多取代苯的自位硝化和溴化反应

IPSO Nitration and Bromination of the Multi-substituted Benzene

系统地研究了苯环取代烷基和侧链取代基对自位硝化反应的影响，详细地讨论了脱溴甲基的机理。多取代苯的正常硝化与自位硝化是一对竞争反应；侧链酯基的存在是自位硝化的必要条件；苯环上取代基的空间效应是自位硝化的充分条件。六元环过渡态很可能为自位硝化反应提供了一个能量上有利的途径。

In order to study substituent effects on the substitution reactions of 10 multi-sub-stituted benzene,the relative yields of the products from their nitration and bromination in different solventsare investigated.The esteric group(X=-CH2OAc) of compound 2(R1=t-Bu and R2=-Me)vields 60%-60% ipso-product 2b(X=NO2,R1=t-Bu and R2=Me) for the nitration in a solution of 65% nitric acid and Ac2O, and 84% ipso-roduct 3b(X=Br, R1=-t-Bu and R2=Me)for the bromination in a solution of 65% nitric acid,NaBr and EtOH. Under the same conditions.the esteric group(X=CH2ONO2) of the compound 4(R1=t-Bu and R2=Me)gives 30% 2b and 95% 3b respectively. An interesting bromination of compound 3(R1=t-Bu,R2=Me and X=CH2Br)in a solution of 65% nitric acid and EtOH yields 95% ipso-product3b. Firstiy, this bromination proceeds practically through a intermediate 4 which resulted from the replacement of Br￣- by NO and then the esteric group(X=CH2ONO2)of intermediate 4 is substituted by Br2 resulting from the oxidation of Br￣- by nitric acid. No matter what the alkyl groups,R1 and R2, are, no ipso-product was found from the nitration of thecompounds,where X=-CH2Cl and-CH2Br in acetic anhydride,which means that the esteric groups,-CH2OAc and-CH2ONO2,and only them are ipso-directors in the case of thepresented compounds, The esteric groups would seem to make it possible for the reactions toproceed through the six-membered cyclic transition states such as: Steric effect of the alkyl groups helps the esteric group to yield ipso-product.the group,X=-CH2OAc, of compounds 8(R1=R2=Me) gives only 5% ipso-product for the nitration.