Effect of substituents on the 13C chemical shifts of the azomethine carbon atom of N-benzylideneanilines and 2-N-arylimino-2-p-nitrophenylethanenitriles

Abstract

The ¹³C chemical shifts of the azomethine carbon atoms have been determined for a series of meta- and para-substituted benzylideneanilines PhCHNC₆H₄R³(1)–(8), p-NO₂C₆H₄CHNC₆H₄R³(9)–(17), R¹C₆H₄CHNPh (18)–(26), andp-NO₂C₆H₄C(CN)NC₆H₄R³(27)–(32) in CDCl₃ solution. For the compounds (1)–(17), good ρ⁺σ⁺ correlations are observed; the ρ⁺ values are 2.73 for compounds (1)–(8) and 3.52 for the compounds (9)–(17). However, the shifts with m- and p-nitro substituents show small but appreciable lower-field deviations. For the compounds (18)–(26) plots of chemical shifts vs.σ⁺ constants give two lines intersecting at σ⁺= 0, explicable by the overlapping effects. For the nitriles (27)–(32) the plots give two lines which do not intersect, but plots of chemical shifts vs. the reciprocals of the λ_max of the K band of the electronic spectra are found to be a straight line. This is explicable in terms of unusually strong resonance electron donation from the phenylimino system. Reaction constants (ρ⁺) changed from 2.49 for (1)–(3) through 3.28 for (9)–(11) to 9.39 for (27)–(29). Apart from the dihedral angle, the sensitivities of chemical shifts to the electronic effects of the substituents in the aniline ring are affected strongly by the degree of electron demand of the carbon atom of the CN moiety.