Enols of 2‐nitro‐ and related 2‐substituted malonamides |
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Authors: | Ahmad Basheer Masaaki Mishima Zvi Rappoport |
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Affiliation: | 1. Institute of Chemistry and the Lise Meitner Minerva Center for Computational Quantum Chemistry, The Hebrew University, Jerusalem 91904, Israel;2. Institute for Materials, Chemistry and Engineering, Kyushu University, Hakozaki, Higashi‐ku, Fukuoka 812‐8581, Japan |
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Abstract: | The structures of 2‐substituted malonamides, YCH(CONR1R2)CONR3R4 (Y = Br, SO2Me, CONH2, COMe, and NO2) were investigated. When Y = Br, R1R2 = R3R4 = HEt; Y = SO2Me, R1–R4 = H and for Y = CONH2 or CONHPh, R1–R4 = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO2Me or CONH2, R1–R4 = 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‐d6, but Kenol 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‐d6. DFT calculations gave the following order of pKenol 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…O?C is less favored than the C?O? H…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd. |
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Keywords: | enols nitromalonamides solvent effects substituent effects calculations |
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