Switching Conjugation Is the Predominant Factor Contributing to Complete Reversal of Amide <i>cis</i>–<i>trans</i> (Z-E) Preference through <i>N</i>-Methylation

<i>N</i>-Methylation strongly influences the <i>cis-trans</i> (Z-E) preference of amides, especially in benzanilide. While the <i>trans</i> (E) conformation is preferred in NH-benzanilide, <i>N</i>-methylation shifts the balance to favor the <i>cis</i> (Z) form. This switch, often attributed to steric hindrance, is frequently utilized in molecular design. However, our findings suggest an alternative mechanism. Through bond model analysis (BMA), we explored the structural preferences driven by bond orbital interactions and assessed the stability of <i>cis-trans</i> (Z-E) preferences in response to electronic perturbations via electron-donating or -withdrawing groups on the benzene rings. Our results reveal that <i>N</i>-methylation rebalances conjugation: NH-benzanilides prefer the <i>trans</i> (E) form due to phenyl-amine conjugation, while N-Me benzanilides favor the <i>cis</i> form due to phenyl-carbonyl conjugation.