Mechanistic investigation on scandium-catalyzed C-H addition of pyridines to olefins

摘要

This paper reports computational studies on the ortho alkylation of pyridines via C-H addition to olefins catalyzed by cationic half-sandwich rare-earth alkyl species. A detailed mechanism concerning the generation of catalytically active species and C-H addition has been computationally investigated at the molecular and electronic levels. The results support the mechanism based on experiments, which involves the initial generation of a metal pyridyl active species, followed by the coordination and insertion of an olefin and the subsequent pyridine C-H activation by a metal-carbon bond. The o-methyl sp ~3 C-H activation product of α-picoline has been also calculated, and the results suggest that the sp ~3 C-H activation product mainly results from the conversion of the sp ~2 C-H activation product of α-picoline rather than from the direct reaction of the cationic species (η ~5-C _5Me _5)Sc(CH _2C _6H _4NMe _2-o) ~+ with α-picoline, and such a conversion is reversible. The reaction rate of the whole process is controlled by the generation of active species and an insertion step. The formation of the branched product is both kinetically and energetically favorable over that of the linear product, which is in agreement with the experimental observation. Both steric and electronic factors account for the regioselectivity. An analysis of energy decomposition provides new insights into the stability of the 1-hexene insertion transition states involved in such processes. A comparison between the successive olefin insertion and the C-H activation of pyridine has also been computationally carried out. In addition, it is predicted that the cationic scandium pyridyl species (η ~5- C _5Me _5)Sc(MeC _5H _3N) ~+ has a shorter induction period than the initial aminobenzyl analogue (precursor) (η ~5-C _5Me _5)Sc(CH _2C _6H _4NMe _2-o) ~+ for the initiation step of ethylene polymerization.