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Synthesis of ketones by oxidation of benzylic substrates
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Cost-effective and widely applicable protocols for controlled and predictably
selective oxidation of methyl-/alkylarenes to corresponding value-added
carbonyls have been developed, using a surfactant-based oxodiperoxo molybdenum
catalyst in water and hydrogen peroxide (H2O2) as an
environmentally benign green oxidant without any external base, additive, or
cocatalyst.
P. Thiruvengetam, D. K. Chand, J. Org. Chem., 2022, 87,
4061-4077.
The presence of KBr enabled a direct benzylic oxidation of alkylarenes via C-H
bond abstraction using oxone as oxidant under mild conditions. This reaction
proceeded with excellent selectivity by thermal oxidation or photooxidation to
provide a broad range of aryl ketones in high yields.
K. Moriyama, M. Takemura, H. Togo, Org. Lett., 2012,
14, 2414-2417.
The use of NaClO/TEMPO/Co(OAc)2 enabled a benzylic oxidation of alkyl
arenes to yield various aromatic aldehydes and ketones in very good yields. The
reaction reactivity, selectivity, and scope of the reaction were investigated.
C. Jin, L. Zhang, W. Su, Synlett, 2011,
1435-1438.
Ru complexes with tridentate hybrid ligands showed a very strong catalytic
activity in water at room temperature for benzylic C-H oxidation. Whereas NHCs
with a stronger donor ability stabilize the Ru center; nitrogen ligands with a
relatively weaker donor ability release from the Ru center, so that they induce
a reaction.
C.-B. Bo, Q. Bu, X. Li, G. Ma, D. Wei, C. Guo, B. Dai, N. Liu, J. Org. Chem., 2020, 85,
4324-4334.
The cationic complex [(pymox-Me2)RuCl2]+BF4-
is a highly effective catalyst for the C-H bond oxidation of aryl alkanes in
water using tert-butyl
hydroperoxide as oxidant to yield various aryl ketones at room temperature in
water as solvent. A solvent-caged oxygen rebounding mechanism via a Ru(IV)-oxo
intermediate species is suggested.
C. S. Yi, K.-H. Kwon, D. W. Lee, Org. Lett., 2009,
11, 1567-1569.
o-Iodoxybenzoic acid (IBX) was found to be highly effective in oxidations
adjacent to carbonyl and benzylic functionalities to form either α,β-unsaturated carbonyl compounds
or conjugated aromatic carbonyl systems. Fine-tuning of the
reaction conditions allowed remarkably selective transformations within
multifunctional substrates.
K. C. Nicolaou, T. Montagnon, P. S. Baran, Y.-L. Zhong, J. Am. Chem. Soc., 2002, 124, 2245-2258.
Permanganate supported on active manganese dioxide can
be used effectively for the oxidation of
arenes, alcohols and sulfides under heterogeneous
or solvent-free conditions.
A. Shaabania, P. Mirzaeia, S. Naderia, D. G. Leeb, Tetrahedron, 2004, 60, 11415-11420.
The bismuth and picolinic acid-catalyzed oxidation of alkyl arenes with tert-butyl hydroperoxide in pyridine and acetic acid gave benzylic
ketones in good yields. Alternatively, oxidation of methyl
arenes gave the corresponding substituted benzoic acids. A radical mechanism is
discussed.
Y. Bonvin, E. Callens, I. Larrosa, D. A. Henderson, J. Oldham, A. J. Burton,
A. G. M. Barrett, Org. Lett.,
2005, 7, 4549-4552.
Autoxidative sp3 C-H transformation of diarylmethanes in the presence
of oxygen mediated by t-BuONa provides diaryl ketones in very good yields
under mild reaction conditions, without transition metal catalysts or additional
chemical oxidants.
J.-S. Li, F. Yang, Q. Yang, Z.-W. Li, Z.-Q. Chen, Y.-D. Da, P.-M. Huang, C.
Chen, Y. Zhang, L-Z. Huang,
Synlett, 2017, 28, 994-998.
CrO3 is an efficient catalyst for benzylic oxidation with periodic
acid as the terminal oxidant in acetonitrile. Substituted electron-poor toluenes
and diarylmethanes were oxidized to the corresponding substituted benzoic acids
and ketones in excellent yields. Benzyl ethers such as isochroman and phthalan
were converted to 3,4-dihydroisocoumarin and phthalide in quantitative yields.
S. Yamazaki,
Org. Lett., 1999, 1, 2129-2132.
Aryl radicals generated in situ from arene diazonium fluoroborates promoted by
ascorbic acid enable a convenient and general oxidative arylation of vinyl
arenes in air at room temperature in the absence of any additive and visible
light irradiation. Various 2-aryl acetophenones have been obtained in good
yields.
B. Majhi, D. Kundu, B. C. Ranu, J. Org. Chem.,
2015,
80, 7739-7745.
A metal-free and green catalytic system enables an oxyfluorination of olefins
for the synthesis of α-fluoroketones which is an important building block for
organic synthesis. Moreover, this reaction system exhibits great functional
group tolerance.
Q. Yang, L.-L. Mao, B. Yang, S.-D. Yang, Org. Lett., 2014,
16, 3460-3463.
Visible-light-induced photocatalysis enables a highly efficient decarboxylative
oxidation of carboxylic acids with molecular oxygen as a green oxidant and
copper as a co-catalyst. This reaction worked smoothly on various type of acids,
and could potentially be used in modifications of natural products.
M. K. Zaman, S. N. Khan, Y. Cai, Z. Sun, Synlett, 2023,
34,
2029-2033.
Related
Reactivity at the homobenzylic position can be accomplished using dual organic
photoredox/cobalt catalysis. Through a two-part catalytic system, alkyl arenes
undergo dehydrogenation followed by an anti-Markovnikov Wacker-type oxidation to
provide benzyl ketone products. This formal homobenzylic oxidation is
accomplished with high atom economy without the use of directing groups.
J. B. McManus, J. D. Griffin, A. R. White, D. A. Nicewicz, J. Am. Chem. Soc.,
2020, 142, 10325-10330.