Concave reagents, 9 Alcoholysis of diphenylketene catalyzed by concave pyridines and open‐chain analogs

Pseudo‐first‐order rate constants k (obs) for the (concave) pyridine‐catalyzed addition of ethanol to diphenylketene have been determined photometrically. The catalytic activity of the concave pyridines 1 or 2 is determined by their basicity and by their size. This effects a difference in k (obs) of up to three orders of magnitude. The decrease in catalytic activity by the concave shielding can be overcome by introduction of basicity‐increasing substituents into the pyridine ring (i.e. 1q ). An amine–alcohol complex is probably the reactive species in this catalysis as the slope β of 0.3‐0.5 in a Brønsted plot supports.