The isomers and conformers of some push-pull enamines studied by vibrational and NMR spectroscopy and by ab initio calculations

The isomers and conformers of six push-pull enamines: 3-dimethylamino-, 3-methylamino- and 3-amino-2-acetyl propenenitrile [(H3C)(2)N-CH=C(CN)(COCH3), H3C-NH-CH=C(CN)(COCH3) and H2N-CH=C(CN)(COCH3)] and 3-dimethylamino-, 3-methylamino- and 3-amino-2-methylsulfonyl propenenitrile [(H3C)(2)N-CH=C(CN)(SO2CH3), H3C-NH-CH=C(CN)(SO2CH3) and H2N-CH=C(CN)(SO2CH3)] have been studied experimentally by vibrational and NMR spectroscopy and theoretically by the ab initio calculations at MP2 level in 6-31G** basis set. The IR and Raman spectra of all compounds as a solid and solute in various solvents have been recorded in the region 4000-50 cm(-1). The NMR spectra were obtained in chloroform and DMSO at room temperature. All six compounds have been prepared by the same way. NMR spectra revealed that both dimethylamino compounds were prepared as a pure E isomers whereas in the case of methylamino compounds the 3-methylamino-2-methylsulfonyl propenenitrile was prepared also as a pure E isomer but 3-methylamino-2-acetyl propenenitrile as a pure Z isomer. Also 3-amino-2-methylsulfonyl propenenitrile was obtained as a pure the E isomer, but 3-amino-2-acetyl propenenitrile as a mixture of both E and Z isomers. Confomational possibilities of studied compounds are given only by the rotation of the acetyl and methylamino groups. Vibrational spectra revealed existence of two conformers with Z and E orientation of acetyl group for 3-dimethylamino-2-acetyl propenenitrile. Two conformers with anti or syn orientation of methylamino group for 3-methylamino-2-methylsulfonyl propenenitrile have been confirmed by vibrational and NMR spectra, but only one conformer with anti orientation of methylamino group for 3-methylamino-2-acetyl propenenitrile in chloroform solution and in solid phase was found. For latter compound the additional isomer/conformer was detected in more polar solvents (acetonitrile/DMSO). These experimental findings have been supported by ab initio solvent effect calculations. (c) 2005 Elsevier B.V. All rights reserved.