10.1002/chem.201704502
Chemistry - A European Journal
COMMUNICATION
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Table 3. Titration experiments for selected ChB donors with
tetraoctylammonium bromide at room temperature..
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no.
1
Catalyst
6N-Oct/Se-Me
6N-Me/Se-Oct
Solvent
CD3CN
Anion
Br−
K [M-1]a
317
2
CD3CN
CD3CN
Br−
351
[4]
[5]
i
3
syn-7N-Oct/Se- Pr
Br−
341
[6]
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[a] K = binding constant.
The bindings constants obtained for all three ChB donors were
relatively similar at around 300 M-1. While the data of 6N-Oct/Se-Me
and 6N-me/Se-Oct (317 M-1 and 351 M-1) may be in line with their
inactivity in the test reaction, the virtually identical binding
constant of syn-7N-Oct/Se-iPr (341 M-1) is obviously in stark contrast
to the catalysis study. While this likely indicates that, inter alia, the
relative binding strengths to bromide in CD3CN are different from
the ones to chloride in THF, the data still provides a rough first
estimate chalcogen bonding strength for these systems.
[8]
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In conclusion, the first application of selenium-based ChB donors
as Lewis acidic organocatalysts was presented. While
bisimidazolium-derived
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ChB
donors
were
inactive,
bisbenzimidazilium-based ones provided up to 92% yield in the
benchmark reaction of 1-chloroisochroman with a silyl ketene
acetal. Comparison experiments with the analogous HB donors
ruled out other possible modes of activation next to chalcogen
bonding. The syn-atropisomers were reproducibly more active
than the related anti-isomers, pointing towards an at least partially
multidentate binding of chloride by the most active ChB donors.
Compared to a brominated XB donor, the rate acceleration by the
XB donor was about 7-fold stronger. All these findings provide a
solid basis for the further development of chalcogen bonding
organocatalysis.
Even though the interaction may be generally weaker than HB or
XB, it also features some unique advantages. Maybe the most
notable one with respect to the catalysis presented herein is the
presence of a second substituent on the electrophilic atom which
is orientated at a 90° angle to the electrophilic axis. This close
proximity to the substrate may offer additional control not possible
with HB or XB. Studies towards the utilization of this effect are
currently underway.
[17] C. Y. Legault, CYLview, 1.0b, Université de Sherbrooke, 2009,
[18] Even though we focused on 1H NMR in this study to obtain the binding
constants in an efficient manner, additional valuable information could
also be gained by 77Se NMR and 13C NMR (by the chemical shift changes
of the carbon atoms bound to Se).
Acknowledgements
P.W. and S.M.H. gratefully acknowledge funding from the
European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (grant
agreement No 638337). The authors also thank the Deutsche
Forschungsgemeinschaft (Cluster of Excellence RESOLV, EXC
1069) and the Fonds der Chemischen Industrie for financial
support.
Keywords: chalcogen bonding • Lewis acids • organocatalysis •
noncovalent interactions • nucleophilic substitution
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