Best Synthetic Methods: Oxidation
Efficient new methods for oxidation are always welcome. The Dess-Martin periodinane has become the workhorse for alcohol to aldehyde or ketone conversion in organic research labs around the world. Viktor V. Zhdankin of the University of Minnesota, Duluth has described (Chem. Commun. 2004, 106. ) a complementary family of reagents. Oxidation of an ester 1 of o-iodobenzoic acid with NaOCl delivers 2. Depending on the ester, the reagent 2 is soluble and an effective oxidant, with KBr catalysis, in a wide range of organic solvents. Presumably, the spent oxidant can be recovered and recycled.
Chromium-based oxidants, noteworthy for their specificity and ease of use, continue to be popular. Enayatollah Mottaghinejad of Azad University of Iran, Tehran has found (Tetrahedron Lett. 2004, 45, 8823. ) that barium dichromate, easily prepared, smoothly oxidizes alcohols to aldehydes and ketones in refluxing acetonitrile.
A recent report (J. Org. Chem. 2004, 69, 8510. ) by Paul G. Williard of Brown University and Ruggero Curci of UniversitÓ di Bara of the oxidation of 5 to 6 serves as a timely reminder that the widely-used epoxidation reagent dimethyl dioxirane is also useful for the oxidation of secondary alcohols to ketones.
α,β-Unsaturated aldehydes can also be prepared by the net oxidation of allylic halides. Paul B. Jones of Wake Forest University has put forward (Org. Lett. 2004, 6, 3767. ) an elegant new method for effecting this transformation, based on nucleophilic displacement by the phenol 8. Photolysis of the stable aryl ether 9 delivers the unstable aldehyde 10.
Allylic oxidation offers another route to ketones. Michael P. Doyle of the University of Maryland has found (J. Am. Chem. Soc. 2004, 126, 13622. ) that Rh caprolactam is a very active (0.1 mol %) catalyst for this conversion.
Although the transformation is not an oxidation, Eric N. Jacobsen of Harvard University has developed (J. Am. Chem. Soc. 2004, 126, 14724. ) an elegant route to enantiomerically-pure alcohols, based on the conjugate addition of an oxime such as 13 to an α,β-unsaturated imide such as 14.