Participation of electrophilic groups with the dehydrogenation of 4-substituted piperidines

Dehydrogenation of 2-(1-piperidinyl)-benzaldehydes 1-3 using mercury(II)-ED TA generated the lactams 4-6, indicating a reversible reaction of a carbino lamine intermediate with the formyl group. The yields and oxidation rates d ecreased by if-substitution in the piperidine moiety. The 2-(1-piperidinyl)-acetophenones 11, 16-19 showed a similar behavior wit h mercury (II)-EDTA but gave rise to a product pattern. The trans-benzoquin olizidones 12, 20, 23, 26. 29 resulted from the cyclic iminium compounds re acting with the acetyl group as nucleophile. By another oxidation these spe cies were partially transformed to the quinolinones 13, 21, 24, 27, 30. An intermediate electrophilic neighboring of the carbonyl group with the cycli c hemiaminals led finally to the lactams 14, 22, 25, 28, 31. Mechanisms for the reactions are proposed.