Synthesis of pyridines from alkynes and nitriles catalyzed by polymer-anchored rhodium complexes

Abstract

Cyclopentadienylrhodium(I) complexes have been attached to polymers by first binding cyclopentadiene to polystyrene resins in a variety of crosslink densities and then converting the resulting species to the cyclopentadiene anion. On subsequent reaction of the resin-bound anions with rhodium compounds of general formula [RhXL 2] n (X = chloro or acac, L = C 2H 4 or CO; L 2 = 1,5-C 8H 12), various rhodium-containing resins were obtained which were then analyzed by X-ray fluorescence and IR spectroscopies. These resin-attached cyclopentadienylrhodium complexes were active in the cocyclization of alkynes and nitriles to pyridine derivatives, at temperatures between 90 and 170 °C. In all cases, variable amounts of benzene derivatives were also formed, owing to the parallel self-trimerization of alkyne. A detailed study was carried out of the activity, chemo- and regio-selectivities of these catalysts as a function of the reaction parameters. The activity was seen to depend markedly upon the nature of the rhodium ancillary ligands and the crosslink density of the support: the ethylene complexes anchored to lightly crosslinked resins were the most active, all other variables being the same. Analogies and differences between heterogeneously and homogeneously rhodium-catalyzed pyridine syntheses are discussed.