(<i>E,Z</i>) Equilibria, 14. Substituent‐Induced Chemical Shifts along the C = N Bond of Schiff Bases

Abstract

Sterically congested N ‐(1,1,3,3‐tetraalkyl‐2‐indanylidene)‐amines 8–11 , N ‐(cyclopentylidene)anilines 13–17 , and two of their salts are described, together with a short synthesis of 2‐imino‐1,1,3,3‐tetramethylindan ( 5 ). Some of these imines show rapid ( E,Z ) equilibration. Positively and negatively charged nitrogen functions (in 6 and 7 ) cause opposite 1 H‐ and 13 C‐NMR chemical shift effects along the C = N bond. Chemical shifts are almost equally affected by the lone electron pair and by the imino N–H bond. Substituent‐induced chemical shifts (SCS) have been assigned for all syn and anti positions with respect to methyl, phenyl, and 2,6‐dimethylphenyl groups at the imino nitrogen atom. The structurally well‐defined, rigid imines recommend themselves as new models for the calibration of theoretical approaches to syn/anti ‐differentiating SCS.