Experimental and Computational Studies of Anti-Bredt Amidinium Salts

Experimental and computational investigations of anti‐Bredt amidinium salts are presented. Calculations show that the pyramidalization of an amino group can significantly destabilize the formal carbocation center of amidiniums, due to the decreased π donation. In some cases, the unfavorable ‐I effect of nitrogen surpasses its beneficial +M effect, and amidiniums become less stable than iminiums. It is shown that although 1‐aza‐3‐azonia[3.3.1]bicyclo‐non‐2‐enes can be isolated, they feature a nonclassical reactivity, which is more typical for iminium than amidinium salts, such as pronounced electrophilicity and azomethineylide instead of carbene formation. Bent out of shape: Strained bicyclic N‐bridgehead amidiniums can be significantly destabilized due to the decreased π donation to the formal carbocation center. The 1‐aza‐3‐azonia[3.3.1]bicyclo‐non‐2‐enes were found to meet the perfect balance: they can be isolated, but they still feature a nonclassical reactivity due to their distorted amino group (see scheme).