[2,3]-THIA-WITTIG REARRANGEMENTS OF ALPHA-LITHIATED SULFIDES VIA DE-AROMATIZED CYCLOHEXADIENE INTERMEDIATES PROCEED WITH INVERSION OF CONFIGURATION AT THE CARBANIONIC CENTER

The nBuLi-induced tin/lithium exchange reactions of the diastereomeric gamma-[(methoxyethoxy)methoxy]-alpha-(tributylstannyl) sulfides anti- and syn-8 delivered the alpha-(lithioalkyl) benzyl sulfides anti- and syn-11, respectively. Within 1 h at -78-degrees-C, these species underwent [2,3]-thia-Wittig rearrangements in THF via the de-aromatized cyclohexadiene intermediates syn- and anti-12. Tautomerization and protonation yielded 81:19 (85%) and 19:81 mixtures (96%) of the ortho-functionalized toluenes syn- and anti-13. Probably, both Wittig rearrangements proceeded with 100% inversion of configuration at the carbanionic stereocenter, and the overall loss of stereoselectivity is due to an epimerization anti-11 reversible syn-11 of the lithio sulfide intermediates at a rate which is 3.3 times slower than the rearrangement.