Polarizing the Nazarov Cyclization: The Impact of Dienone Substitution Pattern on Reactivity and Selectivity
作者:Wei He、Ildiko R. Herrick、Tulay A. Atesin、Patrick A. Caruana、Colleen A. Kellenberger、Alison J. Frontier
DOI:10.1021/ja077162g
日期:2008.1.1
The impact of dienone substitution on the Nazarovcyclization has been examined in detail. Substrates bearing different substituents at each of four positions on the dienone backbone were systematically probed in order to identify trends leading to higher reactivity and better selectivity. Desymmetrization of the pentadienyl cation and oxyallyl cation intermediates through placement of polarizing groups
Preorganization in the Nazarov cyclization: the role of adjacent coordination sites in the highly Lewis acidic catalyst [IrMe(CO)(dppe)(DIB)](BAr4f)2
作者:Mesfin Janka、Wei He、Alison J. Frontier、Christine Flaschenriem、Richard Eisenberg
DOI:10.1016/j.tet.2005.03.114
日期:2005.6
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.