Theoretical Studies of Transition Structures and Stereoselectivities of the [2,3]-Wittig Rearrangement of Sulfur Ylides

Transition structures of the [2,3]-Wittig rearrangements of allylsulfonium methylide and three substituted cases have been located with the 3-2lG(*) and 6-31+G* basis sets. The transition structures have envelope conformations with the partially formed C-C bond nearly eclipsed with the partially broken C-S bond. The forming C-C bond is only formed to a small extent, corresponding to an early transition structure. The lone pair on sulfur prefers to be exo (away from the allyl fragment), primarily to minimize electrostatic repulsions. A formyl group at the anionic center is more stable in the endo configuration, while a methyl substituent prefers the exo configuration. The stereoselectivity of ring expansions of sulfur ylides can be rationalized by the combination of ring strain and the sulfur lone pair exo preference.