Electronic Nature of α-Methoxy, Amino, Cyano, and Mercapto Nitrones

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.