Lewis Acid-Catalyzed Hydrostannation of Acetylenes. Regio- and Stereoselective Trans-Addition of Tributyltin Hydride and Dibutyltin Dihydride

Lewis acids such as ZrCl4 or HfCl4 catalyze the hydrostannation of acetylenes 1 by tributyltin hydride to produce the cis vinylstannanes 2 by regio- and stereoselective anti-hydrostannation. The hydrostannation of acetylenes using dibutyltin dihydride was also catalyzed by ZrCl4 to give the stereodefined Z-Z divinyltin derivatives 4 by an anti-hydrostannation pathway. The use of nonpolar solvents such as toluene or hexane was essential for obtaining high stereoselectivity and chemical yield. Since ZrCl4 and HfCl4 are not soluble in such solvents, the hydrostannations were carried out in a heterogeneous system. The reactions of internal acetylenes with Bu3SnH proceeded smoothly, although the use of stoichiometric amounts of ZrCl4 gave better results. The ZrCl4-catalyzed hydrostannation at 0 °C gave better yields and stereoselectivities than the reaction at room temperature. To help clalify the reason, the reaction of Bu3SnH with ZrCl4 was monitored by 1H and 119Sn NMR spectroscopy, and it was found that Bu3SnH reacted with ZrCl4 at room temperature to afford a mixture of tributyltin hydride, dibutyltin dihydride, and tetrabutyltin.