European Journal of Organic Chemistry
10.1002/ejoc.201800434
FULL PAPER
[a,b]
Table 3. Enantioselective Diels–Alder Reactions of cyclohexa-1,3-diene (2) and cyclopentadiene (27) and chalcone derivatives catalyzed by (S,S)-22.
[
c]
[b]
[d]
[e]
Entry
1
R
Diene
Chalcone
Adduct
Conv. (%)
Endo:exo
Yield (%)
Ee (%)
endo
exo
2
3
4
>99
>95:5
86
-
53
[
f]
2
2
2
25
26
3
28
29
30
>99
>99
89
>95:5
>95:5
30:70
68
70
17
-
-
65
7
3
4
27
44
0
[
f]
5
27
25
31
>99
23:77
10
41
0
[
a] All reactions were performed according to General Procedure 4 at a dienophile concentration of 0.5 M. [b] Diastereomeric (trans:cis) and endo:exo ratios
1
determined by GLC or H NMR analysis of the crude material prior to purification. [c] Determined by GLC analysis using triphenylmethane as the internal
standard. [d] Analytically pure cycloadduct after flash column chromatography on silica gel. [e] Determined by HPLC analysis using a chiral stationary phase. [f]
Dienophile concentration of 0.3 M.
contributions and discussions. M.O. is indebted to the Einstein
Foundation (Berlin) for an endowed professorship.
Conclusion
Keywords: asymmetric catalysis
• chirality • Diels–Alder
We prepared a series of chiral intramolecular silicon–sulfur
Lewis pairs with biphenyl- and binaphthyl silepine backbones
and different aryl thioether groups. The new sulfur-stabilized
silicon cations were fully characterized by NMR spectroscopy
and probed as catalysts in Diels–Alder reactions of chalcones.
The systematic variations of those structural units revealed the
subtle their interdependence. For the phenyl-substituted
thioether, the binaphthyl motif with the fixed chiral axis was not
necessary, and the level of enantioselection was the same with
the conformationally flexible biphenyl-based catalyst [34% ee for
reaction • Lewis acids • silicon cation
[1]
2]
V. H. G. Rohde, P. Pommerening, H. F. T. Klare, M. Oestreich,
Organometallics 2014, 33, 3618–3628.
a) G. K. S. Prakash, S. Bae, Q. Wang, G. Rasul,G. A. Olah, J. Org.
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[
[3]
a) V. H. G. Rohde, M. F. Müller, M. Oestreich, Organometallics 2015,
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[
4]
Cyclohexa-1,3-diene Diels‒Alder reactions catalyzed by stabilized
silicon cations: a) K. Hara, R. Akiyama, M. Sawamura, Org. Lett. 2005,
7, 5621–5623; b) H. F. T. Klare, K. Bergander, M. Oestreich, Angew.
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Mück-Lichtenfeld, S. Grimme, M. Oestreich, J. Am. Chem. Soc. 2012,
(S,S)-1 and 35% ee for (RS,S)-10]. This situation dramatically
changed with bulkier aryl substituents at the thioether of
biphenyl-based (RS,S)-21 and (RS,S)-23. With meta,meta-
disubstitution as in (RS,S)-21, the enantiomeric excess
remained stable at 35% ee but collapsed with ortho,ortho-
disubstitution as in (RS,S)-23. In turn, the benchmark
enantiomeric excess of 34% ee obtained with (S,S)-1 did
improve with the corresponding binaphthyl-based catalysts [48%
ee for (S,S)-24 and 53% ee for (S,S)-22]. These cyclohexa-1,3-
diene Diels–Alder reactions were endo selective but exo
cycloadducts formed predominantly with cyclopentadiene as
dienophile. This is another curious example of an unusual exo
selective Diels–Alder reaction where no enantioinduction is
134, 4421–4428; d) A. R. Nödling, K. Müther, V. H. G. Rohde, G. Hilt, M.
Oestreich, Organometallics 2014, 33, 302–308; e) R. K. Schmidt, H. F.
T. Klare, R. Fröhlich, M. Oestreich, Chem. Eur. J. 2016, 22, 5376–5383.
Additional examples of cyclohexa-1,3-diene Diels‒Alder reactions with
chalcone as dienophile: a) A. C. Kinsman, M. A. Kerr, Org. Lett. 2000, 2,
[5]
[6]
3
517–3520 (ultrahigh pressure); b) M. Karthikeyan, R. Kamakshi, V.
Sridar, B. S. R. Reddy, Synth. Commun. 2003, 33, 4199–4204
microwave irradiation); c) Z. Liu, R. Ganguly, D. Vidović, Dalton Trans.
(
2017, 46, 753–759 (aluminum Lewis acid).
For other enantioselective Diels‒Alder reactions catalyzed by chiral
silicon Lewis acids, see for example: a) Y. Sakaguchi, Y. Iwade, T.
Sekikawa, T. Minami, Y. Hatanaka, Chem. Commun. 2013, 49, 11173–
11175; b) T. Gatzenmeier, M. van Gemmeren, Y. Xie, D. Höfler, M.
Leutzsch, B. List, Science 2016, 351, 949–952.
[
11b]
found.
[
7]
For a related ring closure, see: M. Kienle, A. Unsinn, P. Knochel,
Angew. Chem. Int. Ed. 2010, 49, 4751–4754.
Acknowledgements
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[9]
This
research
was
supported
by
the
Deutsche
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Forschungsgemeinschaft (Oe 249/12-1). We thank Dr. Enrique
E. Maroto Martínez (postdoctoral fellow funded by the Alexander
von Humboldt Foundation, 2015‒2017) for his experimental
115, 2936–2942; d) Y.-h. Lam, P. H.-Y. Cheong, J. M. Blasco Mata, S.
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