"Click" bis-triazoles as neutral C-H?anion-acceptor organocatalysts

Article abstract of DOI:10.1002/chem.201203360

A new player on the field! "Click" bis-triazoles have been introduced as a highly efficient new class of neutral C-H?anion-binding organocatalysts (see scheme). DFT and NMR studies were used to confirm this activation modus and identify the optimal catalyst. Copyright

Full text of DOI:10.1002/chem.201203360

COMMUNICATION
DOI: 10.1002/chem.201203360
À
“Click” Bis-Triazoles as Neutral C H···Anion-Acceptor Organocatalysts
Stephan Beckendorf, Sçren Asmus, Christian Mꢀck-Lichtenfeld, and
Olga Garcꢁa MancheÇo*[a]
Anion-recognition processes play a crucial role in nature,
because anions are ubiquitous in biological systems in which
specific molecular receptor–anion interactions are vital.[1]
The most established approach for anion recognition is the
use of positively charged receptors, leading to important se-
lectivity limitations due to their electrostatic interaction
nature.[1] Alternatively, the appealing selective recognition
of anions by neutral molecules still remains a challenge. Be-
À
sides the conventional hydrogen-bond donors based on N
H groups such as in (thio)ureas,[2] the typically considered
À
weak interaction of C H bonds represents a promising
source of neutral H-bond donors for anion recognition. In
this regard, triazole-based anion acceptors have recently
been reported as new neutral structures for the development
of anion receptors.[3,4] Nevertheless, only their anion-binding
properties and some analytical applications such as anion-
sensing have been investigated so far.[4] An interesting unex-
Figure 1. Triazoles as new anion-acceptor organocatalysts.
anion-selective than the typically used (thio)urea-based cat-
alysts (Figure 1, bottom). Thus, by merging all these fea-
tures, we have herein brought the usually neglected 1,2,3-tri-
azole functionality[8–10] as a new valuable player to the field
of anion-acceptor catalysis for the first time.
À
plored general feature is the possibility of using neutral C
H-based anion receptors as H-bond donors for catalysis.
Considering the easy accessibility of triazoles by employing
the well-established CuI-catalyzed Huisgen 1,3-dipolar cyclo-
addition of azides and alkynes,[5] a representative example
of the powerful “click”-chemistry,[6] the design and develop-
ment of highly selective triazole-based anion receptor orga-
nocatalysts is extremely attractive.
À
To demonstrate if triazole-based all C H···anion interac-
tions are strong enough for conducting efficient catalysis
and get a general structure–activity trend for a rational
design of these kinds of acceptors, comparative catalyst
structure–anion-binding capacity–reactivity studies were car-
ried out. Thus, we first determined the binding constants
(K) of modular varied “click”-triazoles with anions such as a
chloride by NMR titration with the corresponding ammoni-
um salt (Figure 2). For our studies, the 1,3-bis-triazolyl ben-
zene motif[11] was chosen as the key structure. Due to syn-
thetic accessibility, 1,3-bis(1H-1,2,3-triazol-4-yl)benzenes
such as in BisTri1–5, possessing alkyl and electronically
varied aromatic groups, were prepared (see the Supporting
Information). Alternatively, a regioisomer 1,3-bis(1H-1,2,3-
triazol-1-yl)benzene BisTri6 was also synthesized for compa-
rative reasons. The 1N-isopropyl-substituted triazole BisTri1
showed a substantially lower chloride anion-binding affinity
(Kacetone: (34Æ3)mÀ1) than the phenyl substituted derivative
BisTri2 (Kacetone: (342Æ37)mÀ1). For that reason we further
focused on aryl-substituted structures and the influence of
their electronics on the binding behavior. Whereas the elec-
tron-poor 3,5-bis(trifluoromethyl)phenyl substituent in
BisTri3 led to a significantly higher affinity (Kacetone: (458Æ
With the fruitful concept of neutral anion-acceptor orga-
nocatalysis in mind (Figure 1, top),[2,7] in which an ionic elec-
trophile is activated towards nucleophilic attack by H-bond-
ing to its counteranion and is essentially dominated by thio-
À
urea N H-based acceptors, we decided to explore a new use
of triazole receptors for non-covalent anion-binding organo-
À
catalysis. Although C H···anion interactions can intuitively
À
be expected to be weaker than N H···anion interactions,
1,2,3-triazole-based catalysts are generally synthetically
easily accessible, stable, functional group compatible and,
due to their anion-selective pocket,[4] they might be more
[a] S. Beckendorf,+ S. Asmus,+ Dr. C. Mꢀck-Lichtenfeld,++
Dr. O. Garcꢁa MancheÇo
Institute of Organic Chemistry, University of Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
Fax : (+49)251-83-33202
E-mail: olga.garcia@uni-muenster.de
[+] These authors contributed equally to this work.
++] This author carried out the computational studies.
56)mÀ1),[12] electron-rich substituents such as 4-N,N’-di
ACHTUNGTRENNUNGmeth-
[
AHCTUNGTREGyNNUN laminophenyl triazole BisTri4 gave a lower binding capaci-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203360.
ty (Kacetone: (152Æ13)mÀ1). In contrast to this trend, the elec-
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Figure 3. Optimized structure of the simplified deoxygenated catalyst·ClÀ
complexes deO-BisTri3·ClÀ and deO-BisTri5·ClÀ.
adopts a nearly perfect coplanar geometry when it binds to
the chloride anion. The binding energy in acetone DE0K is
À9.1 kcal·molÀ1, thermodynamical corrections[16] give a free
enthalpy of association (DG298K) of À1.6 kcal·molÀ1. It is
noteworthy that, in agreement with the experimental results,
the less-stable complex BisTri5 is also predicted to have a
smaller binding energy and free enthalpy (DE0K =À7.9 kcal
molÀ1, DG298K =À0.7 kcalmolÀ1).
Next, the binding constants of the best anion receptor
BisTri3 in more apolar solvents, which are often used in H-
bonding catalysis, were studied. Whereas a weaker binding
was observed in dichloromethane (KCH Cl : (108Æ9)mÀ1), the
2
2
use of THF (KTHF: (1417Æ171)mÀ1) or a 1:1 mixture of tol-
uene/THF (Ktoluene/THF (1:1): (791Æ49)mÀ1)[17] was beneficial,
leading to a substantial increase in the binding constant.[18]
Although no general relationship between the solvent polar-
ity and K could be identified, this study indicated THF as
the solvent of choice. Subsequently, the BisTri1–5 com-
pounds were challenged as organocatalysts in N-alkylation
reactions of amines. For this purpose, the alkylation of ben-
zylamine (1a) with 4-dimethylamino-N-triphenylmethyl-pyr-
idinium chloride (2),[19] which is already an ionic precursor
of the relatively stable trityl carbocation,[20,21] was chosen as
the model reaction. We found no formation of the desired
product 3a in the blank reaction between 1a and 2 at 458C
in THF.[18] On the other hand, when the reaction was con-
ducted in the presence of 10 mol% of BisTri3, compound
3a was then formed and isolated in 85% yield after 72 h.[22]
Afterwards, a temperature screening was carried out: It was
observed that temperatures below 458C were not capable to
get efficient reaction rates, and temperatures above 658C
led to the slow decomposition of the electrophile 2, even in
the absence of the catalyst (see the Supporting Informa-
tion).
Having demonstrated that the “click” triazoles can act as
H-bond donor organocatalysts, the catalytic activity of the
different catalyst structures BisTri1–5 was next studied by
recording the reaction profile for the formation of 3a at
458C (Figure 4). To our delight, we could observe a clear
trend with the tested structures in which the higher anion-
binding affinity provided a superior catalyst. Thus, the best
catalyst BisTri3 led to the product 3a in a good 90% yield
(GC) after a reaction time of 72 h.
Figure 2. Chloride anion-binding constants (K) of BisTri1–6.
tron-poor pentafluorophenyl-substituted triazole BisTri5
bonded noticeably weaker than expected (Kacetone: (111Æ
12)mÀ1). This behavior shows that the ortho positions of the
substituted aryl groups are also important. In agreement
with previous reports,[3,4] these ortho protons can participate
and attractively interact with the anion.[13] On the other
hand, considering the fact that the other regioisomers, 1,3-
bis(1H-1,2,3-triazol-1-yl)benzenes, are generally considered
to be more potent anion-acceptors,[3,4] the binding constant
of BisTri6 with chloride was also determined. Interestingly
the chloride acceptor capacity of BisTri6 (Kacetone: (152Æ
18)mÀ1) was considerably lower than for its related BisTri3
regioisomer.
Computational density functional theory studies[13] (B-
LYP-D3/def2-TZVP+COSMO)[14] of simplified, deoxygen-
ated complexes confirmed this cooperative participation of
the ortho H-atoms in the binding to the chloride anion
(Figure 3).[15] Moreover, they also indicated a stronger H-
bond interaction of the triazole H-atoms compared with the
other ClÀ/H-bonding interactions. Thus, significantly shorter
À
H Cl bond distances (2.40–2.43 ꢃ vs. 2.74–2.84 ꢃ in deO-
BisTri3·ClÀ) were found in the optimized structure. The
DFT calculations also showed that the BisTri3 receptor
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“Click” Bis-Triazoles
COMMUNICATION
Figure 4. Catalytic alkylation model reaction and comparative reaction profile.
Moved by these exciting results, the alkylation reaction
was next explored with a range of amine nucleophiles
(Table 1). It was found that the electrophile 2 could react in
the presence of BisTri3 with a broad range of amines, in-
cluding primary benzyl 1a, vinyl 1b, linear alkyl 1c and a-
branched amines 1d and 1e, as well as secondary amines
such as pyrrolidine (1g) and morpholine (1h). In general,
good yields of the alkylated amines 3 were isolated (71–
85%), except for the morpholine derivative 3h (44%),
which suffered from low stability on silica.[23]
Finally, an anion-binding selectivity study with the new
anion acceptor BisTri3 was carried out. The performance of
the catalyst, with respect to the established thiourea 4[24]
(with the same Ar substitution) in anion recognition pro-
A
ACHTUNGTRENNUNGtyl-
AHCTUNGTRENNUNG
(Figure 5). Thus, the competitive binding of the acceptor to
the bromide anion of TBABr will lead to an unproductive
pathway, whereas the binding to the chloride anion of re-
agent 2 will activate it, allowing the formation of the desired
N-alkylated product 3a. In the case of the reaction catalyzed
by thiourea 4, the formation of 3a was more strongly hin-
dered than the triazole-based-catalyzed reaction. According-
ly, the thiourea catalyst 4 might better allocate a bromide
anion and therefore partially block its recognition pocket.
This is also consistent with a higher selectivity for chloride
over bromide anions of the triazole catalyst BisTri3.[25]
These results point towards the possibility of anion differen-
tiation with these kinds of triazole-based catalysts.
Table 1. Scope of the N-alkylation reaction catalyzed by BisTri3.[a]
In summary, “click” bis-triazoles (BisTri) have been intro-
duced as promising new players in the field of neutral anion
acceptor organocatalysis. Detailed halogen anion-binding af-
finity and activity studies were carried out. These studies
were key to identify the optimal catalyst and to prove that
[a] Reaction conditions: 1 (0.56 mmol, 2 equiv), 2 (0.28 mmol, 1 equiv)
and BisTri3 (10 mol%) in THF (0.2m) at 458C for 72 h. Yields of isolated
3 are given. [b] Formation of 3 not detected by GC in the non-catalyzed
reaction. [c] Compound 3 f was formed and isolated in <1% in the non-
catalyzed reaction.
À
simple C H bonds can be effectively used for anion-binding
catalysis. Furthermore, these triazole structures, which are
already attractive due to their stability and synthetic accessi-
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O. Garcꢁa MancheÇo et al.
Figure 5. Anion-recognition selectivity study.
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Acknowledgements
The Deutsche Forschungsgemeinschaft (SFB 858), the Fonds der Chemi-
schen Industrie and Prof. Frank Glorius are gratefully acknowledged for
generous support. S. B. thanks the Deutsche Forschungsgemeinschaft
within the SFB 858 for a predoctoral contract. We also thank the OC-
Mꢀnster NMR department, in particular to Karin Voß, for the kind sup-
port.
Keywords: alkylation · anion acceptor · click chemistry ·
NMR spectroscopy · organocatalysis · triazoles
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“Click” Bis-Triazoles
COMMUNICATION
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[18] At present, we do not have an explanation of this observation,
À
which is in contrast to the usual binding affinity of conventional N
H anion acceptors that generally show in CH2Cl2 a higher K value
than in solvents like THF.
[19] O. Hernandez, S. K. Chaudhary, R. H. Cox, J. Porter, Tetrahedron
Lett. 1981, 22, 1491–1494.
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[21] The direct use of trityl chloride for NMR anion-binding experiments
in [D8]THF with BisTri3 showed no change in the NMR signals, sug-
gesting no chloride binding with the catalyst.
[22] The reaction in the presence of 5, 10, and 20 mol% of BisTri3 was
studied. The use of 10 mol% of catalyst was chosen as compromise
considering catalytic loading, reaction time, and conversion; see the
Supporting Information.
[23] In all control experiments without catalyst, no product formation
was detected by GC-FID or GC-MS. Only small traces (<1% yield)
were isolated in the case of compound 3 f, which was incompatible
to GC analysis.
[11] This motif was initially identified as an anion-acceptor functionality
by Craig et al. (though only used for their binding studies with
anions such as halides, carboxylates, sulfates, and nitrates), see
ref. [4d]. However, in our case, important changes in the backbone
were carried out. The triazole regioisomers were synthesized and
the polyethylene glycol carboxylate group at the CAr-5 position, pre-
viously used by the authors for solubility issues, was exchanged for a
(2-methoxypropan-2-yl)acetylene group to maintain the solubility
levels and to avoid building up an ion-pair receptor instead of the
desired anion acceptor (see, for example: S. K. Kim, J. L. Sessler,
Chem. Soc. Rev. 2010, 39, 3784–3809).
[24] Unfortunately, the NMR-titration method employed for the deter-
mination of the binding constant K for the BisTri catalysts cannot
À
be used for the thiourea 4. As was expected, this N H donor cata-
lyst 4 showed a higher binding affinity to chloride anion, but a clear
not 1:1 host/guest binding model (see the Supporting Information).
Therefore, an accurate determination of its binding constant K, even
at lower host concentrations, cannot be achieved by this approach
(mathematic solution with SigmaPlot) and a more complex mathe-
matic treatment is required.
[25] The binding affinity of BisTri3 to bromide (K
(BrÀ): (438Æ
11)mÀ1) and iodide (KTHF(IÀ): (319Æ24)mÀ1) was notably lower than
for chloride (K
(ClÀ): (1417Æ171)mÀ1); see the Supporting Infor-
[12] In analogy to the known effect on the thiourea catalysts, the intro-
duction of electron-poor 3,5-bis(trifluoromethyl)phenyl groups gives
the highest binding capability, most probably due to the enhance of
mation.
Received: September 19, 2012
Revised: November 8, 2012
Published online: December 20, 2012
À
acidity of the C H bonds at the triazoles and these aromatic units
implicated in the anion recognition. Additionally, the effect of the
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1585