Isomerization of Functionalized Olefins Using the Dinuclear Catalyst [PdI(μ-Br)(PtBu3)]2: A Mechanistic Study

Computational Assessment of the Mechanistic Journey in Chan-Lam-Based Arylation of Imidazoles

Cu(II)-catalyzed C-N bond formation reactions remain one of most widely practiced and powerful protocols for the synthesis of value-added chemicals, bioactive molecules, and materials. Despite numerous experimental contributions, the overall mechanistic understanding of the C-N coupling reaction based on the Chan-Lam (CL) reaction methodology is still limited and underdeveloped, particularly with respect to the use of different substrates and catalytic species. Herein, we report an in-depth DFT-based study on the mechanism of N-arylation of imidazoles following Collman's experimental setup. Our findings unfold for the first time the ligand-based CL coupling catalyzed by the [Cu(II)(OH)TMEDA]2Cl2 complex. The transmetalation step with an energy span of 26.2 kcal mol-1 is rate-determining, while the subsequent disproportionation and reductive elimination are relatively facile (δE = 16.4 kcal mol-1) in comparison to the CL amination of secondary amines. The final oxidative catalyst regeneration results in the presence of O2, accompanying an energy span of 12.8 kcal mol-1, where hydrogen transfer from the coordinated water allows the reduction of superoxo linkage. Couplings performed in the presence of a combination of bidentate sp3-N ligands with single and double -(CH2)- spacer units afford a kinetically facile transformation (24.5 kcal mol-1). Furthermore, our results agree with the experimental outcomes of regioselective couplings of substituted imidazoles.

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

Electron Shuttle in N-Heteroaromatic Ni Catalysts for Alkene Isomerization

Simple N-heteroaromatic Ni(II) precatalysts, (L)NiMe₂ (L = bipy, bipym), were used for alkene isomerization. With an original reduction method using a simple borane (HB(Cat)), a low-valent Ni center was formed readily and showed good conversion when a reducing divalent lanthanide fragment, Cp*₂Yb, was coordinated to the (bipym)NiMe₂ complex, a performance not achieved by the monometallic (bipy)NiMe₂ analogue. Experimental mechanistic investigations and computational studies revealed that the redox non-innocence of the L ligand triggered an electron shuttle process, allowing the elusive formation of Ni(I) species that were central to the isomerization process. Additionally, the reaction occurred with a preference for mono-isomerization rather than chain-walking isomerization. The presence of the low-valent ytterbium fragment, which contributed to the formation of the electron shuttle, strongly stabilized the catalysts, allowing catalytic loading as low as 0.5%. A series of alkenes with various architectures have been tested. The possibility to easily tune the various components of the heterobimetallic catalyst reported here, the ligand L and the divalent lanthanide fragment, opens perspectives for further applications in catalysis induced by Ni(I) species.

Regiodivergent Synthesis of Methylene and Methyl Ring-Fused Isoquinolinones: Base-Promoted Isomerization of N-Allyl Amides

Methylene and methyl tricyclic isoquinolinones were selectively prepared using a palladium(II)-catalyzed aerobic aza-Wacker reaction, followed by a base- and temperature-controlled Heck reaction catalyzed by palladium(0). Exo- to endo-double-bond migration in isoquinolinones was achieved with 93-99% yields by treatment of the Heck products with Cs₂CO₃ in dimethyl sulfoxide (DMSO) at 150 °C. A probable mechanism for Cs₂CO₃-promoted olefin isomerization was proposed and examined using D-isotope labeling experiments. Finally, yuanamide, a 13-methyl-8-oxoprotoberberine alkaloid, was synthesized using the palladium-catalyzed aza-Wacker/Heck/migration sequence.