The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y–CH2C(O)-N(OMe)Me (Y = F 1, 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, (c1, c2) and (g1, g2) for 2, and (c) and (g1, g2) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c1 + c2) population prevails over the (g1 + g2) one for 2, and the (g1 + g2) 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 νCO region, in n-hexane, precludes the estimative of relative populations of the (c, g1, g2) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the νCO band component relative intensities for 1–3. NBO analysis showed that the nN→π*CO orbital interaction is the main factor which stabilizes the gauche (g, g1, g2) conformers for 1–4 into a larger extent relative to the cis (c, c1, c2) ones. The nY→π*CO, σC-Y→π*CO, πCO→σ∗C-Y and π∗CO→σ∗C-Y 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 Yδ−(4)…Oδ−(9) and Yδ−(4)…Nδ−(7) short contacts in the gauche (g, g1, g2) 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, g1, g2) and cis (c, c1, c2) 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.
Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies
DAL COLLE, Maurizio
2010
Abstract
The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y–CH2C(O)-N(OMe)Me (Y = F 1, 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, (c1, c2) and (g1, g2) for 2, and (c) and (g1, g2) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c1 + c2) population prevails over the (g1 + g2) one for 2, and the (g1 + g2) 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 νCO region, in n-hexane, precludes the estimative of relative populations of the (c, g1, g2) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the νCO band component relative intensities for 1–3. NBO analysis showed that the nN→π*CO orbital interaction is the main factor which stabilizes the gauche (g, g1, g2) conformers for 1–4 into a larger extent relative to the cis (c, c1, c2) ones. The nY→π*CO, σC-Y→π*CO, πCO→σ∗C-Y and π∗CO→σ∗C-Y 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 Yδ−(4)…Oδ−(9) and Yδ−(4)…Nδ−(7) short contacts in the gauche (g, g1, g2) 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, g1, g2) and cis (c, c1, c2) 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.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
