Synthesis and X-Ray Structure of Bis{S-(1-lithio-3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine– Tetrahydrofuran (1/2)} Complemented by Model ab initio Calculations of α-Lithiosulfoximines

The gas-phase structure of N,S,S-trimethylsulfoximine 1 and of its monolithiated isomers 2–4 was calculated by ab initio methods. It was found that a Li? C? S? N four-membered chelate 2 is the most stable isomer. The second minimum 4 shows N? Li? O complexation and is only slightly higher in energy. Li-Contacts with the C(α) atom and the sulfoximine O-atom in 3 are energetically disfavored by 6.1 kcal/mol. The two transition states 5 and 6 suggest an interconversion mechanism of 2 to 3 with 4 as an intermediate. A comparison of 4 with the crystal structure of the THF solvate 7 , which was prepared by the addition of BuLi to (±)-S-(3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine ( 8 ) at low temperature in THF, demonstrates that the coordination geometry in the solid state is in good agreement with the calculated structure. The (1-lithioallyl)sulfoximine 7 crystallized as a centrosymmetric dimeric aggregate featuring an eight-membered ring with the atomic sequence (Li? N? S? O)₂. The O-atoms of two THF molecules and the sulfoximine O- and N-atoms are coordinated to the Li-atom in a tetrahedral orientation. After metallation, a significant shortening of the S? C(α) bond is observed. Remarkably, only one of the two possible diastereoisomeric enantiomer pairs is found in the solid state.