Chromium Compounds
Chromium (VI) compounds are highly toxic for both acute and chronic exposures and can cause cancer, but chromium (III) is an essential element in human health and its toxicity is moderate. Care must be taken when handling chromium (VI) reagents (for example PDC) in finely powdered form, because such compounds have extremely high toxicity from inhalation and oral exposure.
If chromium compounds are used in excess, typically for the oxidation of alcohols to carbonyl compounds and carboxylic acids, any remaining chromium (VI) species can be quenched by the addition of 2-propanol. A color change from a typical orange to deep green indicates the complete quenching.
See also: Chromium Trioxide, Collins Reagent, Jones Reagent, Sarett Reagent, Pyridinium Chlorochromate, Pyridinium Dichromate
A list of alternative and somewhat more environmentally friendly oxidants for the oxidation of alcohols is given here for the preparations of aldehydes, ketones and carboxylic acids.
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.).
Recent Literature
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.
S. Zhang, L. Xu, M. L. Trudell, Synthesis, 2005,
1757-1760.
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.
A facile and quantitative preparation of carboxylic acids by a
pyridinium chlorochromate (PCC) catalyzed (2 mol%) oxidation of primary
alcohols and aldehydes using 2.2 equivalents and 1.1 equivalents of H5IO6,
respectively, in acetonitrile is described here.
M. Hunsen,
Synthesis, 2005, 2487-2490.
M. Hunsen,
Synthesis, 2005, 2487-2490.
A domino oxidation of primary alcohols gives α,β-unsaturated compounds using the
combination of PCC-NaOAc and stabilized Wittig reagents.
J. Shet, V. Desai, S. Tilve, Synthesis,
2004,
1859-1863.
A rapid and efficient oxidative deamination of various α-aminophosphonates
allows the synthesis of α-ketophosphonates using ZnCr2O7 •
3 H2O under solvent-free conditions at room temperature. This method
is also applicable to the rapid and highly selective oxidation of various amines
to aldehydes and ketones in very good yields.
S. Sobhani, M. F. Maleki, Synlett, 2010, 382-386.
