Chemistry - A European Journal
10.1002/chem.202101138
RESEARCH ARTICLE
Table 1. Comparison of catalytic activity of 1-(donor)
n
and Si(catCl
)
2
-(donor)
2
in the reductive alkyl ether formation and the deoxygenation of benzophenone. P1
[
a]
dialkyl ether for aldehydes, and diphenylmethane for benzophenone, P2 carbonyl hydrosilylation product. General conditions: 24 h, 150 µmol substrate, 1 mol%
[
b]
1
3 3 2 2
catalyst and 225 µmol HSiEt or 2 mol% catalyst and 450 µmol HSiEt in 0.5 mL CD Cl . Yields determined via H NMR integration against an internal standard.
Catalyst /
Substrate
Si(catCF3
Conv.
)
2
•(sulfolane)
2
Si(catCl)
Conv.
2
•(sulfolane)
2
Si(catCl)
Conv.
2
•(CH
3
CN)
2
Si(catCF3
) •OCPh
2 2
P1
P2
P1
P2
P1
6%
P2
Conv.
98%
82%
78%
P1
P2
pCH
pFC
Ph
3
C
6
H
4
CHO[a]
CHO[a]
CO[b]
98%
97%
96%
99%
< 0.5%
31%
26%
25%
27%
10%
24%
3%
7%
11%
16%
< 0.5%
97%
80%
77%
<0.5%
6
H
4
> 99%
> 99%
3%
-
16%
-
5%
5%
-
2%
-
2
16%
Whereas with 1-(sulfolane)
2
, the pristine formation of dialkyl
the
Lewis acid that ranges among the strongest fluoride ion
acceptors currently accessible in the condensed phase. This is a
notable step for silicon, a central element that was associated
with moderate Lewis acidity due to the lack of obvious acceptor
orbitals. It allows stabilizing adducts with very weak donors,
such as the first Lewis pair of a neutral Lewis acid with a
disiloxane. Second, this Lewis acid allows expanding the
catalytic portfolio of neutral silanes onto the first silicon catalyzed
deoxygenation reactions of aldehydes, ketones, amides and
phosphine oxides, as well as the carbonyl-olefin metathesis. The
possibility of controlling the steric and electronic profile of Lewis
acids by defined ligand-variations while maintaining their “super”
aptitude to maneuver through high-energy reaction pathways will
propel a transfer of Lewis superacids into synthetically relevant
ethers P1 was observed, with Si(catCl)2•(sulfolane)
,
2
unselective formation of both P1 and hydrosilylation product P2
was encountered. The conversions with Si(cat )2•(sulfolane)
are higher than the ones for the corresponding bis-acetonitrile
adduct, but the selectivity remains low (col3 vs. col4). In contrast
Cl
2
X
to the previously reported Si(cat )
2
, which form their bis-adducts
in trans orientation,[ the cisoid binding mode, as observed for
4]
3
OPEt with 1 (Figure 4), might preorganize two substrates for
intramolecular ether formation. Beyond, the cis-adduct arranges
the substrates trans with a catecholate, which might cause more
efficient substrate activation. Indeed, a cisoid coordination was
indicated for aldehydes by four peaks in the 19F-NMR spectra
upon
mixing
1-(sulfolane)
2
with
10
eq.
of
cyclohexanecarboxaldehyd (see SI).
Finally, benzophenone’s deoxygenation in the coordination
sphere of 1 was considered a promising route to donor-free 1.
fields. Finally, the new compound H
2
catCF3 will stimulate other
fields such as hydrogen bond donor catalysis,[ redox-active
21]
ligands,[22] or weakly coordinating anions.[9b]
2
Thus, the mono-benzophenone adduct 1-(OCPh ) was prepared
at a 200 mg scale and fully characterized (see Supporting
Information for SCXRD). To the best of our knowledge, it
represents the first Lewis adduct of a silane with a ketone, and it
gives experimental evidence for the mode of carbonyl activation
Acknowledgements
We thank Prof. H.-J. Himmel for his constant support, and the
DFG (GR5007/2-1) for financial support. Dr. Jürgen Graf and
Prof. Markus Enders are thanked for their efforts with NMR
spectroscopy. Dr. Marcel Schorpp is acknowledged for support
in SCXRD structure refinement. The federal state Baden-
Württemberg is acknowledged for providing computational
resources (BWFor/BWUni).
proposed during the catalytic hydrosilylation also with
F
.[3a] Deoxygenation of benzophenone with
Si(cat )2•(acetonitrile)
2
Et
separated from the reaction mixture. Instead, the formation of an
adduct between 1 and the reaction product Et SiOSiEt was
3
SiH occurred quantitatively, but donor-free 1 could not be
3
3
observed (see section 2.5 in the SI). Although not isolable in the
crystalline state, multinuclear NMR spectroscopy and
computations strongly support such an adduct. To the best of
Keywords: Lewis superacids • silicon • deoxygenation •
our knowledge, this is the first example of a neutral Lewis acid
pairing with a disiloxane, an extremely weak donor.[
12, 20]
From a
catechol • adducts with weak Lewis donors
catalytic perspective, the mono-benzophenone adduct
-(OCPh ) promised to serve as a precatalyst under reductive
conditions. Thus, the representative set of catalytic silane
reductions was performed with 1-(OCPh ) and compared with 1-
sulfolane) (Table 1, col4 vs col2). Interestingly, the catalytic
activity of 1-(OCPh turned out as inferior compared to
-(sulfolane) particularly for the deoxygenation of
benzophenone itself. This observation indicates that the donor-
free Lewis acid 1 is a less effective catalyst, but sulfolane might
take an active role in the catalytic cycle. Although further
mechanistic studies are required, this handle offers exciting
opportunities to control the activity and the selectivity within this
class of Lewis superacids.
[
1]
a) A. G. Myers, S. E. Kephart, H. Chen, J. Am. Chem. Soc. 1992, 114,
922-7923; b) H. Yamamoto, Lewis acids in organic synthesis, Wiley-
1
2
7
VCH, Weinheim, 2002; c) J. W. A. Kinnaird, P. Y. Ng, K. Kubota, X.
Wang, J. L. Leighton, J. Am. Chem. Soc. 2002, 124, 7920-7921; d) A. D.
Dilman, S. L. Ioffe, Chem. Rev. 2003, 103, 733-772; e) H. Yamamoto, K.
Ishihara, Acid catalysis in modern organic synthesis., Wiley-VCH,
Weinheim, 2008; f) S. E. Denmark, G. L. Beutner, Angew. Chem. Int.
Ed. 2008, 47, 1560-1638; g) S. Steinhauer, J. Bader, H.-G. Stammler,
N. Ignat‘ev, B. Hoge, Angew. Chem. Int. Ed. 2014, 53, 5206-5209; h) B.
Waerder, M. Pieper, L. A. Körte, T. A. Kinder, A. Mix, B. Neumann, H.-
G. Stammler, N. W. Mitzel, Angew. Chem. Int. Ed. 2015, 54, 13416-
2
(
2
2
)
1
2
,
1
1
3419; i) S. A. Weicker, D. W. Stephan, Chem. Eur. J. 2015, 21, 13027-
3034; j) M. Wiesemann, B. Hoge, Chem. Eur. J. 2018, 24, 16457-
16471.
[2]
3]
L. Greb, Chem. Eur. J. 2018, 24, 17881-17896.
[
a) A. L. Liberman-Martin, R. G. Bergman, T. D. Tilley, J. Am. Chem.
Soc. 2015, 137, 5328-5331; b) R. Maskey, M. Schädler, C. Legler, L.
Greb, Angew. Chem. Int. Ed. 2018, 57, 1717-1720.
[4]
[5]
[6]
[7]
D. Hartmann, M. Schädler, L. Greb, Chem. Sci. 2019, 10, 7379-7388.
D. Hartmann, L. Greb, Angew. Chem. Int. Ed. 2020, 59, 22510-22513.
H. C. Brown, Y. Okamoto, J. Am. Chem. Soc. 1958, 80, 4979-4987.
A. Kütt, V. Movchun, T. Rodima, T. Dansauer, E. B. Rusanov, I. Leito, I.
Kaljurand, J. Koppel, V. Pihl, I. Koppel, G. Ovsjannikov, L. Toom, M.
Mishima, M. Medebielle, E. Lork, G.-V. Röschenthaler, I. A. Koppel, A.
A. Kolomeitsev, J. Org. Chem. 2008, 73, 2607-2620.
Conclusion
The present contribution serves the advancement of Lewis acids
in a two-fold sense. First, we introduce a neutral silicon-based
4
This article is protected by copyright. All rights reserved.