Isomerizational and conformational study of methyl-2-cyano-3-methoxyacrylate and methyl-2-cyano-3-aminoacrylate and its N-methyl derivatives

Abstract

The isomers and conformers of four push–pull compounds: methyl-2-cyano-3-methoxyacrylate (MCMA) H 3C O CH C(CN)(COOCH 3), methyl-2-cyano-3-aminoacrylate (MCAA) H 2N CH C(CN)(COOCH 3), methyl-2-cyano-3-methylaminoacrylate (MCMAA) H 3C NH CH C(CN)(COOCH 3) and methyl-2-cyano-3-dimethylaminoacrylate (MCDMAA) (H 3C) 2N CH C(CN)(COOCH 3) have been studied experimentally by vibrational and NMR spectroscopy and theoretically by the ab initio calculations at MP2 level in 6-311++G ** 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, acetonitrile and DMSO at room temperature. Because both electron-withdrawing groups are different, all studied compounds can exist as E and Z isomers and then conformational possibilities are given by the rotation of the methylester and methoxy or methylamino groups. NMR spectra revealed that both MCMA and MCDMAA compounds without the possibility of intramolecular hydrogen bonding were prepared as a pure E isomer whereas in the case of the compounds with the possibility of intramolecular bonding MCAA and MCMAA a mixture of both E and Z isomers was obtained. X-ray analysis shows the presence of two EZ and EE conformers in solid MCMA. For this compound the possible second conformer was detected by NMR in more polar solvent DMSO. Vibrational spectra revealed the existence of two EZa and EEa conformers with Z and E orientation of methylester group and with anti orientation of dimethylamino group for MCDMAA. For MCAA and MCMAA the Z isomer with Z orientation of methylester group and with intramolecular hydrogen bond is the most stable one. In more polar surrounding (DMSO) the isomerization of ZZ or ZZa conformers of MCAA and MCMAA, respectively to E isomers occurred. These experimental findings have been supported by ab initio solvent effect calculations.