NMR spectroscopic study of the (Z)/(E)-isomerism of 1-aryl-3-arylamino-2-propen-1-ones in solution and in the crystalline state

Conjugate Addition of 3-Buytn-2-one to Anilines in Ethanol: Alkene Geometric Insights through In Situ FTIR Monitoring

“…These findings indicate that enaminones 3a−l exist exclusively in the Z-configuration in the solid state. 21 UV−vis analysis also showed that 3a in chloroform exhibited a significantly longer wavelength, lower intensity absorbance band (343 nm) compared to 3m (316 nm). 16 These analyses further supported the notion that the subtle shoulder at 1669 cm −1 in Figure 1 originates from the E-isomer of 3f.…”

“…Interestingly, when the 1 H NMR spectrum of 3a was recorded in chloroform- d 1 immediately after SiO 2 chromatography, the compound had isomerized to a 1:1 mixture of the Z - and E -isomers. The broad signal for the N–H proton of the E -isomer was evident at much higher field (7.13 ppm) compared to the Z-isomer, indicating that this proton was not significantly hydrogen bonded. , Furthermore, when the 1 H NMR spectrum of a second fraction of 3a was recorded 1 h after chromatography, the Z / E -ratio had returned to 97:3, where it remained indefinitely in chloroform- d 1 . This observation provided clear evidence that enaminones 3a – 3l exist in configurational equilibria that favor the Z -isomer, and that the observed Z / E -ratio is not governed by the initial conjugate addition reaction. , Passing the products through SiO 2 likely causes protonation of 3a , presumably via the ketone oxygen, which disrupts the internal hydrogen bond required for stabilization of the Z -isomer and thus results in the temporary adoption of the sterically, and electronically favored E -isomer.…”

“…In polar protic methanol-d 4 the equilibrium was maintained at a 1:1 ratio of the Z-and E-isomers, and in the dipolar aprotic DMSO-d 6 the E-form predominated (30:70). 14,20,21 The fascinating effect of hydrogen bonding is highlighted in Figure 4. 20,21 After 5 min; the E-form is favored in all solvents except for chloroform-d 1 , presumably due to kinetically controlled hydrogen bond formation with the solvent via the ketone oxygen.…”

“…When these equilibria were followed over 6 days, the Z -form was favored in the nonpolar aprotic solvent, toluene- d 8 ( Z : E , 97:3), and polar aprotic solvents; acetone- d 6 (84:16) and MeCN- d 3 (86:14). In polar protic methanol- d 4 the equilibrium was maintained at a 1:1 ratio of the Z - and E -isomers, and in the dipolar aprotic DMSO- d 6 the E -form predominated (30:70). ,, …”

“…In DMSO-d 6 in particular, the E-form of 3a exhibited a lower field N−H proton (9.65 ppm) than in all other solvents; a characteristic that tallies with the low Z-isomer proportion in DMSO-d 6 , and indicates significant hydrogen bonding to the solvent. 16,21 The equilibrium proportion of the E configuration in acetone-d 6 and MeCN-d 3 is around four times greater than that observed in chloroform-d 1 and toluene-d 8 . Given the even higher E proportions observed in methanol-d 4 and DMSO-d 6 , we can infer that the position of the configurational equilibrium in the enaminone products 3a−l can essentially be related to the hydrogen bond accepting ability of the solvent.…”