Bulky iodotriazolium tetrafluoroborates as highly active halogen-bonding-donor catalysts

Article abstract of DOI:10.1039/C8CC05309J

Steric bulk around catalytic centers is one of the most important factors determining catalytic reactivities and selectivities. This paper reports the synthesis of structurally diverse 5-iodo-3-methyl-1,2,3-triazolium salts and the evaluation of their cat

Full text of DOI:10.1039/C8CC05309J

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Bulky iodotriazolium tetrafluoroborates as highly active halogen-
bonding-donor catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Ryosuke Haraguchi,* Shun Hoshino, Munenori Sakai, Sho-go Tanazawa, Yoshitugu Morita,
Teruyuki Komatsu and Shin-ichi Fukuzawa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Steric bulk around catalytic centers is one of the most important and selectivities.7 Herein, we prepared structurally diverse
factors determining catalytic reactivities and selectivities. This triazolium-based XB-donors and evaluated their catalytic
paper reports the synthesis of structurally diverse 5-iodo-3- activities for the aza-Diels-Alder reaction of imines and 2-
methyl-1,2,3-triazolium salts and the evaluation of their catalytic siloxy-1,3-butadienes (Scheme 1). We found that catalytic
activities as halogen-bonding donors for the aza-Diels-Alder efficiency strongly depends on the steric bulk around the
reaction of 2-siloxy-1,3-butadiene with imines. We found that catalytic center. The origin of the catalytic reactivities of
steric hindrance around an iodide atom in halogen-bonding different XB-donors is discussed using density functional
theory (DFT) and 13C NMR spectra.
donors significantly impacted catalytic efficiency.
Halogen-bonding (XB), which is a non-covalent interaction
between electrophilic organic halides (XB-donors) and Lewis
bases (XB-acceptors),1 has attracted great attention because
this interaction features high directionality. Computational
studies have revealed that most XB-donors have an
electropositive region (σ-hole) on the elongation of a
Ar1
Ph
R
TBSO
XB-donor (5.0 mol%)
N
N
N
+
I
CH2Cl2, 30 °C, 14 h
TBSO
N
Ph
H
BF4
N
Ar2
R
carbon−halogen covalent bond, which interacts with
a
Steric bulk around catalytic centers is
crucial for catalytic efficiency
nucleophilic region in XB-acceptors, resulting in strict
linearity.2 Due to its excellent molecular recognition ability, XB
has been used in rational molecular design for supramolecular
chemistry3 and crystal engineering.4
XB-donor
Scheme 1. Evaluation of the relationship between steric bulk of XB-donors 1 and
their catalytic efficiency for the aza-Diels-Alder reaction.
In view of highly structural diversity of triazole derivatives,
we selected 5-iodo-1,2,3-triazolium salts as XB-donor catalysts.
The corresponding precursors are readily prepared through
[3+2] cycloaddition reaction of azides with alkynes.8 Huber et
al. reported the catalytic and stoichiometric application of
The magnitude of the σ-hole of XB-donors, which affects
the strength of XB, depends on the following electronic
factors: (1) polarizability of the halogen atom (I>Br>Cl>>F) and
(2) electron-withdrawing ability of the group or atom bonded
to the halogen. Therefore, a variety of iodide-based XB-donors
bearing strong electron-withdrawing moieties such as
perfluoroaryls or cationic heterocycles have been developed to
form strong XB with Lewis basic substrates. These have been
successfully applied in organic synthesis5 and organocatalysis.6
However, the effect of the sterics of the XB-donors on the
catalytic efficiency has yet to be studied in detail, though steric
bulk around catalytic centers in transition-metal catalysts or
organocatalysts play a central role in their catalytic reactivities
triazolium-based XB-donors for
a halide-abstraction-type
reaction, however, their catalytic activity has not been studied
systematically.5c,5h,6h
The synthetic route to 5-iodo-3-methyl-1,2,3-triazolium
tetrafluoroborates is shown in Scheme 2. Triazoles 1a and 1c
were prepared from the corresponding alkynes and azides
under the following reaction conditions (method A): 0.4
equivalents of copper(II) sulfate pentahydrate and 0.8
equivalents of sodium ascorbate in the mixture of tert-butyl
alcohol and water (v/v = 1/1). However, triazoles 1b, 1d, and
1e, which have one or two 2,6-diisopropylphenyl (Dip) groups,
could not be prepared under the identical conditions, probably
due to steric hindrance. In contrast, our previously reported
method for the synthesis of triazoles with bulky substituents
(method B),9 which uses a combination of one equivalent each
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of copper(I) chloride and 2-ethynylpyridine, successfully gave bonding may have an impact the catalytic efficiency in the case
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DOI: 10.1039/C8CC05309J
the sterically hindered triazoles 1b, 1d, and 1e. Lithiation of of weak XB-donors like 3f and 3g.
triazoles 1 and subsequent iodination provided 5-iodo-1,2,3-
Table 1. Evaluation of the catalytic activities of XB-donors 3 for the aza-Diels-Alder
triazoles 2, which were reacted with trimethyloxonium
tetrafluoroborate to afford structurally diverse XB-donors 3.
The cationic halogen-bonding-donors 3 exhibited excellent
tolerance against moisture and oxygen, and can be stored
under air without decomposition.
reaction of 4a with 5aa,b
Ph
H
Ph
An
Catalyst (5.0 mol%)
N
N
+
CH2Cl2, 30 °C, 14 h
An
TBSO
4a
TBSO
5a
6aa
Yield (%)b
Entry
7
8
9
10
11
Catalyst
Yield (%)c
method A
Entry
Catalyst
None
3a
3b
3c
CuSO4•5H2O (0.4 eq)
sodium ascorbate (0.8 eq)
1
2
3
4
5
6
0
3f
3g
I2
3h
2c
19
41
17
47
26
93
64
49
Ar1
tert-BuOH/H2O, 25 °C, 16 h
or
N
N
N3
+
H
Ar2
Ar1
9
method B
N
Ar2
CuCl (1.0 eq)
2-ethynylpyridine (1.0 eq)
3d
3e
trace
1
THF/H2O, 100 °C, 24 h
aStandard conditions: 4a (0.30 mmol), 5a (0.20 mmol), XB-donor 3 (0.010 mmol)
b
c
in CH2Cl2 (0.2 mL) at 30 °C for 14 h. An = 4-methoxyphenyl (4-anisyl).
Ar1
Ar1
Determined by 1H NMR analysis of the crude reaction mixture using
dibromomethane as the internal standard.
n-BuLi (1.8 eq)
I2 (2.0 eq)
Me3OBF4 (1.3 eq)
CH2Cl2, rt, 16 h
N
N
N
I
I
N
THF, -78 °C to rt, 15 h
N
Ar2
N
Ar2
BF4
2
3
N
N
N
N
N
N
N
N
XB-donors
Ar1
Ar2
Br
Cl
H
I
Mes =
Dip =
N
N
N
N
BF4
BF4
BF4
3a
3b
3c
3d
3e
Ph
Ph
Mes
Dip
Mes Mes
Mes Dip
Dip Dip
3g
3f
3h
2c
Scheme 2. Preparation of 5-iodo-3-methyl-1,2,3-triazolium tetrafluoroborates 3.
100
In order to evaluate the catalytic activity of XB-donors 3,
we chose the aza-Diels-Alder reaction of imine 5a and 2-siloxy-
1,3-butadiene 4a, which is less reactive than the Danishefsky
diene, as a model reaction.10 First, we confirmed that the
cycloadduct 6aa was not obtained when the reaction was
performed without catalysts at 30 °C for 14 h (Table 1, entry 1).
Treatment of 4a with 5a in the presence of 5.0 mol% of 3a
afforded 6aa in 47% yield (Table 1, entry 2). Replacement of
the mesityl substituent at the 1-nitrogen atom in 3a by the 2,6-
diisopropylphenyl group reduced the yield of 6aa (Table 1,
entry 3). Notably, when XB-donor 3c, which has mesityl
substituents at the 1 and 4 positions of a triazolium ring, was
used as a catalyst, 6aa was obtained in 93% yield (Table 1,
entry 4). However, the bulkier catalysts 3d and 3e exhibited
poorer catalytic performance (Table 1, entries 5 and 6). In
addition, the high catalytic performance of 3c was supported
by the results of our kinetic study (Fig 1). These results
suggested that the appropriate steric hindrance around the
iodide atom in XB-donors is crucial to highly catalytic efficiency
in the aza-Diels-Alder reaction. In the case of 3f and 3g, which
are the bromide and chloride analogues of 3c, 6aa was
obtained in 19% and 41% yields, respectively (Table 1, entries
7 and 8). Since this result is not consistent with the expected
trend in halogen bonding strength (Br>Cl), other non-covalent
interactions such as π-π interactions, ion pairing, and hydrogen
80
60
40
20
0
3 a
3b
3 c
3d
0
5
10
15
20
t (h)
Fig. 1 Yields versus reaction time profile using various XB-donors.
When molecular iodine, a well-known XB-donor,11 was
employed, the product was obtained in only 17% yield (Table 1,
entry 9). Using the hydrogen bonding analogue 3h resulted in
lower catalytic activity than that observed for 3c. In addition,
5-iodo-3-methyl-1,2,3-triazole (2c), which has no cationic
moiety, did not exhibit substantial catalytic activity. These
2 | J. Name., 2012, 00, 1-3
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results supported that halogen bonding worked as a key cationic moiety could enhance the electronV-wiewitAhrtdicrleaOwnilninge
interaction for activation of substrate.12,13
abilities of cationic heterocycles in XB-donors. Although
DOI: 10.1039/C8CC05309J
To understand the origin of steric effects of XB-donors on remains uncertain why the catalytic efficiency of bulky XB-
the catalytic efficiency, DFT calculations were carried out on donor 3d and 3e is lower than that of 3c despite their
various XB-donors 3 using the M062X method with the 6- electropositive character, we now postulate that, in 3d and 3e,
31G+(d,p) basis set for C, H, N and with the LANL2DZ basis set steric hindrance increases the activation energy of the
for I. The DFT-optimized structures of cationic moieties in 3a- transition state of C−C or C−N bond forming steps.
3c and selected dihedral angles in 3a-3e are shown in Figure 2
We next examined the scope of imines in the aza-Diels-
and Table 2, respectively. In the case of 3a and 3b, the dihedral Alder reaction using 4a as a nucleophile and 3c as a catalyst
angles between the phenyl rings (Ar1) and triazolium rings are (Table 3). Imines 5 bearing an electron-donating (5a and 5b) or
around 60°, which indicates the slight presence of π- electron-withdrawing group (5d) on the phenyl ring were
conjugation between these aromatic systems. The DFT- successfully employed in halogen-bonding catalysis. The
calculated LUMOs of 3a and 3b also suggests overlap in the π- installation of a bromide did not inhibit the reaction (entry 5).
orbitals of Ar1 and triazolium rings (see the Supporting Although the 2,4,6-trimethylphenyl group blocked the reaction
Information for details). This conjugation leads to the from proceeding at all (entry 6), the imine 5g with a t-Bu
delocalization of positive charge, which decreases the substituent was used successfully (entry 7). Moreover, as
electron-withdrawing ability of the triazolium rings.
shown in Scheme 2, when the 2-siloxy-1,3-butadiene bearing a
phenyl substituent 4b was used in the halogen-bonding
catalysis, the cycloadduct 6ba was obtained in high yield with
moderate diastereomeric ratio (dr = 3.3:1). In addition, 3c
catalyzed the Mannich-type reaction of silyl enol ether 7 with
5c to afford 8 in moderate yield.
Table 3. Scope of Iminesa
Ph
H
Ph
R
XB-donor 3c (5.0 mol%)
CH2Cl2, 30 °C, 14 h
N
N
+
R
TBSO
4a
TBSO
5
6
Entry
R
Substrate
Product
Yield (%)b
1
2
3
4
5
6
7
4-OMe-C6H4
4-Me-C6H4
Ph
4-CF3-C6H4
4-Br-C6H4
2,4,6-(CH3)3C6H2
t-Bu
5a
5b
5c
5d
5e
5f
6aa
6ab
6ac
6ad
6ae
6af
90
81
62
71
70
0
Fig. 2 DFT-optimized structure of cationic parts in 3a (left), 3b (center), and 3c
(right) calculated by M062X/6-31G+(d,p) for C, H, N and LANL2DZ for I.
Table 2. Selected dihedral angles (deg) and chemical shifts at C5 position in XB-donors
3 (Trz = triazolium ring)
5g
6ag
35
a
Standard conditions: 4a (0.30 mmol), 5 (0.20 mmol), XB-donor 3c (0.010 mmol)
in CH2Cl2 (0.20 mL) at 30 °C for 14 h. b Isolated yields.
XB-donors
Trz-Ar1 (deg)a
57.36
Trz-Ar2 (deg)a
88.86
δ at C5 (ppm)b
90.5
3a
3b
3c
3d
3e
Ph
Ph
55.53
89.74
89.42
79.83
89.60
89.44
89.54
82.53
91.7
91.8
93.4
95.0
Ph
Ph
An
XB-donor 3c (5.0 mol%)
CH2Cl2, 30 °C, 14 h
N
N
+
An
H
TBSO
TBSO
4b
(1.5 eq)
5a
6ba
80%, dr = 3.3:1
a
Dihedral angles between Ar1 or Ar2 and a triazolium ring calculated by
M062X/6-31G+(d,p)/LANL2DZ. b Chemical shift of an iodide-attached carbon in
the 13C NMR spectrum.
Ph
H
OTMS
O
NHPh
XB-donor 3c (5.0 mol%)
CH2Cl2, 30 °C, 14 h
N
+
In contrast, each of the mesityl groups in 3c is nearly
perpendicular to the triazolium ring (Fig. 2, right). This may be
a consequence of minimizing steric repulsion between the
iodide atom and methyl groups in 3c. Consequently, the
triazolium ring of 3c is not conjugated with Ar1 and Ar2,
inducing localization of positive charge on the triazolium ring.
Actually, the chemical shifts of the halogenated carbons in the
13C NMR spectra were further downfield in the examples with
bulky XB-donors, 3c-3e, than in 3a and 3b (Table 2). These
results indicate that introducing bulky wingtip groups on the
Ph
Ph
Ph
Ph
5c
8
39%
7
(1.5 eq)
Scheme. 2 Other nucleophiles for the reaction with imines.
In summary, we synthesized a structurally diverse set of 5-
iodo-1,2,3-triazolium tetrafluoroborates and evaluated the
catalytic efficiency for the aza-Diels-Alder reaction of 2-siloxy-
1,3-butadiene with imines. We disclosed that the steric
environment around the catalytic center in XB-donors
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significantly impacted their catalytic activities. The origin of
these steric effects was analyzed using DFT calculations and 13
C
NMR data. Introducing bulky substituents at the triazolium
rings enhanced the electron withdrawing ability, leading to the
high catalytic efficiency of 3c. Additional mechanistic study and
expansion of substrate scope to less Lewis basic substrates
such as aldehydes and ketones are currently underway in our
laboratory.
This research was financially supported by a Grant-in-Aid
for Young Scientists (B) (JP 80781369) and The Institute of
Science and Engineering, Chuo University.
Conflicts of interest
There are no conflicts to declare.
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Ching-Fa, Tetrahedron 2005, 61, 11751; (e) M. Breugst, E.
Detmar and D. von der Heiden, ACS Catal., 2016, 6, 3203.
12 In the previous report on the halogen-bonding catalyzed aza-
Diels-Alder reaction by Takeda and Minakata et al. (Ref. 6f),
they confirmed that halogen-bonding interaction occurred
between halogen-bonding-donors (iodobenzimidazolium
salts) and substrates (imines) by NMR titration experiments.
13 Our DFT calculation study revealed that the stabilization
energy of halogen bonding between 3c and 5a is
−5.0
kcal/mol. Moreover, the bond length of C=N in imine moiety
of halogen-boding complex 3c-5a is longer that of the
original imine 5a (1.273 Å vs 1.268 Å), indicating that
halogen-bonding-donors work as Lewis acid catalysts to
electrophilically activate imines in the present reaction.
Please see the supporting information for details.
6
(a) A. Bruckmann, M. A. Pena and P. C. Bolm, Synlett, 2008,
900; (b) O. Coulembier, F. Meyer and P. Dubois, Polym.
Chem., 2010, 1, 434; (c) F. Kniep, S. H. Jungbauer, Q. Zhang, S.
4 | J. Name., 2012, 00, 1-3
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