Synthesis and Stereochemical Behavior of Unsymmetrical Tetraarylbismuthonium Salts

Abstract

The synthesis and stereochemical behavior of unsymmetrical tetraarylbismuthonium salts were investigated. Two methods (tin and boron methods) for synthesizing unsymmetrically substituted tetraarylbismuthonium salts were developed. In the tin method, successive treatment of triarylbismuth difluorides (1; Ar13BiF2) with trimethylsilyl cyanide and aryltri-n-butylstannanes (2; Bu3SnAr2) in the presence of a Lewis acid (BF3·OEt2 or Me3SiOTf) in boiling dichloromethane afforded tetraarylbismuthonium salts (3, [Ar13Ar2Bi+][BF4-], or 5, [Ar13Ar2Bi+][OTf-]) in 44−85% yield. In contrast, in the boron method, similar treatment of 1 with arylboronic acids (4; Ar2B(OH)2) in the presence of BF3·OEt2 afforded the tetrafluoroborates 3 in 55−99% yield at room temperature. Both methods were used to synthesize unsymmetrical tetraarylbismuthonium salts (9; [Ar1Ar2Ar3Ar4Bi+][BF4-]) starting from unsymmetrical triarylbismuth difluorides 8. The boron method was also used to synthesize unsymmetrical tetraarylbismuthonium tetrafluoroborates 14 bearing an oxazoline group at the ortho position ([(4-MeOC6H4)(4-CF3C6H4)(2-OxC6H4)ArBi+][BF4-]; Ox = 4,4-dimethyl-3,4-dihydrooxazol-2-yl; a, Ar = 4-MeC6H4; b, Ar = 2-C4H3S). The reaction of bismuthonium tetrafluoroborate 14b with ammonium tosylate afforded bismuthonium tosylate 15, which, on treatment with sodium halides, was converted to the corresponding halides 16 ([(4-MeOC6H4)(4-CF3C6H4)(2-OxC6H4)(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 underwent ligand coupling in solution. The X-ray diffraction analysis of a tetrafluoroborate 20 ([(4-MeC6H4)3(2-OxC6H4)Bi+][BF4-]) revealed a distorted tetrahedral geometry at the bismuth center, which was coordinated by the neighboring oxazoline nitrogen atom. The stereochemical behavior of the synthesized bismuthonium salts was investigated. The signals due to the diastereotopic geminal methyl groups on the oxazoline ring of 14 and 15 did not coalesce in the 1H NMR spectra in 1,2-dichlorobenzene-d4 (up to 150 °C), in DMSO-d6 (up to 135 °C), and in pyridine-d5 (up to 110 °C), whereas those of chloride 16a and bromide 16b coalesced in pyridine-d5, 1,2-dichlorobenzene-d4, chlorobenzene-d5, and toluene-d8. The coalescence temperature (Tc) depended on the nucleophilicity of the counteranions as well as on the polarity of the solvents; Tc decreased as the nucleophilicity of the counteranions increased or as the polarity of the solvent decreased. Thus, the configuration at bismuth in tetrafluoroborates 14 and tosylate 15 was found to be stable, whereas that in halides 16 strongly depended on the solvent polarity. The observed permutation of bismuthonium halides, [Ar1Ar2Ar3Ar4Bi+][X-], can be explained by a pseudorotation mechanism involving a nonionic pentacoordinate species of the type Ar1Ar2Ar3Ar4BiX.