Synthesis, structure and stability of E/Z-isomers of novel conjugated enamines prepared from 9-arylmethyl- or 9-arylpropenyl-9H-carbazole with arylmethyleneanilines

Abstract

Active methylene groups, substituted by 9H-carbazol-9-yl (Carb) and aryl or 2-phenylethenyl groups, condense with arylmethyleneanilines in DMF at 75 °C in the presence of Bu^tOK to form the corresponding enamines [(Carb)(Ar¹)C=C(Ar²)H] and dienamines [(Ar³)HCC(Carb)CHCHPh] in almost quantitative yield. The ¹H and ¹³C NMR spectra for the enamine 1′Z-isomers [16 (Ar¹ = Ar² = 4-fluorophenyl), 17 (Ar¹ = 4-fluorophenyl, Ar² = 4-tert-butoxyphenyl), 19 (Ar¹ = Ar² = 4- tert-butoxyphenyl)], dienamine 1′Z-isomers [14a (Ar³ = 1-naphthyl), 14b (Ar³ = 4-methoxyphenyl), 14c (Ar³ = Ph)] and 1′E-isomers [15a (Ar³ = 1-naphthyl), 15c (Ar³ = Ph)] and precursors are assigned with the aid of COSY, HMBC, and HMQC techniques. The geometrical isomerism of the different dienamines 14–15 is established by ³J_C-H NMR couplings and that of enamine 12 by a difference NOE experiment. X-Ray crystal structures for 16, 14a and 15c corroborate the isomerism results deduced by NMR studies. Dienamines 14a and 15a hydrolyse to the ketone under relatively strong acid conditions [AcOH–HCl–H₂O (18∶1∶1 v/v)] under reflux over 7 h. There is an equilibrium between 14c and 15c in 1,2,4-trichlorobenzene at 180 ± 1 °C with K = 15c/14c = 0.77 as estimated from the kinetic rate profiles from HPLC data acquired over 4 days. However, under the same conditions, 14a and 15a undergo an equilibration concurrent with a reaction (monitored over 9 days) giving apparently a carbazolyl-substituted phenylphenanthrene. In contrast, enamine 16 is thermally stable with no detectable change after boiling for 4 days in 1,2,4-trichlorobenzene.