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Ketene-forming eliminations from C4H3(S)CH2C(0)0-C6H3-2-X-4-N02 (1) promoted by R 2 NH in MeCN have been studied kinetically. The reactions are second-order and exhibit Bronsted β=0.51-0.62 and lβlgl= 0.47-0.53. Hence, an E2 mechanism is evident. The Bronsted β increased from 0.33 to 0.53 and |βlg| 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 Cβ-H and C α -OAr bond cleavage. When the /?-aryl group was changed from thienyl to phenyl in MeCN, the /? value increased from 0.53 to 0.73 and lβ lg l decreased from 0.53 to 0.43. This indicates that the transition state became skewed toward more Cβ-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.

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