Protonation of β-Cyanoenamines:  NMR Spectroscopic and Calculational Studies of Ketenimine Amidinium Ion Formation

• Published in: Journal of the American Chemical Society Vol. 120; no. 49; pp. 12942 - 12949
• Main Authors: Dwyer, Tammy J, Jasien, Paul G, Kirkowski, Scott M, Buxton, Charles M, Arruda, Jeannie M, Mitchell, Gregory W
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 16.12.1998; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; ACTIVATION; PUSH-PULL ETHYLENES; POLARIZED ETHYLENES; KINETICS; ENAMINES; SINGLE BONDS; ROTATIONAL BARRIERS
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja983016m

NMR spectroscopy and quantum mechanical calculations have been used to investigate the protonation of β-cyanoenamines. One-dimensional variable-temperature spectroscopy, two-dimensional exchange spectroscopy (2D EXSY), and density functional calculations provide convincing evidence that the favored site of protonation in 1,1-bis(dimethylamino)-2-cyano-2-p-X-phenyl-substituted ethylenes (X = NO2, CF3, Br, F, or CH3) is the nitrogen of the cyano group. This results in the formation of a ketenimine amidinium ion in solution. When X is less electron withdrawing than −NO2, an equilibrium mixture is formed consisting of 60−93% CN-protonated enamine with the remainder protonated at the β-carbon. Calculational results also support predominantly CN-protonation in these systems. Both experimental and calculational results show no evidence of protonation at the amino nitrogen.