Enamine–imine tautomerism of benzyl- and phenacyl-quinolines

Abstract

Measurements of tautomeric constants K_T for enamine–imine equilibria of 2-benzyl-, 2-phenacyl-, and 4-phenacyl quinolines give values of log K_T of 8.7, –1.1, and 1.8 respectively, which may be compared with values for the corresponding 2- and 4-pyridines. For the stable enamine of 2-phenacylquinoline a test of the assumptions on which evaluations of K_T are usually based, that N-methyl and N-H enamines have the same pK_a values and extinction coefficients, shows differences of two units in pK_a values and 30% in extinction coefficients. For the N-methyl enamine, however, the n.m.r. spectrum points to a trans-arrangement of nitrogen and carbonyl group rather than the hydrogen-bonded cis conformation likely for the N-H compound. Interference with nitrogen-carbonyl conjugation through interaction of the N-methyl group with a hydrogen at the 8-position of the remote quinolyl ring is suggested as destabilising the cis-conformation in the N-methyl case.