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

Abstract

Ketene-forming eliminations from C4H3(S)CH2C(O)O-C6H3-2-X-4-NO2 (1) promoted by R2NH in MeCN have been studied kinetically. The reactions are second-order and exhibit Bronsted β =0.51-0.62 and |βlg|= 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)NH2+ 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 Cα -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 |βlg| decreased from 0.53 to 0.43. This indicates that the transition state became skewed toward more Cβ -H bond breaking with less Cα-OAr bond cleavage. Noteworthy is the greater double bond stabilizing ability of the thienyl group in the ketene-forming transition state.