Mechanistic Study On Iridium-Catalyzed Intramolecular C(sp~2)-H Bond Activation Of 1,1-Disubstituted Alkenes

Transition metal-catalyzed hydroalkenylation of alkenes provides one of the most efficient protocls for the construction of C-C bond.In this thesis,the mechanism and the origins of the directing group-controlled selectivity of Ir-catalyzed intramolecular hydroalkenylation of 1,1-disubstituted alkenes were investigated by means of density functional theory calculations.The computations show that after an initial chelation-assisted C-H oxidative addition to form the Ir(III)hydride intermediate,the reaction can take place through the widely accepted Chalk-Harrod type mechanism,i.e.,migratory insertion into the Ir-H bond followed by the C-C reductive elimination,to afford the endo-cyclization product.On the other hand,the exo-selective hydroalkenylation may proceed through an unconventional modified Chalk-Harrod type mechanism,involving the migratory insertion into the Ir-C bond and C-H reductive elimination.The selectivity of the reaction was found to be caused by the competition between the migratory insertion into the Ir-C bond and C-C reductive elimination.The results show that the directing group has a rather small effect on the migratory insertion into the Ir-C bond.On the other hand,the C-C reductive elimination turns out to be very sensitive to the coordinating ability of the directing group.It was found that the C-C reductive elimination with an electron-poor metal center is faster than that with an electron-rich metal center.As a result,the C-C reductive elimination is favored with the weak coordinating ketone group,but disfavored with the strong coordinating amide group,which thus enables the experimentally-observed selectivity switch.The results will provide important implications for a better understanding of related reactions.