mechanics. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
for the M-(R,R,R) enantiomer. The same conclusions could
1
be drawn from the L
A
region for host M-(S,S,S)-1b/P-
(R,R,R)-1b.
OL047469+
Vanadium complexes are known to be efficient in oxida-
16
tion of allylic alcohols to epoxides, in oxidation of sulfides
to sulfoxides,17 and in oxidation of disulfides. Vanadium
is also present in haloperoxidase enzymes that catalyze
(
19) Crans, D. C.; Smee, J. J.; Gaidamauskas, E.; Yang, L. Chem. ReV.
2004, 104, 849-902.
20) Ligtenbarg, A. G. J.; Hage, R.; Feringa, B. L. Coord. Chem. ReV.
003, 237, 89-101.
21) See the special issue on vanadium: Coord. Chem. ReV. 2003, 237
(1-2).
18
(
2
(
(16) (a) Michaelson, R. C.; Palermo, R. E.; Sharpless, K. B. J. Am. Chem.
Soc. 1977, 99, 1990-1992. (b) Bolm, C. Coord. Chem. ReV. 2003, 237,
(22) In the case of hemicryptophane 1b, we observed complexation, but
1
2
45-256 and references therein.
17) Bolm, C.; Bienewald, F. Angew. Chem., Int. Ed. Engl. 1995, 34,
640-2641.
18) Cogan, D. A.; Liu, G.; Kim, K.; Backes, B. J.; Ellman, J. A. J. Am.
Chem. Soc. 1998, 120, 8011-8019.
two different compounds were detected from the H NMR spectrum. This
(
might be due a less favorable configuration of the complex with respect to
the other, leading to the formation of different species.
(23) MOPAC with AM1/d. Commercial package CAChe 5.0; Fujitsu,
Ltd.: Japan, 2000-2002.
2
(
1210
Org. Lett., Vol. 7, No. 7, 2005