TY - JOUR
T1 - Oxidation of alkylarenes by nitrate catalyzed by polyoxophosphomolybdates
T2 - Synthetic applications and mechanistic insights
AU - Khenkin, Alexander M.
AU - Neumann, Ronny
PY - 2004/5/26
Y1 - 2004/5/26
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=2442697671&partnerID=8YFLogxK
U2 - 10.1021/ja031710i
DO - 10.1021/ja031710i
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C2 - 15149233
AN - SCOPUS:2442697671
SN - 0002-7863
VL - 126
SP - 6356
EP - 6362
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 20
ER -