Aromatic reactivity. Part XL. Additional substituent effects in protodetrimethylsilylation

Abstract

Rates of cleavage, k_rel, relative to that of phenyltrimethylsilane have been measured spectrophotometrically at 50·0° for some X·C₆H₄·SiMe₃ compounds. In methanol–aqueous perchloric acid (5 : 2, v/v) values of k_rel for various substituents (X) are: m-Bu^t 3·86, m-SiMe₃ 1·63 (the observed relative rate has been halved to allow for the availability of two equivalent aryl–SiMe₃ bonds), m-OH 0·87, m-OPh 0·36, m-SMe 0·19, and p-SMe 78. In acetic acid–aqueous sulphuric acid (4 : 3, v/v), values of 10⁴k_rel are m-Cl 119, m-Br 117, m-CO₂Me 11, m-CF₃ 21, p-CO₂Me 18·8, m-SO₃H 14·5, p-SO₃H 9·0, m-NO₂ 3·7, and p-NO₂ 1·4. The value of k_rel for the p-CO₂H group varies with the strength of the sulphuric acid, possibly because of hydrogen bonding between the group and the acidic medium. For both media, values of log k_rel show an excellent linear correlation with the quantity [σ+ 0·65 (σ⁺–σ)].

The order of deactivation by strongly electron-withdrawing substituents, reported for the first time for an electrophilic aromatic substitution, is the same for meta- and para-substituents, viz. NO₂ > Me₃N⁺ > SO₃H > CF₃ > p-CO₂H > CO₂Me > Cl.