Synthesis, Structure, and Reactions of Triaryl(methyl)bismuthonium Salts

Treatment of triarylbismuth difluorides 2 (Ar₃BiF₂; a, Ar = Ph; b, Ar = 4-MeC₆H₄; c, Ar = 4-MeOC₆H₄; d, Ar = 2-MeOC₆H₄) with methylboronic acid (3) in the presence of BF₃·OEt₂ in CH₂Cl₂ afforded the corresponding triaryl(methyl)bismuthonium tetrafluoroborates 4a−d ([Ar₃MeBi⁺][BF₄^-]) in 42−91% yield. X-ray crystallographic analysis of compound 4d revealed that the bismuth center possesses a distorted tetrahedral geometry with C−Bi−C bond angles of 106.1(3)−113.6(3)° and Bi−C bond lengths of 2.182(7)−2.195(8) Å. Compound 4a transferred the methyl group to Ph₃E (E = P, As, Sb), tris(4-methylphenyl)bismuthine, ROH (R = Me, Et, i-Pr, PhCH₂), water, sodium benzenesulfinate, sodium benzoate, N,N-dimethylformamide (DMF), and thioacetamide to give the corresponding methylated products with a good recovery of triphenylbismuthine. The pseudo-first-order rate constant (k_obsd = 2.9 × 10^-4 s^-1) observed for the reaction between 4a and benzyl alcohol (5d) was about twice as large as that (k_obsd = 1.3 × 10^-4 s^-1) between MeOTf and 5d (in CDCl₃ at 23 °C; [4a] or [MeOTf] = 0.062 M; [5d] = 0.97 M). The observed reactivity of 4a clearly demonstrates the high nucleofugality of the triphenylbismuthonio group.