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

• Published in: Journal of the American Chemical Society Vol. 118; no. 19; pp. 4622 - 4630
• Main Authors: Raabe, Gerhard, Gais, Hans-Joachim, Fleischhauer, Jörg
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.05.1996
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja953034t

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.