DFT Case Study on the Comparison of Ruthenium-Catalyzed C–H Allylation, C–H Alkenylation, and Hydroarylation

Abstract

Density functional calculations at the B3LYP-D3+IDSCRF/TZP-DKH­(-dfg) level of theory have been performed to understand the mechanism of ruthenium-catalyzed C–H allylation reported in the literature in depth. The plausible pathway consisted of four sequential processes, including C–H activation, migratory insertion, amide extrusion, and recovery of the catalyst, in which C–H activation was identified as the rate-determining step. The amide extrusion step could be promoted kinetically by trifluoroacetic acid since its mediation lowered the free-energy barrier from 32.1 to 12.2 kcal/mol. Additional calculations have been performed to explore other common pathways between arenes and alkenes, such as C–H alkenylation and hydroarylation. A comparison of the amide extrusion and β-H elimination steps established the following reactivity sequence of the leaving groups: protonated amide group > β-H group > unprotonated amide group. The suppression of hydroarylation was attributed to the sluggishness of the Ru–C protonation step as compared to the amide extrusion step. This study can unveil factors favoring the C–H allylation reaction.

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