The synthesis of some highly strained pyrylium and N-benzylpyridinium salts and kinetics of their reactions with piperidine

Abstract

5,8-Dimethyl-1-tetralone (11) yields the hindered pyrylium salts (12)—(14), which give the corresponding pyridines and pyridinium salts with ammonia and amines. Chroman-4-one affords the oxoniaphenanthrene perchlorate (17). The additional steric hindrance from the C-methyl groups in (28) and (16) decreases the rate of S_N2 displacement. 2,4-Diphenylbenzo[h]chromenylium (7) and 5,6-dihydro-7-phenyldi-benzoxanthylium tetrafluoroborates (8) do not give the corresponding pyridinium salts with aliphatic amines.

Benzo[f]chromanone with αβ-unsaturated ketones yielded tetracyclic pyridinium salts (33) and (34). Acenaphthenone derived pyrylium salts (39), (40), and (46), were prepared from benzylideneacetophenone, styryl t-butyl ketone, and benzo[f]chromanone. Kinetic measurements on derived N-benzylpyridinium salts demonstrated that the αβ-fusion of an acenaphthene ring decreased the tendency of a pyridine to act as a leaving group compared to the corresponding 2-phenylpyridine.