Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides

Abstract

Reasons for the C NMR γ-gauche effect in sulfoxides, i.e., the distinct shielding of a carbon β to a gauche-oriented sulfoxide group were investigated. Several calculated and measuredC NMR data of open chain or thiane-derived sulfoxides revealed that an upfield shift is only observed for that γ-gauche position, in which the respective carbon is anti to the sulfoxide's sulfur lone pair. Carbons in γ-gauche position, which are synclinal to the lone pair, are not affected. The magnetic anisotropy of the S═O group was examined by generation of iso-chemical-shielding surfaces (ICSSs) and magnetically induced current maps. Stereoelectronic interactions were determined with natural bond orbital (NBO) and natural chemical shielding (NCS) analyses. The γ-gauche effect is best described by stereoelectronic interactions, especially those of the sulfur's lone pair with antibonding orbitals to a β-carbon in antiperiplanar orientation. An explanation based on steric interactions, which has frequently been referred to, is not suitable to describe the observed shielding effects. Furthermore, a description of the bonding situation in the S═O group is given. It can be understood as S-O triple bond, where the bond order is significantly reduced by antibonding contributions in some of the occupied molecular orbitals.

Oxidation of Dithia Compounds: Comparative Experimental and Theoretical Studies on 1,3‐Bis(methylthio)propane, Bis(methylthio)methane, and meso‐4,6‐Dimethyl‐1,3‐dithiane

“…Competitive oxidation of monosulfoxides 15 and 16. [a,b] Oxidant [c] 15 16 17 18 Recovered material (yield of oxidation equivalents [d] ) [%]…”

“…R f = 0.43 (CH 2 Cl 2 /MeOH, 10 : 1); 1 H NMR (500 MHz, DMSO-d 6 ): δ = 2.05-2.12 (m, 2 H, 2-H 2 ), 2.56 (s, 3 H, SOCH 3 )" 2.77 (dt, 2 J = 13.0 Hz, 3 J = 7.6 Hz, 1 H, 1-H a ); 2.90 (dt, 2 J = 13.0 Hz, 3 J = 7.9 Hz, 1 H, 1-H b ), 3.00 (s, 3 H, SO 2 CH 3 ), 3.24-3.29 (m, 2 H, 1-H 2 ); 13 C NMR (125 MHz, DMSO-d 6 ): δ = 15.7 (CH 2 ), 22.2 (CH 3 ), 37.9 (CH 2 ), 50.9 (CH 3 ), 52.3 (CH 2 ); IR (ATR): ῦ (cm À 1 ) = 2996 (s), 2912 (vs), 1429 (vs), 1352 (vs), 1288 (s), 1258 (vs), 1191 (s), 1113 (m), 1041 (s), 791 (s). ; MS (EI, 280 °C): m/z (%) = 185 (2) [(M + 1) + ], 184 (14) [M + ], 121 (100), 89 (10), 79 (52), 63 (57), 57 (18); HRMS (EI): calcd. for C 5 H 12 O 3 32 S 2 : 184.0222; found: 184.0221.…”

“…At this stage it could be mentioned that the usually given double bonds in sulfoxides and sulfones should be understood as simplified depictions. The character of these bonds has been investigated previously by us [18] and by others [19] . They are much better described as single bonds with charge separation and an unambiguous depiction would give a resonance hybrid of contributing formulae A and B (Figure 2).…”

Interconnection of sulfides and sulfoxides in medicinal chemistry

“…The formerly discussed participation of free d orbitals of the sulfur atom in bond formation has been erased from current texts but the character of the of the SO bond is still under investigation and open questions prevail. Jung and Podlech recently performed both natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses of the model sulfoxide DMSO in order to rationalize the shielding or deshielding effect on vicinal carbon atoms in NMR spectra. The NBO analysis yielded a list of six localized C–H σ‐bonds, two C–S σ‐bonds, one S–O σ‐bond, and four lone pairs.…”

NOESY and DFT-GIAO Calculations Reveal Pervasive Errors in C20 Configurations of Taraxastane-3,20-diols: Proposals to Improve NMR Structure Determinations

“…Here we establish the C20 configurations of taraxastane-3,20-diols 1 and 2 and implicitly those of their metabolites 3–6 (Figure ) by three methods: DFT-GIAO NMR predictions, γ-gauche effects on 13 C NMR chemical shifts, , and NOESY correlations …”