Chiral Lithiated Allylic α‐Sulfonyl Carbanions: Experimental and Computational Study of Their Structure, Configurational Stability, and Enantioselective Synthesis

X‐ray crystal structure analysis of the lithiated allylic α‐sulfonyl carbanions [CH 2 CHC(Me)SO 2 Ph]Li ⋅ diglyme, [ c C 6 H 8 SO 2 t Bu]Li ⋅ PMDETA and [ c C 7 H 10 SO 2 t Bu]Li ⋅ PMDETA showed dimeric and monomeric CIPs, having nearly planar anionic C atoms, only OLi bonds, almost planar allylic units with strong CC bond length alternation and the s‐trans conformation around C1C2. They adopt a C1S conformation, which is similar to the one generally found for alkyl and aryl substituted α‐sulfonyl carbanions. Cryoscopy of [EtCHCHC(Et)SO 2 t Bu]Li in THF at 164 K revealed an equilibrium between monomers and dimers in a ratio of 83:17, which is similar to the one found by low temperature NMR spectroscopy. According to NMR spectroscopy the lone‐pair orbital at C1 strongly interacts with the CC double bond. Low temperature 6 Li, 1 H NOE experiments of [EtCHCHC(Et)SO 2 t Bu]Li in THF point to an equilibrium between monomeric CIPs having only OLi bonds and CIPs having both OLi and C1Li bonds. Ab initio calculation of [MeCHCHC(Me)SO 2 Me]Li ⋅ (Me 2 O) 2 gave three isomeric CIPs having the s‐trans conformation and three isomeric CIPs having the s‐cis conformation around the C1C2 bond. All s‐trans isomers are more stable than the s‐cis isomers. At all levels of theory the s‐trans isomer having OLi and C1Li bonds is the most stable one followed by the isomer which has two OLi bonds. The allylic unit of the C,O,Li isomer shows strong bond length alternation and the C1 atom is in contrast to the O,Li isomer significantly pyramidalized. According to NBO analysis of the s‐trans and s‐cis isomers, the interaction of the lone pair at C1 with the π* orbital of the CC double bond is energetically much more favorable than that with the “empty” orbitals at the Li atom. The C1S and C1C2 conformations are determined by the stereoelectronic effects n C –σ SR * interaction and allylic conjugation. 1 H DNMR spectroscopy of racemic [EtCHCHC(Et)SO 2 t Bu]Li, [ i PrCHCHC( i Pr)SO 2 t Bu]Li and [EtCHC(Me)C(Et)SO 2 t Bu]Li in [D 8 ]THF gave estimated barriers of enantiomerization of Δ G ≠ =13.2 kcal mol −1 (270 K), 14.2 kcal mol −1 (291 K) and 14.2 kcal mol −1 (295 K), respectively. Deprotonation of sulfone ( R )‐EtCHCHCH(Et)SO 2 t Bu (94 % ee ) with n BuLi in THF at −105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion ( M )‐[EtCHCHC(Et)SO 2 t Bu]Li, the deuteration and alkylation of which with CF 3 CO 2 D and MeOCH 2 I, respectively, proceeded with high enantioselectivities. Time‐dependent deuteration of the enantioenriched carbanion ( M )‐[EtCHCHC(Et)SO 2 t Bu]Li in THF gave a racemization barrier of Δ G ≠ =12.5 kcal mol −1 (168 K), which translates to a calculated half‐time of racemization of t 1/2 =12 min at −105 °C.