Previous spectroscopic (IR, 13CNMR,UV and UPS), X-ray studies performed in our laboratories on some β-carbonylsulfoxides XC(O)CH2S(O)R (X=Me, Ar, NR2, OR and SR; R= Me, Ar) [1-5], have shown that their gauche conformer is the most stable, but the cis conformer prevails for the α-sulfinylacetophenones, XC(O)CH2S(O)R [X=Ph; R=Me, Et, iPr, and Ph] [6,7] and X= p-Y-Ph (for Y = OME, Me, H, Cl, and Br; R=Et)] [8], except for R= tBu for the former series, and (Y = CN and NO2; R=Et) for the latter one for which the gauche conformer is the most stable one. The stabilization of the gauche conformers has been attributed to the πCO/σ*C-S and, to a minor extent, to the π*CO/σC-S orbital interactions, while the peculiar stabilization of the cis conformers of the α-alkylsulfinylacetophenones relative to the α-alkyl- and α-aryl-sulfinylacetones has been ascribed to the larger relaxation of the benzene ring π electron charge (due to the πPh-πCO conjugation), which originates stronger intramolecular electrostatic and charge transfer interactions between Cδ+=Oδ- and Sδ+=Oδ- dipoles. In the MeC(O)X series [9,10] there is a progressive decrease of the experimental carbonyl oxygen lone pair (nO) ionization energy going from ester (Ei=10.45eV) to thioester (Ei=9.64eV) to butanone (Ei=9.46eV), to acetophenone (Ei=9.34eV) and to amide (Ei=9.20eV) for X= OEt, SEt, Et, Ph and NEt2, respectively, which in turn is accompanied by a corresponding increase of the negative charge on the carbonyl oxygen atom in this direction. Therefore it seems reasonable to expect a larger stabilization of the cis conformer of the α-sulfinyl-amides relative to the same conformation of the α-sulfinylacetophenones, due to stronger Cδ+=Oδ-….Sδ+=Oδ- orbital and electrostatic interactions which should operate in the cis conformation of the sulfinyl-amides. Aiming to throw more light on the nature of the different electronic interactions which may stabilize the cis and gauche conformers of the α-sulfinylamides, this paper reports the IR study of some N,N-diethyl-2-[(4-substituted) phenylsulfinyl] acetamides Et2NC(O)CH2S(O)Ph-Y, bearing in 4- electron-donating, hydrogen and electron-withdrawing substituents i.e. Y= OMe 1,Me 2, H 3,Cl 4, Br 5, NO2 6 (Scheme 1) along with ab initio calculations of 3 and X-ray diffraction analysis of compound 6. These compounds were chosen taking into account that the orbital and Coulombic interactions, which could operate in their cis and gauche conformers, should be affected by changes in the conjugation involving the 4-substituent at the phenylsulfinyl group, and consequently should influence the stabilization of the referred conformers.
Spectroscopic and theoretical studies of some N,N-diethyl-2-[(4-substituted)phenylsulfinyl] acetamides
DAL COLLE, Maurizio
2007
Abstract
Previous spectroscopic (IR, 13CNMR,UV and UPS), X-ray studies performed in our laboratories on some β-carbonylsulfoxides XC(O)CH2S(O)R (X=Me, Ar, NR2, OR and SR; R= Me, Ar) [1-5], have shown that their gauche conformer is the most stable, but the cis conformer prevails for the α-sulfinylacetophenones, XC(O)CH2S(O)R [X=Ph; R=Me, Et, iPr, and Ph] [6,7] and X= p-Y-Ph (for Y = OME, Me, H, Cl, and Br; R=Et)] [8], except for R= tBu for the former series, and (Y = CN and NO2; R=Et) for the latter one for which the gauche conformer is the most stable one. The stabilization of the gauche conformers has been attributed to the πCO/σ*C-S and, to a minor extent, to the π*CO/σC-S orbital interactions, while the peculiar stabilization of the cis conformers of the α-alkylsulfinylacetophenones relative to the α-alkyl- and α-aryl-sulfinylacetones has been ascribed to the larger relaxation of the benzene ring π electron charge (due to the πPh-πCO conjugation), which originates stronger intramolecular electrostatic and charge transfer interactions between Cδ+=Oδ- and Sδ+=Oδ- dipoles. In the MeC(O)X series [9,10] there is a progressive decrease of the experimental carbonyl oxygen lone pair (nO) ionization energy going from ester (Ei=10.45eV) to thioester (Ei=9.64eV) to butanone (Ei=9.46eV), to acetophenone (Ei=9.34eV) and to amide (Ei=9.20eV) for X= OEt, SEt, Et, Ph and NEt2, respectively, which in turn is accompanied by a corresponding increase of the negative charge on the carbonyl oxygen atom in this direction. Therefore it seems reasonable to expect a larger stabilization of the cis conformer of the α-sulfinyl-amides relative to the same conformation of the α-sulfinylacetophenones, due to stronger Cδ+=Oδ-….Sδ+=Oδ- orbital and electrostatic interactions which should operate in the cis conformation of the sulfinyl-amides. Aiming to throw more light on the nature of the different electronic interactions which may stabilize the cis and gauche conformers of the α-sulfinylamides, this paper reports the IR study of some N,N-diethyl-2-[(4-substituted) phenylsulfinyl] acetamides Et2NC(O)CH2S(O)Ph-Y, bearing in 4- electron-donating, hydrogen and electron-withdrawing substituents i.e. Y= OMe 1,Me 2, H 3,Cl 4, Br 5, NO2 6 (Scheme 1) along with ab initio calculations of 3 and X-ray diffraction analysis of compound 6. These compounds were chosen taking into account that the orbital and Coulombic interactions, which could operate in their cis and gauche conformers, should be affected by changes in the conjugation involving the 4-substituent at the phenylsulfinyl group, and consequently should influence the stabilization of the referred conformers.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
