Abstract

Development of effective cyclization reactions for the synthesis of carbocycles and heterocycles has been the subject of extensive study because of their relevance to medicines and other functional materials. Among the numerous methods available, the transition metal-promoted cyclization reaction has become one of the most popular. Although considerable efforts are still being devoted to the development of more efficient and practical catalytic systems, identification of true catalysts or utilization of new catalyst precursors is also important since all the key catalytic properties are inherently dependent upon the real catalytic species. Moreover, these studies may lead to development of new catalytic systems. Recently, we have found generation of an anionic rhodium species, [Rh(COD)Cl2] −, from the reaction of dimeric [Rh(COD)Cl]2 with dppe. 4 Thus, reaction between [Rh(COD)Cl]2 and dppe (1 equiv per Rh atom) in THF gave a yellow precipitate, and subsequent heating of the yellow solid above 120 C led to isolation of the well-known orange solid, [Rh(dppe)Cl]2. By the way, the exact composition of the yellow precipitate, a synthetic intermediate for [Rh(dppe)Cl]2, had not been reported until we recently solved its molecular structure by an X-ray crystallographic analysis (Figure 1). Thus, it was revealed that the intermediate yellow solid before heating was an ionic complex of [Rh(dppe)2] cation and [Rh(COD)Cl2] anion. Synthesis of the ionic complex would be attributed to a disproportionation reaction of the starting rhodium dimer. According to the literatures, the first report of this anion with a cation other than the present [Rh(dppe)2] cation appeared in 1957, but its crystal structure was solved thirty years later. While we studied chemistry of [Rh(dppe)2][Rh(COD)Cl2] (1), we realized this anion could be used as a catalyst precursor in many catalytic reactions. Herein, we report that [4+2] cycloaddition reactions with catalysts from a [Rh(COD)Cl2] anion precursor. The anionic rhodium species was prepared not only by the reaction of [Rh(COD)Cl]2 with dppe but also by the reaction of [Rh(COD)Cl]2 with PPNCl [PPN = bis(triphenylphosphoranyliene)ammonium] to study catalytic activity of Rh anion precursor. In order to confirm usefulness of the [Rh(COD)Cl2] anion as a catalyst precursor, catalytic intramolecular [4+2] cycloaddition of a dienyne was investigated as shown in Table 1. Both the complex 1 and an initially in-situ generated species from a reaction of [Rh(COD)Cl]2 with dppe were inactive as catalyst precursors in the intramolecular [4+2] cycloaddition reaction (entries 1 and 2). However, addition

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