Ionic Activation of Tin Hydrides

Organotin hydrides are advantageous reducing agents in terms of their facile availability, moderate stability and reactivity. In most cases, the dehalogenation of organic halides by Bu3SnH has taken place under radical conditions. In contrast, ionic reactions have been conventionally performed by using MeOH solvent, silica-gel, high pressure, palladium and Lewis acid catalysts. These ionic reactions have accomplished mild conditions, and high chemo-, regio- and stereoselectivites have been obtained, however, the ionic reactions have not received much attention. Another characteristic feature of tin hydrides is that active metal hydrides have been prepared to induce selective reductions where tin hydrides are efficient precursors of active metal species. Moreover, we have developed novel type of tin hydrides by modifying tin center by introducing ligands and halogen substituents such as Bu3SnH-Ligand, Bu2SnClH-HMPA, Bu2SnFHHMPA, Bu2SnIH-LiI and Bu2SnIH-MgBr2. Noteworthy is their ionic reduction modes that are markedly different from those of the original Bu3SnH reduction using radical conditions. This paper summarizes the ionic tin hydride reductions for representative functionalities such as aldehydes, ketones, epoxides, imines, unsaturated carbonyls and alkynes. Initially, the control of reducing ability has been performed by various ionic activation of tin hydrides. In particular, tin hydride complexes have achieved the chemo- and stereoselective reductions of carbonyl groups. In the reduction of epoxides, imines and unsaturated carbonyl compounds, the increased nucleophilicity of the Sn-halogen bond in halogenotin hydride complexes have played an very important role for high chemo- and refgioselective reactions. In ionic hydrostannation of alkynes, regio- and stereoselectivities are summarized for representative substrates. Particularly, the first a- stannylation of simple aliphatic alkynes could be achieved with an ate type of tin hydride ate complex.