| Abstract |
Cu-I-catalyzed cycloaddition (CA) of the ketonitrones, Ph2C=N+(R ) O- (R = Me, CH2Ph), to the disubstituted cyanamides, NCNR2 (R = Me-2, Et-2, (CH2)(4), (CH2)(5), (CH2)(4)O, C9H10, (CH2Ph)(2), Ph(Me)), gives the corresponding 5-amino-substituted 2,3-dihydro-1,2,4-oxadiazoles (15 examples) in good to moderate yields. The reaction proceeds under mild conditions (CH2Cl2, RT or 45 degrees C) and requires 10 mol \% of [Cu(NCMe)(4)] (BF4) as the catalyst. The somewhat reduced yields are due to the individual properties of 2,3-dihydro-1,2,4-oxadiazoles, which easily undergo ring opening via N. O bond splitting. Results of density functional theory calculations reveal that the CA of ketonitrones to CuIbound cyanamides is a concerted process, and the copper-catalyzed reaction is controlled by the predominant contribution of the HOMOdipole-LUMOdipolarophile interaction (group I by Sustmann s classification). The metal-involving process is much more asynchronous and profitable from both kinetic and thermodynamic viewpoints than the hypothetical metal-free reaction. |
