Aromatic desilylation reactions.

2015-11-19T08:45:14Z (GMT) by Patricia M. Greasley
The literature on cleavages of the aryl-silicon bond by electrophilic reagents is reviewed. The dependence of the rates of cleavage of p-methoxy-and p-methyl- phenyltrimethylsilanes, by 24.4 moles % ethanol-water-HC1, on the acidity of the medium was studied and a plot of log k against the Ho, acidity function was found to be a straight line of slope 1.15. When increasing amounts of water were added to trifluoro-acetic acid the rate of cleavage of p-chlorophenyltrimethylsilane was found to Pass through a maximum at a ratio of 6.3 molecules of trifluoroacetic acid to 1 of water. The acidity function measured with 2,4-dichloro-6-nitroaniline, was shown to pass through a similar maximum at a trifluoroacetic acid to water ratio 5:1. P-Chlorophenyltrimethyl-silane was cleaved 7.2 times more slowly by deuterotrifluoroacetic acid than by ordinary acid. Compounds of the type p-R3M.C6H4 (where M=Si, Ge, Sn and Pb) were cleaved by hydrogen chloride in dioxan/H2O and dioxan/D2O mixtures. The rates were slower in the deuterium containing medium and the isotope effect kH/kD increased in the order (M = Si > (o-Tolyl)3Si-. Five new biphenylyltrimethylsilanes, 41-X C6H4C6H4SiMe3-4, were prepared. The rates of removal of the trimethylsilyl group in CH3CO2H-H2SO4-H2O media, decrease in the expected order (X = MeO→Me>h>C1>Br>CO2Me>CO2H>NO2). The effects of substituents are smaller than those found for the cleavage of the corresponding C6H4SiMe3, compounds. The rates can be more satisfactorily related to 6 than 6+ constants, but the information obtained cannot be used to decide definitely whether whether tautomeric effects are transmitted directly from one ring to the other. The rates of cleavage of 28 substituted phenyltrimethylsilanes, in MeOH-HC1O4-H2O and CH3CO2H-H2SO4-H2O media, were examined. Quantitative values of the relative effects of substituents on reactivity were obtained, and these have been analysed in terms of 6+ constants, and by means of the Tsuno-Yukawa equation. Possible mechanisms for the cleavages are discussed in the light of the above results.

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