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, k_s, increase with base concentration, especially at higher concentrations. For ArSnMe₃ compounds the values of 10²k_a([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 10⁶k_a([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 k_a([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.