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B. Orlinska / Tetrahedron Letters 51 (2010) 4100–4102
4102
including oxidation of alkyl radicals.7 The redox potential can also
be influenced by ligand type, which can explain the different ob-
served behaviours of the various salts.6
pyl-aromatics, only oxidation of 1,3,5-triisopropylbenzene in the
presence of Cu(OAc)2/NHPI was described and this resulted in a
complex mixture.20
The influence of temperature on cumene oxidation in the pres-
ence of NHPI and the aforementioned metal salts was investigated
in the temperature range 50–90 °C. At 50 °C, long induction periods
were observed (approximately 40 min). When the reaction tem-
perature was increased from 60 to 90 °C, cumene conversion de-
creased. At 90 °C, oxidation processes were quickly inhibited.
This observed quenching of cumene conversion is most likely
caused by thermal decomposition of NHPI,4 as well as by an in-
creased number of reactions that inhibit the process. It is assumed
that phenol, a free radical process inhibitor, can be formed by the
acid rearrangement of cumene hydroperoxide. These acids (e.g.,
formic acid) can be obtained from the oxidation of the methyl rad-
ical formed during b-scission of the 1-methyl-1-phenylethyloxy
radical into acetophenone. Therefore, acetophenone formation
can be connected to acid formation and process inhibition. It has
been established that a rise in temperature leads to increased b-
scission of alkoxyl radicals to the ketone.23–25 In our studies this
tendency was confirmed. For example, the ketone to alcohol ratio
in cumene oxidation products obtained in the presence of NHPI/
CuCl2ꢀ2H2O reached values of 0.6 and 2.2 at 50 °C and 90 °C,
respectively.
The results presented herein for cumene oxidation could be
used to develop a method for alcohol or ketone synthesis from
other isopropyl-aromatics. It has been demonstrated that the use
of NHPI in combination with copper salts at 60 °C results in high
alcohol amount whereas ketone is the major product at 90 °C.
Unfortunately, it was observed that temperature increases beyond
a certain point decrease the cumene conversion, which might limit
the direct synthesis of ketones from isopropyl-aromatics.
We next intend to investigate the oxidations of 2-isopropyl-
naphthalene, p-isopropylanisole, 2,6-diisopropylnaphthalene,
4,40-diisopropylbiphenyl and 2-methoxy-6-isopropylnaphthalene
in the presence of an NHPI/copper salt catalyst. Alcohols and ke-
tones obtained by this method can be used in the production of
valuable products (fragrances, e.g., 2-acetylnaphthalene,26 phar-
maceuticals, e.g., 4-methoxyacetophenone,27 polyesters and liquid
crystals21).
Acknowledgment
Financial help from the Ministry of Science and Higher Educa-
tion of Poland (Grant No. Nr N N209 153836) is gratefully
acknowledged.
References and notes
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This Letter reports a catalytic system composed of a copper salt
and NHPI which has been rarely used previously. Among isopro-