ORTHO-ESTER HYDROLYSIS - DIRECT EVIDENCE FOR A 3-STAGE REACTION-MECHANISM
ORTHO-ESTER HYDROLYSIS - DIRECT EVIDENCE FOR A 3-STAGE REACTION-MECHANISM
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DOI：
10.1021/ja00504a030
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发表时间：
1979-01-01
影响因子：
15
通讯作者：
POWELL, MF
中科院分区：
文献类型：
--
作者：
AHMAD, M;BERGSTROM, RG;POWELL, MF
Direct evidence is supplied for the existence of 1, 3-dioxolenium ion and hydrogen ortho ester intermediates in the acid-catalyzed hydrolysis of a series of 2-aryl-(and 2-cyclopropyl-) 2-alkoxy-l, 3-dioxolanes. These two intermediates require a three-stage reaction path:(1) loss of the exocyclic alkoxy group from the substrate to generate the dioxolenium ion,(2) reac-tion of this ion with water to form hydrogen ortho ester, and (3) breakdown of the hydrogen ortho ester to carboxylic acid prod-ucts. Rate constants for all three stages and equilibrium constants for stage 2 were measured in some cases and were estimated in others. Differential substituenteffects of phenyl and cyclopropyl on stages 1 and 3 are shown tobe responsible for the changes in rate-determining step which these systems undergo between low (stage 3 rate determining) and high (stage 1 rate determining) pH. The purely aliphatic ortho esters 2-methoxy-1, 3-dioxolane and 2-methyl-2-methoxy-1, 3-dioxolane were found not to give this change in rate-determining step.It is now generally accepted that the acid-catalyzed hy-drolysis of ortho esters consists of three separate reaction stages:(1) generation of a dialkoxy carbonium ion,(2) hy-dration of this ion to a hydrogen ortho ester, and (3) breakdown of the latter to alcohol and carboxylic acidester products (eq
