A route to 1,2,4-oxadiazoles and their complexes via platinum-mediated 1,3-dipolar cycloaddition of nitrile oxides to organonitriles

A significant activation of the Cequivalent toN 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 [PtCl4{N=C(R)ON=CC6H2R'(3)}] (R = Me, R' = Me (1); R = Et, R' = Me (2); R = Et, R' = We (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 [PtCl2{N=C(R)ON=CC,H2R'(3)}] (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 H-1, C-13{H-1}, and Pt-195 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.