Chemistry - A European Journal
10.1002/chem.202004712
COMMUNICATION
evoked by binding of QCl to 2 outperforms the anodic shift of
~
(
1.0 V for p-quinones caused by Pd-cages encapsulation
Figure 1C),[ or of ~0.6 V with Q coordinated to dicationic
1]
Cl
hydrogen-bond donors (Figure 1D),[2] and illustrates the
effectiveness of Lewis superacid-binding strategy for redox
amplification.
EPR-spectroscopic monitoring of reactions with substrates of
known redox potentials served to bracket the potential of 1.[
Indeed, successful oxidations were observed by mixing 1 with
13]
+
thianthrene (0.84 V vs. Fc/Fc ) and tris(4-bromophenyl)amine
+
(
0.72 V vs. Fc/Fc , “magic blue”) in CH
2
Cl
2
. An upper bound was
encompassed by the reaction with tris(2,4-dibromo-
+
phenyl)amine (1.12 V vs. Fc/Fc ). Although the formation of the
corresponding aminium radical cation could not be detected by
EPR spectroscopy, substantial line broadening of the 1H-NMR
aromatic signals of the phenylamine indicated electron transfer
processes occurring. Accordingly, the experimental findings
ranged the redox potential of 1 between tris(2,4-dibromo-
+
phenyl)amine and thianthrene (0.86 – 1.12 V vs. Fc/Fc ), in line
with the electroanalytical results. Next, the oxidation of alkyl-
substituted benzenes was followed by EPR spectroscopy. Given
the fleeting nature of those radical cations, their isolation was
not attempted.[ Formation of the corresponding radical cations
was observed for hexamethylbenzene, pentamethylbenzene,
and 1,2,3,4-tetramethylbenzene by the appearance of
characteristic hyperfine coupling patterns (Figures S31/S32).
With mesitylene (1,3,5-trimethylbenzene), no reaction occurred.
Since the oxidation potentials of alkyl-benzenes are poorly
Figure 3. a) Redox-catalytic transformation of 4 into 5, either with 5 mol% of 1
14]
and QCl (+ 30 min premixing). b)
or with in situ formed 1 from 5 mol% SiI
4
Dehydrogenative coupling of 6 by redox catalyst 1.
To probe the generality of 1 to act as redox-catalyst, other
reactions were tested. Methoxy substituted terphenyl
6
underwent quantitative dehydrogenative coupling with 10 mol%
of 1 after 24h at r.t. (Figure 3b). Moreover, dihydroanthracene
was oxidized to anthracene, but the product reacted faster than
the starting material, resulting in secondary oxidation products
documented, they were determined in CH
2 2
Cl with 0.1 M
NBu PF as electrolyte (Table S1). Remarkably, these values
4
6
were exceeding the oxidative power of 1. We explain this
observation by the possibility of 1 to engage in proton-coupled
electron transfer and the rapid follow chemistry of the oxidation
products. Interestingly, the π-π-stacked complexes observed
between 1 and less strong electron donors, such as benzene,
(
Figure S47).
In conclusion, we describe the redox chemistry of silicon
trisdioxolenes and complete its redox-series. In doing so, we
establish the strategy of Lewis acid-induced redox amplification
of ortho-quinones. Cyclic voltammetry and guiding redox
can be considered as precursor complexes for the herein
observed electron transfer.[4]
+
reactions disclose a potential of E1/2 ≈ 1.0 V vs. Fc/Fc ,
After investigations on the redox chemistry of 1 applied in
stoichiometric fashion, we were keen to know whether 1 holds
the potential to act as a redox catalyst. As a model reaction, an
+
rendering 1 as strong an oxidant as "magic blue" or NO .
However, in contrast to such oxidants that might possess
limitations due to their charged nature, non-innocence, or a
tedious synthesis, 1 stands out as a neutral oxidant that is
obtained by the simple combination of the two commercially
intramolecular oxidative lactonization was chosen (Figure 3A).[
2]
Quantitative conversion of 2-(4-methoxybenzyl)benzoic acid 4
into the corresponding lactone 5 was achieved with a catalyst
loading of 5 mol% of 1 and 1.3 eq. of QCl at r.t. in less than 2h.
Thus, 1 shows improved efficiency than the hydrogen-bond
donor system (Figure 1D), well in line with the more positive
Cl
available compounds SiI4 and Q . Proof-of-concept reactions
exemplify 1 as an efficient redox catalyst, that can be generated
in situ. We foresee the combination of QCl and SiI
and, more
4
Cl [2]
redox-shifting effect for Q . The mechanistic proposal consists
generally, the concept of Lewis acid-binding induced redox
amplification as a powerful tool for challenging transformations
and catalysis, wherever high oxidation potentials are needed.
+
-
of a net 2H /2e delivery from the substrate to 1, with the formed
C
l
C
l
o-tetrachlorocatechol H
2
C
a
t
b
e
i
n
g
r
e
p
l
a
c
e
d
b
y
a
n
o
t
h
e
r
Q
,
redelivering 1.[2] This assumption finds support by the respective
Cl
signals of 1 eq of H
2
Cat in the NMR spectra (Figure S43).
Given the straightforward synthesis of 1 by the combination of Acknowledgements
Cl
SiI
4
with Q , we attempted to form the catalyst 1 in situ.
Remarkably, transformation of 4 to 5 succeeded in similar
We gratefully thank Prof. H.-J. Himmel for his support, the FCI,
the DFG (GR5007/2-1) and the HeiKa Research Bridge for
Cl
efficiency by premixing 5 mol% of SiI
4
and 1.3 eq. of Q before
adding the substrate (Figure 3a). It is by no means required to
pre-isolate 1.
financial
support
and
the
BWFor/BWUniCluster
for
computational resources.
Keywords: electron transfer • main group elements • oxidation •
quinones • redox catalysis
3
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