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

The structures of 2‐substituted malonamides, YCH(CONR 1 R 2 )CONR 3 R 4 (Y = Br, SO 2 Me, CONH 2 , COMe, and NO 2 ) were investigated. When Y = Br, R 1 R 2  = R 3 R 4  = HEt; Y = SO 2 Me, R 1 –R 4  = H and for Y = CONH 2 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 = SO 2 Me or CONH 2 , R 1 –R 4  = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O … O distance = 2.3730 Å at 100 K) and d (OH) ≠ d(O … 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 p K enol values for Y: H > CONH 2  > COMe ≥ COMe (on acetyl) ≥ MeSO 2  > CN > NO 2 in the gas phase, CHCl 3 , and DMSO. The enol on the CO group is preferred to the aci ‐nitro compound, and the NOH … OC is less favored than the COH … OC hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.