Oxidation benzylic position, chromium

CrOs. Chromium oxide can also oxidize benzylic positions., as in the Etard reaction (see sec. 3.8.E). When 391 was treated with chromium trioxide in acetic acid, an 85% yield of 392 was obtained, as part of Pan s synthesis of (-)-6,7-dehydrofeiruginyl methyl ether. l... 

Chromium is useful as an oxidation catalyst, especially with t-BuOOH or O2 as the oxidant. When a Cr precursor [e.g., a Cr(VI) compound] is used, alcohols can be oxidized to ketones with t-BuOOH. Moreover, CH2 groups with relatively weak C-H bonds, for instance, in allylic or benzylic positions, are easily converted to carbonyl groups in the presence of Cr and r-BuOOH or 62. Often these reactions are free radical autoxidations, in which alkylhydroperoxide and alcohol products react further to form ketones (2, 4, 6). Relatively little is known about the active Cr species in these reactions, but it is plausible that high valent, neutral Cr compounds such as alkylchromates(VI) are involved. 

Chromium(VI) oxide is also an efficient catalyst for oxidation at the benzylic position with periodic acid as the terminal oxidant in acetonitrile. Substituted toluenes with an electron-withdrawing group at the 4- or 3-position and diarylmethanes such as Ph2CH2 and fiuorene are oxidized to the respective substituted benzoic acids and ketones in excellent yields [1332], Periodic acid in the presence of catalytic CrOj can be used for the oxidation of arenes, such as naphthalenes and anthracene, to the corresponding quinones for example,... 

NFSi as an Oxidizing Reagent. It has been demonstrated that the benzylic position of arene chromium tricarbonyl complex could be deprotonated by strong bases and the resulting anion... 

Some further examples of stereoselective deprotonation/alkylation reactions of tricarbonyl-chromium complexed (V-methyl tetrahydroisoquinolines have been reported27. Starting with the enantiomerically pure (35)-methyl tetrahydroisoquinoline reaction with hexacarbonyl-chromium led to a mixture of endo- (40%) and exo- (60%) complexes, which were deprotonated with butyllithium and subsequently methylated with iodomethane. In this way methylation occurred firstly at the 4- and secondly at the 1-position. In all cases, the methyl group entered anti to the chromium complexed face. After separation of the alkylated complexes by chromatography and oxidative decomplexation, the enantiomerically pure diastereomers (—)-(l 5,35,47 )-and ( + )-(17 ,35,45)-1,2,3,4-tetrahydro-l,2,3,4-tetramethylisoquinolme were obtained, benzylic amines such as tetrahydroisoquinoline to 2-amino-4,5-dihydrooxazoles. Deprotona... 

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