Preorganization in the Nazarov cyclization:: the role of adjacent coordination sites in the highly Lewis acidic catalyst [IrMe(CO)(dppe)(DIB)](BAr4f)2

The dicationic Ir(III) complex [IrMe(CO)(dppe)(DIB)](BAr(4)(f))(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. (31)P 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 group, 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 conformation is not achieved. and cyclization is not observed. In one case, monodentate binding of substrate occurs, and the rate of cyclization is significantly slower than when O.O'-chelation is possible. The viability of O,O'-chelation is shown by the crystal structure determination of a model substrate-catalyst complex. (c) 2005 Published by Elsevier Ltd.