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Monday, May 26, 2008
Douglass F. Taber
University of Delaware

Best Synthetic Methods: Oxidation

Although the enantioselective oxidation of alkyl aryl sulfides is well developed, much less is known about dialkyl sulfides. Tsutomu Katsuki of Kyushu University has designed (J. Am. Chem. Soc. 2007, 129, 8940. ) an Fe(salan) complex that combines with aqueous H2O2 to oxidize alkyl methyl sulfides in high ee.

The oxidation of alcohols to aldehydes and ketones is one of the most widely practiced of synthetic transformations. Ge Wang of the University of Science and Technology in Beijing has developed (Chem. Lett. 2007, 36, 1236. ) a Mo catalyst that used aqueous H2O2 to effect this transformation. Secondary alcohols are oxidized more rapidly than primary alcohols. Vinod K. Singh of the Indian Institute of Technology, Kanpur, has found (Synth. Comm. 2007, 37, 4099. ) that the solid, inexpensive 6 can take the place of oxalyl chloride in the Swern oxidation. Viktor V. Zhdankin of the University of Minnesota, Duluth has devised (J. Org. Chem. 2007, 72, 8149. ) a polymer-bound hypervalent iodine reagent that is easily separated after use, and reoxidized for reuse.

Enones such as 11 are versatile intermediates for organic synthesis. Makoto Tokunaga, now at Kyushu University, and Yasushi Tsuji, now at Kyoto University, have found (Tetrahedron Lett. 2007, 48, 6860. ) a Pd catalyst that, in the presence of O2, will oxidize a cyclic ketone such as 10 to the enone.

The direct oxidation of an alcohol to the acid is not always an efficient process, so the conversion of 12 to 13 would often be carried out over at least three steps. David Milstein of the Weizmann Institute of Science has devised (Science 2007, 317, 790. ) a Ru catalyst that effected the transformation in a single step, generating H2 as a byproduct as the oxidation proceeded.

The oxidation of an aldehyde to the corresponding amide is also a useful transformation. Noritaka Mizuno of the University of Tokyo has designed (Angew. Chem. Int. Ed. 2007, 46, 5202. ) a Rh catalyst that can combine, in water, the aldehyde 14 and NH2OH to give the primary amide 15. Johann Chan of Amgen Inc., Thousand Oaks, CA has found (J. Am. Chem. Soc. 2007, 129, 14106. ) a different Rh catalyst that mediated the oxidation of a sulfonamide to the nitrene, which under the reaction conditions inserted into the aldehyde H to give the amide 17.

Krishnacharya G. Akamanchi of the Institute of Chemical Technology, Matunga, Mumbai has shown (Tetrahedron Lett. 2007, 48, 5661. ) that t-butyl hypochlorite and NaN3 will convert an aldehyde 18 to the acyl azide 19. The acyl azide 19 can be carried on to the nitrile 20, or, on warming, to the inverted isocyanate 21.

D. F. Taber, Org. Chem. Highlights 2008, May 26.