A Mechanistic Study on Acyl Transfer Reaction of Aryl Substituted Benzoates Between Aryloxides

Abstract

Acyl transfer 반응의 메카니즘에 대한 체계적인 연구를 수행하기 위하여 DMSO - H_(2)O혼합용매계에서 aryl benzoate와 aryloxide의 친핵성 치환 반응속도를 분광 광도법으로 측정하였다. 기질의 acyl부분과 친핵체 phenoxide의 치환체를 변화시켰을 때 얻어진 속도상수들은 Bronsted상관관계를 좋은 직선으로 만족시키는 것으로 보아 치환체의 electronic effect에 상당히 민감함을 알 수 있다. 반면에 leaving phenoxide에서의 치환체 변화는 반응성에 별 영향을 보이지 못했다. 이렇게 얻어진 Bronsted coefficient β_(acyl), β_(nuc), β_(lg) 값들의 크기와 부호의 비교로부터 본 acyl transfer반응은 친핵체의 공격이 속도결정단계인 stepwise 메카니즘을 거치는 것으로 해석되었다. β_(acyl)과 β_(nuc) 각각의 값은 Reactivity-Selectivity Principle로 부터 기대하였던 바와는 반대로 반응성이 증가할수록 β 값들이 증가하고 있다. 즉, 예를 들어4-CH_(3)-C_(6)H_(4)O^(-)가 4-CN-C_(6)H_(4)O^(-)보다 반응성이 좋으면서도 β_(acyl)값이 각각 -2.51과 -2.10으로 나타나고 있다. 이러한 사실은 Reactivity-Selectivity Principle이 본 연구 system에서는 적용되지 않음을 의미한다.;Second order rate constants (k_(2)) have been measured spectrophotometrically for reactions of 4-nitrophenyl substituted benzoates with various aryloxides and aryl benzoates with p-chlorophenoxide. Values of k_(2) obey linear Bronsted correlation when the structures of either the acyl group or the attacking phenolate ion are varied. The reactivity has exhibited significant dependences on the electronic nature of the substituent in the acyl moiety of the substrate and in the nucleophilic phenoxide, while the substituent in the leaving phenoxide has little influenced the reactivity. The Bronsted coefficient β values so obtained support that the present acyl transfer reaction proceeds via a stepwise mechanism in which the nucleophilic attack would be the rate-determining step. Interestingly, the magnitude of the β_(acyl) and β_(nuc) increases with increasing reactivity, for example, the β_(acyl) values are -2.55 and -2.06 for 4-CH_(3)-C_(6)H_(4)O^(-), and 4-CN-C_(6)H_(4)O^(-), respectively, although the former is more reactive than the latter. It implies that the reactivity selectivity principle is not operative in the present system. The failure of the reactivity selectivity principle is attributed to a change in transition state structure upon the substituent variation in the present acyl transfer reaction.