Enols of Substituted Cyanomalonamides
datasetposted on 06.07.2007, 00:00 by Ahmad Basheer, Hiroshi Yamataka, Salai Cheettu Ammal, Zvi Rappoport
Twenty open-chain mono-, di-, and trialkyl and aryl-N-substituted cyanomalonamides R2R1NCOCH(CN)CONHR3 were prepared. In solution, signals for both amide and a single enol are mostly observed, despite the potential for E and Z isomeric enols. The equilibrium (KEnol) values between the amides and the enols were determined in different solvents by NMR spectra. They decrease on increasing the polarity of the solvent in the order CDCl3 ∼ C6D6 > THF-d8 > (CD3)2CO > CD3CN > DMF-d7 > DMSO-d6. For the R1R2NCOCH(CN)CONHR3 system when R1 = R2 = H, Me or R1 = H, R2 = Me, KEnol for R3 follows the order: C6F5 > Ph ≥ An ≥ i-Pr ≥ t-Bu, and for R1, R2:H, H > Me, H > Me, Me in all solvents. A unique feature is the appreciable % enol in DMSO-d6 when R1 = R2 = H, in contrast with enol systems with other electron-withdrawing groups (EWGs). Calculations (B3LYP/6-31G**) corroborate the higher KEnol values for less alkyl-substituted systems, showing that in the most stable conformer when R1 = H, R2 = R3 = Me the N-hydrogens are closer to the CN group. The order of promoting substituents for enol of amide formation is CONH2 > CO2CH2CF3 > CO2Me > CONHMe. The solid-state structures of the isolated species, determined by X-ray crystallography, were either amides or enols, and a higher KEnol(CDCl3) value does not ensure a solid enol structure. In no system were both solid species isolated. The X-ray structures of the enols were temperature-dependent. In most cases, the difference between the O−H and O···H bond lengths at low temperature were appreciable, but they become closer at the higher temperature. Similar tendency for either the CC/C−C or the C−O/CO bonds was observed. This is ascribed to a hydrogen shift between two regioisomeric enols in an asymmetric double-well potential, which becomes faster at a higher temperature. Calculations show that the enol structures are nonsymmetrical, resembling the lower temperature structures, even when they are chemically symmetrical, but the energy differences between the two regioisomers are <1 kcal. The hydrogen bonds in the enol moiety are strong, with O···O distances <2.45 Å, and are resonance-assisted hydrogen bonds. IR spectra in solution and the solid state qualitatively corroborate the NMR and X-ray structure determination.