The rates of base cleavage of substituted 2-thienyltrimethylsilanes. AB initio calculations of the acidities of monosubstituted thiophens

Abstract

Rates of cleavage in NaOMe-MeOH at 50°C have been determined for the mono-substituted 2-trimethylsilylthiophens X·C 4H 2S·SiMe 3-2 with X = H, 3-NO 2, 3-Br, 4-Br-, and 5-NO 2, -CN, -COPh, -Me, -OMe and -NMe 2, and for one disubstituted compound, 4,5-Br 2-C 4HS-SiMe 3-2. For several compounds the rate and product isotope effects have also been determined. The energies involved in the process X·C 4H 3S → X·C 4H 2S − for deprotonation at the 2-position have been calculated by an ab initio method (STO-3G), and the results used in analysis of the substituent effects in the cleavage of the XC 4H 2S· SiMe 3-2 compounds. The results are consistent with the view the aryl anions separate in the rate determining step of the cleavage. The isotope effects are all close to unity, whereas by comparison with results for XC 6H 4CH 2SiMe 3 and related species of similar reactivity, values in the region of 10 would be expected for the 3- and 5-NO 2 compounds if the product isotope effect for a carbanion R depended only in the p K a of RH. The difference between the thienyl- and benzyl-type anions is attributed to the absence of conjugative delocalization of the charge in the former. Approximate p K a values are derived for the 2-positions of the X-C 4H 3S compounds.