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

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 C α −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 C α −S bond is shortened and the S−CF 3 (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 C α −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 (n C −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 C α −S rotational barrier which is determined by n C −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.