Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies

Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies

Author Olivato, Paulo R. Google Scholar
Gomes, Roberto da Silva Google Scholar
Rodrigues, Alessandro Google Scholar
Reis, Adriana K. C. A. Autor UNIFESP Google Scholar
Domingues, Nelson L. C. Google Scholar
Rittner, Roberto Google Scholar
Dal Colle, Maurizio Google Scholar
Institution Universidade de São Paulo (USP)
Universidade Federal de São Paulo (UNIFESP)
Fundacao Univ Fed Grande Dourados
Universidade Estadual de Campinas (UNICAMP)
Univ Ferrara
Abstract The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y-CH(2)C(O)-N(OMe)Me (Y = F1, OMe 2, OPh 3, Cl 4), supported by B3LYP/6-311++G(3df, 3pd) calculations along with the NBO analysis for 1-4, indicated the existence of cis-gauche conformers i.e. (c) and (g) for 1 and 3, (c(1), c(2)) and (g(1), g(2)) for 2, and (c) and (g(1), g(2)) for 4. in the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c(1) + c(2)) population prevails over the (g(1) + g(2)) one for 2, and the (g(1) + g(2)) conformer population is more abundant than (c) one for 4. in n-hexane solution, the cis conformer is more abundant for 1-3. the occurrence of Fermi resonance in the nu(CO) region, in n-hexane, precludes the estimative of relative populations of the (c, g(1), g(2)) conformers for 4. the SCI-PCM calculations agree with the solvent effect on the nu(CO) band component relative intensities for 1-3. NBO analysis showed that the n(N) -> pi.(CO), orbital interaction is the main factor which stabilizes the gauche (g, g(1), g(2)) conformers for 1-4 into a larger extent relative to the cis (c, c(1), c(2)) ones. the n(y) -> pi(.)(Co,) sigma(C-Y) -> pi.(CO,) pi(CO) -> sigma(C-Y) and 7co orbital interactions still contribute, but into a minor extent for the stabilization of the gauche conformers relative to the cis ones. the existence of some pyramidalization at the nitrogen atom of the Weinreb amides 1-4 is responsible for the occurrence of Y(delta)-(4)center dot center dot center dot O(delta)-(9) and Y(delta)-(4)center dot center dot center dot N(delta)-(7) short contacts in the gauche (g, g(1), g(2)) conformers, which originates strong repulsive Coulombic interactions, acting in opposition to the large orbital stabilization of the gauche conformer with respect to the cis one. Therefore, a delicate balance of the Coulombic and orbital interactions seems to be responsible for the observed stabilization of the gauche (g, g(1), g(2)) and cis (c, c(1), c(2)) conformers, both in the gas phase and in the solution for 1-4. However, the cis conformer predominance, in non polar solvents, for the 2-substituted N-methoxy-N-methyl acetamides 1-3, bearing in a first raw (fluorine and oxygen) atoms, is in the opposite direction to the gauche conformer preference for the corresponding 2-substituted N,N-dialkyl-acetamides. (C) 2010 Elsevier B.V. All rights reserved.
Keywords Conformational analysis
Infrared spectroscopy
Theoretical calculations
2-Substituted N-methoxy-N-methylacetamides
Language English
Sponsor Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
University of Ferrara
Date 2010-08-10
Published in Journal of Molecular Structure. Amsterdam: Elsevier B.V., v. 977, n. 1-3, p. 106-116, 2010.
ISSN 0022-2860 (Sherpa/Romeo, impact factor)
Publisher Elsevier B.V.
Extent 106-116
Access rights Closed access
Type Article
Web of Science ID WOS:000280498600016

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