oxidation with a combination of HBr and H2O2 has also
been reported,14 it is impractical to use only a limited
number of substrates because this leads to the noticeably
low yield and low selectivity of the desired products. We
recently found that the activated bromonium-like species
obtained by the oxidative umpolung of alkali metal bro-
mides with Oxone could efficiently promote the bromo-
amination of N-alkenyl sulfonamides without producing
the corresponding organic waste.15 We report here a heavy
metal-free direct and selective benzylic oxidation of alky-
larenes via CꢀH abstraction using an alkali metal bro-
mide/oxidant system, focused toward meeting the goals of
green chemistry (Scheme 1). This oxidation efficiently
Scheme 2. Direct Benzylic Oxidation of Alkylarenes 1a and 1b
Scheme 3. Thermal Direct Benzylic Oxidation of Alkylarenes 1
(Method A)
Scheme 1. Direct Benzylic Oxidation by Generation of Bromo-
radical from Metal Bromides
proceeded through different reaction pathways under
thermal conditions in polar solvent and photochemical
conditions in halogenated solvents.
First, we screened a series of bromo reagents, oxidants,
and solvent for direct benzilic oxidation of alkylarenes
(1a and 1b) (Table S1 in the Supporting Information).
The optimum reactions were found to involve 1a, KBr
(0.5 equiv), and Oxone (3.0 equiv) in MeNO2 at 50 °C
(Method A), and 1b, KBr (1.2 equiv), and Oxone (3.0 equiv)
in a mixture of CH2Cl2 and H2O at room temperature
under visible light irradiation (Method B) (Scheme 2).
To explore the scope of the direct benzylic oxidation,
various alkylarenes 1 were examined using alkali metal bro-
mides under optimized Method A conditions (Scheme 3).
a Reaction was carried out with KBr (1.2 equiv). bReaction was
carried out at room temperature. cReaction was carried out at 40 °C.
When p-substituted alkylarenes bearing Br (1c), t-Bu (1d),
and AcO (1e) were used, oxidative products 2c, 2d, and 2e
were obtained in good yields (83ꢀ>99%). The reaction
of n-butylbenzene (1f), n-octylbenzene (1g), and isobu-
tylbenzene (1h) gave corresponding ketones 2f, 2g, and 2h
in 86%, 87%, and 90% yields, respectively. Diaryl-
methanes (1i and 1j) were also efficiently converted into
diarylketones (2i and 2j) in quantitative yields, respec-
tively. Furthermore, various alkylarenes bearing func-
tional groups, such as ester (1k and 1l), sulfonyl (1m),
nitro (1n), nitrile (1o), and imide (1p and 1q), provided
desired products 2kꢀ2q in high yields (75ꢀ99%),
respectively.
To our surprise, photooxidation (Method B) was much
more effective than thermal oxidation (Method A) for the
direct benzylic oxidation of alkylarenes 1 bearing an
electron-withdrawing group (Table S1 in the Supporting
Information). Next, we investigated the photochemical
direct benzylic oxidation of alkylarenes using alkali metal
bromides under the optimized Method B conditions
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