HYLSILYL-S-PHENYLSULFONIMIDOYL)-3-METHYL-2-BUTENYL LITHIUM-2(12-CROWN-4) - STRUCTURE OF A LITHIATED ALLYLIC SULFOXIMINE IN THE CRYSTAL AND COMPARISON WITH MODEL AB-INITIO CALCULATIONS

The crystal structures of the lithiated allylic sulfoximine 1a(12-c-4) (2), which is a solvent separated contact ion pair, and of the allylic sulfoximine 3b were determined. The anion of 1a(12-c-4)(2), adopts a conformation in which the lone pair orbital at the C-alpha-atom is gau che to the O- and the N-atom. The C-alpha-S bond is considerably short ened. Similar to alpha-sulfonyl carbanions the conformation of alpha-s ulfonimidoyl carbanions seems to be determined by negative hyperconjug ation. Ab initio calculations (6-31+G) of various conformers of the a nion 4 (a-c) suggest a low configurational stability of the C-alpha-at om of 1.