Deuterium isotope effects on 13C chemical shifts of enaminones

Abstract

Deuterium isotope effects on 13C chemical shifts have been studied in a series of substituted N-alkyl and N-phenyl keto-enamines. The intramolecularly hydrogen bonded Z-forms show the largest two-bond isotope effects, 2δC-1 (ND). Methyl-substitution at C-1 leads to a larger two-bond isotope effect in the N-phenyl-substituted derivatives. This effect is ascribed to steric compression. Space-filling substituents at the ortho-position of the N-phenyl ring lead to a decrease of the two-bond isotope effect. A correlation is found between 2δC-1(ND) and 3δC-2′(ND). The latter becomes negative in the sterically hindered cases. 3δC-2′(ND) may therefore be used as a gauge of the twist of the phenyl ring. o-Hydroxy substitution of the CO-phenyl rings enables intramolecular hydrogen bonding to the carbonyl group. This kind of hydrogen bond with two donors to one acceptor leads to smaller 2δC-2(ND) and 2δC-2″(OD) isotope effects equivalent to weaker hydrogen bonds for the Z-isomer. This is ascribed to competition for the acceptor. For the E-isomer 2δC(OD) is enhanced. The same feature is seen for N,N-dimethylamino enamines. This increase is ascribed to delocalization of the nitrogen lone-pair onto the carbonyl oxygen, thereby strengthening the hydrogen bond and thus leading to larger two-bond, 2δC(OD), isotope effects. © Acta Chemica Scandinavica 1997.