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Description:
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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 . |