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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/2456 |
| Date | 29 August 2005 |
| Creators | Cui, Xiuhua |
| Contributors | Burgess, Kevin |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1608679 bytes, electronic, application/pdf, born digital |
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