Investigation and Comparison of the Mechanistic Steps in the [(Cp*MCl₂)₂] (Cp*=C₅Me₅; M=Rh, Ir)‐Catalyzed Oxidative Annulation of Isoquinolones with Alkynes

Abstract

The mechanism of the [(Cp*MCl(2))(2)] (M = Rh, Ir)-catalyzed oxidative annulation reaction of isoquinolones with alkynes was investigated in detail. In the first acetate-assisted C-H-activation process (cyclometalated step) and the subsequent mono-alkyne insertion into the M-C bonds of the cyclometalated compounds, both Rh and Ir complexes participated well. However, the desired final products, dibenzo[a,g]quinolizin-8-one derivatives, were only formed in high yield when the Rh species participated in the final oxidative coupling of the C-N bond. Moreover, a Rh(I) sandwich intermediate was isolated during this transformation. The iridium complexes were found to be inactive in the oxidative coupling processes. All of the relevant intermediates were fully characterized and determined by single-crystal X-ray diffraction analysis. Based on this mechanistic study, a Rh(III)→Rh(I)→Rh(III) catalytic cycle was proposed for this reaction.

HFIP‐Assisted C−H Functionalization by Cp*CoIII: Access to Key Reactive Cobaltacycles and Implication in Catalysis

“…[1] These cobalt catalysts can not only emulate the same reaction patterns of analogous rhodium and iridium systems,b ut also exhibit au nique reactivity because of the inherent properties of this first-row metal, properties such as its low electronegativity,h ard nature,o rs mall radius. [1] Despite the remarkable progress achieved in this field since the seminal work by Kanai, Matsunaga, and co-workers in 2013, [2] there is still al ack of fundamental understanding of these Cp*Co III -catalyzed transformations.I ns harp contrast to analogous rhodium systems, [3] thei nvestigation of the underlying reaction mechanisms of Cp*Co-catalyzed directed C À Hf unctionalizations has been hampered by the difficulty of capturing transient key reaction intermediates,s ince,i nm ost cases,t he formation of CÀHa ctivated Cp*Co III metallacycles is proposed to be reversible. [1,4] To tackle this situation, our group has recently reported the employment of acetonitrile as astabilizing ligand to access ad irect analogue of al ong-sought cyclometalated cobalt(III) complex (1 ppy -MeCN;F igure 1), by al igandassisted oxidative addition, [5] to bring light into the mechanistic insights of C À Ho xidative alkyne annulations.I nspired by these results,wewondered whether it would be possible to overcome the reversibility of the CÀHc obaltation by taking advantage of the unique ability of MeCN to stabilize otherwise highly reactive cobalt species.H erein, we provide ad irect synthetic route to two of the most widely invoked cationic metalacyclic intermediates in Cp*Co III -catalyzed CÀ Hf unctionalization reactions by CÀHb ond cleavage.O ur studies not only demonstrate the intermediacyo ft his type species in the oxidative alkyne annulation and alkyne insertion benchmark transformations,b ut also reveal the crucial role of fluorinated alcohols,s uch as 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), to improve the efficiency, not only of the CÀHa ctivation step,b ut also of catalytic transformations using diphenylacetylene as coupling partner.…”

Computational Characterization of the Mechanism for the Oxidative Coupling of Benzoic Acid and Alkynes by Rhodium/Copper and Rhodium/Silver Systems

“…Resulting five-membered rhodacycle 8 is stable (0.3 kcal mol À1 )a nd has been proposed and detected in others imilar reactions. [28] This intermediate evolves into intermediate 9 by dihedralr otationo ft he aromatic ring through an accessible transition state at 19.0 kcal mol À1 .I ni ntermediate 9,t here is a chlorideb ridge between the Rh and Cu atoms;t his will be important in the reductivee limination step and gives the complex enough flexibility to coordinate the alkyne in subsequent steps.…”

Cooperative Reductive Elimination: The Missing Piece in the Oxidative‐Coupling Mechanistic Puzzle

“…[9] Thef act that some of the acetate ligands are replaced by chloride in the active transition state also agrees with the observed influence of acetate concentration on the nature of the species present in solution. [12,13] Theparticipation of more than one metal center in the reduction is reminiscent of the bimetallic reductive elimination observed with dinuclear Pd III complexes, [17] but with substantial differences.T he role of the additive is also different from that computationally determined for AgOAc and CsF in Pd II CÀHa ctivation chemistry. [18] In conclusion, we have described an ew cooperative reductive-elimination mechanism that satisfactorily explains the specific role of the copper diacetate oxidant in the rhodium-catalyzed oxidative coupling of benzoic acid and alkynes.T he oxidizing agent participates directly in the reductive-elimination process by taking one electron already in the reductive-elimination transition state itself.Inthis way, the rhodium(I) oxidation state is never reached, which constitutes ac lear deviation from the commonly observed reductive-elimination pattern at transition-metal centers.W e consider that this cooperative reductive-eliminationm echanism may be operating in other systems where external oxidizing agents are involved.…”

“…[12] Thep resence of additional acetate,i ntroduced as NaOAc, was also shown to be critical for the isolation of intermediates in the oxidative coupling of isoquinolones with alkynes. [13] Theo xidant role of the copper center was not examined in detail in either of these experimental studies or in the computational analyses [14] that have been carried out on related processes with Rh III /Cu II systems.Herein, we computationally examine the specific role of Cu II in the mechanism of this process.…”