Palladium-catalyzed construction of quaternary carbon centers with propargylic electrophiles

Rh(II)/Pd(0) Dual-Catalyzed Regio-Divergent Three-Component Propargylic Substitution

Regio-divergent propargylic substitution to generate functionally diverse products from identical starting materials remains a formidable challenge, probably due to the unpredictable regiochemical complexity. In practically, the synthesis of α-quaternary propargylic-substituted products is still much less developed, and preprepared nucleophiles are generally applied in this type of reaction with propargylic substrates, which limits the reaction efficiency and diversity of the obtained products. Herein, we disclose unprecedented three-component propargylic substitution of α-diazo esters with amines and propargylic carbonates under dirhodium/palladium dual catalysis. The key to the success of this multicomponent propargylic substitution is to avoid two-component side reactions through a tandem process of dirhodium(II)-catalyzed carbene insertion and palladium-catalyzed regiodivergent propargylic substitution. The judicious selection of a diphosphine (dppf) or monophosphine (tBuBrettphos) as the ligand is crucial for the reaction to generate different products in a switchable way, α-quaternary 1,3-dienyl or propargylated products, with high regio- and chemoselectivities.

Cyclocarbopalladation/Stille Cascade: Stereoselective Access to Quaternary Functionalized Carbons

Exploration of the 4-exo-dig cyclocarbopalladation in the discovery of new and original scaffolds afforded some unexpected results. The search for a way to produce seven-membered ring systems led to polycyclic molecules bearing a tetrasubstituted carbon. The triple bond that substitutes the cyclohexene ring on the starting compound is crucial for a high stereoselectivity. This observation has been confirmed by the reaction of a nonsubstituted cyclohexene ring resulting in poor stereoselectivity and low yields.

Mechanistic Studies of the Pd- and Pt-Catalyzed Selective Cyclization of Propargyl/Allenyl Complexes

Following the discovery of an unusual transition-metal-catalyzed reaction, the elucidation of the underlying mechanism is essential to understand the characteristic reactivity of the metal. We previously reported a synthetic method for tricyclic indoles using Pt-catalyzed Friedel-Crafts-type C-H coupling. In this reaction, the Pt catalyst selectively formed a seven-membered ring, but the Pd catalyst only afforded a six-membered ring. However, the reasons for the different selectivities caused by Pd and Pt were unclear. We performed density functional theory (DFT) calculations and experimental studies to reveal the origin of the different behaviors of the two metals. The calculations revealed that the formation of the six- and seven-membered rings proceeds via η¹-allenyl and η³-propargyl/allenyl complexes, respectively. A molecular orbital analysis of the η³-propargyl/allenyl complex revealed that, for the platinum complex, the energy required to convert the unoccupied molecular orbital on the reactive carbon into the lowest unoccupied molecular orbital (LUMO) was lower than that for the palladium complex. In addition, DFT calculations revealed that the combination of platinum and bis[2-(diphenylphosphino)phenyl] ether (DPEphos) reduced the activation energy of the seven-membered cyclization in comparison with palladium or PPh₃. Additional experimental studies, including NMR studies and stoichiometric reactions, support the aforementioned examination.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

Theoretical Calculations on the Mechanism of Enantioselective Copper(I)-Catalyzed Addition of Enynes to Ketones

Computational investigations on the bisphospholanoethane (BPE)-ligated Cu-catalyzed enantioselective addition of enynes to ketones were performed with the density functional theory (DFT) method. Two BPE-mesitylcopper (CuMes) catalysts, BPE-CuMes and (S,S)-Ph-BPE–CuMes, were employed to probe the reaction mechanism with the emphasis on stereoselectivity. The calculations on the BPE-CuMes system indicate that the active metallized enyne intermediate acts as the catalyst for the catalytic cycle. The catalytic cycle involves two steps: (1) ketone addition to the alkene moiety of the metallized enyne; and (2) metallization of the enyne followed by the release of product with the recovery of the active metallized enyne intermediate. The first step accounts for the distribution of the products, and therefore is the stereo-controlling step in chiral systems. In the chiral (S,S)-Ph-BPE–CuMes system, the steric hindrance is vital for the distribution of products and responsible for the stereoselectivity of this reaction. The steric hindrance between the phenyl ring of the two substrates and groups at the chiral centers in the ligand skeleton is identified as the original of the stereoselectivity for the titled reaction.

Enantioselective synthesis of tetrahydrocyclopenta[b]indole bearing a chiral quaternary carbon center via Pd(ii)–SPRIX-catalyzed C–H activation

Pd(ii)–SPRIX promotes the first highly enantioselective cyclization of 3-alkenylindoles into the corresponding indoles bearing a chiral quaternary carbon center via C–H activation and an assisting effect by the allyl substituent.