Synthesis and stereochemical behavior of unsymmetrical tetraarylbismuthonium salts

The synthesis and stereochemical behavior of unsymmetrical tetraarylbismuth onium salts were investigated. Two methods (tin and boron methods) for synt hesizing unsymmetrically substituted tetraarylbismuthonium salts were devel oped. In the tin method, successive treatment of triarylbismuth difluorides (1; (Ar3BiF2)-Bi-1) with trimethylsilyl cyanide and aryltri-n-butylstannan es (2; Bu3SnAr2) in the presence of a Lewis acid (BF3. OEt2 or Me3SiOTf) in boiling dichloromethane afforded tetraarylbismuthonium salts (3, [(Ar3ArBi +)-Ar-1-Bi-2][BF4-], or 5, [(Ar3ArBi+)-Ar-1-Bi-2][OTf-]) in 44-85% yield. I n contrast, in the boron method, similar treatment of 1 with arylboronic ac ids (4; (ArB)-B-2(OH)(2)) in the presence of BF3. OEt2 afforded the tetrafl uoroborates 3 in 55-99% yield at room temperature. Both methods were used t o synthesize unsymmetrical tetraarylbismuthonium salts (9; (ArArArArBi+)-Ar -1-Ar-2-Ar-3-Bi-4][BF4-]) starting from unsymmetrical triarylbismuth difluo rides 8. The boron method was also used. to synthesize unsymmetrical tetraa rylbismuthonium tetrafluoroborates 14 bearing an oxazoline group at the ort ho position ([(4-MeOC6H4)(4-CF3C6H4)(2-OxC(6)H(4))ArBi+][BF4-]; Ox = 4,4-di methyl-3,4-dihydrooxazol-2-yl; a, Ar = 4-MeC6H4; b, Ar = 2-C4H3S). The reac tion of bismuthonium tetrafluoroborate 14b with ammonium tosylate afforded bismuthonium tosylate 15, which, on treatment with sodium halides, was conv erted to the corresponding halides 16 ([(4-MeOC6H4)(4-CF3C6H4)(2-OxC(6)H(4) )(2-C4H3S)Bi+][X-]; a, X = Cl; b, X = Br; c, X = I). Tetrafluoroborates 14 and tosylate 15 are thermally stable, but halides 16 are unstable and under went ligand coupling in solution. The X-ray diffraction analysis of a tetra fluoroborate 20 ([(4-MeC6H4)(3)(2-OxC(6)H(4))Bi+][BF4-]) revealed a distort ed tetrahedral geometry at the bismuth center, which was coordinated by the neighboring oxazoline nitrogen atom. The stereochemical behavior of the sy nthesized bismuthonium salts was investigated, The signals due to the diast ereotopic geminal methyl groups on the oxazoline ring of 14 and 15 did not coalesce in the H-1 NMR spectra in 1,2;dichlorobenzene-d(4) (up to 150 degr ees C), in DMSO-d(6) (up to 135 degrees C), and in pyridine-d(5) (up to 110 degrees C), whereas those of chloride 16a and bromide 16b coalesced in pyr idine-d(5), 1,2-dichlorobenzene-d(4), chlorobenzene-d(5), and toluene-d(8). The coalescence temperature (T-c) depended on the nucleophilicity of the c ounteranions as well as on the polarity of the solvents; T-c decreased as t he nucleophilicity of the counteranions increased or as the polarity of the solvent decreased. Thus, the configuration at bismuth in tetrafluoroborate s 14 and tosylate 15 was found to be stable, whereas that in halides 16 str ongly depended on the solvent polarity. The observed permutation of bismuthonium halides, [(ArArArArBi+)-Ar-1-Ar-2- Ar-3-Bi-4][X-], can be explained by a pseudorotation mechanism involving a nonionic pentacoordinate species of the type (ArArArArBiX)-Ar-1-Ar-2-Ar-3-B i-4.