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

Isolation and structure determination of a tetrameric sulfonyl dilithio methandiide in solution based on crystal structure analysis and 6Li/13C NMR spectroscopic data

Dilithio sulfonyl methandiides are a synthetically and structurally highly interesting group of functionalized geminal dianions. Although very desirable, knowledge of the structure of dilithio methandiides in solution was lacking up to now. Herein, we describe the isolation and determination of the structure of tetrameric dilithio (trimethylsilyl)(phenylsulfonyl) methandiide in solution and in the crystal. The elucidation of the structure of the tetramer is based on crystal structure analysis and ¹³C/⁶Li NMR spectroscopic data. A characteristic feature of the structure of the tetramer is the C₂ symmetric C–Li chain, composed of four doubly Li-coordinated dianionic carbon and five Li atoms. Three Li atoms are devoid of a contact to a dianionic C atom. The tetramer, the dianionic C atoms of which undergo fast exchange, is in THF solution in fast equilibrium with a further aggregate, which is stable only at low temperatures.

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.

A mechanistic study on multifunctional Fei-Phos ligand-controlled asymmetric palladium-catalyzed allylic substitutions

Mechanistic studies were performed for the clarification of the role of multifunctional Fei-Phos in allylic substitution reactions, in which it is proved to be a diphosphine-controlled catalytic asymmetric allylic alkylation.