Acceleration Mechanisms of C–H Bond Functionalization Catalyzed by Electron-Deficient CpRh(III) Complexes

Abstract

A rhodium(III) complex bearing a 1,3-bis(ethoxycarbonyl)-substituted or an unsubstituted cyclopentadienyl ligand (Cp E or Cp) significantly accelerates a variety of oxidative C−H bond functionalization reactions. However, the driving force of the acceleration compared with a conventionally used Cp*Rh(III) complex has not been elucidated. Herein, we performed density functional theory (DFT) calculations of the rhodium(III)-catalyzed oxidative C−H bond olefination and annulation reactions using Cp*, Cp, and Cp E ligands, which revealed that the Cp E Rh(III) complex stabilizes transition states of not only a C−H bond activation step but also rate-determining reductive elimination and insertion steps by strong orbital interactions. For the sterically demanding substrates, the less sterically hindered CpRh(III) complex can stabilize the transition states of the reductive elimination step more than the Cp E Rh(III) complex. Moreover, the whole reaction pathways were calculated to elucidate the mechanism and selectivity of the oxidative [4 + 2] and [2 + 2 + 2] annulation reactions under cationic and neutral conditions, respectively.

Formation of a Naphthalene Framework by Rhodium(III)-Catalyzed Double C–H Functionalization of Arenes with Alkynes: Impact of a Supporting Ligand and an Acid Additive

“…In contrast, both the electron-deficient complex [Cp′RhCl 2 ] 2 (Cp′ = 1,3-(CO 2 Et) 2 -C 5 Me 3 ) reported by Tanaka 14 and the non-methylated complex [CpRhI 2 ] n , first introduced by our group, 15 effectively promoted the reaction with formation of 3aa in 90 and 96% yield (entries 5 and 6), respectively. The enhanced catalytic activity can be explained by higher electrophilicity of rhodium in these complexes, 16 which facilitates the ratedetermining CH activation step (for mechanism, see below). When the reaction temperature is decreased to 100 °C, the yield of 3aa drops dramatically to 10% for [Cp′RhCl 2 ] 2 , while for [CpRhI 2 ] n it still remains acceptable (50%).…”

Oxidative [4 + 2] annulation of 1-naphthols with alkynes accelerated by an electron-deficient rhodium(iii) catalysts

“…5). 34 When the Cp E Rh( iii ) complex is applied, the activation energies of both rate-determining CMD ( IM1 → IM2 , ΔΔ G ‡ = –4.6 kcal mol −1 ) and reductive elimination ( IM4′ → IM5 , ΔΔ G ‡ = –3.2 kcal mol −1 ) steps were found to be significantly lowered, compared to those when using the Cp*Rh( iii ) complex. In addition, the higher Lewis acidity of the Cp E Rh( iii ) complex facilitated the smooth ligand exchange and the successive alkyne insertion, with only 16.0 kcal mol −1 ( IM2 → TS2 ) in total, while the Cp*Rh( iii ) complex required higher energy (21.4 kcal mol −1 ).…”

Triphenylcyclopentadienyl Rhodium Complexes in Catalytic C–H Annulations. Application for Synthesis of Natural Isocoumarins

“…For example, we found earlier that the use of an unsubstituted complex [CpRhI 2 ] n leads to a dramatic change in the selectivity of annulation of benzoic acids with internal alkynes, shifting the reaction pathway toward the formation of naphthalenes instead of isocoumarins . Recently, Nagashima and Tanaka have shown by calculation that the presence of an electron-withdrawing COOEt group in the Cp ring can decrease the activation barriers for the C–H bond activation step …”