Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The electronic nature of various C-substituted nitrones was investigated by IR spectroscopy and ¹³C NMR as well as MNDO calculations. These include α-methoxy nitrones (imidate N-oxides) RC(OMe)=N(O)t-Bu with R = p-MeOC₆H₄ (Ia), C₆H₅ (Ib), p-NO₂C₆H₄ (Ic), and H (Id) and nitrones YCH=N(O)t-Bu with Y = CN (IIIa), n-BuS (IIIb), C₆H₅CH₂NH (IIIc). Upfield ¹³C shifts of C(α), the iminyl (C=N) carbon, of imidate N-oxides I versus the corresponding imidates are less than the usual upfield shifts of imine N-oxides versus imines, suggesting less buildup of electron density on C(α) in the case of alcoxy nitrones. Charge density and π bond order values from MNDO calculations for C-methoxy-C-phenyl nitrones versus model systems confirm this result and indicate a more localized C=N π bond in nitrones bearing an α-methoxy group. For N-tert-butyl nitrones with an α heteroatom (nitrogen or sulfur), phenyl, or cyano group, C(α) shifts move downfield for π-donating groups and upfield for π-accepting groups. This "reverse substituent effect" as well as C=N stretching frequencies can also be readily explained by C=N π bond containment by lone pair groups. The reported enhanced cycloaddition reactivity of α-alkoxy nitrones and their electrochemical behavior are discussed in terms of HOMO energy levels.