The First Examples of Transition Metal-Mediated 1,3-Dipole Formation

The First Examples of Transition Metal-Mediated 1,3-Dipole Formation 

The cycloaddition reaction of dipoles has been known since the late eighteenth century however, before Huisgeris introduction of the concept of a 1,3-dipole, these reactions were considered to proceed via a diradical mechanism [16]. One of the earhest examples of metal-catalyzed 1,3-dipole formation involved the controlled decomposition of an a-dia- 

Modem Rhodium-Catalyzed Organic Reactions. Edited by P. Andrew Evans Copyright 2005 WILEY-VCH Verlag GmbH Co. KGa.A, Weinheim ISBN 3-527-30683-8 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

Ri = alkyl, aryl, fluro-alkyl, sulfoxyl Rg = H, alkyl 

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