The thermal decomposition of quaternary ammonium hydroxides. Part 5. The importance of conformational factors in β-eliminations from quaternary hydroxides derived from piperidines, morpholines, and decahydroquinolines

Abstract

Classical work on the thermal decomposition of the 1-methylmethohydroxides of piperidine and C-methyl-piperidines has been confirmed, although the elimination products from 2- and 3-methylpiperidines contain small proportions of isomeric alkenes. The products of thermal decomposition of 3-oxa-6-azoniaspiro[5,5]undecane hydroxide show that β-elimination occurs several times faster in a morpholine ring than in a piperidine ring. The requirement for easy elimination in six-membered rings is the anti-coplanarity of the four centres H_β, C_β, C_α and N⁺. Strikingly, molecules which lack this requirement, such as the 1-methylmethohydroxide of cis-2,6-dimethylmorpholine gave no elimination whatever on thermal decomposition. On the other hand, thermal decomposition of the 1-methylmethohydroxide of trans-3,5-dimethylpiperidine gave ca. 50%β-elimination. Elimination of the β-hydrogens of an α-attached methyl group are strongly preferred to elimination of ring β-hydrogens, as in the degradation of cis-2,6-dimethylpiperidine, but a β-attached oxygen atom allows the corresponding morpholine compound to undergo 16% elimination of a ring hydrogen. The 1-methylmethohydroxides of substituted trans- and cis-decahydroquinolines (type 1) undergo elimination of a β-hydrogen in a 2-alkyl group, as expected from conformational considerations. However, a cis-molecule in which the type 2 conformation is dominant such as the 1-methylmethohydroxide of cis(4H,4aH), cis(4aH,8aH)-decahydro-2,2,4-trimethylquinoline, undergoes an appreciable proportion of elimination of the correctly oriented 8ax-H in the cyclohexane ring. In all cases reported, the direction of elimination is readily explained by reference to stereochemical considerations and to the steric and inductive effects of β-attached substituents.