Janka M, He W, Frontier A, Flashenriem C, Eisenberg R. Preorganization in the Nazarov Cyclization: The Role of Adjacent Coordination Sites in the Highly Lewis Acidic Catalyst [IrMe(CO)(dppe)(DIB)](BAr4)2. TETRAHEDRON 61: 6193-6206, 2005 (invited paper).

Abstract The dicationic Ir(III) complex [IrMe(CO)(dppe)(DIB)](BAr4f)2 where dppe=bis(diphenylphosphino)ethane and DIB=o-diiodobenzene possesses adjacent labile sites and is found to be a very active catalyst for the Nazarov cyclization. 31P NMR spectroscopy provides evidence for substrate–catalyst binding by chelation, and this is found to be the resting state of the system during catalysis. The efficiency of the cyclization is attributed to the electrophilicity of the Ir(III) complex and substrate activation via O,O ′ -chelation which employs two substrate carbonyl groups or one carbonyl and an ether function, and encourages the s-trans/s-trans conformation required for cyclization. When two point binding occurs through an oxygen atom and one of the vinyl groups, the s-trans/s-trans ......