Ab Initio Study of the Effect of Fluorination upon the Structure and Configurational Stability of α-Sulfonyl Carbanions:  The Role of Negative Hyperconjugation

Abstract

Ab initio calculations of the (methylsulfonyl)methyl anion (1), the ((trifluoromethyl)sulfonyl)methyl anion (1T), the (fluorosulfonyl)methyl anion (1F), and the (methylsulfonyl)isopropyl anion (1D) at the HF/6-31+G*//HF/6-31+G* level revealed a significant effect of fluorine substitution upon the structure and energy of α-sulfonyl carbanions. The Cα−S bond in 1T and 1F is shorter and the pyramidalization of the anionic carbon atom is less than in 1. In the anions 1T and 1F the Cα−S bond is shortened and the S−CF3(F) and the S−O bonds are lengthened as compared to the sulfones 3T and 3F. For all anions the staggered conformation (1, 1T, 1F, 1D) is energetically more stable than the eclipsed conformation (2, 2T, 2F, 2D). At the MP2/6-31+G*//HF/6-31+G* level the energy difference between the staggered and the eclipsed conformation is much larger for the fluorinated anions 1T and 1F than for 1 and 1D. The rotational barriers about the Cα−S bond of the fluorinated species 1T and 1F are in accordance with related experimental results significantly higher than the barriers of the non-fluorinated species 1 and 1D. A Fourier series analysis of the rotational potential curves shows the dominance of a positive V 2 term (conjugative overlap effects) which is much higher for 1T and 1F than for 1 and 1D. Negative hyperconjugation (nC−σ*SR) is an important mechanism which determines the conformation of the anions and particularly of the fluorinated anions. In the dimethyl anion 1D, which has a strongly pyramidalized anionic carbon atom, the V 3 term (steric and torsional effects) also contributes significantly to the rotational barrier. The configurational stability of chiral α-sulfonyl carbanions thus depends on the height of the Cα−S rotational barrier which is determined by nC−σ*SR interaction and the steric contribution of the substituents. The calculations suggest that S-(trifluoromethyl) substitution of other S-stabilized carbanions should lead also to derivatives of a higher configurational stability.

Syntheses and structures of lithiated sulfones Li[CR(R′)SO2Ph] – C versus O bound lithium. [{Li{CH(Me)SO2Ph}(thf)}∞] – The structure of a C-bound derivative

“…In 13b and 13b AE 2Me 2 O the lone pair at the C atom is gauche to both oxygen atoms. Thus, a negative hyperconjugation ðn C -r à SPh Þ between the anionic lone pair at C a and the orbitals of the S-Ph bond seems to be of major importance for the carbanion stabilization [16][17][18]. In 13a and 13a AE 2Me 2 O the torsional angles C3-S-C1-Li (À134.9/ À 143.7°) point to reduced negative hyperconjugation.…”

A Persistent α‐Fluorocarbanion and Its Analogues: Preparation, Characterization, and Computational Study

“…Besides these two differences, the structure of II-N is essentially consistent with the properties of a-sulfonyl carbanions described by Gais and co-workers. [14] For instance, the average SÀC17 bond length (1.716 ) in II-N is noticeably shorter than the corresponding one in sulfone [13] whereas the SÀO bonds of II-N are slightly longer. Further NMR spectroscopy studies of II with different counterions (Li + (II-Li), Na + (II-Na), and nBu 4 N + (II-N)) in [D 6 ]DMSO reveal the liquid-phase behavior of the anion (Table 1) …”

The Ambident Reactivity of 2,4,6‐Tris(trifluoromethanesulfonyl)anisole in Methanol: Using the SO2CF3 Group as a Tool to Reach the Superelectrophilic Dimension in σ‐Complexation Processes

“…The ratios k The mode of activation of the SO 2 CF 3 group: How the SO 2 CF 3 group exerts its exceptional capability to stabilise a negative charge has recently received much attention. [41][42][43][44][45] Based on the information provided through structural, thermodynamic and kinetic investigations of the ionisation of a number of a-SO 2 CF 3 -activated carbon acids, it has been convincingly demonstrated that the electron-withdrawing effect of the SO 2 CF 3 group must be primarily understood in terms of polarisability effects [resonance structure R1, Eq. (16)], with no significant contribution of other modes of stabilisation such as negative hyperconjugation (R2) or d-p p bonding (R3).…”