Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates

Synthetic applications and mechanistic insights

Alexander M. Khenkin, Ronny Neumann

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H3+xPVxMo12-xO40 (x = 0-2). H 5PV2Mo10O40 was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant. The conversion to the aldehyde/ketone could be increased by the addition of water to the reaction mixture. As evidenced by IR and 15N NMR spectroscopy, initially the nitrate salt reacted with H5PV2Mo10O 40 to yield a NVO2 +[H 4PV2Mo10O40] intermediate. In an electron-transfer reaction, the proposed NVO2 +[H4PV2Mo10O40] complex reacts with the alkylarene substrate to yield a radical-cation-based donor-acceptor intermediate, NIVO2[H4PV 2Mo10O40]-ArCH2R+.. Concurrent proton transfer yields an alkylarene radical, ArCHR., and NO2. Alternatively, it is possible that the NVO 2 +[H4PV2Mo10O 40] complex abstracts a hydrogen atom from alkylarene substrate to directly yield ArCHR. and NO2. The electron transfer-proton transfer and hydrogen abstraction scenarios are supported by the correlation of the reaction rate with the ionization potential and the bond dissociation energy at the benzylic positions of the alkylarene, respectively, the high kinetic isotope effect determined for substrates deuterated at the benzylic position, and the reaction order in the catalyst. Product selectivity in the oxidation of phenylcyclopropane tends to support the electron transfer-proton transfer pathway. The ArCHR. and NO2 radical species undergo heterocoupling to yield a benzylic nitrite, which undergoes hydrolysis or acetolysis and subsequent reactions to yield benzylic acetates and corresponding aldehydes or ketones as final products.

Original languageEnglish
Pages (from-to)6356-6362
Number of pages7
JournalJournal of the American Chemical Society
Volume126
Issue number20
DOIs
Publication statusPublished - May 26 2004

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Proton transfer
Nitrates
Protons
Electrons
Ketones
Aldehydes
Oxidation
Hydrogen
Acetates
Substrates
Salts
Nitric Acid
Molybdenum
Ionization potential
Catalyst selectivity
Nitric acid
Nitrites
Carboxylic Acids
Carboxylic acids
Oxidants

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates : Synthetic applications and mechanistic insights. / Khenkin, Alexander M.; Neumann, Ronny.

In: Journal of the American Chemical Society, Vol. 126, No. 20, 26.05.2004, p. 6356-6362.

Research output: Contribution to journalArticle

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abstract = "Alkylarenes were catalytically and selectively oxidized to the corresponding benzylic acetates and carbonyl products by nitrate salts in acetic acid in the presence of Keggin type molybdenum-based heteropolyacids, H3+xPVxMo12-xO40 (x = 0-2). H 5PV2Mo10O40 was especially effective. For methylarenes there was no over-oxidation to the carboxylic acid contrary to what was observed for nitric acid as oxidant. The conversion to the aldehyde/ketone could be increased by the addition of water to the reaction mixture. As evidenced by IR and 15N NMR spectroscopy, initially the nitrate salt reacted with H5PV2Mo10O 40 to yield a NVO2 +[H 4PV2Mo10O40] intermediate. In an electron-transfer reaction, the proposed NVO2 +[H4PV2Mo10O40] complex reacts with the alkylarene substrate to yield a radical-cation-based donor-acceptor intermediate, NIVO2[H4PV 2Mo10O40]-ArCH2R+.. Concurrent proton transfer yields an alkylarene radical, ArCHR., and NO2. Alternatively, it is possible that the NVO 2 +[H4PV2Mo10O 40] complex abstracts a hydrogen atom from alkylarene substrate to directly yield ArCHR. and NO2. The electron transfer-proton transfer and hydrogen abstraction scenarios are supported by the correlation of the reaction rate with the ionization potential and the bond dissociation energy at the benzylic positions of the alkylarene, respectively, the high kinetic isotope effect determined for substrates deuterated at the benzylic position, and the reaction order in the catalyst. Product selectivity in the oxidation of phenylcyclopropane tends to support the electron transfer-proton transfer pathway. The ArCHR. and NO2 radical species undergo heterocoupling to yield a benzylic nitrite, which undergoes hydrolysis or acetolysis and subsequent reactions to yield benzylic acetates and corresponding aldehydes or ketones as final products.",
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