Bis(benzylamine) monomers: One-pot preparation and application in dendrimer scaffolds for removing pyrene from aqueous environments

Article abstract of DOI:10.3762/bjoc.9.266

Bisimine and bisamine AB2 monomers have been synthesized from 3,5-diaminobenzoic acid and benzaldehyde derivatives without the need for protective groups or purification. This monomer preparation is universal for various electron-donating and electronwith

Full text of DOI:10.3762/bjoc.9.266

Bis(benzylamine) monomers: One-pot preparation
and application in dendrimer scaffolds for removing
pyrene from aqueous environments
Olivia N. Monaco, Sarah C. Tomas, Meghan K. Kirrane and Amy M. Balija*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2320–2327.
Department of Chemistry, Fordham University, 441 E. Fordham Road,
Bronx, NY 10458, USA
doi:10.3762/bjoc.9.266
Received: 01 July 2013
Email:
Accepted: 07 October 2013
Published: 31 October 2013
Amy M. Balija* - balija@fordham.edu
*
Corresponding author
Associate Editor: H. Ritter
Keywords:
© 2013 Monaco et al; licensee Beilstein-Institut.
License and terms: see end of document.
amines; dendrimer; fluorescence studies; imines; pyrene
Abstract
Bisimine and bisamine AB2 monomers have been synthesized from 3,5-diaminobenzoic acid and benzaldehyde derivatives without
the need for protective groups or purification. This monomer preparation is universal for various electron-donating and electron-
withdrawing benzaldehyde substrates. To demonstrate the versatility of these previously unreported AB2 monomers in the forma-
tion of high molecular weight structures, novel first-generation dendrimers and hybrid second-generation dendrimers have been
synthesized. Using fluorescence spectroscopy, pyrene was shown to be removed from an aqueous environment upon exposure to
thin dendrimer films, with the first-generation dendrimer removing 70% of the pyrene within 30 min and the hybrid second-genera-
tion dendrimers removing 38–52%. Inclusion formation constants were calculated to be on the order of 109–1011 M−1 and are
comparable to the values of previously reported macromolecules. These results illustrate that size may not influence pyrene removal
as effectively as composition.
Introduction
Highly-branched polymeric systems provide an attractive route taining the ability to incorporate functional groups at precise
for removing pollutants from water due to their interior cavities locations, which may be ideal for removing specific pollutants.
and their ease of formation [1]. While several promising However, the use of dendrimers may be hindered without the
approaches have been reported [2], these polymeric water- development of new, more convenient approaches to prepare
purification systems utilize a patching method, in which the these systems with minimal purification [5]. Although several
periphery of known architectures is modified with specific func- strategies offer solutions towards preparing dendrimers more
tionalities for water treatment processes. Alternatively, efficiently [6-10], these methods are limited to specific func-
dendrimers [3,4] provide the branched architecture while main- tionalities. There remains an unmet need for novel, flexible syn-
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
thetic pathways to broaden the functional groups utilized to Alternatively, the condensation of 3,5-diamino benzoic acid
fine-tune the periphery, core, and branching sites of dendrimers. with 2.1 equiv of benzaldehyde (2a) in methanol at room
temperature followed by in-situ reduction with NaBH4 and
The objective of the research disclosed is the development of a acidification with 2 N HCl resulted in the precipitation of
straightforward synthesis for novel dendrimers that can effec- bisamine 4a. Bisamine products were obtained with electron-
tively remove small organic molecule pollutants from water. deficient, electron-rich, and sterically hindered benzaldehydes
Furthermore, the ability to fine-tune the dendrimer easily is (Table 2, Compounds 4b–h). The formation of 4f was difficult,
desired. The focus of this report is on three areas of progress: resulting in lower overall yields and longer reaction times,
(
1) the synthesis of new AB2 monomers without the use of possibly due to the electron-withdrawing properties of the nitro
protective groups and with minimal purification; (2) the group [13]. Replacing NaBH4 with other hydride sources such
incorporation of these monomers into dendrimeric architectures; as NaBH3CN did not impact the reduction rate of 4f. To ensure
and (3) the examination of how effectively these new a clean isolation of bisamines 4, the benzaldehyde starting ma-
dendrimers remove a representative organic pollutant from an terial needed to be free from oxidized aldehyde byproducts.
aqueous solution.
Using elevated temperatures to promote the formation of
bisamines 4 was unsuccessful. This result was in contrast to
comparable systems [14], which required heat to form benzyl
Results and Discussion
Condensation of 3,5-diamino benzoic acid (1) with 2.5 equiv of amine products exclusively. A preliminary examination using
benzaldehyde (2a) in methanol at room temperature resulted in 1H NMR spectroscopy suggested that twelve hours were needed
the precipitation of bisimine monomer 3a, which was isolated to completely reduce the proposed intermediate imine 3. A mix-
cleanly by filtration after 20 minutes [11,12]. Electron-deficient, ture of products was obtained when 1 was replaced with methyl
electron-rich, and sterically hindered benzaldehydes 2b–g could 3,5-diaminobenzoate.
also be utilized to prepare the corresponding bisimine products
(
Table 1, compounds 3b–g). Minimal impact on the overall The single-pot process of preparing derivatized bisimine and
yield of the reaction was observed upon varying the benzalde- bisamine products from 3,5-diaminobenzoic acid (1) and
hyde concentration or substituting absolute ethanol for benzaldehyde derivatives eliminates the use of protective
methanol; however, elevated temperatures hindered the ability groups, removes the need for purification, and follows the
to cleanly obtain bisimines 3.
general principles of atom economy [15]. This simple one-step
Table 1: Formation of bisimine compounds 3a–g.
entry
benzaldehyde
product
R
yield (%)a
1
2
3
4
5
6
7
2a
2b
2c
2d
2e
2f
3a
3b
3c
3d
3e
3f
H
78
81
72
86
61
93
78
OCH3
CH3
Fb
Clb
NO2
2g
3g
1-napthaldehyde
aIsolated yields. b90% pure by 1H NMR spectroscopy.
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
Table 2: Formation of bisamine compounds 4a–h.
entry
benzaldehyde
product
R
H
yield (%)a
53
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
4a
4b
4c
4d
4e
4f
OCH3
79
35
48
34
50
18
38
CH3
F
Cl
Br
NO2
2g
2h
4g
4h
1-napthaldehyde
aProduct precipitated out of solution upon addition of 2 N HCl. The solid was filtered and washed with cold methanol.
approach is in contrast to the synthesis of the oxygen analogue While various applications are possible with monomers 3 and 4,
of 4a, the well-known AB2 bis(benzyl) ether monomer, which the AB2 structure of these compounds is ideal for incorporation
is prepared in three steps from 3,5-dihydroxybenzoic acid and into dendritic systems [17,18]. As a proof of concept, the
benzyl bromide. When synthesizing the bis(benzyl) ether mono- preparation of first-generation dendrimers was attempted with
mer, the carboxylic acid group must be masked, and the inter- monomers 3a and 4a. Although imine-based dendrimers had
mediates must be purified through column chromatography been previously reported [19,20], bisimine 3a readily degraded
[
16]. Furthermore, although many benzyl bromide compounds to its starting materials under the condensation conditions
are commercially available, they are typically lachrymators and necessary to form the dendrimer. Alternatively, the bisamine
are considered hazardous as compared with their benzaldehyde scaffold 4 was proposed to be superior to monomer 3 for
counterparts. Therefore, the bisimine and bisamine synthesis incorporation into dendrimers due to its improved stability.
disclosed in this paper are ideal for the generation of functional- Condensation of 4a with triphenol core 5 [16] resulted in the
ized systems with a higher molecular weight without the use of formation of first-generation dendrimer 6 (Scheme 1). Though
hazardous reagents or generating large quantities of waste as successfully prepared, the dendrimer decomposed in most
compared with previously published systems.
organic solvents, making it difficult to purify and analyze.
Attempts to prepare other first-generation dendrimers were not
Bisimine 3a was stable to ambient conditions in air over four pursued due the perceived instability.
months but degraded to release benzaldehyde within two hours
in CHCl3, CH2Cl2, and THF as noted by the red shift in the To obtain a more stable system, 3,5-bis(benzyloxy)benzalde-
UV–vis absorption spectrum. 1H NMR spectroscopic analysis hyde 7 was designed such that upon condensation with 3,5-
confirmed the disappearance of the imine peak upon exposure diaminobenzoic acid, a hybrid bisamine/benzyl ether product
of bisimine 3 to acid. Hydrolysis occurred at different rates with would be obtained. It was proposed that the benzyl ether groups
varying functionalities on 3; however, a quantitative compari- would mask the bisamine functionalities and result in increased
son of how the substituents affected hydrolysis could not be compound stability. Structure 7 was successfully prepared
adequately obtained. Bisamine 4 was found to be more stable in through a multi-step synthetic scheme [21]. Condensation of 1
air although it began to decompose after exposure to organic with 2.1 equivalents of benzaldehyde 7 followed by in situ
solvents for one hour.
reduction with NaBH4 and acidification with 2 N HCl provided
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
Scheme 1: Synthesis of first generation bisamine based dendrimer 6.
dendron 8 (Scheme 2). Compound 8, itself an AB2 monomer diameter of 2.1 Å [22]. It was proposed that the cores with
similar to compounds 4a–4g, was stable in CDCl3 as visualized higher molecular weight would influence the overall properties
by 1H NMR spectroscopy.
of the corresponding dendrimer more than the smaller core.
Standard coupling conditions [16] of 8 with triphenol cores
Due to the improved stability of 8 in organic solvents, second- 5, 9 and 10 resulted in second-generation dendrimers 11–13
generation dendrimers containing the dendron and triphenol (Scheme 3), which were successfully purified by size
cores 5, 9, and 10 (Schemes 1 and 3) were designed. The tri- exclusion chromatography (SEC). The products were
phenol cores were utilized to examine how the core diameter determined to be >90% pure through 1H NMR spectroscopy
and shape influenced the chemical and physical properties of and their molecular weights were confirmed by MALDI–TOF.
the resulting dendrimers. Cores 5 and 9 were calculated to exist The resulting novel dendrimers contained six amino groups
in a helical conformation with an average diameter of 5.7 Å within the interior and twelve ether functionalities along the
while 10 was present as a planar structure with an average periphery.
Scheme 2: Synthesis of dendron 8.
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
Scheme 3: Synthesis of hybrid dendrimers 11–13.
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
Studies then were performed to determine whether the
dendrimers could remain intact upon exposure to various
external conditions. To investigate their stability in organic
solvents, dendrimers 11–13 were separately dissolved in chloro-
form, and their 1H NMR spectra were obtained. While the first-
generation dendrimer 6 decomposed within one hour, the
second-generation dendrimers 11–13 were stable under similar
conditions. Additional studies focused on the stability of the
dendrimers in water at pH 7. Thin solid films of 6 and 11 were
prepared on the bottom of a beaker and water at pH 7 was
added to the beaker. After three days, the water was removed
and the 1H NMR spectra were obtained. No changes in the
spectra were observed, which indicated that the dendrimers
remained intact in an aqueous solution. Therefore, pure second-
generation hybrid dendrimers could be isolated and used in
different solvent environments without being degraded.
Figure 1: Fluorescence of a saturated aqueous solution of pyrene
after 30 min exposure to dendrimers 11–13.
Due to their hydrophobic periphery and larger size, hybrid sure of first-generation dendrimer 6 to a saturated aqueous
dendrimers 11–13 were envisioned as potential scaffolds to pyrene solution lead to a 74% decrease in pyrene fluorescence
encapsulate small molecule organic pollutants such as poly- intensity after 30 minutes and 90% decrease in pyrene fluores-
cyclic aromatic hydrocarbons (PAHs) [23-25]. The presence of cence intensity after two days.
PAHs in drinking water is a growing concern due to their
carcinogenic properties and persistence in the environment This decrease in the pyrene fluorescence intensity upon expo-
[
26,27]. Current methods to remove these hydrocarbons are sure to the dendrimers may suggest that a hydrophobic, non-
becoming ineffective. To examine the ability of the hybrid specific interaction is occurring between the pyrene and
dendrimers to remove small organic pollutants, saturated dendrimer film. Over time, the dendrimer film becomes satu-
aqueous solutions of pyrene [28], a representative PAH, were rated with the pyrene, resulting in its inability to remove addi-
introduced to beakers containing thin films of the hydrophobic tional molecules. Although the size of the dendrimer core did
dendrimers [29,30]. Aliquots were taken at 30 min, 60 min, and not dramatically influence the pyrene fluorescence signal after
two days, and the pyrene fluorescence intensity was determined 30 min in dendrimer 13, the smaller-core dendrimer was not as
by fluorescence spectroscopy [31]. After 30 min, a 38–52% effective over two days. Furthermore, first-generation
decrease in the pyrene fluorescence intensity was recorded with dendrimer 6 resulted in a higher initial change in the pyrene
dendrimers 11–13 (Figure 1), a change that was not observed in signal, which is hypothesized to result from the greater density
the absence of the dendrimers.
of amine groups, which could favorably interact with the PAHs
32].
[
The intensity of the pyrene fluorescence continued to decrease
upon longer exposure to the dendrimer films. After two days, Pyrene inclusion formation constants were calculated using the
dendrimers 11 and 12, which contained the cores with larger measurement results obtained with the thin film experiments
diameters, showed a 95% decrease in pyrene fluorescence after two days (Table 3). The inclusion values were large,
intensity, while the exposure to dendrimer 13 resulted in a 70% ranging from 109 to 1011 M−1, and are comparable to previous
reduction. No additional decrease in the fluorescence intensity results obtained with cyclodextrin polymers (109 M−1) [33] and
of pyrene was observed over time. As a comparison, the expo- alkylated fifth-generation diaminobutane poly(propylene imine)
Table 3: Inclusion formation constants K (M−1) and Gibbs free energies ΔG° (kcal/mol) of dendrimers 6 and 11–13 and pyrene.
entry
compound
K (M−1)
ΔG° (kcal/mol)
capacity (mol pyrene/g dendrimer)
1
2
3
4
6
2.44 × 1010
1.02 × 1011
1.62 × 1011
6.09 × 109
−14.1
−15.0
−15.3
−13.3
6.34 × 10−7
3.12 × 10−7
3.31 × 10−7
3.34 × 10−7
11
12
13
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Beilstein J. Org. Chem. 2013, 9, 2320–2327.
dendrimers (108 M−1) [28-30]. The large magnitude is proposed relative to core 10. While the larger dendrimers were effective
to be due to the thermodynamically favorable process of pyrene at removing pyrene over two days, the first-generation
moving from a hydrophilic to a hydrophobic environment. A dendrimer 6 extracted more pyrene after 30 min than 11–13.
similar conclusion can be drawn from calculating the Gibbs free Pyrene inclusion constants for the dendrimers 6 and 11–13 were
energies of each system, which lie between −13 and −15 kcal/ comparable to other polymeric purification systems, suggesting
mol and further demonstrate the favorable interaction of the that large polymeric systems are not necessary to remove
dendrimers with pyrene in an aqueous environment [29,30]. The pyrene. Rather, the composition of the purification system
capacity of the dendrimers to remove pyrene was estimated to appears to play an important role. Current efforts are focused on
be between 3.12 × 10−7 to 6.34 × 10−7 mol of pyrene per gram incorporating the bisamine dendron motif into additional supra-
of dendrimer. Current levels of pyrene in industrialized coun- molecular structures such as star polymers and hyperbranched
tries range from 1.48 × 10−12 to 1.98 × 10−10 mol of pyrene per systems and examining how fine-tuning the steric and elec-
liter of water [34]. Qualitatively, relatively small amounts of tronic nature of the bisamine AB2 monomers influences the
dendrimer (<1 g) should effectively remove pyrene from 1 L of removal of pyrene from water. The results of these studies will
stagnant water.
be reported in due course.
Overall, the relatively small dendrimers 6 and 11–13 gave
similar results compared to polymers with higher molecular
weight and therefore, large polymeric systems may not be
necessary to effectively remove PAHs from water. Furthermore,
the flexible preparation of AB2 bisamine monomers using
different benzaldehyde starting materials allows dendrimer
properties to be readily tuned to remove pollutants more effec-
tively. This is more difficult to accomplish with commercially
available dendrimers and polymeric systems. Although not
applicable for large scale production due to its instability in
Supporting Information
Supporting Information File 1
Full experimental synthetic procedures for compounds
3
a–g, 4a–h,6–8, and 11–13, and pyrene fluorescence
spectral data.
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-266-S1.pdf]
[
organic solvents, first-generation dendrimer 6 does illustrate the Acknowledgements
importance of the bisamine monomer in removing pyrene.
We thank Tom Carberry and Marcus Weck from New York
University for their help in obtaining the MALDI–TOF data and
the Dean’s Office at Fordham University for its generous finan-
Conclusion
Described is the synthesis of bisimine and bisamine AB2 cial support. The Q-Tof Ultima mass spectrometer (University
monomers through a one-step process, which requires no purifi- of Illinois at Urbana-Champaign) was purchased in part with a
cation and no manipulations of protective groups. Unlike the grant from the NSF, Division of Biological Infrastructure (DBI-
analogous benzyl ether AB2 compounds previously reported, 0100085) and the Bruker MALDI–TOF/TOF UltrafleXtreme
these monomers were formed at room temperature and were MS Spectrometer (New York University) was acquired through
filtered and washed to obtain the desired product. The reaction the support of the NSF (CHE-0958457).
was found to be general for aromatic aldehydes except strong
electron-donating benzaldehydes that resulted in unstable References
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