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 [R12Sn(C≡C–SiMe3)2; R12 = –(CH2)5– (a), R1 = nBu (b), nOct (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 (1H, 11B, 13C, 19F, 29Si, 119Sn NMR) was used to characterize the intermediates and the stannoles as the final products. Some of the NMR parameters (11B, 13C, 29Si) of the intermediates were calculated by DFT methods using the optimized gas‐phase geometries.