Aromatic reactivity. Part LXI. Reactivities and solvent isotope effects in the base cleavage of triorgano-silicon, -germanium, and -tin groups from furan, thiophen, benzofuran, and benzothiophen rings

Abstract

First-order constants have been measured for the cleavage of some heterocyclic ArMR3 compounds by methanolic sodium methoxide at 50°C, along with the values, p.i.e., of the product ratio ArH : ArD obtaine on cleavage in 1 : 1 MeOH–MeOD, and in some cases the values, r.i.e., of the ratio of the rates of cleavage in MeOH and MeOD, respectively. The values of the specific rate constants, ks, increase with base concentration, especially at higher concentrations. For ArSnMe3 compounds the values of 102ka([MeONa]= 0.001 5M) p.i.e. any r.i.e. are (Ar =) 2-furyl, 18.0, 4.3, 2.0; 2-thienyl, 11.4, 3.95. 1.90; 2-benzofuryl, 41, 3.5, 1.46; and 2-benzothenyl, 23.8. 3.6,–. The corresponding values of 106ka([NaOMe]= 1M), p.i.e. and r.i.e. for SiMe3 compounds are (Ar) 2-furyl, 2.75,1.20, 0.495; 2-thienyl. 7.5, 1.05, –; 2-benzofuryl, 98, 1.2, 0.46; and 2-benzothienyl, 117. 1.2, 0.46. The r.i.e.: p.i.e. ratios fall in the range 0.40–0.49. The ArSiMe3 are ca. 130–190 times as reactive as the ArSiEt3 compounds. For ArSnR3 compounds the corresponding ratios lie in the range 22–32, while the ratio for R = Me and Pri is ca. 16 000 for Ar = 2-thienyl and 8 000 for Ar = 2-benzothienyl. For 2-benzothienyltrimethlgermanehylgermane the values of ka([MeONa]= 2M) and p.i.e. are 0.017 and 1.19. and the germanium compound is estimated to be ca. 1 300 times less reactive than its silicon analogue. The results are consistent with substantial electrophilic assistance by proton transfer to the leaving carbon atom in the rate-determining step of the cleavece of the ArSnR3 and little, if any, such assistance in cleavage of the ArSiR3 and ArGeMe3 compounds.