13C NMR of methyl N‐methylthiobenzimidates. A novel approach to the interpretation of the relative 13C‐E/Z shifts in amides and related compounds

The 13C NMR shifts of the E/Z mixtures of twelve, mainly ortho substituted, methyl N‐methylthio‐benzimidates have been completely assigned. The angles of twist about the aryl‐imino‐carbon bond were determined by means of the C‐1 aryl shifts. Depending on the bulk of the ortho substituents the values vary from 18° in methyl 2‐fluoro‐N‐methylthiobenzibenzimidate (Z) to 73° in methyl 4‐bromo‐N,2,6‐trimethylthio‐benzimidate (E). An explanation for the 13C‐E/Z‐N‐alkyl and ‐S‐alkyl resonances is facilitated by a new model which is initially based on the influence of electric fields. As this model is derived from amides, thioamides etc. it is possible to interpret the 13C‐E/Z shifts in all related classes of compounds. The role of steric compression does not seem to be as important as was previously assumed.