Enols of 2-nitro- and related 2-substituted malonamides

The structures of 2-substituted malonamides, YCH((CONRR2)-R-1)(CONRR4)-R-3 (Y = Br, SO2Me, CONH2, COMe, and NO2) were investigated. When Y = Br, (RR2)-R-1 = (RR4)-R-3 = HEt; Y = SO2Me, R-1-R-4 = H and for Y = CONH2 or CONHPh, R-1-R-4 = Me, the structure in solution is that of the amide tautomer. X-ray crystallography shows solid-state amide structures for Y = SO2Me or CONH2, R-1-R-4 = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O center dot center dot center dot O distance = 2.3730 angstrom at 100 K) and d(OH) not equal d(O center dot center dot center dot H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO-d(6), but K-enol values decrease on increasing the solvent polarity. The N,N'-dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non-polar solvents, and only the amide in DMSO-d(6). DFT calculations gave the following order of pK(enol) values for Y: H > CONH2 > COMe >= COMe (on acetyl) >= MeSO2 > CN > NO2 in the gas phase, CHCl3, and DMSO. The enol on the C=O group is preferred to the aci-nitro compound, and the N-O-H center dot center dot center dot O=C is less favored than the C=O-H center dot center dot center dot O=C hydrogen bond. Copyright (C) 2009 John Wiley & Sons, Ltd.