Paulo R. Olivato

Conformational studies of α-substituted carbonyl compounds. Part 1. Conformation and electronic interaction in hetero-substituted acetones by infrared and ultraviolet spectroscopy

νCO Frequencies and intensities and n→π* transition energies were measured for some heterosubstituted ketones (XCH2COMe : X = F, Cl, Br, I, NMe2, OMe, SMe, or SEt) and compared with the corresponding unsubstituted ketone. The stability of the gauche-conformers is discussed in terms of hyperconjugative interactions between σC – X and πCO orbitals and repulsive interactions between CO and C – X dipoles. The carbonyl frequency shifts, induced by inductive (ΔνI), field (ΔνF), and hyperconjugative effects (ΔνH), are estimated separately. A close relationship is shown to exist between the ΔνH values and the energies of the n→π* transition.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF SOME (ALKYLTHIO)-SUBSTITUTED PROPANONES AND THEIR MONO- AND DI-OXIDATED DERIVATIVES

Abstract The analysis of the carbonyl frequency shifts hyperconjugative parameters (ΔV H) along with the Non-Additivity Effect of the α-methylene carbon chemical shifts for the ethylthio- (1), ethylsulfinyl-(2) and ethylsulfonyl- (3) propanones indicated the occurrence of both π∗ CO/σ C-SOn and πCO/σ∗C -SOn interaction in the ground state of their gauche conformers. The bathochromicaly shifted n → π∗CO transition of the title compounds in relation to the parent compound indicated the existence of the π∗CO/σ∗C-SOn interaction in the excited state of their gauche rotamers.

Experimental and theoretical study of the intramolecular interactions determining the conformation of β-carbonyl sulfoxides

Information on the geometrical and electronic structures of α-methylsulfinylacetophenone, C6H5C(O)CH2S(O)CH3 2, have been obtained from X-ray diffraction analysis, UV photoelectron spectroscopy and ab initio 6-31G** calculations. A comparison of the results with those obtained from the spectra and the computations on α-methylthioacetophenone, C6H5C(O)CH2SCH3 1 and α-methylsulfonylacetophenone, C6H5C(O)CH2SO2CH3 3, together with previous results on β-keto sulfides and β-keto sulfones indicates that the CH2–S(O) bond in 2 is quasi-cis to the carbonyl group in the gas and solid phase, at variance with the other β-carbonyl thioderivatives which adopt a gauche conformation. Eigenvector analysis, electron charge distribution at various atoms and/or groups and geometric parameters indicate that the cis conformation of 2 is stabilized by a strong non-bonded interaction between the negatively charged carbonyl oxygen and the positively charged sulfur atom from which it is separated by a distance (2.8–2.9 A) much shorter than the sum of the van der Waals radii. The predominant charge transfer interaction in 3 and related sulfones occurs in the opposite direction (OSO2→ CCO). The inversion of the direction of the charge transfer (and the change of the cis/gauche orientation of the thio group) from sulfone to sulfoxide is associated with an increase of electron affinity of the thio group in the latter, and could explain its smaller thermal stability. Ab initio 3-21G* calculations on several conformations of the bis-thioderivatives C6H5C(O)CH(SCH3)S(O)CH3 4, C6H5C(O)CH(SR)SO2R (R = Me 5 and Ph 6) and C6H5C(O)CH(SOCH3)SO2CH3 7, together with X-ray diffraction (4, 6 and 7) and photoelectron spectroscopy (4) analyses confirmed the cis(SOR) and gauche(SR and SO2R) preferred orientation of the thio groups with respect to the carbonyl group as observed in the monosubstituted derivatives. In 4 and 7 the SSO atom is about 30° out of the cis plane [O(1)–C(2)–C(3)].

Conformational and electronic interaction studies of α-substituted carbonyl compounds. Part 9. ω-Hetero-substituted acetophenones

νCO frequencies and intensities, n→π*CO transition energies and α-methylene carbon chemical shifts were measured for some ω-hetero-substituted acetophenones (φCOCH2X: X = F, OMe, NMe2, NEt2, Cl, Br, SEt, and I), and their conformations were estimated with the help of molecular-mechanics calculations. The stability of the gauche rotamers is discussed in terms of hyperconjugative interactions between σCX and πCO* orbitals. The carbonyl frequency shifts of the cis rotamers are interpreted as due to the substituent field effect, while the corresponding shifts for the gauche rotamers have been ascribed to an interplay of the inductive and hyperconjugative effects. The abnormal negative νCOgauche shifts and the n→π*CO band bathochromic shifts of the ω-fluoro- and ω-methoxy-acetophenones, and the observed shielding effect on the α-methylene carbon atom of the studied acetophenones, are discussed in terms of π*CO/σCX and π*CO/X1p orbital overlapping. The higher stabilization of the ω-ethylthioacetophenone gauche rotamer, the largest negative νCOgauche shift, and the lower shielding effect for its methylene carbon have been interpreted as due to the simultaneous occurrence of π*CO/σCS and πCO/σCS orbital interactions.

Conformational Studies of α-Substituted Carbonyl Compounds by I. R. Spectroscopy. II. α-Heterosubstituted N, N-Diethylacetamides

Abstract The α-heterosubstituted N, N−diethylacetamides(1–8) cis/gauche conformations population ratio have been estimated through the analysis of the carbonyl group stretching vibration in the carbon tetrachloride and in chloroform. The substituent steric properties determine the cis/gauche ratio which decreases as the substituent size increases. For most of the studied amides, the gauche rotamer carbonyl band exhibits larger shifts to lower frequencies than in the corresponding α-heterosubstituted propanones1, but for the α-chloro-(4) and for the α-bromo-N,N−diethylacetamide(5) a reverse trend is observed. This behavior can be related to changes in the πC=O C—X Dihedral angle, an hyperconjugative effect being sugges ted to explain the frequency shifts of the gauche rotamers.

Conformational and electronic interaction studies of 2-fluoro-substituted N, N-dimethylacetamides

Abstract The ν CO IR analysis of the 2-fluoro-substituted N,N -dimethylacetamides RC(O)NMe 2 [RFCH 2 ( 1 ), F 2 CH ( 2 ) and F 3 C ( 3 )] and of the parent N,N -dimethylacetamide [RCH 3 ( 4 )] in solvents of increasing polarity suggests the occurrence of Fermi resonance for 2 , 3 and 4 and of cis – gauche rotational isomerism for 1 . The 1st overtone analysis along with ab initio calculations at MP2/6−31+G(d,p) level are in agreement with the existence of the cis and gauche conformers for 1 and indicate the occurrence of cis – gauche and gauche – gauche conformers for 2 and of a single cis – gauche – gauche rotamer for 3 . The stabilisation of the gauche rotamer with respect to the cis rotamer for 1 and of the gauche – gauche rotamer over the cis – gauche rotamer for 2 , along with the unique cis – gauche – gauche rotamer for 3 is discussed in terms of a −I inductive effect of the CF n group, a Repulsive Field Effect between the CO and C−F dipoles and the orbital interaction between one or two fluorine 2p lone pairs (in the gauche conformation) and the π CO ∗ orbital. Eigenvector analysis shows that the fluorine 2p orbital coefficient at the LUMO increases progressively, going from the cis – gauche – gauche rotamer of 3 to the cis – gauche rotamer of 2 , to the gauche – gauche rotamer of 2 and to the gauche rotamer of 1 . Carbonyl frequency gauche shifts (Δ ν ) are in line with this trend and support the π CO ∗ / n F orbital increases in the same direction.

Preferred conformations of methylthio-acetone and -acetophenone, methylsulphonyl-acetone and -acetophenone, from molecular-mechanics and dipole-moment techniques

Abstract Preferred conformations for the title compounds are determined from calculated molecular-mechanics energies in the gaseous phase and analysis of the dipole moments measured in solvents of low dielectric permittivity, In all cases, the gauche conformer having the MeS bond situated far away from the Me(C) (or Ph(C)) group, and nearly bisecting one of the C(O)CH valleys, is predominant.

Interaction between the carbonyl group and a sulphur atom. Part 9. The relationship between conformation and ground- and excited-state interactions in some α-sulphur-substituted cycloalkanones

The conformations of some α-sulphur-substituted cycloalkanones are estimated approximately from the i.r. spectra. The decrease of the carbonyl stretching frequency in these compounds, by comparison with the corresponding unsubstituted ketones (ΔνCO), is reported and interpreted as due to hyperconjugative σC(2)–S–πCO interaction on the basis of the dependence on the position of C(2)–S bond. Similarly, hyperconjugative interaction in the excited state is proposed to account for the bathochromic shift of the n→π* band, at 300 nm, of the same compounds. The shorter wavelength u.v. absorption band, at 250 nm, is assigned to charge transfer from sulphur to the carbonyl group owing to its dependence on the position of the 3p orbital of sulphur relative to the carbonyl group.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XIV. α-(ARYLSULFONYL)-p-SUBSTITUTED ACETOPHENONES[1]

Abstract The νCO IR analysis of α-(p-phenylsulfonyl)-p-substituted acetophenones X-⊘C(O)CH2S(O)2⊘-Y 1–8, being X and Y = NO2, H and OMe substituents, supported by Molecular Mechanics data of the α-methylsulfonylacetophenone (model compound), indicates the existence of the cis/gauche rotational isomerism. The less polar gouche 2 rotamer is the more stable (conc. ≅ 90%) and the more polar cis rotamer is the less stable one along the series. Compounds 4 (X = Y = H) and 3 (X = H, Y = OMe) present another less stable and rather polar gauche 1 rotamer. The almost constant carbonyl gauche 2 shifts (Δνg2) together with the quasi-invariability of the cis/gauche ratios, when X varies and Y is fixed, indicates a very nearly constant summing up of π∗CO/[sgrave]C-SCO2 and π∗CO/[sgrave]C-SCO2 orbital interactions. The progressive stabilization of the gauche 2 rotamer when X is fixed and Y changes from NO2 to OMe substituents, along with the parallel decrease of the positive sulfonyl asymmetric frequency shifts (ΔνSO2(a...

Conformational and electronic interaction studies of some para-substituted S-phenyl α-ethylsulfonylthioacetates

Abstract The preferred conformations of some p -substituted S -phenyl α-ethylsulfonylthioacetates, p -X-Ph-SC(O)CH 2 SO 2 Et (X=NO 2 1 , Cl 2 , Br 3 , H 4 , Me 5 and OMe 6 ) are determined through ν CO infrared analysis and ab initio HF/6-31G** calculations, which show the occurrence of two stable conformations, i.e. the gauche 2 ( syn ) and the gauche 1 ( anti ), with the former being the most stable. The lower frequency and more intense component of the doublet for the carbonyl absorption is then attributed to gauche 2 ( syn ) while the higher frequency and less intense component is attributed to the gauche 1 ( anti ) conformer. Experimental and theoretical data of the corresponding p -substituted- S -phenyl thioacetates (X=NO 2 10 , Cl 11 , Br 12 , H 13 , Me 14 and OMe 15 ), not bearing an α-ethylsulfonyl group, lead to similar results, confirming the frequency assignments. The progressive increase of the gauche 2 ( syn )/ gauche 1 ( s - anti ) infrared population ratio for compounds 1 – 6 , on going from electron attracting to electron donating substituents at the para position, along with larger negative carbonyl frequency shifts (Δ ν ) for the gauche 2 ( syn ) conformer relative to the gauche 1 ( anti ) conformer, and the ab initio data for the corresponding thioacetate methylsulfones 7 – 9 are interpreted in terms of the predominance of the summing up of the O (CO) δ− ⋯S (SO 2 ) δ+ and H (SO 2 Me) δ+ ⋯O (CO) δ− interactions over the O (SO 2 ) δ− ⋯C (CO) δ+ interaction which stabilises the gauche 2 ( syn ) conformer into a larger extent than the gauche 1 ( anti ) conformer. X-ray single crystal diffraction analyses for the S -phenyl- ( 4 ) and p -methyl- S -phenyl α-ethylsulfonylthioacetates ( 5 ) indicate that these compounds exist in the solid state as a slightly distorted gauche 2 ( syn ) conformation, which is stabilised by dipole moment coupling along with intermolecular hydrogen bond interactions.