The mechanism and regioselectivity of iridium-mediated cleavage of aromatic C-C bonds in a series of monomethylated, dimethylated, and trimethylated benzenes without the activation of weaker C-H and C-C bonds are clarified using density functional theory (DFT) calculations. The calculations explained why the reactivity of the coordinated arene in the observed C-C bond cleavage reaction decreases as the degree of substitution decreases.

Thrimurtulu et al. recently reported unprecedented cobalt-catalyzed annulation of allenes with benzamide (N. Thrimurtulu, A. Dey, D. Maiti, C. M. R. Volla, Angew. Chem., Int. Ed., 2016, 55, 12361-12365). In this reaction, the substituent on the allene controls the regioselectivity for the formation of either dihydroisoquinolin-1(2H)-one or isoquinolin-1(2H)-one. In the present study, density functional theory calculations were performed to investigate the detailed reaction mechanism and the origin of the experimentally observed regioselectivity. A systematic search shows that the electronic and steric effects of the substituent on the allene determine which of the two allene insertions is followed, and thus determine the regioselectivity. The bulky diphenylphosphonate and two phenyl substituents of the allenylphosphonate and diarylallene favor C1[double bond, length as m-dash]C2 insertion, which eventually leads to the formation of isoquinolin-1(2H)-one. In contrast, for the arylallene, which has a relatively electron-rich C2[double bond, length as m-dash]C3 bond, C2[double bond, length as m-dash]C3 insertion is favored and eventually leads to the formation of dihydroisoquinolin-1(2H)-one. The calculations also explain why annulation rather than hydroarylation of benzamide with allenylphosphonate occurs with a cobalt catalyst.

Theoretical studies reveal the ligand role in the Rh(i)-catalyzed alkyne–isatin decarbonylative coupling and account for the origin of chemo- and region-selectivity.

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Metal-free boron Lewis acids, tris(pentafluorophenyl)borane B(C 6 F 5 ) 3 , have the advantages of low toxicity and low cost and are a promising catalyst. A density functional theory (DFT) calculation was used to clarify the mechanism and the origin of the diastereoselective cyclopropanation of aryldiazodiacetate and styrene derivatives catalyzed by B(C 6 F 5 ) 3 . Four pathways were calculated: B(C 6 F 5 ) 3 -catalyzed N-, C-, and O-bound boron-activated aryldiazodiacetate and without B(C 6 F 5 ) 3 catalysis. By calculating and comparing the energy barriers, the most possible reaction mechanism was proposed, that is, first, B(C 6 F 5 ) 3 catalyzed O-bound boron to activate aryldiazodiacetate, followed by the removal of a N 2 molecule, and finally, styrene nucleophilic attack occurred to produce [2+1] cyclopropane products. N 2 removal is the rate-limiting step, and this step determines the preference of a given mechanism. The calculated results are in agreement with experimental observations. The origin of diastereoselectivity is further explained on the basis of the favorable mechanism. The steric hindrance interference between the styrene aryl group and the large tri(pentafluorophenyl)borane B(C 6 F 5 ) 3 and the favorable π–π stacking interaction between the benzene rings combined to cause the high diastereoselectivity, which resulted in lower energy of the transition state (TS) corresponding to the reaction mechanism. The calculated results not only provide a more detailed explanation of the mechanism for the experimental study but also have certain reference and guiding significance for other catalytic cyclopropanation reactions.