Quantitative chirality sensing of amines and amino alcohols via Schiff base formation with a stereodynamic UV/CD probe

Article abstract of DOI:10.1039/c5ob02529j

A stereodynamic chemosensor that can be used for simultaneous determination of the absolute configuration, enantiomeric composition and total concentration of chiral amines and amino alcohols based on two fast optical measurements was prepared and tested. The free sensor is CD-silent and produces characteristic blue-shifted UV and CD signals upon substrate binding via Schiff base formation. The potential in high-throughput screening applications and for rational sensor developments are discussed.

Full text of DOI:10.1039/c5ob02529j

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Quantitative chirality sensing of amines and
amino alcohols via Schiff base formation with a
stereodynamic UV/CD probe†
Cite this: DOI: 10.1039/c5ob02529j
Received 10th December 2015,
Accepted 11th January 2016
Zeus A. De los Santos, Ransheng Ding and Christian Wolf*
DOI: 10.1039/c5ob02529j
www.rsc.org/obc
A stereodynamic chemosensor that can be used for simultaneous (HTS) methodologies that enable simultaneous analysis of the
determination of the absolute configuration, enantiomeric enantiomeric composition of large number of chiral
a
composition and total concentration of chiral amines and amino samples. Optical chirality sensing with small molecular
alcohols based on two fast optical measurements was prepared probes3 and supramolecular systems4 has been identified as a
and tested. The free sensor is CD-silent and produces characteristic promising alternative to traditional enantioselective chromato-
blue-shifted UV and CD signals upon substrate binding via Schiff graphy or electrophoresis. Because of the general significance
base formation. The potential in high-throughput screening appli- of chiral amines, the development of chemosensors that
cations and for rational sensor developments are discussed.
utilize Schiff base formation to capture a chiral substrate for
subsequent ee analysis has been of particular interest. Several
probe designs exhibiting an aldehyde or ketone moiety for this
purpose have been reported by Kim, Hong and Chin,5 Anslyn,6
Pu,7 Joyce8 and our group (Fig. 1).9 Typically, the covalent
binding of the chiral amine affects the circular dichroism or
fluorescence readout of the chemosensor, and this change is
then used to determine the enantiomeric composition of the
target compound. Remaining drawbacks of some of these
chemosensors are limited substrate scope and the formation
of more than one reaction product which can complicate the
ee analysis.
Introduction
The important role of asymmetric synthesis and stereochemi-
cal analysis of chiral compounds in the pharmaceutical,
environmental and forensic sciences or other interdisciplinary
research settings is staggering and continues to grow. The
widespread availability of high-throughput equipment acceler-
ates overall progress and it reduces labor and material costs, in
particular when small-scale reactions are carried out. Today,
the production of numerous chiral samples via automated
parallel asymmetric synthesis is routine in many laboratories.
Despite the efficient use of high-throughput technology in
reaction discovery and development programs, the analysis of
the absolute configuration and enantiomeric excess (ee) of
chiral compounds has remained relatively time-consuming
and laborious. Several groups have introduced robust GC and
HPLC methods suitable for the study of one-pot multi-sub-
strate reactions.1 Some of the typical shortcomings of chiral
chromatography, including time-inefficiency and solvent waste
production, however, remain unless high-speed separations
can be achieved.2
The mismatch in parallel synthesis and analysis capabilities
has inspired the introduction of high-throughput screening
Department of Chemistry, Georgetown University, Washington, D.C., USA.
E-mail: cw27@georgetown.edu
†Electronic supplementary information (ESI) available. CCDC 1440658–1440660.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ob02529j
Fig. 1 Structures of optical sensors used for covalent binding and
enantioselective detection of chiral amines, amino alcohols and amino
acids.
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slow evaporation of concentrated acetonitrile and chloroform
solutions, respectively. As expected, crystallographic analysis
Results and discussion
We now wish to report a readily available chiroptical sensor 1 confirmed that the amides exhibit a near planar structure and
that is designed to trap chiral amino compounds as Schiff an intramolecular hydrogen bond in the case of 1.10 A closer
base for quantitative ee and concentration determination. The look at the crystal structure of 2 revealed that the two aromatic
receptor used in this study consists of a benzaldehyde unit moieties are coplanar with the amide unit being twisted out of
and an adjacent naphthamide moiety which activates the that plane. Probably because of the intramolecular hydrogen
formyl group toward imine formation by intramolecular hydro- bond, compound 1 shows less molecular twisting.
gen bonding (Scheme 1). At the onset of this study we hypo-
With sensor 1 in hand, we selected a series of 11 amines
thesized that the 2-amidobenzaldehyde component (red) would and amino alcohols 3–13 to evaluate the use of this readily
function as a rigid substrate binding site. Schiff base formation available probe for chirality sensing (Fig. 2). We first investi-
would then result in a nonenantioselective change in the UV gated the imine formation which needs to be quantitative for
absorption of 1 (to be used for total concentration analysis) and the intended analytical purposes and should preferably occur
a stereochemical bias in the 2-naphthamide CD reporter unit under mild conditions. Proton NMR analysis using amine 6
(blue). This chirality imprinting from the bound amino com- showed complete reaction within 5 hours at room temperature.
pound onto the 2-naphthamide unit was expected to furnish a The time necessary for quantitative Schiff base formation can
characteristic CD signal which would provide information about easily be reduced if necessary. For example, we found that the
the absolute configuration and ee of the substrate.
condensation reaction takes less than 40 minutes in the pres-
We were able to prepare 1 in two high yielding steps ence of Ti(Oi-Pr)4 which may serve as both an activating Lewis
(Scheme 2). Acylation of 2-amino benzylalcohol with acid and a hygroscopic reagent to accelerate the imine for-
2-naphthoyl chloride gave direct access to amide 2 without the mation and favor quantitative conversion, respectively (ESI†).
need to protect the primary alcohol. Oxidation with pyridi- We were excited to observe strong CD readouts of the imines
nium chlorochromate (PCC) provided 1 in almost quantitative formed in situ above 300 nm at submillimolar concentrations.
amounts. We were able to grow single crystals of 1 and 2 by Representative examples obtained with the imines prepared
from the enantiomers of 3 and 10 are shown in Fig. 3. Similar
results were observed with the other amino compounds and
we note that the free substrates are CD silent in the region of
interest (ESI†). Interestingly, all imines derived from the (R)-
enantiomers of the monoamines produce a negative CD signal
while the (S)-enantiomers induce the opposite chiroptical
sensor response. Because of the variations in the amino
alcohol structures exhibiting either acyclic or cyclic scaffolds
with one or two chiral centers a general relationship between
the sign of the CD readout of 1 and the absolute configuration
of the bound substrate cannot be construed at this point. In
contrast to the strong CD signals obtained with sensor 1 we
observed only a moderate chiroptical response with the corres-
ponding 1-naphthamide derivative.11
Scheme 1 Structure of 1 and illustration of the chiroptical sensing
concept.
Scheme 2 Synthesis and X-ray structures of 2 and sensor 1. Selected torsion angles [°] and bond lengths [Å] for 1: C1–C2–N1–C3 175.5, N1–C3–
C4–C5: −15.3, hydrogen bond: 1.973. For 2: C1–C2–N1–C3: 146.5, N1–C3–C4–C5: −150.1.
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from the calibration shown in Fig. 4 and the measured Cotton
effect amplitude at 320 nm, the enantiomeric excess of these
samples was determined (Table 1). Gratifyingly, the experi-
mentally obtained sensing data were within a few percent of
the actual values. Basically the same approach was applied to
UV chemosensing of five solutions containing 3 at varying con-
centrations (Table 2). Again, the sensing results were of
sufficient accuracy for HTS purposes. To prove the reproduci-
bility and practicality, the concentration analysis of these
samples was conducted with calibration curves obtained at
different days. The sensing results show little variation which
highlights that a single regression analysis suffices and recali-
bration is not necessary (ESI†).
A single crystal was obtained by slow evaporation of a con-
centrated solution of the imine derived from sensor 1 and
(R)-3 in CD3CN (Fig. 5).12 Crystallographic analysis confirmed the
expected intramolecular hydrogen bonding motif between the
imine nitrogen and the adjacent amide proton. Although solid
state packing forces are likely to affect the conformational
structures of the sensor and the imine derivative, it is note-
worthy that the latter exhibits enhanced twisting on both sides
of the amide function which might in part explain the distinct
chiroptical readouts measured in solution. Comparison of the
CD signals in acetonitrile, dichloromethane, diethyl ether and
methanol, however, showed minor solvent effects (ESI†). The
intensity of the CD readout of the imine of (R)-3, which can be
used as a measurement of the chiral amplification process,
remains almost unchanged and decreases by approximately
10% in methanol. This small variation indicates that the con-
formation in the crystal structure and the intramolecular
hydrogen bonding motif might not be predominant in
solution.
Fig. 2 Amines and amino alcohols tested (only one enantiomer is
shown).
An important feature of the chiroptical analysis (CD and
UV) with 1 is that purification of the imines or any work-up
steps are not required and all measurements can be performed
directly using the crude condensation product mixtures in
acetonitrile. Further analysis with nonracemic samples of 3
showed a perfectly linear relationship between the amplitude
of the CD signals of the imines of 1 and the ee of the free
amine (Fig. 4). The change in the UV signature of 1 upon
addition of varying amounts of 3 was then analyzed. We were
pleased to find that the substrate fixation coincides with
characteristic CD and UV changes. As shown in Fig. 4, the sub-
strate binding event results in a hypsochromic shift and a
linear increase in the UV absorption at 310 nm.
Finally, we decided to investigate if the rationale underlying
the chiroptical amine and amino alcohol sensing with 1 could,
in principle, be extended to aldehydes. To date, chirality
sensing of aldehydes and ketones has remained very challen-
ging.13 We previously found that the concept of isostericity has
remarkable potential in this regard.13a Accordingly, fixation of
a chiral aldehyde such as citronellal, 14, by the aniline deriva-
Having established that 1 undergoes smooth condensation
with amino compounds yielding distinct CD and UV responses
that change linearly with the ee and total amount of the sub-
strate encouraged us to further test its value for quantitative
stereochemical analysis. Five nonracemic samples of 3 were
prepared and then treated with sensor 1 to generate the corres-
ponding imine. Using the linear regression equation obtained
Fig. 3 CD Spectra of the imines derived from 1 and the enantiomers of 3 (left) and 10 (right). The CD analysis was conducted with sample concen-
trations of 3.0 × 10−4 M in CH3CN. The chiroptical responses collected with the imines derived from the (R)-configured substrates are shown in blue
and those from the (S)-enantiomers are in red.
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Fig. 4 Top: CD spectra of the imine obtained from 1 and varying ee compositions of 3 and linear relationship between the CD amplitude at 320 nm
and the sample ee. Bottom: UV spectra of 1 upon addition of 3 in varying molar ratio and linear regression analysis of the UV change at 310 nm.
Table 1 Experimentally determined ee’s of five nonracemic samples of
3 using the imine CD maximum at 320 nm
Sample composition
Abs. config.
Chiroptical sensing results
Actual %ee
Abs. config.a
Calc. %eeb
R
R
R
S
S
S
87.0
76.0
12.0
26.0
68.0
89.0
R
R
R
S
S
S
87.6
72.3
8.4
20.7
66.9
89.1
Fig. 5 Crystallographic analysis of the imine derived from sensor 1 and
(R)-3. Selected torsion angles [°] and bond lengths [Å]: C1–C2–N1–C3:
−162.2, N1–C3–C4–C5: 155.8, (CO)NH⋯N hydrogen bond: 2.018.
a Based on the sign of the CD response. b Based on the amplitude of
the CD response.
Table 2 UV sensing of the concentration of 3 a
tive 15 would afford an isosteric Schiff base motif that in
analogy to the sensing study described above with 1 could
result in chirality imprinting onto the adjacent naphthamide
unit and a chiroptical sensor readout (Scheme 1). Similarly to
1, N-(2-aminophenyl)-2-naphthamide, 15, is readily available
and was prepared in one step in 87% yield. Condensation with
the enantiomers of 14 required elevated temperatures. We
were pleased, however, to observe a measurable chiroptical
Actual conc.
(10−3 M)
UV sensing (1)
UV sensing (2)
(10−3 M)
(10−3 M)
1.5
5.5
7.0
8.5
9.0
1.3
5.6
6.1
8.3
8.9
1.6
5.7
6.1
8.3
8.8
a UV sensing data were calculated with two different calibration curves.
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Scheme 3 Chirality imprinting on 1 and rationale for the sensing of
citronellal, 14, with 15.
response from 15 which proves the value of the isostericity
concept for the development of new chirality sensors (ESI†)
(Scheme 3).
Conclusions
In summary, we have demonstrated the usefulness of N-(2-for-
mylphenyl)-2-naphthamide, 1, for quantitative sensing of
chiral amines and amino alcohols. Schiff base formation
results in spontaneous chirality amplification yielding a dis-
tinct CD signal that is successfully correlated to the absolute
configuration and ee of a series of compounds. The free
sensor 1 is CD-silent and therefore does not generate any
chiroptical background noise. The imine formation also coincides
with a UV change that is nonenantioselective and can be used
to determine the total amount of a chiral analyte. Altogether,
this readily available chiroptical sensor allows simultaneous
determination of the total concentration and enantiomeric
composition of amines and amino alcohols. The potential of
the binding motif and the architecture of this sensor for iso-
steric aldehyde sensing was confirmed and briefly discussed.
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Acknowledgements
We gratefully acknowledge financial support from the U.S.
National Science Foundation (CHE-1464547).
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