Experimental and Theoretical Investigation of the Enantiomerization of Lithium α‐ tert ‐Butylsulfonyl Carbanion Salts and the Determination of Their Structures in Solution and in the Crystal

Dynamic NMR (DNMR) spectroscopy of [R 1 C(R 2 )SO 2 R 3 ]Li (R 1 , R 2 = alkyl, phenyl; R 3 = Ph, t Bu, adamantyl, CEt 3 ) in [D 8 ]THF has shown that the S ‐ t Bu, S ‐adamantyl, and S ‐CEt 3 derivatives have a significantly higher enantiomerization barrier than their S ‐Ph analogues. C α –S bond rotation is most likely the rate‐determining step of the enantiomerization of the salts bearing a bulky group at the S atom and two substituents at the C α atom. Ab initio calculations on [Me(Ph)SO 2 t Bu] – gave information about the two C α –S rotational barriers, which are dominated by steric effects. Cryoscopy of [R 1 C(R 2 )SO 2 t Bu]Li in THF at –108 °C revealed the existence of monomers and dimers. X‐ray crystal structure analysis of the monomers and dimers of [R 1 C(R 2 )SO 2 t Bu]Li · L n (R 1 = Me, Et, t BuCH 2 , PhCH 2 , t Bu; R 2 = Ph, L = THF, 12‐crown‐4, PMDTA) and [R 1 C(R 2 )SO 2 Ph]Li · 2diglyme [R 1 = R 2 = Me, Et; R 1 –R 2 = (CH 2 ) 5 ] showed them to be O–Li contact ion pairs (CIPs). The monomers and dimers have a C α –S conformation in which the lone‐pair orbital at the C α atom bisects the O–S–O angle and a significantly shortened C α –S bond. The C α atom of [R 1 C(R 2 )SO 2 R 3 ]Li · L n (R 1 = Ph; R 3 = Ph, t Bu) is planar, whereas the C α atom of [R 1 C(R 2 )SO 2 R 3 ]Li · L n (R 1 = R 2 = alkyl) is strongly pyramidalized in the case of R 3 = Ph and most likely planar for R 3 = t Bu. Ab initio calculations on [MeC(Me)SO 2 R] – gave a pyramidalized C α atom for R = Me and a nearly planar one for R = CF 3 and t Bu. The [R 1 C(R 2 )SO 2 t Bu]Li salts were characterized by 1 H, 13 C, and 6 Li NMR spectroscopy. 1 H{ 1 H} and 6 Li{ 1 H} NOE experiments are in accordance with the existence of O–Li CIPs. 1 H and 13 C NMR spectroscopy of [R 1 C(R 2 )SO 2 t Bu]Li in [D 8 ]THF at low temperatures showed equilibrium mixtures of up to five different species being most likely monomeric and dimeric O–Li CIPs with different configurations. According to 7 Li NMR spectroscopy, the addition of HMPA to [MeC(Ph)SO 2 t Bu]Li in [D 8 ]THF at low temperatures causes the formation of the separated ion pair [MeC(Ph)SO 2 t Bu]Li(HMPA) 4 .