The preferred conformations of p-substituted α-ethylsulfonylacetophenones, Y-PhC(O)CH2SO2Et (Y=OMe 1, Me 2, H 3, F 4, Cl 5, Br 6, CN 7 and NO2 8) are determined by νCO IR analysis, HF/6-31G** computations (1, 3, 5, 7 and 8) and X-ray diffraction for 5 and 7. The match between IR frequencies and intensities of the carbonyl doublet components in carbon tetrachloride (first overtone) and the results of the calculations indicates that the lower frequency component corresponds to the more stable gauche (g) conformer, while the higher frequency component is related to the quasi-cis (q-c) one. The gauche conformer (g) (ca. 85%) prevails over the quasi-cis conformer (q-c) population, and the q-c/g population ratio increases on going from electron-attracting to electron-donating substituents, both in the gas phase and in solution. These trends are discussed in terms of Oδ-(SO2)...Cδ+(CO) charge transfer (CT), Oδ-(CO)...Sδ+(SO2) (Coulombic interaction), Hδ+[CH2(Et)]...Oδ-(CO), Hδ+(2')(o-Ph)...Oδ-(CO) and Hδ+(6')(o-Ph)...Oδ-(SO2) hydrogen bonds and electrostatic interactions, along with the πCO/σ*C-S and π*CO/σC-S orbital interactions, which stabilise the g rotamer. The stabilisation of the q-c rotamer through Oδ-(CO)...Sδ+(SO2) (CT), Hδ+(2')(o-Ph)...Oδ-(CO) and Hδ+[CH2(Et)]...Oδ-(CO) hydrogen bond interactions is counterbalanced by Oδ-(CO)...Oδ-(SO2) electrostatic repulsion, which destabilises significantly this rotamer. The X-ray diffraction analyses show that 5 and 7, in the solid, adopt the cis (c') conformation, which is stabilised through intermolecular hydrogen bonds involving the acidic α-methylene Hδ+(3A), H(3B) and m-phenyl Hδ+(5')[m-Ph] hydrogen atoms and the Oδ-(6), Oδ-(7) sulfonyl and Oδ-(1) carbonyl oxygen atoms.
Spectroscopic and theoretical studies of some p-substituted alpha-ethylsulfonylacetophenones
DAL COLLE, Maurizio
2004
Abstract
The preferred conformations of p-substituted α-ethylsulfonylacetophenones, Y-PhC(O)CH2SO2Et (Y=OMe 1, Me 2, H 3, F 4, Cl 5, Br 6, CN 7 and NO2 8) are determined by νCO IR analysis, HF/6-31G** computations (1, 3, 5, 7 and 8) and X-ray diffraction for 5 and 7. The match between IR frequencies and intensities of the carbonyl doublet components in carbon tetrachloride (first overtone) and the results of the calculations indicates that the lower frequency component corresponds to the more stable gauche (g) conformer, while the higher frequency component is related to the quasi-cis (q-c) one. The gauche conformer (g) (ca. 85%) prevails over the quasi-cis conformer (q-c) population, and the q-c/g population ratio increases on going from electron-attracting to electron-donating substituents, both in the gas phase and in solution. These trends are discussed in terms of Oδ-(SO2)...Cδ+(CO) charge transfer (CT), Oδ-(CO)...Sδ+(SO2) (Coulombic interaction), Hδ+[CH2(Et)]...Oδ-(CO), Hδ+(2')(o-Ph)...Oδ-(CO) and Hδ+(6')(o-Ph)...Oδ-(SO2) hydrogen bonds and electrostatic interactions, along with the πCO/σ*C-S and π*CO/σC-S orbital interactions, which stabilise the g rotamer. The stabilisation of the q-c rotamer through Oδ-(CO)...Sδ+(SO2) (CT), Hδ+(2')(o-Ph)...Oδ-(CO) and Hδ+[CH2(Et)]...Oδ-(CO) hydrogen bond interactions is counterbalanced by Oδ-(CO)...Oδ-(SO2) electrostatic repulsion, which destabilises significantly this rotamer. The X-ray diffraction analyses show that 5 and 7, in the solid, adopt the cis (c') conformation, which is stabilised through intermolecular hydrogen bonds involving the acidic α-methylene Hδ+(3A), H(3B) and m-phenyl Hδ+(5')[m-Ph] hydrogen atoms and the Oδ-(6), Oδ-(7) sulfonyl and Oδ-(1) carbonyl oxygen atoms.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
