Kinetically controlled simplification of a multiresponsive [10 × 10] dynamic imine library

Article abstract of DOI:10.1039/c6cc06772g

Kinetically controlled self-sorting processes in complex synthetic mixtures represent an important model for behaviours of biological networks, which operate far from equilibrium and without interference among simultaneous metabolic pathways. However, most of the previously reported kinetic self-sorting protocols dealt with small dynamic libraries and a single external stimulus. Here, we report the iterative simplification of a large imine dynamic combinatorial library (DCL) constructed from 10 aldehydes and 10 anilines, under the sequential influence of an oxidant, an adsorbent, and an increase in temperature. Six components of this initial DCL are mechanically isolated and amplified at least three-fold relative to their equilibrium distributions at the outset of the sorting process.

Full text of DOI:10.1039/c6cc06772g

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Kinetically controlled simplification of a
multiresponsive [10 Â 10] dynamic imine library†
Cite this:DOI: 10.1039/c6cc06772g
ˇ
Chia-Wei Hsu and Ognjen S. Miljanic*
´
Received 17th August 2016,
Accepted 22nd September 2016
DOI: 10.1039/c6cc06772g
www.rsc.org/chemcomm
Kinetically controlled self-sorting processes in complex synthetic of smart materials, including multiresponsive polymers and
mixtures represent an important model for behaviours of biological gels,4 and low-symmetry self-assembled structures.5
networks, which operate far from equilibrium and without inter-
We6 and others7 have studied kinetically controlled sorting
ference among simultaneous metabolic pathways. However, most processes in DCLs. Our protocols utilized chemical oxidation,8 as
of the previously reported kinetic self-sorting protocols dealt with well as physical processes such as distillation,9 precipitation,10 or
small dynamic libraries and a single external stimulus. Here, we report adsorption,11 to simplify complex mixtures with as many as
the iterative simplification of a large imine dynamic combinatorial 25 members into a handful (five or less) of pure products. In all
library (DCL) constructed from 10 aldehydes and 10 anilines, under of these protocols, an irreversible stimulus amplified the imine
the sequential influence of an oxidant, an adsorbent, and an increase component of the DCL that best responded to it; that imine
in temperature. Six components of this initial DCL are mechanically subsequently extracted its constituent aldehyde and amine from
isolated and amplified at least three-fold relative to their equilibrium all other DCL members that shared them. Iterative application of
distributions at the outset of the sorting process.
these stimuli resulted in the sorting of DCLs with n2 members
into just n final products.
The growing field of systems chemistry1 deals with the behaviors of
In this contribution, we report a dramatic increase in com-
complex chemical systems, focusing on the properties that cannot plexity of such a sorting methodology: sequential fractionation
be traced back to an individual component of the mixture but are of a large imine DCL12—constructed from 10 aldehyde and
instead only observed in the complex mixture. Some of these 10 aniline precursors—into three discrete imine subsets,
properties are unexpected, and often referred to as emergent.2 during the operation of three external stimuli. Notably, as two
Many of the emergent phenomena are observed in living systems, of the three stimuli are physical in nature, sorting is coupled
and behaviors of synthetic systems are often likened to their living with separation techniques, resulting in mechanical isolation
counterparts; chief among these is the adaptability to external of the amplified imines.
stimuli. In the context of response to external stimuli, dynamic
We constructed a [10 Â 10] imine DCL by heating (65 1C,
combinatorial libraries (DCLs)3—that is, collections of molecules 12 h) equimolar amounts (0.56 mmol each) of substituted
that can interconvert through a reversible chemical reaction—are benzaldehydes A–J (Scheme 1) and anilines 1–10 in PhMe
particularly well suited for this study, as the formalism of the solution, with H2O removal. After its equilibration,13 the com-
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Le Chatelier principle allows for stimuli responsiveness even at position of this initial library was assessed using a combination
the level of a simple reaction.
of gas chromatography/mass spectrometry (GC/MS) and 1H NMR
To truly begin approaching living systems, the complexity of spectroscopy. In the gas chromatogram, 53 peaks attributed to
1
synthetic dynamic libraries needs to be increased both in terms imines could be identified, while the H NMR spectrum of the
of a number of components (biological cells have thousands of initial mixture revealed 64 imine-diagnostic NQC–H resonances.
chemicals) and their multi-responsiveness—that is, the ability While the existence of all 100 theoretically possible imines could
to be addressed by a number of different stimuli. Such an not be confirmed, it is reasonable to assume that the number of
increase in complexity of response could also advance the field formed species is higher than 64, as extensive peak overlapping
(in both GC and NMR) likely concealed many imines.
The initial irreversible stimulus applied to the mixture of
A1–J10 was oxidation. In our 2012 protocol,8 iodine was used as
Department of Chemistry, University of Houston, 3583 Cullen Blvd. Room 112,
a weak oxidant that selectively oxidized electron-rich imines
Houston, TX 77204-5003, USA. E-mail: miljanic@uh.edu
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc06772g derived from anilines which had an –OH or –NH2 group in the
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Scheme 1 Synthesis and composition of the starting [10 Â 10] imine DCL.
Scheme 3 Column chromatography of DCL composed of B2–J10 on
silica gel amplifies imines B2–C3.
ortho-position relative to the imine linkage. In the hundred-
imine DCL studied here, this protocol was expected to selectively
target imine A1, which was derived from the electron-richest
aldehyde A and phenylenediamine (1). Indeed, upon the slow
addition of a 0.05 M solution of I2 in PhMe (12 h, addition rate
1 mL hÀ1, 65 1C), benzimidazole A1ox precipitated as the exclusive
product in 74% yield (0.41 mmol, Scheme 2). This finding was
mixture was pre-adsorbed onto oven-dried silica gel (2.2 g), and
the solid mixture was carefully placed on top of a chromatography
column (inner diameter: 12.7 mm), which was previously loaded
with a 20 cm-high layer of oven-dried silica gel. The column was
wrapped with heating tape and heated to B50 1C. The imine
mixture was eluted first with a 10 : 1 mixture of hexane and
PhMe (v/v), and then with an 8 : 1 mixture of the same solvents
(Scheme 3).
The first eluted fraction contained a [2 Â 2] sub-library of
least polar imines, composed of B2 (0.13 mmol, 46%), B3
(0.16 mmol, 57%), C2 (0.14 mmol, 50%), and C3 (0.17 mmol,
59%).15 According to the GC-based estimate of the relative
distributions of imine components in the initial A1–J10 library,
imines B2–C3 were present in the amounts of 0.044 mmol (B2),
0.021 mmol (B3), 0.020 mmol (C2), and 0.049 mmol (C3). After
the column chromatography, all of these imines were ampli-
fied, with the amplification factors being 3.0 for B2, 7.6 for B3,
7.0 for C2, and 3.5 for C3. It should be noted that minor
amounts of some other imines (G2: 0.05 mmol, G3: 0.06 mmol,
G5: 0.03 mmol, G7: 0.003 mmol, B6: 0.018 mmol, and C6:
0.014 mmol) were also observed in the first eluted fraction,
suggesting that aldehyde G was close in its polarity to B and C.
The dominant elution of imines B2–C3 led to the dramatic
lowering of concentrations (but not complete disappearance) of
all other imines that shared either the aldehyde or the aniline
component with them.
1
remarkable as the H NMR spectrum and GC/MS analysis of the
initial mixture suggested that the concentration of A1 was below
the detection limit of either technique (presumably on account of
the weak electrophilicity of the electron-rich aldehyde A and low
resonance stabilization of the doubly donor-substituted imine
A1). Thus, this reaction generated a product whose immediate
precursor virtually did not exist in the starting DCL! Removal of A1
from the DCL also resulted in the disappearance of imines A2–A10
(which shared the aldehyde component with A1) and B1–J1, which
shared the phenylenediamine component with A1. This first
oxidation step removed 19 imines from the DCL, resulting in a
smaller 81-component DCL.14
After the completion of this first oxidation step, the residual
imines (composed of aldehydes B–J and anilines 2–10) were
re-equilibrated by heating in PhMe for 12 h. The new B2–J10 DCL
was shown by 1H NMR spectroscopy to contain at least 61 imines,14
and was next subjected to sorting on silica gel based on its members’
affinity for this adsorbent (which correlates with polarity). The crude
In the final step of this iterative self-sorting procedure, the
mixture of residual imines was subjected to vacuum distilla-
tion. Prior to this step, the imines were left to reequilibrate, by
heating for 12 h in PhMe in a flask equipped with a Dean–Stark
adapter. PhMe was removed, leaving behind an oily dark brown
mixture, which was set to distill in vacuo (50–120 1C, 0.15 mmHg).
After 12 d, the light imines were distilled away, leaving as the
distillation residue only the least volatile imine D4, which was
isolated in 61% yield (0.34 mmol), along with minor amounts of
J4 (0.13 mmol) and H4 (0.008 mmol). In the initial library, imine
D4 was present in the amount of only 0.092 mmol, corresponding
to approx. 3.7-fold amplification (Scheme 4).
Scheme 2 Slow addition of iodine into the DCL composed of A1–J10
leads to the isolation of A1ox in 74% yield and formation of a simplified
81-imine DCL composed of B2–J10.
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Scheme 4 Slow distillation of imine DCL D4–J10 removes all but the
least volatile imine D4 from the distillation flask; D4 is amplified in the
process and isolated in 61% yield.
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6 (a) Q. Ji, R. C. Lirag and O. S. Miljanic, Chem. Soc. Rev., 2014,
In conclusion, we have demonstrated that sequential applica-
tion of three irreversible stimuli—chemical oxidation, chromato-
graphy on silica gel, and vacuum distillation—can result in iterative
simplification of an imine DCL with one hundred components into
just six preferred products. This set of experiments also allowed
side-by-side comparison of the fidelity of these three self-sorting
techniques. Chemical oxidation results in the highest amplification
of its preferred substrate from the most complex initial library.
Distillation and chromatography showed more moderate but still
synthetically useful amplification factors. Our future work will
explore whether multiple chemical stimuli can be applied to a
high-complexity DCL. We will seek to answer the question: how
many different chemical reactions can find their preferred substrate
in ‘‘messy’’ precursor mixtures analogous to A1–J10 DCL?
Results of our studies will be reported in due course.
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under the influence of external stimuli. We nevertheless use the term
‘‘self-sorting’’ to describe them, as the mixtures still sort because of
their inherent properties (exhibited under a given stimulus) rather
than an intervention of an intelligent ‘‘sorter’’.
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8 K. Osowska and O. S. Miljanic, J. Am. Chem. Soc., 2011, 133, 724.
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This research was supported by the University of Houston and
its Grant to Advance and Enhance Research, the National Science
Foundation (award CHE-1151292), and the Welch Foundation
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(c) K. Osowska and O. S. Miljanic, Angew. Chem., Int. Ed., 2011,
50, 8345. See also: (d) A. Herrmann, Chem. – Eur. J., 2012, 18, 8568;
(e) A. Herrmann, Org. Biomol. Chem., 2009, 7, 3195.
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(award E-1768). O. S. M. is a Cottrell Scholar of the Research
Corporation for Science Advancement. Ms Anh N. Ngo (UH) is
gratefully acknowledged for assistance in GC/MS measurements.
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10 R. C. Lirag, K. Osowska and O. S. Miljanic, Org. Biomol. Chem., 2012,
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11 C.-W. Hsu and O. S. Miljanic, Angew. Chem., Int. Ed., 2015, 54, 2219.
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13 Equilibration of this and subsequent DCLs was confirmed by
following the changes in the DCLs’ 1H NMR spectra: once no further
changes were observed, it was assumed that the mixture was at
equilibrium. Typical equilibration times were 6–12 h in PhMe at
elevated temperatures.
14 Same caveat applies to this DCL as to the initial one: the existence of
all 81 imines could not be unambiguously confirmed, but is highly
logical.
15 Yields of these compounds were determined using 1,3,5-trimethoxy-
benzene as an internal standard in 1H NMR spectroscopy, and are
reported relative to the theoretical situation in which B2, B3, C2,
and C3 were all in 1 : 1 : 1 : 1 ratio, with 0.28 mmol of each imine.
Notes and references
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S. Otto, Dynamic Combinatorial Chemistry, Wiley-VCH, Weinheim,
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