Current Organic Chemistry - Volume 6, Issue 8, 2002

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.

Synthetic organic chemists have widely exploited nitrones for the last 50 years as precious substrates for the assembly of complex nitrogen containing frameworks through inter and intramolecular 1,3-dipolar cycloadditions. In the last decade the interest for nitrones was further on increased by a second opportunity, offered by the nucleophilic addition of organometallic reagents to give N,N-disubstituted hydroxylamines.Grignard, lithium reagents and allylic zinc derivatives react very efficiently with nitrones in a wide temperature range, both the reaction rate and the stereochemical outcome of the process can be significantly modified by the addition of nitrone-chelating Lewis acids. The allylation of nitrones is also carried out under Sakurai conditions in the presence of trimethylsilyltriflate, and different results are obtained using allylic silanes or stannanes. Homoallylic hydroxylamines, generated by the allylation of nitrones, are exploited in 5-exo-trig iodocyclizations to give 5-iodomethyl isoxazolidines, useful precursors of hydroxylated acyclic and cyclic amino compounds. Enolates and silyl enol ethers under Mukayama conditions are also investigated. In particular, 2-trimethylsilyloxyfuran reacts with nitrones, in the presence of nitrone activators, to give 3-substituted tetrahydrofuro[2,3-d]isoxazol-5(2H)-ones, easily transformed into a variety of polyhydroxylated piperidine and indolizidine azasugars. The most recent solutions to achieve the control of stereochemistry in nucleophilic additions to nitrones will be examined.

This review summarizes recent developments that have been reported on organic reactions using indium metal (or salts) in aqueous media. These organic reactions include the carbon-carbon bond formation such as Barbier-type reactions and aldol reactions, reduction, reactions using indium salts in aqueous media, and their synthetic applications.

Dendrimers constitute a new class of macromolecules having a hyperbranched and perfectly defined structure. The attractive beauty of these nanosized compounds induces since ten years an exponential development of the number of publications devoted to this topic. Most dendrimers are purely organic compounds, but heteroatom-containing dendrimers also play a role in the development of this field. Among them, dendrimers having phosphorus derivatives, either at each branching point or only in part of the structure (either on the surface, or at the core, or at special places within the structure), occupy a special place. Indeed, the well-known ability of phosphorus derivatives, especially phosphines, to react with organometallic compounds is also applicable to dendritic derivatives of phosphorus. This particular aspect of phosphorus dendrimers, which undergoes a significant development since 5 years, is the subject of this review. The organometallic derivatives can be grafted either to the surface of the dendrimer, or at its core, or throughout the structure, or only at particular places within the structure. Direct phosphorus-metal linkages, but also phosphorus-metal linkages through one atom will be considered. Most of these macromolecules were synthesized for studying their catalytic properties, this important application is emphasized at the end of this review.