Ketene-forming elimination reactions from aryl thienylacetates promoted by R₂NH in MeCN. Effects of base-solvent and β-aryl group

Ketene-forming eliminations from C₄H₃(S)CH ₂C(O)O-C₆H₃-2-X-4-NO₂ (1) promoted by R₂NH in MeCN 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.