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is supported by the observation of an absorption band at 295 nm
1
5
+
3+
that increases over time (see Fig. 2). RFTH
3
–2Sc can react
1
6
with dioxygen, regenerating 3 while producing H O (Fig. S8†).
In addition, RFTH –2Sc (3) may also be regenerated by the
direct reaction of RFTH2 –2Sc with O2 (step iv, Fig. S7†).
2
2
+
3+
2
ꢀ
+
3+
This process may conceivably be facilitated by Lewis acid
2
0
coordination.
We presume that the mechanism of the catalytic oxidation
of benzyl alcohols (Table S1 (ESI†), entries 5 and 6) is analogous
to the one previously suggested by Fukuzumi et al. for the
8
oxidation p-chlorobenzyl alcohol. The proposed catalytic
cycle involves an initial electron transfer from the substrate
1
3+
to ( RFT–2Sc )*, followed by proton transfer forming the
Scheme 3 Proposed mechanism for the photocatalytic aerobic oxidation
of ethylbenzene (1) to acetophenone (2) with RFT in presence of Sc -ions
and HCl.
ꢀ
hydroxybenzyl radical (p-R-C
RFT radical anion ( RFTH –2Sc )*. Subsequent H atom transfer
6
H
4
3+
CHOH ) and the protonated
3+
2
ꢀ
3
+
between these species yields the aldehyde and RFTH
2
–2Sc .
In summary, RFT/scandium triflate is an efficient photoca-
substrate and the photoexcited flavin metal complex ( RFTH –2Sc )* talytic system for the aerobic oxidation of alkylbenzenes and
in its singlet state (step i). This electron transfer produces the electron deficient benzyl alcohols. The results show that the
1
+
3+
8
ethylbenzene radical cation 4 and the protonated flavin radical well-known effect of Lewis acid coordination on the redox
2
ꢀ
3+
2
ꢀ
3+
8,9
complex RFTH –2Sc . It seems likely that the RFTH –2Sc
potential of flavins can be exploited to improve their photo-
–2Sc , while the catalytic properties. An extension of this principle, and an
strongly acidic ethylbenzene radical cation 4 is deprotonated to exploration of the effects of other metal ions including redox-
2
ꢀ
+
3+
complex is then protonated to yield RFTH
2
1
4 2
ꢀ
+
3+
the benzyl radical 5 (step ii).§ RFTH
to broad absorptions at lmax = 400–550 nm similar to those of
the uncoordinated dihydroflavin radical cation RFTH
2
–2Sc should give rise active ones, is hand.
We thank Dr Michael Sp o¨ rner and Helmut Sch u¨ ller for
2
ꢀ
+ 15
2
.
Such assistance with ESR measurements and Prof. Burkhard K o¨ nig
a broad band is indeed observed in the UV-vis spectrum of for stimulating discussions. Support by the DFG Graduate
the reaction mixture under argon (Fig. 2). In addition, the ESR Program ‘‘Chemical Photocatalysis’’ (GRK 1626) is gratefully
spectrum of the reaction mixture of 1, RFT, Sc(OTf)
obtained while irradiating at 440 nm exhibits a signal at g = 2.0033
Fig. 2, inset), which is in line with the expected spectrum for
3
and HClO
4
acknowledged.
(
Notes and references
ꢀ
2
–2Sc . The presence of the scandium(III) ions appears
+
3+ 8
RFTH
‡
No structural information is presently available on the scandium(III)
to have only a slight effect on the shape of the ESR spectrum. The
hyperfine coupling constants obtained by computer simulation
complexes in solution, though it seems likely that these correspond to
neutral or cationic complexes or clusters with the general composition
ꢀ
+ 15
m+
are similar to the values reported for free RFTH2
.
The hyper- [Sc X (RFTH ) ] (X = OTf or Cl). We choose to designate the species
x
y
n z
m+
3+
involved in the catalytic mechanism as RFTH
§ The pK
the pK
n
–2Sc for simplicity.
ꢀ
radical is approximately 2, while
fine coupling constants obtained by computer simulation are
+
2
ꢀ
+ 16
a 2
of the closely-related RFTH
similar to those reported for free RFTH
2
.
The ESR spectrum
17
a
of a toluene radical cation in MeCN is estimated to À12 to À13.
of a mixture of 1, RFT, Sc(OTf) and HCl (instead of HClO ,
3
4
1
(a) S. Ghisla and V. Massey, Eur. J. Biochem., 1989, 181, 1–17;
b) Flavins: Photochemistry and Photobiology, ed. E. Silva and
A. M. Edwards, Royal Society of Chemistry, Cambridge, 2006, vol. 6.
Fig. S6†) is more complicated and thus defied a satisfactory
(
simulation so far. This is presumably due to the formation of
ꢀ
+
3+
ꢀ
3+
an equilibrium between RFTH
the weaker acid HCl.
2
–2Sc and RFTH –2Sc with
2 (a) H. Schmaderer, P. Hilgers, R. Lechner and B. K o¨ nig, Adv. Synth.
Catal., 2009, 351, 163–174; (b) J. Svoboda, H. Schmaderer and
B. K o¨ nig, Chem. – Eur. J., 2008, 14, 1854–1865; (c) R. Cibulka, R. Vasold
and B. K o¨ nig, Chem. – Eur. J., 2004, 10, 6223–6231; (d) Chemical Photo-
catalysis, ed. B. K ¨o nig, S. K u¨ mmel, R. Cibulka and B. K o¨ nig, de Gruyter,
Berlin, 2013, pp. 44–61.
There are at least two conceivable pathways that connect the
benzyl radical 5 with the final product 2 (Scheme 3). One
2
ꢀ
+
3+
possibility is that RFTH
2
–2Sc recombines with 5 to form
3
4
R. Lechner and B. K o¨ nig, Synthesis, 2010, 1712–1718.
J. Dad’ov ´a , E. Svobodov ´a , M. Sikorski, B. K o¨ nig and R. Cibulka,
ChemCatChem, 2012, 4, 620–623.
a covalent RFT-benzyl radical adduct (not shown in Scheme 3),
which rapidly collapses under irradiation in air to product 2
+
3+
19
5 U. Megerle, M. Wenninger, R.-J. Kutta, R. Lechner, B. K o¨ nig, B. Dick
and RFTH –2Sc (3). However, this pathway seems less likely
based on the UV-vis spectra of the reaction mixture, where
characteristic broad absorptions are expected for such an
adduct at lmax = 600–630 nm. An alternative pathway is the
conversion of 5 into the benzylperoxyl radical 6, which subse-
and E. Riedle, Phys. Chem. Chem. Phys., 2011, 13, 8869.
6
7
(a) J. Rosenthal, T. D. Luckett, J. M. Hodgkiss and D. G. Nocera, J. Am.
Chem. Soc., 2006, 128, 6546–6547; (b) K. Ohkubo and S. Fukuzumi,
Org. Lett., 2000, 2, 3647–3650; (c) K. Ohkubo, K. Suga, K. Morikawa
and S. Fukuzumi, J. Am. Chem. Soc., 2003, 125, 12850–12859.
R. Lechner, S. K u¨ mmel and B. K ¨o nig, Photochem. Photobiol. Sci., 2010,
9, 1367.
1
8
quently transforms into 2 via the benzyl hydroperoxide. As
2
ꢀ
+
2
ꢀ
+
3+
8 S. Fukuzumi, K. Yasui, T. Suenobu, K. Ohkubo, M. Fujitsuka and
observed for RFTH
2
, RFTH
2
–2Sc may disproportionate
O. Ito, J. Phys. Chem. A, 2001, 105, 10501–10510.
S. Fukuzumi, S. Kuroda and T. Tanaka, J. Am. Chem. Soc., 1985,
107, 3020.
+
3+
into oxidized RFTH –2Sc and the reduced dihydroflavin
9
+
3+
15
3
RFTH –2Sc (step iii). The formation of the latter species
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