Mechanistic Study Of Iridium-Catalyzed Isomerization Of 1?Alkenes To Trans-2-Alkenes

Transition metal-catalyzed olefin isomerization provides one of the most important protocles for the synthesis of internal alkenes.In this thesis,the isomerization process of 1-hexene to E-2-hexane catalyzed by two kinds of iridium pincer complexes,[(^{t Bu}NCP)Ir](complex A)and[(^HNCP)Ir](complex B),were investigated by means of density functional theory calculations.The calculations show that the isomerization proceeds via an insertion-elimination mechanism,instead of the previously proposed1,3-hydrogen transfer mechanism.The reaction catalyzed by complex A begins with the C(sp²)-H oxidative addition of the pyridine ligand to give the key Ir(III)hydride intermediate,from which the metal-alkyl intermediate can be obtain via the insert of C=C bond into Ir-H bond.Finally,this intermediate undergoes a?-H elimination to obtain the isomerized product.The computations show that the?-H elimination controls the regioselectivity and stereoselectivity of the isomerization process,reproducing quite well the experimentally observed regio-and stereoselectivities.The origins of the excellent stereoselectivity could be explained by the steric repulsions in the?-H elimination process.The regioselectivity is mainly thermodynamically controlled,which makes the relatively more stable 2-alkenes be much less reactive compared with the 1-alkenes.By changing catalyst to complex B,both possible pathways are kinetically infeasible,since that the complexation between the terminal olefin and B is highly exergonic.This thus results in the experimental outcomes that the isomerization did not take place in the presence of B.