Selective Synthesis of Stannoles by 1,1-Carboboration of Bis( trimethylsilylethynyl) tin Compounds Using Weakly and Strongly Electrophilic Triorganoboranes: Characterization of a Zwitterionic Intermediate

The triorganoboranes BEt3, BPh3, and B(C6F5)(3) were allowed to react with bis(trimethylsilylethynyl)diorganotin compounds [(R2Sn)-Sn-1(CC-SiMe3)(2); R-2(1) = -(CH2)(5)- (a), R-1 = nBu (b), (n)Oct (c), Ph (d)] to give selectively and quantitatively stannoles. The reactions proceeded by 1,1-carboboration in two consecutive steps (inter- and intramolecular) and intermediates were detected by NMR spectroscopy. In one case, using the strongly electrophilic B(C6F5)(3), a zwitterionic intermediate was isolated and structurally characterized by X-ray diffraction analysis. The question of reversibility of the 1,1-carboboration is addressed. Multinuclear magnetic resonance spectroscopy (H-1, B-11, C-13, F-19, Si-29, Sn-119 NMR) was used to characterize the intermediates and the stannoles as the final products. Some of the NMR parameters (B-11, C-13, Si-29) of the intermediates were calculated by DFT methods using the optimized gas-phase geometries.