Angewandte
Chemie
complexes, which were not particularly soluble in common
organic solvents, can be formulated as 1·(2-butanol) and 1·(3-
3
methyl-2-butanol) . The chiral nature of the included mole-
3
cule is determined by the handedness of host, as further
evidenced by the remarkable inclusion preference of (R)-1
for the R enantiomer of 2-butanol over the S enantiomer
(99.2% ee). Control experiments indicated that the ligand
H L itself can not resolve the enantiomers of 2-butanol under
2
otherwise identical conditions.
Although many artificial hosts have been developed for
chiral recognition of alcohols in the solid state, there are few
examples of the highly enantioselective inclusion of small
[13]
alcohols. In particular, in the case of secondary alcohols, it
is difficult to recognize chirality because the subtle structural
difference between the enantiomers as methyl groups and
hydrogen atoms attached to the chiral carbon atom have to be
discriminated. In this case, success may result from the
combination of the chiral cage with the amphiphilic cavity
interior that is lined with flexible methoxymethyl ethers; this
combination leads to bioanalogous interaction of the host
Figure 3. CD (top) and UV/Vis (bottom) spectra of 1 and 2 in THF.
The CD spectra of (S)- and (R)-2 show a phenyl p–p*
band at 240 nm and do not show exciton coupling of the
phenyl groups, whereas intense positive and negative exciton
splitting patterns of acac groups centered at 296 nm were
observed for (S)- and (R)-2, respectively. Based on the exciton
chirality method for the absolute configuration assignment of
[13g]
with guest species during the crystallization process.
Attempts to obtain single crystals of the inclusion adducts
have been unsuccessful. Further investigations on the reso-
lution behavior and resolution of other racemic organic
species are in progress.
In conclusion, we have presented the diastereoselective
self-assembly of chiral neutral metal–organic cages and
demonstrated their highly enantioselective abilities to resolve
small racemic alcohols by crystallization inclusion. Further
work is aimed at enlarging the open channels and further
functionalization of the interiors of the cages through
modifications of organic linkers for enantioselective process-
es.
[12]
tris(acac) metal complexes,
the positive exciton pattern
indicates that each Ga center in (S)-2 is in the L form, which
is consistent with the crystal structure; the negative pattern
indicates each Ga center in (R)-2 is in the D form. The
configuration of the newly formed stereogenic metal centers
is thus controlled by the configuration of the biphenyl
platform. Exciton coupling of acac chromophores can be
clearly observed for 2, as they may be free from overlapping
[11c]
transitions,
as evidenced by the significant red shift of their
p–p* transition in the absorption spectra.
Upon addition of 2-butanol to apohost 2 in CHCl , the
3
expected formation of the kinetically stable host–guest
1
complex could not observed by H NMR spectroscopy.
Experimental Section
However, the inclusion of 2-butanol could be readily achieved
through cocrystallization of the evacuated host and racemic
alcohols in CHCl3 at room temperature (Scheme 2). The
desorbed 2-butanol from (S)-1 was analyzed by using GC on a
chiral support, which showed that the enantiomeric excess
Synthesis of 1 and 2: A solution of FeCl (10.8 mg, 0.067 mmol) in
3
DMF (1 mL) was added dropwise to a solution of (S)- or (R)-H L
2
(52.6 mg, 0.1 mmol) in DMF (4 mL) at room temperature. The
resulting solution turned dark purple and was stirred for 10 minutes,
after which time the solution was layered with methanol. After two
weeks, dark-red crystals of 1 suitable for X-ray crystallography were
collected by filtration, washed with ethanol and ether, and dried in air.
Yield: 57%. IR (KBr): n˜ = 2918(b), 1575(vs), 1443(s), 1357(s),
(ee) value was 98.8%, and the absolute configuration of (S)-2-
butanol was confirmed by comparison of the retention time
[
13e]
with that of a reported sample.
Similar enantioselective
1271(w), 1155(m), 1081(w), 1030(m), 970(w), 935(w), 817(w),
À1
inclusion behavior was observed for racemic 3-methyl-2-
butanol, whereby (R)-1 exhibited remarkable selective inclu-
sion of the R enantiomer over the S enantiomer. The ee value
of the desorbed guests was 99.5%. On the basis of micro-
analysis and thermogravimetric analysis (TGA), the inclusion
699(w), 682(w), 602(w), 449 cm (w); elemental analysis calcd (%)
for C192
H292Fe N O76 ([Fe L ]·4DMF·24H O): C 56.30, H 7.19; found:
4 4 4 6 2
C 56.01, H 7.14. The guest molecules could be removed by heating in
vacuum to further confirm the composition of the apohost [Fe L ]:
4
6
calcd (%) for C180H216Fe O : C 64.13, H 6.46; found: C 63.98, H 6.40.
4
48
(S)- and (R)-2 were synthesized by following a similar method.
Yield: 51%. IR (KBr): n˜ = 2918(b), 1575(vs), 1443(s), 1357(s),
1
6
271(w), 1155(m), 1081(w), 1030(m), 970(w), 935(w), 817(w),
99(w), 682(w), 602(w), 449 cm (w); elemental analysis calcd (%)
À1
for C192H284Ga N O ([Ga L ]·4DMF·20H O): C 56.53, H 7.02;
4
4
72
4
6
2
found: C 56.60, H 7.00. The guest molecules were also removed by
1
heating, and the resulting apohost characterized: H NMR (400 MHz,
CDCl ): d = 1.75 (s, 6H), 1.99 (s, 6H), 2.03 (s, 6H), 2.25 (s, 6H), 2.85
3
1
3
(
s, 6H), 4.36–4.40 (t, 4H), 6.97 ppm (s, 2H); C NMR (400 MHz,
CDCl ): d = 17.1, 20.2, 27.6, 27.8, 56.4, 98.1, 111.1, 130.6, 132.3, 132.8,
3
133.7, 135.9, 152.4, 190.5, 194.2 ppm. Elemental analysis calcd (%) for
Scheme 2. Selective inclusion of (S)-2-butanol by LLLL-1.
C180H216Ga O : C 63.09, H 6.35; found: C 62.92, H 6.33.
4
48
Angew. Chem. Int. Ed. 2010, 49, 4121 –4124
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4123