A Route to 1,2,4-Oxadiazoles and Their Complexes via Platinum-Mediated 1,3-Dipolar Cycloaddition of Nitrile Oxides to Organonitriles

• Published in: Inorganic chemistry 2003-02, Vol.42 (3), p.896-903
• Main Authors: Bokach, Nadezhda A, Khripoun, Anatolii V, Kukushkin, Vadim Yu, Haukka, Matti, Pombeiro, Armando J. L
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic026103v

A significant activation of the C⋮N group in organonitriles upon their coordination to a platinum(IV) center has been found in the reaction of [PtCl4(RCN)2] (R = Me, Et, CH2Ph) with the nitrile oxides 2,4,6-R‘3C6H2CNO (R‘ = Me, OMe) to give the (1,2,4-oxadiazole)platinum(IV) complexes (R = Me, R‘ = Me (1); R = Et, R‘ = Me (2); R = Et, R‘ = OMe (3); R = CH2Ph, R‘ = Me (4)); the [2 + 3] cycloaddition was performed under mild conditions (unless poor solubility of [PtCl4(RCN)2] precludes the reaction) starting even from complexed acetonitrile and propionitrile, which exhibit low reactivity in the free state. The reaction between complexes 2−4 and 1 equiv of Ph3PCHCO2Me in CH2Cl2 leads to the appropriate platinum(II) complexes (5−7); the reduction failed only in the case of 1 insofar as this complex is insoluble in the most common organic solvents. All the platinum compounds were characterized by elemental analyses, FAB mass spectrometry, and IR and 1H, 13C{1H}, and 195Pt NMR spectroscopies, and three of them also by X-ray crystallography. The oxadiazoles formed in the course of the metal-mediated reaction were liberated almost quantitatively from their Pt(IV) complexes by reaction of the latter (complexes 2−4) with an excess of pyridine in chloroform, giving free 1,2,4-oxadiazoles and trans-[PtCl4(pyridine)2]; the sequence of the Pt(IV)-mediated [2 + 3] cycloaddition and the liberation opens up an alternative route for the preparation of this important class of heterocycles.