Asymmetric hydrogenations of aryl alkenes using imidazol-2-ylidene iridium complexes

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Title: Asymmetric hydrogenations of aryl alkenes using imidazol-2-ylidene iridium complexes
Author: Cui, Xiuhua
Abstract: A library of iridium complexes featuring oxazoline and imidazol -2 -ylidene ligands were synthesized by reaction of a library of imidazoles with a second library of oxazoline iodides . These complexes were active catalysts for hydrogenations of aryl substituted monoenes . Tri - and 1 ,1 -disubstituted alkenes were hydrogenated quantitatively with ee ? ?s up to 99 % at 1 atm hydrogen pressure . Catalyst , substrate , temperature and pressure effects were studied . The iridium complexes were also used for the kinetic study of hydrogenation of 2 ,3 - diphenylbutadiene . This hydrogenation is a stepwise reaction : one double bond was hydrogenated first , then the second one . Both step hydrogenations were zero order in alkene . The consumption of 2 ,3 -diphenylbutadiene was first order in catalyst , and probably first order in hydrogen pressure too . The enantioselectivity for the first step hydrogenation was low . There were match and mismatch catalyst -substrate relationships for the second step hydrogenation , and the enantioselectivities for this step were catalyst controlled . NMR studies indicated that the initiation of the reaction involved both hydrogen and alkene substrate . A competitive experiment was designed to explore the formation of meso -alkane at first step hydrogenation , and the results indicated that the alkane was formed predominantly via an associative mechanism . Four types of conjugate dienes were synthesized and hydrogenated . Different reactivities and selectivities were obtained for each type of dienes . In the best case , a diene was hydrogenated quantitatively with an excellent ent /meso ratio of 20 :1 .0 and 99 % enantioselectivity . The scope , limitation and potential applications of the reactions were discussed . A selection of the dienes was hydrogenated with the Crabtree ? ?s catalyst , for comparison , and the yields , conversions and diastereoselectivities were inferior to those from iridium -oxazoline -imidazol -2 -ylidene catalysts .
URI: http : / /hdl .handle .net /1969 .1 /2456
Date: 2005-08-29

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Asymmetric hydrogenations of aryl alkenes using imidazol-2-ylidene iridium complexes. Available electronically from http : / /hdl .handle .net /1969 .1 /2456 .

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