Chemical reactions mediated by heavy metal ions. 2. Mercury ligation effects on the mercury(II)-promoted hydrolyses of benzaldehyde O-ethyl S-ethyl and S-phenyl acetals
作者:James L. Jensen、David F. Maynard、Gregory R. Shaw、Tyrrell W. Smith
DOI:10.1021/jo00033a016
日期:1992.3
The kinetics of the mercury(II)-promoted hydrolyses of acyclic O,S-acetals of benzaldehyde have been measured to establish that (i) mercury(II) is a reagent, not a catalyst, in the hydrolysis reaction, (ii) the rate-limiting step can be any of the three steps in Scheme I (k(hemi), k(Hg), or k(acetal)), depending on the concentration of mercury(II) relative to the acid, and (iii) the state of complexation of mercury(II) is critical to its reactivity. Part 1 of this series5 established the ''standard state'' for all further work; this paper utilized the same controls and safeguards. The second-order rate constants for hydrolysis of benzaldehyde O-ethyl S-ethyl acetal promoted by Hg2+, HgCl+, HgBr+, or HgI+ is at the diffusion limit (10(9) M-1 s-1). Rate constants for HgCl2, HgBr2, HgI2, HgCl3-, and HgI3- are 10(6), 10(5), 10(2), 10(4), and 50 M-1 s-1, respectively. Rate constants for hydrolysis of benzaldehyde O-ethyl S-phenyl acetal are the same, within experimental error, except for perhaps the least reactive reagents. Thus, although reagents differ in reactivity by 10(7), the reactivity toward a SEt moiety is equal to the reactivity toward a SPh moiety. A detailed mechanism is presented in which the rate-limiting step does not involve carbon-sulfur bond heterolysis, but rather one of the mercury ligation steps.