Mechanistic analysis of iridium(III) catalyzed direct C-H arylations: A DFT study

摘要

In a recent experimental work the Ir complex [Ir(cod)(py)(PCy _3)](PF_6) (that is, Crabtree's catalyst) has been shown to catalyze the C-H arylation of electron-rich heteroarenes with iodoarenes using Ag_2CO_3 as base. For this process, an electrophilic metalation mechanism, (S_EAr) has been proposed as operative mechanism rather than the concerted metalation-deprotonation (CMD) mechanism, widely implicated in Pd-catalyzed arylation reactions. Herein we have investigated the C-H activation step for several (hetero)arenes catalyzed by a Ir~III catalyst and compared the data obtained with the results for the Pd ~II-catalyzed C-H bond activation. The calculations demonstrate that, similar to Pd~II-catalyzed reactions, the Ir~III-catalyzed direct C-H arylation occurs through the CMD pathway which accounts for the experimentally observed regioselectivity. The transition states for Ir ~III-catalyzed direct C-H arylation feature stronger metal-C _((arene)) interactions than those for Pd~II-catalyzed C-H arylation. The calculations also demonstrate that ligands with low trans effect may decrease the activation barrier of the C-H bond cleavage.