Chromium Trioxide (CrO3)
Chromium trioxide is a strong oxidizing agent that is not soluble in most organic solvents and tends to explode in the presence of organic compounds and solvents. In water, it forms chromic acid and anhydrides, from which salts such as sodium dichromate (Na2Cr2O7) and pyridinium dichromate are commercially available.
Chromium trioxide is soluble in tert-butyl alcohol, pyridine (see Sarett Reagent, Collins Reagent) and acetic anhydride, although care must be taken to follow the given procedures, because these solutions tend to explode. A solution of chromium trioxide in aqueous sulfuric acid can be safely mixed with acetone (Jones Reagent). If this mixture is slowly added to an alcohol in acetone, oxidation products such as carbonyl compounds and carboxylic acids can be isolated in good yields (Jones Oxidation).
A full review of chromium-based reagents can be found in the book written by Tojo and Fernández (Oxidation of Alcohols to Aldehydes and Ketones, Springer Berlin, 2006, 1-97.).
Attention: Chromium (VI) compounds are toxic and must be handled with care.
Benzyl alcohols and benzyl TBDMS ethers were efficiently oxidized to the corresponding carbonyl compounds in high yield with periodic acid catalyzed by CrO3 at low temperature (-78 °C). The oxidation procedure was highly functional group tolerant and very selective for the TBDMS group over the TBDPS group.
S. Zhang, L. Xu, M. L. Trudell, Synthesis, 2005, 1757-1760.
A sequential PdCl2/CrO3-promoted Wacker process followed by an acid-mediated dehydration enables the synthesis of β-substituted and β,β-disubstituted α,β-unsaturated methyl ketones from homoallyl alcohols with a terminal double bond, whereas internal homoallyl alcohols delivered regioselectively nonconjugated unsaturated carbonyl compounds under the same protocol.
V. Bethi, R. A. Fernandes, J. Org. Chem., 2016, 81, 8577-8584.
A CrO3-catalyzed oxidation of primary alcohols to carboxylic acids proceeds smoothly with only 1-2 mol % of CrO3 and 2.5 equivalents of H5IO6 in wet MeCN to give the carboxylic acids in excellent yield. No significant racemization is observed for alcohols with adjacent chiral centers. Secondary alcohols are cleanly oxidized to ketones.
M. Zhao, J. Li, Z. Song, R. Desmond, D. M. Tschaen, E. J. J. Grabowski, P. J. Reider, Tetrahedron Lett., 1998, 39, 5323-5326.
Trihaloacetic acids can been converted to trichloromethyl and tribromomethyl ketones in good yield by a catalyzed reaction with aldehydes followed by oxidation. A coupling of organozinc intermediates with trichloroacetyl chloride gives trichloromethyl ketones.
E. J. Corey, J. O. Link, Y. Shao, Tetrahedron Lett., 1992, 33, 3435-3438.
A reaction between dialkyl acetylenedicarboxylates and β-aminoketones promoted by triphenylphosphine allows an efficient one-pot synthesis of polysubstituted 2,5-dihydropyrrole derivatives. The prepared 2,5-dihydropyrroles can be easily oxidized to the corresponding pyrrole derivatives with chromium trioxide.
M. Anary-Abbasinejad, E. Poorhassan, A. Hassanabadi, Synlett, 2009, 1929-1932.