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

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...

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Published inJournal 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
LanguageEnglish
Published WASHINGTON American Chemical Society 16.12.1998
Amer Chemical Soc
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Summary: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.
Bibliography:istex:15F23C0B3FD6CBA8A1915318A5F208D47B041697
ark:/67375/TPS-65F3BS52-K
ISSN:0002-7863
1520-5126
DOI:10.1021/ja983016m