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acetophenones in good yields when HClO4 (30 mol%) is
added to the reaction mixture (entry 7). Toluene derivatives
are first rapidly converted into aldehydes and subsequently
into the corresponding carboxylic acids (entry 8) through an
iron-catalyzed aerobic oxidation process (Chapter S4).
Benzyl alcohols with strongly electron-withdrawing para
substituents (R = NO2, CO2Me, CF3) are also converted into
the corresponding aldehydes with high efficiency (entries 9
and 10). For synthetic applications, an efficient separation of
the catalysts from the product is highly desirable. As shown by
UV/Vis spectroscopy, both catalyst components can be easily
removed by adsorption on basic alumina, whereas < 1% of
the product are lost in this operation (Chapter S4). The
oxygenation of 1 does not proceed in the dark, in the absence
of RFT, or under an N2 atmosphere (Chapter S2). The
presence of water is crucial for a high reaction rate
(Chapter S4). Product yield and substrate conversion are
unaffected when the reaction is carried out in an atmosphere
of pure dioxygen, and the yield of 2 was 60% in CD3CN/D2O
(1:1, v/v; Chapters S2 and S4). A singlet oxygen pathway thus
appears to be unlikely.[21]
Whereas RFT/4 efficiently catalyzes the photooxygena-
tion of alkyl benzenes, RFTalone shows only modest catalytic
activity (Chapter S2). Insights into the striking effect of the
iron cocatalyst were gained from spectroscopic investigations
and monitoring the reaction progress. Fluorescence emission
quenching experiments revealed enhanced quenching of the
excited singlet state, 1RFT*, in the presence of various metal
complexes or metal salts (Chapter S2). However, there seems
to be no correlation between the magnitude of the Stern–
Volmer constants and the catalyst activity, which indicates
that the observed emission quenching is probably unproduc-
tive. An ESI-MS spectrum of RFT/4 (Chapter S4) showed
signals corresponding to [Fe(TPA)(MeCN)]2+ (m/z 193.5)
and RFTH+ (m/z 545.2). Additional signals were apparent
at m/z 445.1, 889.5, and 989.5, which may indicate the
formation of an adduct between RFT and 4 (Chapter S4).
Nonetheless, the UV/Vis and IR spectra of the catalyst
mixture correspond to a superposition of the individual
spectra of 4 and RFT, which suggests that the partial complex
formation observed by ESI-MS, if at all present at the lower
catalytic concentrations, does not affect the ground state of
the photocatalyst.
Figure 2. a) Reaction–time profiles of the photooxygenation of
&
1 (0.02 mmol) to 2 and 3 catalyzed by RFT/MnO2 ( , black) or RFT/4
*
( , red) with RFT (8 mol%) and MnO2 (20 mg) or 4 (0.5 mol%); the
combined yield is defined as the sum of the molar yields of 2 and 3
divided by the molar amount of converted starting material 1; the
relative concentration of RFT was determined by the change in the
absorbance A at lmax =443 nm (inset). b) Reaction–time profile of the
photooxygenation of 1 (0.02 mmol) to 2 and 3 catalyzed by RFT/4 in
~
the absence of additional H2O2 ( , blue) and with addition of 1 equiv
&
H2O2 ( , black) before starting to irradiate at 440 nm; catalyst
loadings: RFT (8 mol%), 4 (8 mol%); the inset shows a magnification
of the reaction–time profile; lines are visual guides only.
Whereas a direct interaction between RFT and the
cocatalyst thus does not seem to be responsible for the
improved catalytic activity, metal-catalyzed H2O2 dispropor-
tionation is important. The rapid photobleaching of RFT
observed in the absence of a cocatalyst is effectively
diminished by the addition of complex 4 (Chapter S4).
Compared to using RTF alone, the yields of 2 (30%) and 3
(12%) improved to 60% and 8%, respectively, with man-
ganese dioxide as the cocatalyst instead of 4 (Chapters S2 and
S4). MnO2 has no oxygenation activity; therefore, this
increase must be attributed to its activity as an H2O2
disproportionation catalyst. The efficiencies of the RFT/4
and RFT/MnO2 combinations can be directly compared when
the concentrations of the cocatalysts are adjusted such that
the bleaching rates of RFT are the same (Figure 2a). Even
when only 0.5 mol% of 4 are used, the combined yield of 2
and 3 is still substantially higher for RFT/4 than for RFT/
MnO2. RFT/MnO2 also performed significantly worse than
RFT/4 in the oxygenation of related alkyl benzenes (Chap-
ter S2). Assuming that MnO2 and 4 are efficient H2O2
disproportionation catalysts under these conditions, it seems
likely that the higher yields of 2 obtained with RFT/4 are due
to the additional oxygenation activity of the iron complex
(Scheme 1, step ii).
An additional monitoring study further supports the
relevance of the oxygenation activity of the iron complex
(Figure 2b). In the presence of RFT and 4, the formation of 2
and 3 is initially slow (< 2% yield after two minutes).
Subsequently, the reaction rate starts to increase rapidly. By
contrast, a high initial rate was observed when H2O2 (1 equiv)
was added immediately before starting the irradiation. In this
Angew. Chem. Int. Ed. 2016, 55, 427 –430
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