Ketene-Forming Elimination Reactions from Aryl Thienylacetates Promoted by R₂NH in MeCN. Effects of Base-Solvent and β-Aryl Group

Abstract

have been studied kinetically. The reactions are second-order and exhibit Brönsted β = 0.51-0.62 and |β lg | = 0.47-0.53. Hence, an E2 mechanism is evident. The Brönsted β increased from 0.33 to 0.53 and |β lg | remained nearly the same by the change of the base-solvent from Bz(i-Pr)NH/Bz(i-Pr)NH 2 + in 70 mol% MeCN(aq) to Bz(i-Pr)NH-MeCN, indicating a change to a more symmetrical transition state with similar extents of C β -H and C α -OAr bond cleavage. When the β-aryl group was changed from thienyl to phenyl in MeCN, the β value increased from 0.53 to 0.73 and |β lg | decreased from 0.53 to 0.43. This indicates that the transition state became skewed toward more C β -H bond breaking with less C α -OAr bond cleavage. Noteworthy is the greater double bond stabilizing ability of the thienyl group in the ketene-forming transition state.