Best Synthetic Methods: Functional Group Transformation
Selective halogenation can be a direct route to high value added materials from inexpensive hydrocarbons. Josť Barluenga of the Universidad de Oviedo has developed (Angew. Chem. Int. Ed. 2005, 44, 5841. ) a procedure for iodination that allows the incorporation of both iodines of I2 into the product. Saturated hydrocarbons can also be iodinated.
Both ketones and active aromatics are easily brominated by a reagent devised (J. Org. Chem. 2005, 70, 4267. ) by Bhisma K. Patel of the Indian Institute of Technology Guwahati. On oxidation with oxone and KBr, the bis pyridinium salt 3 is converted to the bis tribromide 4. Bromination was effected by grinding the solid starting material and 4 together, without added solvent. The spent reagent 3 was easily removed from the brominated product by a simple water wash. It could be recovered and recycled.
Majid M. Heravi of Azzahra University, Tehran has described (Tetrahedron Lett. 2005, 46, 6243. ) the preparation of a DABCO-bromine complex. An alcohol on being stirred at room temperature with the complex is converted to the corresponding aldehyde or ketone. Notably, the easily brominated aldehyde does not react further.
Xiaolai Wang of the Lanzhou Institute of Chemical Physics has devised (Tetrahedron Lett. 2005, 46, 7023. ) a more vigorous oxidizing system, a tungsten catalyst coupled with 30% aqueous hydrogen peroxide. Secondary alcohols are oxidized to ketones, and primary alcohols go on to acids. The latter transformation is currently most commonly carried out in two steps.
Thottumkara K. Vinod of Western Illinois University has found (Org. Lett. 2005, 7, 2933. ) that the commonly-available o-iodobenzoic acid can also serve as a catalyst for the oxidation of primary alcohols to acids and of secondary alcohols to ketones. In this case, oxone is the bulk oxidant.
K. C. Nicolaou of Scripps La Jolla has been exploring organic synthesis applications of the o-iodobenzoic acid-derived Dess-Martin periodinane. He has found (Angew. Chem. Int. Ed. 2005, 44, 5992. ) that amides such as 13 are oxidized to the corresponding imide 14, the net transformation being the oxidation of a primary amine to the acid. A β-alanine derivative such as 15 is oxidized to the (Z)-vinylogous carbamate 16.
Another route to amides is the Beckmann rearrangement of ketoximes such as 17. Hisashi Yamamoto of the University of Chicago and Kazuaki Ishihara of Nagoya University have shown (J. Am. Chem. Soc. 2005, 127, 11240. ) that 5 mol % of the inexpensive cyanuric chloride 18 is sufficient to catalyze this reaction. The reaction is even faster with 2 mol % of 18 and 2 mol % of ZnCl2.