Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 13. Penta- and nona-cyclic derivatives

Abstract

Steric acceleration by α,β-polycyclic fusion in pyridine leaving groups is quantitatively assessed. Constraining phenyl rings in pyridines to near planarity by ethano bridges in phenanthro[2,3-h]quinolinium (39) is less effective in terms of S_N2 displacement reactions by a factor of ca. 20 than in the less sterically hindered benzoquinolinium (2b). However, the corresponding rate for diphenanthro[2,3-c;3′,2′-h]-acridinium (43) is almost the same as that for the dibenzoacridinium (3b) systems. The pendant phenyl groups in systems (12) and (19) decrease S_N2 rates compared with (2a) and (3a) by factors of ca. 2 and ca. 4, respectively. Neither of these structural modifications significantly increases S_N1 rates.