Application of the indenyl effect to the oxidative annulation of 2-phenylimidazole with 1-phenyl-1-propyne catalyzed by (5-ligand) rhodium and iridium species was investigated. (See figure) Seven catalysts were utilized and the greatest activity was found for [(Ind*)RhCl2]2 (Ind* = 1,2,3,5,6,7,8-heptamethylindenyl), which gave quantitative yield of 6-methyl- 5-phenylimidazo[2,1-a]isoquinoline, followed by [(tmi)RhCl2]2 (75% yield; tmi = 1,2,3- trimethylindenyl) and [(pmi)RhCl2]2 (64% yield; pmi = 1,2,3,5,8-pentamethylindenyl). [Cp*RhCl2]2 and [Cp*IrCl2]2 gave significantly lower yields of 51% and 8%, respectively. Two plausible mechanisms have been proposed, one of which involves a ring slipped transition state and may explain the greater activity of the indenyl based catalysts. [(Ind*)RhCl2]2 was also found to stoichiometrically cyclometalate 2-phenyl-1H-imidazole and 1-phenyl- 1H-pyrazole. Insertion of phenylacetylene into the cyclometalated complex (Ind*)Rh(2- phenylimidazole)Cl occurred regioselectively so that the phenyl group was oriented closer to the metal center. Several single crystal X-ray structures were also solved and the slip-fold parameters of the indenyl ligands were determined.

Exploiting the indenyl effect of Rh(III) piano stool compexes for cyclometalation and catalytic C–H activation

ACS Spring 2022

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