Organic Letters
Letter
spectrum of DPMUJ-f2 presented as a perfect symmetrical
mirror image to that of DPMUJ-f1(Figure 1b), confirming the
enantiomeric relationship between the two isolated com-
pounds.
Scheme 1. Chemical Structure of Ru-tri(DPMUJ) and
Amino Acid Derivatives
Unlike previously reported MUJs that possess a small side
ring capable of flipping in and out of the cavity to thus induce
the inversion of MUJs in different solvents or temper-
atures,11c,d the bipyridyl side ring was too bulky for the cavity
of P[5] to allow the rolling-in/out switching of the side ring.
Indeed, the CD spectra of DPMUJs in the different solvents
showed only the intensity’s variation to a certain extent (SI,
Figure S18), whereas the CD sign was never inverted. The
same was true in the variable-temperature CD measurements
consistently existed outside the cavity. We have demonstrated
that negative CD extrema at ca. 310 nm correspond to the Sp
configuration of pillararenes11c,d and vice versa for the Rp
configuration. The configurations of DPMUJ-f1 and DPMUJ-
f2 could thus be assigned as (Sp)-DPMUJ and (Rp)-DPMUJ,
respectively. Because the chiral cavity of DPMUJ is not
occupied by the bulky side ring, it should be available for
complexing a guest.
The DPMUJ’s metal complex Ru-tri(DPMUJ) with a Cl
counterion was then prepared by adding one enantiopure
DPMUJ and ruthenium trichloride monohydrate (molar ratio
3:1) into a Schlenk tube under the protection of nitrogen, and
the mixture was dissolved in dry DMF at 120 °C for 6 h. Then,
the solvent was removed under vacuum, and the residue was
purified by silica-gel flash column chromatography using
acetonitrile/H2O/NaNO3 (V/V/V 200:9:1) as the eluent to
obtain an orange-red solid. The photophysical properties of
compounds DPMUJ and Ru-tri(DPMUJ) were studied by
UV−vis and fluorescence spectroscopy and fluorescence decay
studies (Figure 2 and Table S1). Ru-tri(DPMUJ) showed an
absorption peak at 297 nm in the UV−vis spectrum, which was
slightly bathochromic-shifted relative to compound DPMUJ
(295 nm), and a strong absorption peak around 464 nm
ascribing to the metal-to-ligand charge transfer absorption
band was observed which was apparently bathochromic-shifted
relative to the typical absorption peak of Ru(bpy)3 at 452 nm.
A red emission peaked at 612 nm was observed when exciting
at 473 nm (Figure 2a). The red shift of absorption and
emission spectra could be interpreted in terms of the
coordination of Ru2+ with the DPMUJ moiety to form Ru-
tri(DPMUJ), which enhances the ability for intramolecular
electron transfer. It is noted that the coordination of bipyridine
with Ru2+ could induce metal-centered Δ and Λ chirality,16
and the use of chiral bipyridine precursors may stereo-
selectively lead to the formation of chiral ruthenium tris-
(bipyridine) complexes.17 Indeed, the Ru-tri(DPMUJ) formed
with (Rp)-DPMUJ showed positive CD signals at the
wavelength range from 289 to 325 nm, and negative CD
signals were observed with that given by (Sp)-DPMUJ. Thin
layer chromatography (TLC) analyses of the Ru-tri(DPMUJ)
thus obtained demonstrated a mixture composed of two
points. The Ru-tri(DPMUJ) formed by (Sp)-DPMUJ was
HPLC-separated to give two factions in a ratio of 56.7/43.3.
The major fraction showed a negative exciton-coupling-type
CD signal at the lowest energy (483 nm) and a positive sign
around 431 nm, whereas the minor fraction gave a positive
exciton-coupling-type CD signal at the lowest energy (483 nm)
and a negative sign around 431 nm, demonstrating Δ and Λ
configurations, respectively (Figure 2c).18 The CD spectra of
redox reaction at the electrode surface,15 whereas the chiral
P[5] moiety was expected to bind chiral guests in an
enantioselective fashion. DPMUJ was synthesized by the
acylation of A1/A2-diaminomethyl-P[5], prepared from
hydroquinone bis(2-hydroxyethyl) ether through a four-step
synthesis, with 4,4′-dimethyl-2,2′-bipyridyl in the presence of
1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide and 1-hy-
obtained was purified by silica gel chromatography and
characterized by high-resolution electrospray ionization
(HRESI) mass spectrometry and NMR spectroscopy. Similar
to other P[5]-based MUJs,11c,d the presence of the bipyridyl-
bearing side ring lead to a pair of isolable DPMUJ
enantiomers. The optical resolution of racemic DPMUJ was
successfully carried out by preparative chiral-phase high-
performance liquid chromatography (HPLC) (CHIRALPAK
IA, EA as eluent), which gave two well-separated peaks under
the HPLC separation conditions (Figure 1a). The first
(DPMUJ-f1) and the second (DPMUJ-f2) fractions of
DPMUJ from the HPLC column had identical integration
areas. The collected fractions were studied by CD measure-
ments in chloroform. DPMUJ-f1 exhibited a negative CD peak
at 309 nm and a positive peak at 291 nm, respectively. The CD
Figure 1. (a) Chiral-phase HPLC traces of DPMUJ (CHIRALPAK
IA, EA as eluent) (black), the first fraction (red), and the second
fraction (green) of DPMUJ. (b) Circular dichroism and UV−vis
absorption spectra (100 μM) of DPMUJ-f1 and DPMUJ-f2 in
chloroform at 25 °C.
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Org. Lett. 2021, 23, 3885−3890