SOLVOLYTIC ELIMINATION-REACTIONS OF TERTIARY ALPHA-CSNME2-SUBSTITUTED SYSTEMS

The tertiary benzylic alpha-CSNMe2-substituted p-nitrobenzoates and trifluoroacetates of general structure Ar(CH3)C(CSNMe2)(OCOR), 7 and 8, solvolyze to give exclusively elimination products H2C = C(CSNMe2)Ar. A Hammett study gave a nonlinear correlation. Variation in rate with solvent ionizing power was small for the unsubstituted trifluoroacetate derivative of 8, and the beta-CD3 isotope effect on rate was negligible. There is, however, a large isotope effect (2.5-2.8) in formation of the elimination product when Ph(CH2D)C(CSNMe2)(OCOCF3) solvolyzes. It is concluded that an intermediate must be involved since the product-determining step and the rate-determining step have differing isotope effects. The likely intermediate is an alpha-CSNMe2-substituted cation (as an ion pair), despite the fact that the reaction has few characteristics of a typical E1 reaction. Tertiary norbornyl, cyclohexyl, and 2-propyl alpha-CSNMe2-substituted systems also react to give exclusively elimination products at rates far in excess of alpha-CONMe2 analogues. It is suggested that alpha-CSNMe2 cations are also intermediates and that these cations undergo proton loss at an early ion pair stage. These cations are proposed to derive substantial stabilization by charge delocalization onto sulfur of the thiocarbonyl group. By way of contrast, the secondary system CH3CH(CSNMe2)(OCOCF3), 25, solvolyzes to give mainly a rearranged product CH3CH(CONMe2)(SCOCH3) via a k(DELTA) mechanism involving neighboring thiocarbonyl participation leading to a cyclized ion.