Ene-diamine versus Imine-amine Isomeric Preferences

Cyanide-catalyzed aldimine coupling was employed to synthesize compounds with 1,2-ene-diamine
and α-imine-amine structural motifs:  1,2,N,N‘-tetraphenyletheylene-1,2-diamine (13) and (±)-2,3-di-(2-hydroxyphenyl)-1,2-dihydroquinoxaline (17), respectively. Single-crystal X-ray diffraction
provided solid-state structures and density functional theory calculations were used to probe isomeric
preferences within this and the related hydroxy-ketone/ene-diol system. The ene-diamine and imine-amine core structures were calculated (B3LYP/6-311++G(d,p)) to be essentially identical in energy
(ΔG = 0.2 kcal/mol in favor of the imine-amine, within the error of the calculation). However,
additional effectssuch as π conjugationin 13 render an ene-diamine structure that is slightly
more stable than the imine-amine tautomer (14) (ΔG = 0.2−0.7 kcal/mol, within the error of the
calculation). In contrast, the intramolecular hydrogen bonding present in 17 significantly favors
the imine-amine isomer over the ene-diamine tautomer (18) (ΔG = 7.2−8.9 kcal/mol). For both 13
and 17, the optimized calculated structures (B3LYP/6-31+G(d‘)) are identical to those observed by
single-crystal X-ray diffraction.