Enantioselective sensing of amines based on [1 + 1]-, [2 + 2]-, and [1 + 2]-condensation with fluxional arylacetylene-derived dialdehydes

Article abstract of DOI:10.1021/ol200574x

Four induced circular dichroism (ICD) probes exhibiting a stereodynamic arylacetylene framework and terminal aldehyde units have been prepared. The CD silent sensors generate a strong chiroptical response to substrate-controlled induction of axial chirality upon selective [1 + 1]-, [2 + 2]-, and [1 + 2]-condensation. The intense Cotton effects can be exploited for in situ ICD analysis of the absolute configuration and ee of a wide range of amines.

Full text of DOI:10.1021/ol200574x

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2602–2605
Enantioselective Sensing of Amines
Based on [1 þ 1]-, [2 þ 2]-, and
[1 þ 2]-Condensation with Fluxional
Arylacetylene-Derived Dialdehydes
Daniel P. Iwaniuk and Christian Wolf*
Department of Chemistry, Georgetown University, Washington, D.C. 20057,
United States
cw27@georgetown.edu
Received March 17, 2011
ABSTRACT
Four induced circular dichroism (ICD) probes exhibiting a stereodynamic arylacetylene framework and terminal aldehyde units have been
prepared. The CD silent sensors generate a strong chiroptical response to substrate-controlled induction of axial chirality upon selective [1 þ 1]-,
[2 þ 2]-, and [1 þ 2]-condensation. The intense Cotton effects can be exploited for in situ ICD analysis of the absolute configuration and ee of a
wide range of amines.
Conformationally flexible receptor molecules that adopt
anaxiallyorhelically chiralarrangement upon binding to a
chiral substrate have received increasing attention in recent
years.1 Imprinting of the chiral information of carboxylic
acids or other analytes onto supramolecular assemblies,2
molecular bevel gears,3 propellers,4 and other well-defined
arrangements5 can give rise to remarkable induced circular
dichroism (ICD) signals.6 Using carefully studied ICD
probes that form hydrogen bond adducts7 and metal
complexes8 with distinct chiral amplification, Berova,
Nakanishi, Canary, and others have demonstrated that
the corresponding Cotton effects can be used for reliable
(1) Selected examples: (a) Tumambac, G. E.; Mei, X.; Wolf, C. Eur. J.
Org. Chem. 2004, 3850. (b) Kawai, H.; Katoono, R.; Fujiwara, K.; Tsuji,
T.; Suzuki, T. Chem.;Eur. J. 2005, 11, 815. (c) Katoono, R.; Kawai, H.;
Fujiwara, K.; Suzuki, T. Tetrahedron Lett. 2006, 47, 1513. (d) Hembury,
G. A.; Borovkov, V. V.; Inoue, Y. Chem. Rev. 2008, 108, 1. (e) Kohmoto,
S.; Takeichi, H.; Kishikawa, K.; Masu, H.; Azumaya, I. Tetrahedron
Lett. 2008, 49, 1223. (f) Tartaglia, S.; Pace, F.; Scafato, P.; Rosini, C.
Org. Lett. 2008, 10, 3421. (g) Kim, H.; So, S. M.; Yen, C. P.-H.; Vinhato,
E.; Lough, A. J.; Hong, J.-I.; Kim, H.-J.; Chin, J. Angew. Chem., Int. Ed.
2008, 47, 8657.
(2) Das, N.; Ghosh, A.; Singh, O. M.; Stang, P. J. Org. Lett. 2006, 8,
1701.
(3) Sciebura, J.; Skowronek, P.; Gawronski, J. Angew. Chem., Int.
Ed. 2009, 48, 7069.
(4) Katoono, R.; Kawai, H.; Fujiwara, K.; Suzuki, T. J. Am. Chem.
(6) (a) Gawronski, J.; Grajewski, J. Org. Lett. 2003, 5, 3301. (b)
Berova, N.; Di Bari, L.; Pescitelli, G. Chem. Soc. Rev. 2007, 36, 914.
(7) (a) Katoono, R.; Kawai, H.; Fujiwara, K.; Suzuki, T. Tetrahedron
Lett. 2006, 47, 1513. (b) Waki, M.; Abe, H.; Inouye, M. Angew. Chem.,
Int. Ed. 2007, 46, 3059.
(8) (a) Proni, G.; Pescitelli, G.; Huang, X.; Nakanishi, K.; Berova, N.
J. Am. Chem. Soc. 2003, 125, 12914. (b) Yang, Q.; Olmsted, C.; Borhan,
B. Org. Lett. 2002, 4, 3423. (c) Huang, X.; Fujioka, N.; Pescitelli, G.;
Koehn, F. E.; Williamson, R. T.; Nakanishi, K.; Berova, N. J. Am.
Chem. Soc. 2002, 124, 10320. (d) Ishii, H.; Chen, Y.; Miller, R. A.;
Karady, S.; Nakanishi, K.; Berova, N. Chirality 2005, 17, 305. (e) Balaz,
M.; De Napoli, M.; Holmes, A. E.; Mammana, A.; Nakanishi, K.;
Berova, N.; Purrello, R. Angew. Chem., Int. Ed. 2005, 44, 4006. (f)
Holmes, A. E.; Das, D.; Canary, J. W. J. Am. Chem. Soc. 2007, 129,
1506. (g) Berova, N.; Pescitelli, G.; Petrovic, A. G.; Proni, G. Chem.
Commun. 2009, 5958. (h) Canary, J. W.; Mortezaei, S.; Liang, J. Chem.
Commun. 2010, 46, 5850.
Soc. 2009, 131, 16896.
(5) (a) Waki, M.; Abe, H.; Inouye, M. Angew. Chem., Int. Ed. 2007,
46, 3059. (b) Tartaglia, S.; Pace, F.; Scafato, P.; Rosini, C. Org. Lett.
2008, 10, 3421. (c) Kim, H.; So, S. M.; Yen, C. P.-H.; Vinhato, E.; Lough,
A. J.; Hong, J.-I.; Kim, H.-J.; Chin, J. Angew. Chem., Int. Ed. 2008, 47,
8657. (d) Wezenberg, S. J.; Salassa, G.; Escudero-Adan, E. C.; Benet-
Buchholz, J.; Kleij, A. W. Angew. Chem., Int. Ed. 2011, 50, 713.
r
10.1021/ol200574x
Published on Web 04/19/2011
2011 American Chemical Society

configurational and conformational analysis of a wide
range of substrates. Rosini and Toniolo have successfully
used this concept for the assignment of the absolute
configuration of chiral amino acids, carboxylic acids, and
alcohols that were covalently attached to a stereodynamic
biphenyl probe.9
To date, few compounds bearing a diarylacetylene motif
which typically undergoes fast rotation about the triple
bond at room temperature have been reported.15 The
molecular bias for almost frictionless rotation has been
exploitedforthe development ofmolecular turnstiles16 and
gyroscopes.17 Based on the potential of 1,3-diarylacety-
lenes as versatile stereodynamic units in molecular devices,
probes, and other applications, we decided to evaluate a
series of arylacetylene-based frameworks having a well-
defined intramolecular separation and relative orientation
of terminal formyl groups. The molecular geometry of
these structures would either favor or prevent [1 þ 1]-
cyclocondensation with diamines. We now report that the
design of the CD silent arylacetylene moiety of four
stereodynamic dialdehydes directs the reaction with dia-
mines toward a [1 þ 1]- or [2 þ 2]-assembly that provides a
distinct chiroptical response to a substrate-controlled chir-
al amplification process. We assumed that bridging of 1,
4-di(2-formylphenyl)buta-1,3-diyne, 2, and 1,4-bis(2-form-
ylphenylethynyl)benzene, 3, through cyclocondensation
with a diamine would be geometrically impossible and
thus favor formation of a large macrocyclic tetraimine via
[2 þ 2]-cyclocondensation (Figure 1). By contrast, 1,4-bis-
(2(2-formyl-1-naphthylethynyl)phenylethynyl)benzene, 4,
and 1,4-bis(2(2-formylphenylethynyl)phenylethynyl)anthra-
cene, 5, were expected to favor a [1 þ 1]-assembly.
In recent years, we have investigated the stereodynamic
properties of a wide range of axially chiral compounds10
and used 1,8-diquinolyl- and 1,8-diacridylnaphthalenes as
UV and fluorescence sensors for quantitative analysis of
the enantiomeric composition of carboxylic acids, amino
acids, amines, and amino alcohols.11 These sensorsprovide
accurate information about the ee and amount of a wide
range of substrates often at micromolar concentrations.12
We then realized that rapidly racemizing chromophoric
probes exhibit strong Cotton effects if the binding event locks
the stereodynamic sensor into a single chiral conformation.13
For example, 1,4-bis(2(2-formylphenylethynyl)phenyleth-
ynyl)benzene, 1, reacts with chiral diamines to a highly CD
active macrocycle showing substrate-controlled induction
of three chiral axes upon diimine formation.14
(9) (a) Superchi, S.; Casarini, D.; Laurita, A.; Bavoso, A.; Rosini, C.
Angew. Chem., Int. Ed. 2001, 40, 451. (b) Hosoi, S.; Kamiya, M.; Kiuchi,
F.; Ohta, T. Tetrahedron Lett. 2001, 42, 6315. (c) Mazaleyrat, J.-P.;
Wright, K.; Gaucher, A.; Toulemonde, N.; Wakselman, M.; Oancea, S.;
Peggion, C.; Formaggio, F.; Setnicka, V.; Keiderling, T. A.; Toniolo, C.
J. Am. Chem. Soc. 2004, 126, 12874. (d) Mazaleyrat, J.-P.; Wright, K.;
Gaucher, A.; Toulemonde, N.; Dutot, L.; Wakselman, M.; Broxterman,
Q. B.; Kaptein, B.; Oancea, S.; Peggion, C.; Crisma, M.; Formaggio, F.;
Toniolo, C. Chem.;Eur. J. 2005, 11, 6921. (e) Superchi, S.; Bisaccia, R.;
Casarini, D.; Laurita, A.; Rosini, C. J. Am. Chem. Soc. 2006, 128, 6893.
(f) Dutot, L.; Wright, K.; Gaucher; Wakselman, M.; Mazaleyrat, J.-P.;
De Zotti, M.; Peggion, C.; Formaggio, F.; Toniolo, C. J. Am. Chem. Soc.
2008, 130, 5986.
(10) (a) Wolf, C.; Ghebramariam, B. T. Tetrahedron: Asymmetry
2002, 13, 1153. (b) Wolf, C.; Tumambac, G. E. J. Phys. Chem. A 2003,
107, 815. (c) Tumambac, G. E.; Wolf, C. J. Org. Chem. 2004, 69, 2048.
(d) Wolf, C. Chem. Soc. Rev. 2005, 34, 595. (e) Tumambac, G. E.; Wolf,
C. J. Org. Chem. 2005, 70, 2930. (f) Wolf, C.; Xu, H. Tetrahedron Lett.
2007, 48, 6886. For a general overview of the stereodynamics of chiral
compounds, see: Wolf, C., Ed. Dynamic Stereochemistry of Chiral
Compounds; RSC Publishing: Cambridge, 2008.
(11) (a) Mei, X.; Wolf, C. Chem. Commun. 2004, 2078. (b) Mei, X.;
Wolf, C. J. Am. Chem. Soc. 2004, 126, 14736. (c) Tumambac, G. E.;
Wolf, C. Org. Lett. 2005, 7, 4045. (d) Mei, X.; Martin, R. M.; Wolf, C.
J. Org. Chem. 2006, 71, 2854. (e) Liu, S.; Pestano, J. P. C.; Wolf, C.
J. Org. Chem. 2008, 73, 4267. (f) Mei, X.; Wolf, C. Tetrahedron Lett.
2006, 47, 7901. (g) Wolf, C.; Liu, S.; Reinhardt, B. C. Chem. Commun.
2006, 4242. (h) Mei, X.; Wolf, C. J. Am. Chem. Soc. 2006, 128, 13326.
(i) Liu, S.; Pestano, J. P. C.; Wolf, C. J. Org. Chem. 2008, 73, 4267.
(12) For a discussion of the general significance of molecular sensing,
see: Anslyn, E. V. J. Am. Chem. Soc. 2010, 132, 15833.
Figure 1. Structures of dialdehydes 2 to 5 (top) and [1 þ 1]- and
[2 þ 2]-assembled macrocycles (bottom).
(13) (a) Ghosn, M. W.; Wolf, C. J. Am. Chem. Soc. 2009, 131, 16360.
(b) Wolf, C. Tetrahedron 2010, 66, 3989.
(14) Iwaniuk, D. P.; Wolf, C. J. Am. Chem. Soc. 2011, 133, 2414.
Dialdehyde 2 was readily available through oxidative
homocoupling of 2-ethynylbenzaldehyde as described in
the literature (Scheme 1).18 The elongated analogue 3 was
prepared by Sonogashira coupling of 2-bromobenzalde-
hyde and 1,4-diethynylbenzene in 80% yield. Slow eva-
poration of a solution of 2 in dichloromethane gave single
crystalssuitable forX-ray diffraction. Thecrystallographic
analysis of 3 showed the expected linear geometry and the
lack of steric hindrance to rotation about the arylꢀalkyne
bonds. As discussed above, we expected that the geo-
metry of 2 and 3 would afford tetraimine macrocycles upon
€
(15) (a) Koo Tze Mew, P.; Vogtle, F. Angew. Chem., Int. Ed. Engl.
1979, 18, 159. (b) Toyota, S.; Yamamori, T.; Asakura, M.; Oki, M. Bull.
Chem. Soc. Jpn. 2000, 73, 205. (c) Toyota, S.; Iida, T.; Kunizane, C.;
Tanifuji, N.; Yoshida, Y. Org. Biomol. Chem. 2003, 1, 2298. (d) Toyota,
S.; Makino, T. Tetrahedron Lett. 2003, 44, 7775. (e) Miljanic, O. S.; Han,
S.; Holmes, D.; Schaller, G. R.; Vollhardt, K. P. C. Chem. Commun.
2005, 2606. (f) Nandy, R.; Subramoni, M.; Varghese, B.; Sankararaman,
S. J. Org. Chem. 2007, 72, 938.
(16) Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662.
(17) (a) Dominguez, Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay,
M. A. J. Am. Chem. Soc. 2002, 124, 2398. (b) Dominguez, Z.; Dang, H.;
Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124,
7719. (c) Khuong, T.-A. V.; Dang, H.; Jarowski, P. D.; Maverick, E. F.;
Garcia-Garibay, M. A. J. Am. Chem. Soc. 2007, 129, 839. (d) Jarowski,
P. D.; Houk, K. N.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2007, 129,
3110. (e) Garcia-Garibay, M. A.; Godinez, C. Cryst. Growth Des. 2009,
9, 3124. (f) Toyota, S. Chem. Rev. 2010, 110, 5398.
(18) Ojima, J.; Yanamato, K.; Kato, T.; Wada, K.; Yoneyama, Y.;
Ejiri, E. Bull. Chem. Soc. Jpn. 1986, 59, 2209.
Org. Lett., Vol. 13, No. 10, 2011
2603

ꢀ90.0%, ꢀ46.0%, 30.0%, and 86.0% ee were then pre-
pared and treated with sensor 3 as described above. Using
the linear regression equation calculated from the calibra-
tion curve and the measured CD amplitudes of these
samples, the enantiomeric excess was determined. Experi-
mentally obtained data were within 3.9% of the actual
values; see SI.20
Scheme 1. Synthesis of 2 and 3 and Crystal Structure of 3
[2 þ 2]-cyclocondensation with diamines.19 The reaction
between stoichiometric amounts of either enantiomer of
trans-diaminocyclohexane and 2 was monitored by NMR
spectroscopy showing quantitative disappearance of the
formyl protons. Formation of the expected [2 þ 2]-macro-
cycle was evident from ESI-MS analysis, but the conden-
sation with 1,2-diamino-1,2-diphenylethane yielded
an acyclic product (see Supporting Information (SI)).
Importantly, the 2-derived imines obtained with the
enantiomers of trans-diaminocyclohexane and other
amines gave measurable Cotton effects (Figure 2
and SI).
Figure 3. Structures of amine and diamine substrates.
Palladium catalyzed Sonogashira coupling of 2-bro-
moiodobenzene and 1,4-diethynylbenzene gave dibromide
17 in quantitative amounts (Scheme 2). Cross-coupling
with ethynyltrimethylsilane then furnished 18 in 93% yield
which was deprotected with TBAF to produce dialkyne
19. Finally, coupling of 19 and 2 equiv of 1-bromo-2-
naphthaldehyde in the presence of 10 mol % of Pd(PPh3)4
and CuI produced 4 in 54% yield. A somewhat similar
strategy was used to prepare the branches of compound 5
prior to the construction of the central rod. First, 2-((2-
bromophenyl)ethynyl)benzaldehyde, 20, was obtained in
almost quantitative yields by Sonogashira coupling of 2-iodo-
bromobenzene and 2-ethynylbenzaldehyde. Alkynylation
Scheme 2. Synthesis of Sensors 4 and 5
Figure 2. CD Spectra of the condensation products obtained
from 2 (left) or 3 (right) and (1R,2R)-diaminocyclohexane (blue)
and its (1S,2S)-enantiomer (red). The reactions were performed
at 3.75 mM in chloroform at room temperature. For CD
analysis the samples were diluted to 2.82 ꢁ 10ꢀ4 and 1.32 ꢁ
10ꢀ4 M, respectively.
Encouraged by the results obtained with 2, we continued
to screen the condensation between 3 and several amines
(Figure 3). We were pleased to find that this dialdehyde
undergoes [2 þ 2]-cyclocondensation with all diamines
tested and generates acyclic diimines with amines 10ꢀ16.
Both the enantiomeric tetraimines and diimines formed
show remarkable Cotton effects that can be used for
quantitative ee analysis of the amine substrates (Figure 2
and SI). To demonstrate the use of 3 in enantioselective
sensing applications, a calibration curve was constructed
using scalemic mixtures of diaminocyclohexane, 6. The
corresponding 3-derived macrocyclic tetraimines were pre-
pared in chloroform at 3.75 mM, and the samples were
diluted to 7.48 ꢁ 10ꢀ4 M for CD analysis. Plotting of the
CD amplitudes measured at 300 nm versus %ee revealed a
linear relationship. Four scalemic samples of 6 having
2604
Org. Lett., Vol. 13, No. 10, 2011

with ethynyltrimethylsilane and subsequent deprotection
then gave 21 and 22 in 83% and 62% yield, respectively.
2-(2-Ethynylphenylethynyl)benzaldehyde, 22, was then at-
tached to the anthracene core by palladium catalyzed
cross-coupling with 9,10-dibromoanthracene to give 5 in
60% yield.
and 5 with (1S,2S)-6 showed that this distinctive chiral
amplification is controlled by the central chirality of the
amine substrate, and we anticipated that the induced
chiroptical properties of the diimines formed should pro-
vide quantifiable information on the absolute configura-
tion and the enantiomeric composition of the substrate
(Figure 5). Indeed, we were able to obtain linear calibra-
tion curves using these two sensors and either diamine 8 or
monoamine 13. As described above, the linear regression
equation calculated from these calibration curves and the
measured CD amplitudes of the diimines generated from
scalemic mixtures of amines 8 and 13 gave experimentally
obtained data that were within 7.4% of the actual values;
see Supporting Information.
Figure 4. CD spectra of the [1 þ 1]-macrocycle obtained using 4
and (1R,2R)-6 (blue) or (1S,2S)-6 (red) at 9.75 ꢁ 10ꢀ5 M in
CHCl3 (left). CD spectra of the acyclic [1 þ 2]-cyclocondensa-
tion product produced from 5 and (R)-13 (blue) or (S)-13 (red) at
3.39 ꢁ 10ꢀ4 M in CHCl3 (right).
The suitability of dialdehydes 4 and 5 for enantioselec-
tive sensing of the amines shown in Figure 4 was then
evaluated. As expected, mass spectrometric analysis of the
reaction mixtures obtained with diamimes 6ꢀ9 proved the
formation of [1 þ 1]-macrocycles while the monoamines
generate [1 þ 2]-cyclocondensation products. In situ CD
analysis of the diimines showed strong Cotton effects that
can be attributed to population of a highly CD active
axially chiral conformer (Figure 4). MM2 calculations of
the dimines obtained by cyclocondensation of sensors 4
Figure 5. MM2 Computation of the structures of the diimines
derived from (1S,2S)-6 and 4 (top) or 5 (bottom). For better
clarity, the hydrogens are omitted in the space filling model.
In summary, we have introduced four CD silent
probes exhibiting a stereodynamic arylacetylene frame-
work and two terminal aldehyde units. The well-defined
geometry of these compounds favors either [1 þ 1]- or
[2 þ 2]-cyclocondensation with diamines while mono-
amines generate acyclic [1 þ 2]-diimines. Computational
analysis suggests that the strong Cotton effects observed
upon di- and tetraimine formation are due to substrate-
controlled induction of axial chirality. These sensors
provide a means for metal-free enantioselective CD
analysis of a wide range of chiral amines. The use of
sensitive ICD sensors 2ꢀ5 avoids cumbersome isolation
of reaction products prior to analysis, and it eliminates
the need for an elaborate synthesis of an enantiomeri-
cally pure chiral probe.
(19) Several [2 þ 2]-, [3 þ 3]-, [4 þ 4]-, and [4 þ 6]-cyclocondensation
products have been prepared from 1,2-diaminocyclohexane with iso-
phthalaldehyde, terephthalaldehyde, and similar aldehydes. (a) Korupoju,
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J.; Zavada, J.; Junk, P. C. Tetrahedron: Asymmetry 2001, 12, 127.
(e) Kuhnert, N.; Strassnig, C.; Lopez-Periago, A. M. Tetrahedron: Asym-
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Lett. 2002, 43, 3329. (g) Kuhnert, N.; Rossignolo, G. M.; Lopez-Periago,
A. M. Org. Biomol. Chem. 2003, 1, 1157. (h) Gao, J.; Martell, A. E. Org.
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Acknowledgment. Funding from the National Science
Foundation (CHE-0910604) is gratefully acknowledged.
Supporting Information Available. Synthetic details,
compound characterization, and MS, CD, and NMR
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(20) For CD analysis of amines with chiral metal complexes, see: (a)
Nieto, S.; Lynch, V. M.; Anslyn, E. V.; Kim, H.; Chin, J. J. Am. Chem.
Soc. 2008, 130, 9232. (b) Nieto, S.; Dragna, J. M.; Anslyn, E. V. Chem.;
Eur. J. 2010, 16, 227.
Org. Lett., Vol. 13, No. 10, 2011
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