Mechanism and Regioselectivity of Rh(III)-Catalyzed Intermolecular Annulation of Aryl-Substituted Diazenecarboxylates and Alkenes: DFT Insights

The
mechanism of Rh-catalyzed intermolecular annulation of aryl-substituted
diazenecarboxylates and alkenes was investigated using density functional
theory (DFT) (PCM-M062X/6-311+G­(d,p)//M062X/6-31G­(d)). The acetate
ligand (OAc)-assisted C–H activation via the formation of a
five-membered rhodacycle (I-TS1; ΔG⧧ = 19.4 kcal/mol) is
more favorable compared to that via a four-membered intermediate (II-TS1; ΔG⧧ = 27.8 kcal/mol). Our results also revealed that the
seven-membered intermediate (I-3, ΔGrel = −6.8 kcal/mol) formed after the alkene insertion
could undergo a coordination switch with the adjacent nitrogen atom
(via TScs; ΔG⧧ = 16.5 kcal/mol) to produce a thermodynamically
stable six-membered intermediate (II-3, ΔGrel = −10.4 kcal/mol), eventually leading
to a cyclization process followed by a barrierless ligand-assisted
protonation to yield the final product. The β-hydride elimination
product was found to be kinetically and thermodynamically undesirable.
The rate-determining step is identified as the initial C–H
activation, consistent with the previous kinetic studies. Notably,
DFT studies offered important insights on the ability of the substrate
(diazene carboxylate) to promote the switchable coordination site
selectivity during the reaction to achieve a lower energy pathway.