Origins of regioselectivity of the palladium-catalyzed (aromatic)C-H bond metalation-deprotonation

A comprehensive understanding of the C-H bond cleavage step in direct arylation is important in further development of cross-coupling reactions using transition metal catalysts. Analysis of Pd-catalyzed C-H bond cleavage of a wide range of (hetero)arenes via the concerted metalation-deprotonation (CMD) pathway allows one to quantify various contributions to the activation barriers and to identify activation characteristics of different substituent groups. In general, the CMD activation barriers do not show correlation with C-H bond acidities and metal-aryl bond energies. Regioselectivity of arylation for many (hetero)arenes, especially electron-deficient arenes, can be predicted from the C-H bond acidities. Regioselectivity of arylation for different (hetero)arenes can be understood from the distortion and electronic interaction contributions to the CMD activation barriers. The effects of remote substituents and metal coordination to heteroarenes on reactivity and regioselectivity are discussed. Electron-withdrawing substituents and metal coordination to heteroatoms increase acidities of (hetero)arene C-H bonds, making those bond more reactive in the CMD process. (C) 2012 Elsevier B.V. All rights reserved.