Chirality-Memory Molecule: A D2-Symmetric Fully Substituted Porphyrin as a Conceptually New Chirality Sensor

Full text of DOI:10.1021/ja970431q

J. Am. Chem. Soc. 1997, 119, 5267-5268
5267
Chirality-Memory Molecule: A D
Substituted Porphyrin as a Conceptually New
Chirality Sensor
2
-Symmetric Fully
Scheme 1. Schematic Representation of the Principle of
Chirality Sensing by 1
Yoshio Furusho, Takayuki Kimura, Yukitami Mizuno, and
Takuzo Aida*
Department of Chemistry and Biotechnology
Graduate School of Engineering
The UniVersity of Tokyo
Hongo, Bunkyo-ku, Tokyo 113, Japan
ReceiVed February 10, 1997
Molecules recognize one another when they are interacting,
but on dissociation, they generally lose any trace of the
interaction.1 Herein we report a conceptually new chirality-
2
sensing molecule (1), which reads out the chirality of asym-
metric molecules through self-assembly and memorizes the
acquired information within its skeleton even after the assembly
is broken. Furthermore, the chirality memory, thus imprinted,
can be erased by the action of external stimuli, but it is
automatically retrieved when the stimuli are switched off. We
designed this unique molecule on the basis of a saddle-shaped,
fully substituted porphyrin.
corresponds to racemization. In fact, there was no sign of
optical resolution of 1 in chiral HPLC.
6
Fully substituted porphyrins are highly basic due to their
nonplanarity and bind two molecules of carboxylic acids via
the formation of four hydrogen bonds (Scheme 1).4 When a
hot AcOEt solution of a mixture of 1 and 2 equiv of
,7
(
S)-mandelic acid was allowed to cool to room temperature,
1
fine green needles formed. H NMR spectroscopy (23 °C) of
the product in CDCl3 showed that 1 actually binds two
molecules of mandelic acid to form a diastereoisomeric complex
(
2): The signals due to the mandelate moieties in 2 shifted
8
upfield from those of mandelic acid, due to the strong magnetic
shielding by the porphyrin ring current. In dichloroethane
(DCE), 2 exhibited distinct circular dichroism (CD) bands with
a split Cotton effect around its Soret band (Figure 1). The CD
patterns of the (R)- and (S)-mandelate complexes (2) were
perfect mirror images of each other, thus reflecting the absolute
configuration of mandelic acid.
The mandelate complex (2), when dissolved in AcOH, was
completely transformed into an enantiomeric diacetate complex
Unlike ordinary porphyrins, fully substituted porphyrins are
very unique in that they adopt highly nonplanar conformations
due to the steric repulsion among the neighboring substituents.
9
10
(3) with a release of two molecules of free mandelic acid.
Although 3 no longer has any chiral elements at the carboxylate
moieties, we still observed clear and further enhanced CD bands
3
In particular, octaalkyltetraarylporphyrins show a strong prefer-
ence for a “saddle” conformation with the pyrrole units pointing
(6) For our previous reports on optical resolution of chiral porphyrins,
see: (a) Konishi, K.; Sugino, T.; Aida, T.; Inoue, S. J. Am. Chem. Soc.
4,5
up and down alternately. Therefore, saddle-shaped 1 having
two different aryl groups at the opposite meso-positions should
be chiral with a symmetry group D2. Smith et al. have reported
a rapid saddle-to-saddle macrocyclic inversion for octaalkyl-
1
991, 113, 6487. (b) Konishi, K.; Oda, K.; Nishida, K.; Aida, T.; Inoue,
S. J. Am. Chem. Soc. 1992, 114, 1313. (c) Konishi, K.; Takahata, Y.; Aida,
T.; Inoue, S.; Kuroda, R. J. Am. Chem. Soc. 1993, 115, 1169. (d) Konishi,
K.; Mori, Y.; Aida, T.; Inoue, S. Inorg. Chem. 1995, 34, 1292.
(7) Barkigia, K. M.; Faler, J.; Berber, M. D.; Smith, K. M. Acta
Crystallogr. Sect. C 1995, C51, 511.
4
tetraarylporphyrins at room temperature, which, for chiral 1,
(
8) For 2 ((S)-mandelate): 1H NMR (270 MHz, CDCl3, 23 °C) δ 8.42
(
1) The concept of “molecular hysteresis” has been reported: (a) Sano,
(4H, dd, o-H in C6H5), 7.77-7.93 (6H, m, m- + p-H in C6H5), 7.72 (2H,
t, J ) 8.34 Hz, p-H in C6H3(OMe)2), 6.96 (4H, d, J ) 8.34 Hz, m-H in
C6H3(OMe)2), 6.50-6.62 (6H, m, m- + p-H in mandelate Ph), 5.39 (4H,
d, J ) 6.84 Hz, o-H in mandelate Ph), 3.75 (12H, s, OCH3), 2.03 (12H, s,
â-CH3 closed to C6H3(OMe)2), 1.70 (12H, s, â-CH3 closed to C6H5); UV-
vis (DCE) λmax (ꢀ) in nm ) 471 (230 000), 681 (17 000). For (S)-mandelic
M.; Taube, H. J. Am. Chem. Soc. 1991, 113, 2327. (b) Sano, M.; Taube,
H. Inorg. Chem. 1994, 33, 705.
(
1
2) H NMR (270 MHz, CDCl3, 23 °C): δ 8.14-8.21 (4H, m, o-H in
C6H5), 7.65-7.70 (6H, m, m- + p-H in C6H5), 7.64 (2H, t, J ) 8.4 Hz,
p-H in C6H3(OMe)2), 6.93 (4H, d, J ) 8.4 Hz, m-H in C6H3(OMe)2), 3.69
1
(
12H, s, OCH3), 2.05 (12H, s, â-CH3 closed to C6H3(OMe)2), 1.84 (12H,
acid: H NMR (270 MHz, CDCl3, 23 °C) δ 7.50-7.36 (5H, m, C6H5),
s, â-CH3 closed to C6H5), -1.9 (2H, s, NH). UV-vis (CHCl3/0.1%
Et3N): λmax (ꢀ) in nm ) 447 (170 000), 545 (20 000), 686 (9700).
HRMS: calcd. for C56H55N4O4 (M + H) 847.4223, found 847.4279.
5.27 (1H, s, CH).
1
(9) H NMR (270 MHz, CD3CO2D, 23 °C): δ 8.45-8.49 (4H, m, o-H
in C6H5), 7.98-8.06 (6H, m, m- + p-H in C6H5), 7.94 (2H, t, J ) 8.35 Hz,
p-H in C6H3(OMe)2), 7.24 (4H, d, J ) 8.35 Hz, m-H in C6H3(OMe)2),
4.02 (12H, s, OCH3), 2.18 (12H, s, â-CH3 closed to C6H3(OMe)2), 1.94
(12H, s, â-CH3 closed to C6H5). UV-vis (AcOH): λmax (ꢀ) in nm ) 467
(220 000), 619 (8300), 680 (21 000).
(3) Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Olmstead, M. M.; Smith,
K. M. J. Am. Chem. Soc. 1996, 118, 10918.
4) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner,
M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851.
5) Medforth, C. J.; Hobbs, J. D.; Rodriguez, M. R.; Abraham, R. J.;
Smith, K. M.; Shelnutt, J. A. Inorg. Chem. 1995, 34, 1333.
(
1
(
(10) H NMR (270 MHz, CD3CO2D, 23 °C): δ 7.54-7.58 (2H, m, o-H
in Ph), 7.37-7.48 (3H, m, m- + p-H in Ph), 5.39 (1H, s, CH).
S0002-7863(97)00431-9 CCC: $14.00 © 1997 American Chemical Society

5268 J. Am. Chem. Soc., Vol. 119, No. 22, 1997
Communications to the Editor
Figure 1. Circular dichroism (CD) spectra at 23 °C of the diastereo-
isomeric mandelate complexes (2) in DCE and enantiomeric diacetate
complexes (3, derived from 2) in AcOH.
Figure 2. Photoresponsive change in optical purity of the (S)-mandelate
complex (2) in DCE/AcOEt (9:1) at 23 °C upon irradiation at its Soret
band (λext ) 470 ( 5 nm). The optical purity was evaluated from the
intensity of the circular dichroism (CD) band (450-500 nm) after
conversion into the diacetate complex (3) in AcOH (see ref 12).
(Figure 1). This observation indicates that the diacetate complex
(3), thus formed, is rich in either of the two possible enanti-
omers, as a consequence of the strong inclination of 2 toward
a thermodynamically favorable diastereoisomer (Scheme 1), and
therefore exhibits optical activity. The optical purity of 2 was
regions without interfering absorptions of usual organic mol-
ecules (<300 nm). From this point of view, we investigated
the assembly of 1 with several other chiral carboxylic acids
including amino acid derivatives, such as 2-methylbutanoic acid,
>
98% (diastereoisomeric excess), as estimated from the NMR
11
profile at -50 °C. Therefore, the porphyrin 1 can read out
the chirality of mandelic acid through hydrogen-bonding
interaction and records the acquired information within its D2-
symmetric saddle-shaped skeleton.
2
-hydroxypropanoic acid, 2-methylphenylacetic acid, 2-meth-
oxyphenylacetic acid, camphanic acid, N-(benzyloxycarbonyl)-
alanine, and N-(benzyloxycarbonyl)phenylalanine, and observed
a certain correlation between the sign of the CD bands of 2 (3)
As expected, the optical activity of 2 was instantly erased
upon addition of a base such as Et3N, as a result of neutralization
of 2 into easily invertible 1.4 On the contrary, in AcOH, the
optical activity lasted for a long period of time, where the half-
life at 23 °C was as long as 200 h. As the temperature was
higher, the lifetime of the optical activity of 3 became shorter
15
and the absolute configurations of the acids. In addition to
carboxylic acids, enantiomers of a phosphoric acid such as 1,1′-
binaphthyl-2,2′-diyl hydrogen phosphate were also distinguish-
able.
(
half-lives of 38, 21, and 5 min at 60, 70, and 84 °C,
In conclusion, we have exploited the conceptually new
chirality-sensing molecule (1), which diagnoses and memorizes
the absolute configurations of chiral acids through hydrogen-
bonding interaction, and can then release and retrieve the
chirality memory in response to the switching of external stimuli
such as heat and light. Determination of the absolute configura-
tions of 2 and 3 and their correlation with the circular dichroism
profiles are the subjects worthy of further investigation.
q
respectively), and the activation free energy (∆G 298) for the
macrocyclic inversion in AcOH was evaluated to be 26.1 kcal
mol (∆H ) 29.1 kcal mol , ∆S ) 10.6 cal K mol ).
Quite interestingly, 3 also lost its optical activity upon
excitation by visible light. One of the interesting demonstrations
of this phenomenon is the photoswitching of the optical activity
of the mandelate complex (2) (Figure 2): When the (S)-
mandelate complex (2) in DCE/AcOEt (9:1) was irradiated for
-
1
q
-1
q
-1
-1
1
2
5
min at its Soret band (xenon arc light, λext ) 470 ( 10 nm),
the optical purity of the chiral saddle was decreased to 60%
quantum yield ) 4.7%). However, when the light was switched
Supporting Information Available: Synthetic procedures for 1 and
bis-(S)-mandelate complex (2) and the method for evaluation of the
(
diastereoisomeric excesses of 2 and bis-(S)-2-methoxyphenylacetate
off, the optical purity automatically reverted to the initial value
within 5 min. Such a photoerasure/self-retrieval cycle could
be repeated many times, where the depth of erasing was
controlled by the irradiation time. The self-retrieval (self-
repairing) of the optical activity of 2 is obviously a result of
the reassembly of the racemized porphyrin with (S)-mandelic
acid.
1
complex together with their H NMR spectra (CDCl
3
, -50 °C) (4
pages). See any current masthead page for ordering and Internet access
instructions.
JA970431Q
(13) (a) Beychok, S.; Blout, E. R. J. Mol. Biol. 1961, 3, 769. (b) Calvin,
Porphyrin derivatives have attracted considerable attention
M. J.; Fiel, R. Nucleic Acids Res. 1983, 11, 6121. (c) Foster, M.; Singhal,
A. K.; Smith, M. W.; Marcos, N. G.; Schray, K. J. Biochim. Biophys. Acta
1988, 950, 118. (d) Nishino, N.; Mihara, H.; Hasegawa, R.; Yanai, T.;
Fujimoto, T. J. Chem. Soc., Chem. Commun. 1992, 692. (e) Pasternak, R.
F.; Bustamante, C.; Collings, P. J.; Giannetto, A.; Gibbs, E. J. Am. Chem.
Soc. 1993, 115, 5393. (f) Matile, S.; Berova, N.; Nakanishi, K.; Novkova,
S.; Philipova, I.; Blagoev, B. J. Am. Chem. Soc. 1995, 117, 7021. (g) Matile,
S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W. J. Am. Chem.
Soc. 1996, 118, 5198.
as chirality sensors,13 since they provide highly enhanced
exciton-coupled circular dichroism (CD) bands at visible
1
4
1
(
11) In the H NMR spectrum in CDCl3 at -50 °C, negligibly small
signals assignable to a thermodynamically unfavorable diastereoisomer were
detected at δ 8.27 (o-H in C6H5) and 3.84 (OCH3). For evaluation of the
diastereoisomeric excess, see Supporting Information.
(
12) A quartz cell, containing a DCE/AcOEt (9:1) solution of 2, was
irradiated at 23 °C by a 300 W xenon arc lamp through a band-path filter
470 ( 10 nm) and a thermal-cut filter, and an aliquot of the solution was
(14) (a) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy-
Exciton Coupling in Organic Stereochemistry; University Science Books:
Mill Valley, CA, 1983. (b) Nakanishi, K.; Berova, N. In Circular
Dichroism-Principles and Applications; Nakanishi, K., Berova, N., Woody,
R. W., Eds.; VCH Publishers Inc.: New York, NY, 1994; pp 361-398.
(15) Similar to that for the mandelate complex (2), 3 derived from (R)-
and (S)-dicarboxylate complexes in AcOH showed positive and negative
CD bands at 480 nm, respectively.
(
periodically taken out by a glass pipet under irradiation and immediately
poured into a large volume of AcOH in a bottle wrapped in aluminum foil,
in order to suppress further conformational change of the porphyrin ring.
The AcOH solution of 3, thus obtained, was then subjected to CD
spectroscopy for evaluation of the optical purity.