Experimental and Computational Studies of the Structure of Sulfonimidoyl Vinyllithiums

tBuCH=C(Li)S(O)(NSO2Tol)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 13C,6Li coupling and 6Li,{1H} NOE experiments. The CIP has a six-membered C-Li-O-S-N-S chelate ring structure. 6Li,1H FUCOUP and 6Li,1H HMQC NMR experiments of 1⋅TMEDA revealed a scalar 6Li,1H 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 6Li,{1H} NOE experiments it has a four-membered C-Li-N-S chelate ring structure. 13C NMR spectroscopy showed the C−Li bonds of 1⋅L and 2⋅L to be fluxional. 1H NMR spectroscopy and 1D TOCSY experiments of Ph2C=C(Li)S(O)(NSO2Tol)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)(NSO2Me)Ph gave a structure with a C−Li bond and a six-membered C-Li-O-S-N-S chelate ring. 6Li,1H NOE experiments and cryoscopy of LiCH2S(O)(NSO2Tol)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.