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