1,1-Carboboration of Dialkynyltin Compounds using Tri­organoboranes of Greatly Different Lewis Acid Strength. 1,4-Stannabora-cyclohexa-2,5-dienes and Characterization of Zwitterionic Intermediates


  • Dedicated to Professor Heinrich Nöth on the Occasion of His 85th Birthday


Abstract. Triorganoboranes BR3, Et-9-BBN, BPh3, and B(C6F5)3, were compared in their reactivity towards various dialkynyl(diorgano)tin compounds (R12Sn(C≡C–R2)2 with R12 = –(CH2)5–, R2 = H (a), R1 = nBu, R2 = H (b), R1 = Ph, R2 = H (c), R1 = R2 = nBu (d)). 1,1-Carboboration took place readily in two consecutive steps (inter- and intramolecular), leading either to stannoles or to 1,4-stannabora-cyclohexa-2,5-dienes, or mixtures thereof. The weakest Lewis-acidic triorganoboranes BEt3 and Et-9-BBN afford selectively stannoles with diethynyltin compounds, whereas the strongly electrophilic B(C6F5)3 leads selectively to 1,4-stannabora-cyclohexa-2,5-dienes for all dialkynyltin compounds studied. In several cases, zwitterionic intermediates could be detected by multinuclear magnetic resonance spectroscopy (1H, 11B, 13C, and 119Sn NMR), and the molecular structure of such an intermediate as well as that of the final product, an 1,4-stannabora-cyclohexa-2,5-diene, could be determined by X-ray crystallography.