Anion-Binding Catalysis by Electron-Deficient Pyridinium Cations

Article abstract of DOI:10.1002/anie.201403778

A new activation principle in organocatalysis is presented: halide binding through Coulombic interactions. This mode of catalysis was realized by using 3,5-di(carbomethoxy)pyridinium ions that carry an additional electron-withdrawing substituent on the nitrogen atom, for example, pentafluorobenzyl or cyanomethyl. For the N-pentafluorobenzyl derivative, Coulombic interaction with the pyridinium moiety is complemented in the solid state by anion-π interactions with the perfluorophenyl ring. Bromide and chloride are bound by these cations in a 1:1 stoichiometry. Catalysis of the C-C coupling between 1-chloroisochroman (and related electrophiles) with silyl ketene acetals occurs at -78 °C and at low catalyst loading (2 mol %).

Full text of DOI:10.1002/anie.201403778

.
Angewandte
Communications
DOI: 10.1002/anie.201403778
Organocatalysis
Anion-Binding Catalysis by Electron-Deficient Pyridinium Cations**
Albrecht Berkessel,* Somnath Das, Daniel Pekel, and Jçrg-M. Neudçrfl
Abstract: A new activation principle in organocatalysis is
presented: halide binding through Coulombic interactions.
This mode of catalysis was realized by using 3,5-di(carbo-
methoxy)pyridinium ions that carry an additional electron-
withdrawing substituent on the nitrogen atom, for example,
pentafluorobenzyl or cyanomethyl. For the N-pentafluoro-
benzyl derivative, Coulombic interaction with the pyridinium
moiety is complemented in the solid state by anion–p
interactions with the perfluorophenyl ring. Bromide and
chloride are bound by these cations in a 1:1 stoichiometry.
Catalysis of the CꢀC coupling between 1-chloroisochroman
(
and related electrophiles) with silyl ketene acetals occurs at
ꢀ788C and at low catalyst loading (2 mol%).
[
1]
A
nion-binding catalysis, in which the catalyst facilitates the
release of an anion from one of the starting materials by
binding it in a reversible fashion, is a relatively new concept in
noncovalent organocatalysis. A prominent example was
[
2]
reported by Jacobsen et al. in 2008, with hydrogen bonding
[
3]
as the anion-binding motif. The chiral thiourea 4 shown in
Scheme 1 abstracts chloride from the substrate 1a to furnish
an oxonium cation, which is then alkylated by the silyl ketene
acetal 2a. Since the prochiral oxonium cation intermediate in
Scheme 1. Organocatalytic a-haloether alkylation and anion-binding
motifs: hydrogen bonding, halogen bonding, and Coulombic interac-
tion with electron-deficient pyridinium cations (this work).
ꢀ
the CꢀC bond forming step carries a chiral counterion (4·Cl ),
the alkylation product 3aa is formed not only in high yield but
also with superb enantioselectivity.
There are alternatives to hydrogen bonding in anion
[
4]
binding, in particular halogen bonding (Scheme 1). The
latter has been studied intensively as a structure-determining
factor in crystal engineering, and recent results point to it as
an important force in the interaction between proteins/
donor catalyst 5, just as it does for the hydrogen-bond donor
4. Yet another mode of anion binding hinges on the
[
8,9]
interaction of anions with electron-poor p surfaces
—
a process for which Matile et al. have recently reported first
[5]
[10,11]
enzymes and halogenated ligands (e.g. enzyme inhibitors).
catalytic applications.
It was not until recently that the promotion of chemical
reactions through halogen bonding was proven experimen-
Our goal was to exploit electron-deficient pyridinium
[12]
cations for anion-binding catalysis.
drawn to pyridinium ions because their halide-binding ability
Our attention was
[
6]
tally. In 2013, Huber et al. disclosed that the neutral double
halogen-bond donor 5 catalyzes the reaction between 1-
chloroisochroman (1a) and the silyl ketene acetal 2a
[
13]
is well documented, with regard to both structure
and
[
14]
complex stabilities. Our attempts to effect the 1-chloroiso-
chroman alkylation shown in Scheme 1 with pyridinium ions
[
7]
(
Scheme 1). It is assumed that the promotion of oxonium
ion formation from the substrate 1a by chloride excision
carrying one (6a·BPh ) or two electron-withdrawing substitu-
4
underlies the catalytic activity of the neutral halogen-bond-
ents (6b·BPh ; 6c·BPh ) met with frustration. Therefore,
4
4
further reduction of electron density in the pyridinium core
[
+]
[+]
[
*] Prof. Dr. A. Berkessel, S. Das, D. Pekel, Dr. J.-M. Neudçrfl
Cologne University, Department of Chemistry
Greinstrasse 4, 50939 Cologne (Germany)
E-mail: berkessel@uni-koeln.de
Homepage: http://www.berkessel.de
+
[
] These authors contributed equally to this work.
[
19]
[
**] Support from SusChemSys and by the Fonds der Chemischen
Industrie is gratefully acknowledged. A.B. acknowledges COST
membership (CM0905, Organocatalysis).
was sought by attaching a third electron-withdrawing sub-
stituent. This approach led to the triply substituted pyridi-
nium salts 6d·BPh4, 6e·BPh4, 6 f·BPh4, and 6g·BPh4
(Scheme 2), and afforded the first organocatalysts that
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403778.
1
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Angewandte
Chemie
intramolecular contact between the centroid of the penta-
fluorophenyl ring and the bromide ion. The distance between
the two (dHal–C6F5 = 3.409 ꢀ) is shorter than the sum of the van
der Waals radii (d = 3.52 ꢀ), thus indicating anion–p bonding.
We were also able to generate and crystallize the isostructural
chloride equivalent of the pyridinium bromide 6d·Br, that is,
6
d·Cl (see below), for the generation of 6d·Cl, see Supporting
Information for X-ray data). In the last step of catalyst
preparation, the bromides 6d–g·Br were converted into the
tetraphenylborates 6d–g·BPh , with anion metathesis being
4
enforced by precipitation of the products 6d–g·BPh4 from
aqueous NaBPh . The X-ray crystal structure of 6d·BPh is
4
4
shown in Figure 1c (see the Supporting Information for the
Scheme 2. Preparation of the pyridinium salts 6d–g: a) C F CH Br,
6
5
2
X-ray crystal structures of 6a–c,e–g·BPh ). There is tight
4
4
8
8
9%; b) BrCH CO CH , 83%; c) BrCH CO C H , 94%; d) BrCH CN,
7%; e) tetraphenylborates by anion exchange in aqueous NaBPh4,
2–96%.
2 2 3 2 2 2 5 2
stacking of one of the tetraphenylborate phenyl rings with the
pyridinium unit, and another of the tetraphenylborate phenyl
rings stacks with the C F5 moiety (see the Supporting
6
Information for packing diagrams).
promote 1-chloroisochroman alkylation through Coulombic
anion binding.
As summarized in Scheme 2, the N-benzylation/alkylation
of 3,5-dicarbomethoxypyridine smoothly afforded the pyridi-
nium bromides 6d–g·Br. The X-ray crystal structure of the
As may be expected from their crystal structures, the
bromide 6d·Br and the tetraphenylborate 6d·BPh4 show
significantly different chemical shifts for the resonances of the
2,6-protons on the pyridinium ring, and even more so for the
protons of the benzylic methylene group. An approximately
1
3
,5-dicarbomethoxypyridinium bromide 6d·Br is shown in
2 ppm upfield shift of the benzyl H resonance results from
Figure 1a. As with the other pyridinium halides (see Fig-
ure 1b for the structure of 6g·Br and the Supporting
Information for the X-ray crystal structures of 6e·Br and
the pyridinium iodide 6c·I), a pertinent feature is the charge-
dominated interaction between the bromide ion and the
exchange of bromide for tetraphenylborate (see the Support-
ing Information for the spectral data). To investigate chloride
binding, H NMR spectroscopy was applied in titration
experiments (Figure 2) that 1) showed a thermodynamic
preference for chloride over tetraphenylborate binding, with
1
[
13,14]
pyridinium moiety.
However, there is an additional
a formal association constant for 6d·Cl (from 6d·BPh and
4
1
Figure 2. Changes in the H NMR spectra resulting from the titration
of 6d·BPh against nBu NCl; [D ]THF/CD CN 9:1 (v/v), RT.
4
4
8
3
ꢀ1
TBACl) in the range of 200m (see the Supporting Informa-
tion for titration curves), and 2) confirmed the 1:1 stoichiom-
etry of chloride binding in solution (see the Supporting
Information for Job plots).
Catalytic activity, as summarized in Table 1, was assessed
by adding the nucleophiles 2a–e to a precooled solution of the
catalysts 6d–g·BPh and the electrophiles 1a, 1b, or 1c in
4
THF, and quenching with sodium methoxide (excess in
methanol) after 12 h at ꢀ788C. Note that there is no
measurable background reaction between the most reactive
substrate 1a and any of the silyl ethers 2a–e at ꢀ788C, over
12 h (Table 1, entry 1). After evaporation of the solvent, the
Figure 1. X-Ray crystal structures of 6d·Br (a), 6g·Br (b), and 6d·BPh4
c).
(
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Angewandte
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Table 1: Catalytic activity of the pyridinium salts 6d–g·BPh and 6d·Br
was no evidence for the build-up of reaction intermediates
4
(
Scheme 2) in the reaction of the electrophiles 1a–c with the silyl ether
(see the Supporting Information).
nucleophiles 2a–e.
The results summarized in Table 1 allow the following
conclusions: 1) Most importantly, the triply substituted pyr-
idinium tetrafluoroborates 6d–g·BPh indeed act as efficient
4
catalysts for the alkylation of 1-chloroisochroman (1a) and
also of the acyclic chlorobenzyl ether 1b, with all four of the
silyl ketene acetals (2a–d) tested (Table 1, entries 2–6,8–
1
1,13–15). For the more reactive electrophile 1a in combina-
tion with the silyl ketene acetal 2a, catalyst loading as low as 2
mol% is sufficient to achieve full conversion within a few
hours at ꢀ788C (entry 3). 2) Substrate scope: both chloro-
benzyl ethers 1a and 1b are transformed smoothly. However,
1
-chlorotetralin (1c) is not reactive (i.e. readily ionizable)
enough to be alkylated by the silyl ketene acetal 2a (Table 1,
entry 12). The same holds for the silyl enol ether 2e as the
nucleophile, even with the most reactive electrophile 1a
[
a]
[15]
Entry Electro- Nucleo- Pyridinium Catalyst Conversion Yield
(entry 7). 3) Despite lower steric hindrance at their nucle-
phile 1 phile 2 salt 6
Loading of 1 [%]
mol%]
of 3 [%]
ophilic a-carbon atoms, the a-unsubstituted silyl ketene
acetals 2c and 2d give lower conversions and product yields
relative to their dimethyl (2a) and cyclohexylidene (2b)
analogues (entries 5,6 versus 2,4 and 10,11 versus 8,9). We
attribute this effect to competing and irreversible catalyst
deactivation owing to C4 alkylation of the pyridinium
[
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1c
1a
1a
1a
1a
1a
1a
2a–e
2a
2a
2b
2c
2d
2e
2a
2b
2c
2d
2a
2a
2a
2a
2a
2a
2a
none
6d·BPh4
6d·BPh4
–
0
–
[
[
[
[
b]
b]
b]
b]
5
2
5
5
quant.
quant.
quant.
77
90
89
92
70
6d·BPh4
6d·BPh4
[16]
moiety, an effect that occurs at the low reaction temper-
6d·BPh4
6d·BPh4
6d·BPh4
6d·BPh4
6d·BPh4
5
82
n.d.
–
ature only for the unsubstituted silyl ketene acetals 2c and 2d
and not for 2a and 2b. 4) Catalyst inhibition by halides: as
shown in Table 1, entry 17, the catalytic activity of the
5
0
[
b]
10
10
10
10
10
5
5
5
5
quant.
quant.
83
86
48
56
–
[
b]
[
c]
[b,d]
[b,d]
0
1
2
3
4
5
6
7
8
ca. 80
ca. 80
tetraphenylborate 6d·BPh is strongly inhibited by addition
4
[
c]
6d·BPh4
of tetra-n-butylammonium chloride. Unsurprisingly, the pyr-
idinium bromide 6d·Br is catalytically inactive (Table 1,
entry 16). 5) An additional control experiment confirmed
6d·BPh4
6e·BPh4
6 f·BPh4
0
quant.
quant.
quant.
0
85
87
88
–
–
–
that neither TMS-Cl nor BPh catalyze the reaction between
3
6g·BPh
4
[
e]
1a and 2a (Table 1, entry 18).
6d·Br
6d·BPh
[
f]
5
–
<10
0
Scheme 3 summarizes our mechanistic interpretation of
4
[
g]
none
the catalysis effected by the tetraphenylborates 6d–g·BPh , as
4
exemplified by the substrates 1a and 2a. Step I consists of the
ionization of the substrate 1a to the oxonium cation A. Halide
excision from 1a is effected by Coulombic interaction with
[
a] All reactions were carried out at ꢀ788C in THF for 12 h, as described
in the Experimental Section. [b] Yield of isolated product 3 after column
chromatography. [c] Methanolate induced partial elimination to methyl
cinnamate, thus complicating quantitative analysis. [d] A different
workup was required for product isolation (see the Supporting
Information). [e] Acetonitrile (10 vol%) was added. [f] 10 mol% tetra-n-
the pyridinium catalyst 6·BPh , with concomitant formation
4
butylammonium chloride was added. [g] 10 mol% of TMS-Cl or BPh was
3
added.
ratio of alkylation product 3 to nonreacted electrophile could
1
easily be determined by H NMR integration of, for example,
the proton at the C1 position of the isochroman 3aa (d =
5
5
.17 ppm) versus the C1ꢀH of the methyl acetal (d =
.45 ppm) that result from the quenching of unreacted 1a
with methanolate. As shown in Table 1, the conversions
determined by this method coincide well with the gravimetri-
cally determined product yields following chromatographic
isolation. The alkylation of 1-chloroisochroman (1a) by the
silyl ketene acetal 2a with the catalyst 6d·BPh was addition-
4
1
ally monitored by low-temperature H NMR spectroscopy.
The concentration/time profiles thus obtained confirmed
smooth and parallel conversion of the starting materials,
and concomitant formation of the reaction products. There
Scheme 3. Mechanistic proposal for the alkylation of the a-haloether
1
a by the silyl ketene acetal 2a, catalyzed by the halide-binding
pyridinium cations 6d–g·BPh4.
1
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Angewandte
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of 6·Cl. In fact, when the catalyst 6d·BPh is exposed to 1-
solution of 1-chloroisochroman (1a, 0.10 mmol, 1.00 equiv) in anhy-
4
drous THF was added, followed by another 1.00 mL of anhydrous
THF. The reaction vessel was sealed with a rubber septum and taken
from the glovebox. The solution was cooled to ꢀ788C with stirring.
After 10 min, the silyl ketene acetal 2a (30 mL, 0.15 mmol, 1.50 equiv)
was added. After stirring for another 12 h at ꢀ788C, the reaction was
quenched by adding a solution of sodium methoxide in methanol
(30 wt%, 200 mL, 10.0 equiv) at ꢀ788C. The solution was diluted with
2 mL of an n-pentane/diethyl ether (1:1) mixture, filtered through
silica, and thoroughly eluted with n-pentane/diethyl ether (1:1). The
solvent was evaporated under reduced pressure, and the remaining
crude product was subjected to column chromatography (silica gel, n-
chloroisochroman (1a) alone, in THF at RT, crystalline 6d·Cl
starts to precipitate after approximately one day (material
used for the X-ray crystal structure shown in the Supporting
Information). In step II of the catalytic cycle, CꢀC bond
formation occurs as the oxonium cation A is attacked by the
silyl ketene acetal 2a, thereby resulting in the cationic species
B. The cation B acts as a highly reactive silylating agent and
transforms the chloride 6·Cl back into the active catalyst
6
·BPh , with concomitant formation of the product 3aa and
4
TMS-Cl (step III). Chloride excision from substrate 1a by
cation B, that is, a “shortcut” chain reaction not involving
step III, can be excluded based on control experiments (see
the Supporting Information): AgO CCF or NaBPh4 as
pentane/diethyl ether 9:1). The product 3aa was obtained as a clear
1
liquid (42.0 mg, 0.18 mmol, 90%). H NMR (300 MHz, CDCl ): d =
3
7.19 (m, 3H), 6.97 (m, 1H), 5.17 (s, 1H), 4.16 (ddd, J = 10.7, 5.3,
1.6 Hz, 1H), 3.75 (s, 3H), 3.60 (m, 1H), 3.04 (ddd, J = 15.6, 12.0,
2
3
5
.3 Hz, 1H), 2.54 (d, J = 15.8 Hz, 1H), 1.12 (s, 3H), 1.10 ppm (s, 3H);
chloride traps in the absence of a catalyst 6·BPh induce just
4
1
3
[
17]
C NMR (75 MHz, CDCl ): d = 177.5, 136.4, 134.7, 128.7, 126.4,
3
stoichiometric conversion of the substrate 1a.
1
25.8, 125.7, 80.1, 63.9, 51.9, 49.0, 30.2, 21.1, 20.9 ppm; ESI-MS: 257.1
Yet another alternative pathway may be envisaged in
which a covalent intermediate (7) is generated from the
+
[
M+Na] ; IR (ATR): ~n ¼2953, 2870, 1743, 1693, 1384, 1153, 985,
ꢀ1
746 cm
.
pyridinium catalyst 6·BPh by attack of the nucleophile 2a at
4
CCDC 983184 (6d·Br), 983185 (6d·BPh ), 983186 (6c·I), 983187
4
C4 (Scheme 4). Compounds of type 7 have recently been
(6c·BPh ), 983188 (6d·Cl), 983189 (6b·BPh ), 983190 (6a·BPh ),
4 4 4
1
002110 (6 f·BPh ), 1002111 (6g·Br), 1002112 (6e·Br), und 1002113
4
(
6e·BPh ) contain the supplementary crystallographic data for this
4
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Received: March 27, 2014
Revised: June 2, 2014
Published online: September 10, 2014
Keywords: anion-binding catalysis · anion–p interactions ·
.
organocatalysis · organofluorine compounds ·
pyridinium cations
Scheme 4. C4 Alkylation of catalysts 6·BPh by the silyl ketene acetal
4
2
a.
reported to transfer their C4 substituent to reactive electro-
[
18]
philes, albeit slowly and at temperatures above 1008C.
[
[
1] a) K. Brak, E. N. Jacobsen, Angew. Chem. Int. Ed. 2013, 52, 534 –
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2008, 130, 7198 – 7199.
1
However, the typical H NMR resonances of compound 7
5
were not detectable under in situ monitoring conditions
(
200 K) and nor does isolated 7 undergo any transformation
when exposed to the electrophile 1a, even at room temper-
ature for > 20 h. We therefore conclude that no covalent
intermediate is involved in the catalytic cycle.
[3] a) K. Hof, M. Lippert, P. R. Schreiner in Science of Synthesis
Asymmetric Organocatalysis, Vol. 2 (Eds.: B. List, K. Maruoka),
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G. Resnati), Springer, Berlin, 2008, pp. 1 – 15; b) P. Metrangolo,
G. Resnati, T. Pilati, S. Biella, Structure and Bonding, Vol. 126
In summary, we describe a new motif for organocatalysis:
anion binding by electron-deficient pyridinium cations. To our
knowledge, this is the first case of anion-binding catalysis
effected solely by Coulombic interactions. The first represen-
tatives of this novel type of organocatalyst appear to show
effectiveness similar to hydrogen-bonding thioureas and
exceed halogen-bonding catalysts with regard to reaction
rate. Further work to explore the scope of this novel anion-
binding motif is underway.
(Eds.: P. Metrangolo, G. Resnati), Springer, Berlin, 2008,
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Experimental Section
Alkylation of 1-chloroisochroman (1a) by silyl ketene acetal 2a,
catalyzed by 3,5-dicarbomethoxy N-[(pentafluorophenyl)methyl]pyr-
idinium tetraphenylborate (6d·BPh ): Catalyst 6d·BPh (3.50 mg,
[5] a) M. R. Scholfield, C. M. Vander Zanden, M. Carter, P. S. Ho,
Protein Sci. 2013, 22, 139 – 152; b) L. A. Hardegger, B. Kuhn, B.
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Diez, J. Benz, J.-M. Plancher, G. Hartmann, D. W. Banner, W.
4
4
2
.00 mmol, 0.05 equiv) was dissolved in 1.00 mL anhydrous THF in
a Schlenk flask in a glove box. To this mixture, 100 mL of a 1.0m
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