Aryl Thienylacetates의 케텐-형성 제거반응 연구

Abstract

Ketene-forming eliminations from C₄H₃(S)CH₂C(O)O-C_(6)H₃-2-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

β_(lg)

= 0.47-0.53. Hence, an E2 mechanism is evident. The Bronsted 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)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 C_(β) -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

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 Cα-OAr bond cleavage. Noteworthy is the greater double bond stabilizing ability of the thienyl group in the ketene-forming transition state.