Experimental and Computational Studies of the Structure of Sulfonimidoyl Vinyllithiums

tBuCH=C(Li)S(O)(NSO₂ Tol)Ph⋅L (L=2THF, TMEDA) (1⋅L) in THF solution is a monomer with a C-Li bond according to NMR spectroscopy and cryoscopy. It was identified as CIP through the scalar ¹³ C,⁶ Li coupling and ⁶ Li,{¹ H} NOE experiments. The CIP has a six-membered C-Li-O-S-N-S chelate ring structure. ⁶ Li,¹ H FUCOUP and ⁶ Li,¹ H HMQC NMR experiments of 1⋅TMEDA revealed a scalar ⁶ Li,¹ H coupling across the Li-C=C-H bonds. According to the NMR data the π-bond of 1⋅L is polarized by the negative charge of the anionic C atom. tBuCH=C(Li)S(O)(NMe)Ph (2⋅L) is most likely also a monomer with a C-Li bond. According to ⁶ Li,{¹ H} NOE experiments it has a four-membered C-Li-N-S chelate ring structure. ¹³ C NMR spectroscopy showed the C-Li bonds of 1⋅L and 2⋅L to be fluxional. ¹ H NMR spectroscopy and 1D TOCSY experiments of Ph₂ C=C(Li)S(O)(NSO₂ Tol)Ph revealed topomerization of the phenyl groups, which is attributed to a fast positional exchange of the Li atom and the sulfonimidoyl group. The fluxionality of the C-Li bond and the interchange of the Li atom and the sulfonimidoyl group at the anionic C atom of sulfonimidoyl vinyllithiums, which result in a low configurational stability, most likely involve the formation of O,Li and N,Li CIPs through heterolysis of the C-Li bond. Ab initio calculation of MeCH=C(Li)S(O)(NMe)Ph yielded an energy minimum structure with a C-Li bond, a four-membered C-Li-N-S chelate ring and a strongly expanded C=C-Li bond angle. According to calculation of MeCH=C(Li)S(O)(NMe)Ph, [MeCH=CS(O)(NMe)Ph]^- and MeCH=C(H)S(O)(NMe)Ph deprotonation is not accompanied by a shortening of the C-S bond. Ab initio calculation of MeCH=C(Li)S(O)(NSO₂ Me)Ph gave a structure with a C-Li bond and a six-membered C-Li-O-S-N-S chelate ring. ⁶ Li,¹ H NOE experiments and cryoscopy of LiCH₂ S(O)(NSO₂ Tol)Ph (3) revealed a monomeric CIP with a C-Li bond. The CIP has a six-membered C-Li-O-S-N-S chelate ring structure found in polymeric 3 in the crystal.