Chalcogen???π Bonding Catalysis

Article abstract of DOI:10.1002/anie.202101140

While the presence of sulfur???π bonding interaction is a general phenomenon in the biological systems, the exploitation of this noncovalent force in a chemical process yet remains elusive. Herein, we describe the concept of chalcogen???π bonding catalysis that activates molecules of π systems through the interaction between chalcogen and π-electron cloud. The proof-of-concept studies using a vinylindole-based Diels–Alder benchmark reaction demonstrate that S???π and Se???π bonding interaction can drive the cycloaddition reaction efficiently. Experimental results suggest that a simultaneously double Se???π bonding interaction directs the stereoselectivity in this cycloaddition process.

Full text of DOI:10.1002/anie.202101140

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Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 9395–9400
International Edition: doi.org/10.1002/anie.202101140
German Edition: doi.org/10.1002/ange.202101140
Chalcogen-Bonding Catalysis
Chalcogen···p Bonding Catalysis
Xiangjin Kong, Pan-Pan Zhou, and Yao Wang*
Abstract: While the presence of sulfur···p bonding interaction
is a general phenomenon in the biological systems, the
exploitation of this noncovalent force in a chemical process
yet remains elusive. Herein, we describe the concept of
chalcogen···p bonding catalysis that activates molecules of p
systems through the interaction between chalcogen and p-
electron cloud. The proof-of-concept studies using a vinyl-
indole-based Diels–Alder benchmark reaction demonstrate
that S···p and Se···p bonding interaction can drive the cyclo-
addition reaction efficiently. Experimental results suggest that
a simultaneously double Se···p bonding interaction directs the
stereoselectivity in this cycloaddition process.
T
he noncovalent sulfur···aromatic interaction between the
sulfur atom in methionine and cysteine and aromatic rings in
phenylalanine, tyrosine, and tryptophan provides stabilization
in several biological events such as protein folding as well as
protein-protein/ligand interactions.[1] Based on analysis of X-
ray crystal structures of proteins, Reid et al. suggested that the
distance (d1) from the aromatic ring centroid to the sulfur
atom no more than 6.0 ꢀ was identified as a sulfur···aromatic
interaction, while d1 at approximately 5.3 ꢀ separation
represents the most marked distribution of such interaction
in proteins (Figure 1a).[2] Morgan et al. suggested that the
distance (d2) from the sulfur atom to the p-bonded atom not
exceed 5.0 ꢀ could be considered as a sulfur···aromatic
interaction.[3]
Figure 1. From observation to chalcogen···p bonding catalysis.
aromatic systems that the negative charge distributed above
and below the ring plane while the positive charge located
around the edge of the aromatic ring.[4] Considering the
nature of these noncovalent interactions, we herein suggest to
unambiguously define chalcogen···p bonding interactions as
the noncovalent interactions between chalcogens and p-
electron cloud of aromatic systems, alkenes, alkynes, etc.
along the direction of the axis of covalent bond of chalcogen-
containing molecules.
The interactions between chalcogen-based electron
acceptors and the lone-pair electron donors can exceed
10 kcalmolÀ1.[5] As a result, this type of chalcogen bonding
interactions were capable of playing a role in several chemical
events,[5,6] such as anion recognition[7] and transport,[8] mod-
ulating conformation,[9] and noncovalent catalysis.[10–12] How-
ever, the hitherto known chalcogen bonding interactions with
p systems are rather weak as evidenced by the results that the
maximum stabilization of cysteine···aromatic interaction is
2 kcalmolÀ1 while the methionine···aromatic interaction is
around 1 kcalmolÀ1.[1] As a consequence, the application of
chalcogen···p bonding interaction in a chemical process yet
remains elusive. Herein, we report the proof-of-concept
studies on chalcogen···p bonding catalysis (Figure 1B).
We found that Ch1-type chalcogen bonding donors can
interact with ketones in a considerable strength.[12b] At the
outset of this study, a critical problem is to know the bonding
behavior between Ch1-type chalcogen bonding donors and p
systems. As shown in Figure 2, analysis of the packing model
of the X-ray crystal structures of Ch1 and Ch2 reveals the
presence of both the ring-edge and ring-face chalcogen···ar-
omatic bonding interactions, which is in consistence with both
the notions of Reid and Morgan on a chalcogen···aromatic
bonding interaction.[2,3] In the S···p interaction, the phenyl
ring centroid to the sulfur atom is 3.86 ꢀ along with an angle
The analysis of these noncovalent interactions reveals that
there are two distinct types of sulfur···aromatic interac-
tions.[1–3] One type of these interactions is the sulfur···ring-
edge interaction that the sulfur atom is located near the
aromatic ring plane while the lone pair electrons of sulfur is
proximal to the edge of aromatic ring, thus repelling the
electron-rich p-cloud (Figure 1b). On the contrary, another
type interaction is the sulfur···ring-face interaction that the
sulfur atom is located above or below the aromatic ring plane
to interact with the p-electron cloud (Figure 1c). These
observations are in consistence with the quadrupolar model of
[*] X. Kong, Prof. Dr. Y. Wang
School of Chemistry and Chemical Engineering, Key Laboratory of the
Colloid and Interface Chemistry, Ministry of Education, Shandong
University
Jinan, 250100 (China)
E-mail: yaowang@sdu.edu.cn
Prof. Dr. P.-P. Zhou
College of Chemistry and Chemical Engineering, Key Laboratory of
Special Function Materials and Structure Design of Ministry of
Education, Lanzhou University
Lanzhou, 730000 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.202101140.
À
of P S···ring centroid 176.48 (Figure 2A). In the S···phenyl
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donors (Table 1, entries 2,3). In contrast to aromatic rings,
more marked variation of the chemical shift of 77Se signal of
Ch3 was observed in the presence of a vinyl substitution
(Table 1, entries 4,5). These experiments indicate that the p-
electrons of alkenes could contribute to the strength of Se···p
bonding interactions. Furthermore, the contrast of the
variation of the chemical shift of 77Se signal of Ch3 using
styrene (p3), 2-vinylnaphthalene (p4) and 9-vinylanthracene
(p5) as p-electron donors suggests the stronger p-electron
donating ability, the more marked change of the chemical
shift of 77Se signal (Table 1, entries 4–6). Moreover, the
interaction of Ch3 with ethene-1,1-diyldibenzene (p6),
ethene-1,1,2-triyltribenzene (p7) and 1,1,2,2-tetraphenyle-
thene (p8) reveals that the larger p-electron donor leads to
the more dramatic variation of the chemical shift of 77Se signal
(Table 1, entries 7–9). In contrast to 3-methyl-N-methylindole
(p9), a more marked variation of the chemical shift of 77Se
signal was observed upon using a modified indole p10
(Table 1, entries 10–11). Notably, 31P signal of Ch3 almost
remains unchanged in all these NMR experiments, thus
excluding the possible cation-p contribution. These observa-
tions suggest Se···p bonding interaction leads to the variation
of 77Se signal.
Figure 2. The intermolecular chalcogen···aromatic interactions in syn-
thetic molecules.
ring-edge interaction, the elevation angle of S···phenyl ring
centroid relative to the phenyl ring plane is 8.68 while the
distance from sulfur to ring centroid is 5.51 ꢀ. Analogously,
the aromatic ring centroid to the selenium atom is 3.76 ꢀ
After identifying the bonding behavior of Ch3-type
molecules with p-electron donors, the proof-of-concept
study on chalcogen···p bonding catalysis was implemented
by employing a vinylindole-based Diels–Alder reaction
(Scheme 1).[14] The desirable product 2a was obtained in
83% and 88% yield in the presence of 5 mol% monodentate
À
along with an angle of P Se···ring centroid 171.88. A slightly
smaller elevation angle (8.18) was observed (Figure 2B).
The bonding behavior of Ch3[13] with various p systems
was examined by 31P and 77Se NMR analysis of the mixture of
Ch3 and p-electron donors (Table 1). The Se···p interactions
between Ch3 and aromatic rings were very weak as no
substantial change of the chemical shift of 77Se signal was
observed using benzene and naphthalene as p-electron
Table 1: Se···p bonding interaction of –Ch3 with various p electron
donors.[a]
Entry
Components
Dd of 31P [ppm]
Dd of 77Se [ppm]
1
Ch3
0
0
2
3
4
5
6
7
8
9
Ch3 + p1
Ch3 + p2
Ch3 + p3
Ch3 + p4
Ch3 + p5
Ch3 + p6
Ch3 + p7
Ch3 + p8
Ch3 + p9
Ch3 + p10
0
<0.01
0.04
0.08
0.23
0.29
0.10
0.15
0.22
0.10
0.30
<0.01
0.02
0.02
<0.01
0.02
0.03
0.02
0.01
0.02
10
11
[a] All the NMR experiments were carried out with Ch3 (0.1 mmol) and p
electron donor (0.2 mmol) in 0.6 mL CD2Cl2.
Scheme 1. Chalcogen···p bonding catalysis.
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catalyst Ch1 and Ch2, respectively. The addition of 10 mol%
of 2a was obtained. Then a stronger electron donor, that is,
K2CO3 to the Ch2 catalysis system did not inhibit the catalytic
tetrahydrothiopyran (n2), was employed to inhibit the Se···p
bonding interaction. The addition of 2.0 equiv n2 already
resulted in a complete suppression of the Se···p interaction.
Phosphines represent a class of strong electron donors.[16]
Indeed, the addition of 2 mol% triphenylphosphine (n3) or
XantPhos (n4) to the standard reaction resulted in a complete
suppression of the Se···p interaction.
While the cycloaddition reaction using substrate 1e as
a reactant could be promoted by Brønsted acid (i.e. p-
toluenesulfonic acid) or Lewis acid (i.e. scandium triflate),
however, the controlling of stereoselectivity is not productive
(2.0:1 and 2.7:1 2e/2e’) (Table 3, entries 1-2). A detailed
activity, thus excluding possible role of acid. Upon associating
a less coordinating counteranion (i.e. BArF À, tetrakis(3,5-
4
bis(trifluoromethyl)phenyl)borate), catalyst Ch2’ was more
efficient. The product was obtained in 81% yield using
5 mol% of catalyst Ch2’’. In addition, typical binding
constants of catalyst Ch2’’ with representative p systems
were determined by 13C NMR titrations[15] in CD2Cl2, and
values of 0.79 MÀ1 for benzene, 1.43 MÀ1 for 2-vinylnaphtha-
lene (p4) and 1.61 MÀ1 for substrate analogue p10 were
obtained.
In the presence of a bidentate catalyst Ch3, the catalyst
loading could be lowered to 0.5 mol% to give 90% yield of 2a
within 10 min. Furthermore, 1 mol% bidentate catalyst Ch3’
associating a tetrachlorogallate counteranion or Ch4 bearing
a different scaffold was efficient to catalyze the cycloaddition
reaction. The NMR experiments suggest that Se···p bonding
interaction using an alkene-substituted-electron donor is
much stronger than its non-substituted counterpart. To
further support this observation, catalyst Ch5 was used.
Despite the presence of an intramolecular Se···p interaction
between selenium and indole ring in the solid structure of
Ch5, it still showed comparable efficiency in contrast to the
other bidentate catalysts. In line with the observations in
NMR and X-ray studies, a phosphonium catalyst P1 did not
show catalytic activity, thus suggesting that there is no
interferential cation···p interaction. The investigation of the
electron effect of substrates on this chalcogen···p bonding
catalysis approach was carried out (Scheme 1). While both
the electron-withdrawing (i.e. fluoro, 1b) and electron-
donating (i.e. methoxy, 1c) substituents were amendable to
this catalysis approach, methoxy-substituted substrate 1c was
less reactive. Furthermore, an N-H free substrate 1d gave
91% yield in the presence of 1 mol% catalyst Ch3 within
10 min.
Table 3: Controlling of stereoselectivity.[a]
Entry
Catalyst
t [h]
2e [%][b]
2e’ [%][b]
2e/2e’[c]
1
2
TsOH·H2O
Sc(OTf)3
Ch2
Ch2’
Ch2’’
Ch2’
Ch3
Ch4
Ch5
Ch3 + n2
Ch3 + n3
Ch3 + n3
Ch3 + n3
12
12
6
2
8
10
0.6
1.5
1.5
2
1
1.5
6
65
71
74
73
67
73
96
88
89
87
92
88
85
33
23
24
25
23
24
<5
7
6
10
6
9
13
2.0:1
2.7:1
3[d]
4[d]
5[d]
6
2.4:1
2.2:1
2.4:1
2.9:1
>20:1
15.0:1
14.3:1
9.4:1
15.0:1
9.7:1
7
8
9
10[e]
11[f]
12[f]
13[f]
6.4:1
[a] See the Supporting Information for details. [b] Isolated yield.
1
[c] Determined by H NMR analysis of the reaction mixture. [d] 5 mol%
catalyst was used. [e] 1.0 equiv n2 was used. [f] n3 (0.2 mol% for
entry 11; 0.5 mol% for entry 12; 0.8 mol% for entry 13) was used.
The competitive chalcogen bonding between a lone-pair
electron donor and a p-electron donor was investigated
(Table 2). Upon addition of 1.0 equiv tetrahydropyran (n1) to
the standard reaction condition, the reaction worked almost
with the same efficiency. The addition of 5.0 equiv n1 could
inhibit the catalytic activity to a certain extent and 68% yield
investigation reveals that the bidentate catalysts (Ch3–5) can
control the stereoselectivity in an excellent manner (14.3:1 to
> 20:1) while the monodentate catalysts (Ch2, Ch2’, Ch2’’)
resulted in low stereoselectivity (2.2:1–2.9:1) (Table 3,
entries 3–9). These experiments suggest that a dual Se···p
bonding catalysis approach involving the simultaneous inter-
action of a bidentate catalyst with two reactants is in
operation, thus governing the stereoselectivity in the cyclo-
addition process. In this context, it was envisioned that the
stereoselectivity should decrease to some extent upon addi-
tion of an electron donor because it would competitively bind
to the bidentate catalyst to generate a portion of equilibrating
species with mono activating site. In line with this notion, the
addition of 1.0 equiv n2 gave lower stereoselectivity (Table 3,
entry 10). In the presence of a stronger electron-donor n3, the
diastereomer ratio decreased from > 20:1 to 6.4:1 depending
on the amount of n3 (from 0 to 0.8 mol%, Table 3, entries 11–
13).
Table 2: Competition between Se···n and Se···p bonding interaction.[a]
Entry
Competitor
Competitive bonding
Yield [%]
1
2
3
4
5
6
7
no
no
92
84
68
92
<5
<5
<5
n1 (1.0 equiv)
n1 (5.0 equiv)
n2 (1.0 equiv)
n2 (2.0 equiv)
n3 (2.0 mol%)
n4 (2.0 mol%)
Se···O
Se···O
Se···S
Se···S
Se···P
Se···P
Beyond the observation of different performance on the
controlling of stereoselectivity by bidentate and monodentate
catalysts, the dual catalysis mode was further corroborated by
[a] All the experiments were carried out with Ch3 (1 mol%), 1a
(0.2 mmol) and competitor in 1.0 mL CH2Cl2.
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p-p interaction in the catalysis process, a methyl substituted
reactant 1 f was used. The control experiments showed that
the replacement of phenyl group in substrate 1e with methyl
(1 f) resulted in a dramatic decrease of diastereoselectivity
(i.e. from > 20:1 to 1.7:1, Scheme 2d).
As shown in Figure 3, DFT optimized complexes of
catalyst Ch4 and two 1e molecules were obtained at the
M06-2X/6–31g(d,p) level of theory. The structure of complex
C1 shows dual Se···p bonding interactions of Ch4 and two 1e
molecules with p-p interaction between indole ring in one 1e
and phenyl group in the other 1e. The complex C1 has
a stronger interaction energy than the single Se···p bonding
complex C2 by 8.1 kcalmolÀ1, and the relative Gibbs free
energy for the formation of complex C1 is smaller than that
for the formation of complex C2 by 7.5 kcalmolÀ1, indicating
that the formation of complex C1 is more favorable than the
formation of complex C2 (see the Supporting Information).
Based on the experimental results, the proposed Se···p
bonding catalysis mode was depicted in Scheme 3. The Se···p
interaction that attracts electrons from dienophile would
benefit the reaction process. The dual Se···p bonding inter-
action between a bidentate catalyst and two reactants enables
shifting the intermolecular reaction to take place in an
intramolecular manner, thus facilitating the cycloaddition
process. In mode A, the p-p interaction enables the indole
ring of dienophile donating p-electrons to phenyl of diene,
Scheme 2. Mechanistic investigation.
a series of control experiments (Scheme 2). Upon lowering
the temperature to À408C, the previously effective mono-
dentate catalyst Ch2’ lost catalytic activity. In contrast, the
bidentate catalyst Ch3 was still capable of facilitating this
reaction. It was found that the monodentate catalyst Ch6 was
ineffective to promote this reaction while its bidentate
counterpart Ch7 was efficient (Scheme 2a). In principle, the
bidentate catalyst interacts with two substrates and brings
these two reactants in proximity to give rise to highly catalytic
activity. Therefore, an inappropriate long distance between
the two binding sites of a bidentate catalyst would have
problem in placing the two reactants in a proper position, thus
lowering the catalytic activity. In line with this notion, the
control experiments using bidentate catalysts Ch7–9 demon-
strate that the long distance between the two binding sites of
Ch9 resulted in lower catalytic activity along with decreased
stereoselectivity (Scheme 2b). These experiments suggest
a dual catalysis approach is in operation.
Figure 3. Optimized structure of the bonding complex of catalyst Ch4
and two 1e molecules (hydrogen atoms are omitted for clarity).
As the simultaneous interaction with two substrates is
significant to give high catalytic activity, the increasing of the
steric hindrance of catalyst would inhibit the interaction
between catalyst and substrate. Further studies indicate that
this catalysis mode is susceptible to the variation of the steric
environment of catalyst. In contrast to Ch4, the use of catalyst
Ch10 resulted in lower efficiency (Scheme 2c). Further
experiments were carried out to probe the role of p-p
interaction between phenyl and indole ring. To eliminate this
Scheme 3. Proposed Se···p bonding catalysis mode.
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thus enabling the generation of a more electron-rich diene
along with a more electron-poor dienophile which is favor-
able to the cycloaddition process to generate product 2e. In
contrast to mode A, the opposite p-p interaction in mode B
generates a more electron-poor diene along with a more
electron-rich dienophile, thus playing a negative effect on the
cycloaddition approach to product 2e’. As a result, a diaste-
reomer-selection event takes place. It is a different scenario in
the presence of a monodentate catalyst. The cycloaddition
process favors mode C while the formation of the complex
favors mode D. In mode D, an “undisturbed” indole moiety
donates electrons to a Se···p bonded “electron-deficient”
phenyl ring. On the contrary, in mode C, a Se···p bonded
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“undisturbed” phenyl ring. As a consequence, poor stereose-
lectivity was obtained by a monodentate catalyst.
In conclusion, the proof-of-concept study on chalcogen···p
bonding catalysis was implemented. The results indicate that
the p-electrons of alkenes contribute significantly to the
strength of Se···p bonding interaction. The investigation on
a vinylindole-based Diels–Alder benchmark reaction dem-
onstrates the feasibility of this concept. Significantly, a dual
Se···p bonding interaction model can control the stereoselec-
tivity. We anticipate this concept would find more application
in activation of molecules of p systems.
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Acknowledgements
We gratefully acknowledge the National Natural Science
Foundation of China (22022105, 21971147, 21772113), the
Natural Science Foundation of Shandong Province
(ZR2019JQ08), the Natural Science Foundation of Gansu
Province (20JR5RA289), the Fundamental Research Funds
for the Central Universities (lzujbky-2020-kb06). We thank
Prof. Di Sun and Prof. Peizhou Li at Shandong University for
assistance with the X-ray crystal structure analysis. Technical
support from Shandong University structural constituent and
physical property research facilities is acknowledged.
Conflict of interest
The authors declare no conflict of interest.
Keywords: catalysis · chalcogen bonding ·
chalcogen···p interaction · cycloaddition ·
noncovalent interaction
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Manuscript received: January 25, 2021
Accepted manuscript online: February 2, 2021
Version of record online: March 11, 2021
9400 www.angewandte.org
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