Highly active rhodium catalysts for the [2+2+2] cycloaddition of acetylenes

Abstract

The catalytic activity of the complexes [Rh(π 5-indenyl)L 2] (L = ethylene or cyclooctene; L 2 = 1,5-cyclooctadiene), ( I) - ( III), and [Rh(π 5-fluorenyl) (1,5-cyclooctadiene)], ( IV), in the cyclotrimerization reaction of alkynes to benzene derivatives has been investigated. Compounds I - IV were very active in comparison with the isostructural cyclopentadienyl rhodium complexes. The catalytic activity depended markedly upon the nature of the substituents attached to the carbon-carbon triple bond, decreasing in the order: dimethyl acetylenedicarboxylate > methyl propiolate > diphenylacetylene > 1-hexyne > 3-hexyne > 1-octyne. In almost all cases the reaction has shown a high chemoselectivity. Some influence of the nature of the cyclopentadienyl-like ligand on both the catalytic activity and the regioselectivity has been observed. By reacting diphenylacetylene (dpa) with roughly stoichiometric amounts of I and III, the complexes [Rh 2π 5-indenyl) 2(dpa) 2], ( VI), and [Rh(π 5-indenyl)(π 4-tetraphenylcyclobutadiene)], ( VII), have been obtained, respectively. VI was poorly active and VII inactive in dpa cyclotrimerization. The mechanism of these cyclotrimerization reactions is discussed with reference to the role played by the indenyl and fluorenyl ligands in determining the activity of the catalyst precursors I - IV.