Metal/metal dual catalysis in C–H activation

Palladium-Catalyzed Annulation of Tertiary Anilines with 3-Butenoic Acid via Dual C-H Bond Activation

Catalytic cyclization via dual C-H bond activation has evolved as a powerful strategy for building bi- and polycyclic molecules. Herein, a palladium-catalyzed annulation of tertiary anilines with 3-butenoic acid via N-α-C(sp3)-H and ortho-C(sp2)-H activation is described. The remarkable characteristics of this reaction include excellent diastereoselectivity, broad substrate scope, and good tolerance for some highly sensitive groups. In addition, the KIE experiment suggested that the C-H bond abscission is not the turnover-limiting step.

Acetoxy Group-Directed Regioselective C2 Alkenylation of Indoles via Pd-Ag Bimetallic Catalysis

Regioselective C-H functionalizations of indoles reported to date with directing groups at C3 mainly rely on functional groups that are linked to the indole via C-C bonds. However, groups that are linked to the indole core by C-X linkages are also attractive due to the possibility of further modifications of the C-X bond. Herein, we report a 3-acetoxy directing group for the regioselective C2 alkenylation of indoles via a C-H activation-based, cross-dehydrogenative, oxidative Heck-type reaction. The reaction is catalyzed by Pd(II) and Ag(I) with stoichiometric Cu(II) as the oxidant and provides the 2-alkenylated indoles in yields of 52-84%. The reaction conditions are compatible with several functional groups at different positions as well as different N-protecting groups or free NH groups on the indole core. With respect to the alkene coupling partners, the reactions are successful with acrylates, vinyl sulfates, and phosphates. Specifically designed experiments, as well as density functional theory (DFT) computational studies, reveal that a heterodinuclear [Pd(μ-OAc)3Ag] bimetallic species is the actual catalyst responsible for the C-H alkenylation. A mechanistic path involving this catalytic species was also found to be favorable over other possible pathways for explaining the observed regioselectivity through DFT studies.

Direct Regioselective Hydro(hetero)arylation/Cyclocondensation Reactions of β-(2-Aminophenyl)-α,β-ynones by Means of Transition-Metal Catalysis/Brønsted Acid Synergism: Experimental Results and Computational Insights

Experimental results and computational insights explain the key role of transition-metal catalysis/Brønsted acid synergism in the achievement of the sequential regioselective direct heteroarylation/cyclocondensation reactions of β-(2-aminophenyl)-α,β-ynones with a variety of electron-rich aromatic heterocyclic/arenes to afford quinoline-(hetero)aromatic hybrids. The first approach to the synthesis of 4-(1H-pyrrol-2-yl)quinolines is described. The effectiveness of various transition metals is compared.

Multimetallic-Catalyzed C-C Bond-Forming Reactions: From Serendipity to Strategy

The use of two or more metal catalysts in a reaction is a powerful synthetic strategy to access complex targets efficiently and selectively from simple starting materials. While capable of uniting distinct reactivities, the principles governing multimetallic catalysis are not always intuitive, making the discovery and optimization of new reactions challenging. Here, we outline our perspective on the design elements of multimetallic catalysis using precedent from well-documented C-C bond-forming reactions. These strategies provide insight into the synergy of metal catalysts and compatibility of the individual components of a reaction. Advantages and limitations are discussed to promote further development of the field.

CuX Dual Catalysis: Construction of Oxazolo[2,3-b][1,3]oxazines via a Tandem CuAAC/Ring Cleavage/[4+2+3] Annulation Reaction

CuX as a simple dual catalyst strategy that promotes the tandem transformations of fused oxazolo[2,3-b][1,3]oxazines has been developed. Copper catalyzed terminal ynones, sulfonyl azides, and nitriles for the CuAAC/ring cleavage/[4+2] annulation reaction, while the halogen catalyzed ring cleavage and [2+3] annulation of oxiranes to form the final fused products. This study provides a four-component, one-pot strategy for synthesizing complex fused heterocycles from simple ingredients and expands the application of CuAAC in organic synthesis.