Palladium-Catalyzed Domino Reaction of Two Aryl Iodides involving C-H Activation Processes: Efficient Synthesis of Fused Polycycles

Intermolecular Sonogashira Coupling and Intramolecular 5-Exo-dig Cycloisomerization Cascade: A One-Pot Pathway for Accessing (3-Benzylbenzofuran-2-yl)(phenyl)methanones

Herein, we present an efficient and straightforward strategy enabling access to 2,3-disubstituted benzo[b]furans. The whole synthetic process proceeds via a domino intermolecular Sonogashira coupling of 2-(2-bromophenoxy)-1-phenylethan-1-ones/alkyl 2-(2-bromophenoxy)acetates/2-(2-bromophenoxy)acetonitrile/1-(2-bromophenoxy)propan-2-one with terminal acetylenes followed by an intramolecular carbanion-yne cyclization in a 5-exo-dig manner and subsequent double-bond isomerization. Notably, two C-C bonds have been constructed in a one-pot manner and a wide variety of (3-benzylbenzofuran-2-yl)(phenyl)methanones were accomplished with good functional group tolerance.

Transition Metal-Catalyzed Intermolecular Cascade C-H Activation/Annulation Processes for the Synthesis of Polycycles

Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C-H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C-H activation/annulation, or combination of C-H activation/annulation and further interaction with a proximal group, or merger of C-H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C-H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C-H activation.

Recent Advances in Palladium-Catalyzed Bridging C–H Activation by Using Alkenes, Alkynes or Diazo Compounds as Bridging Reagents

Transition-metal-catalyzed direct inert C–H bond functionalization has attracted much attention over the past decades. However, because of the high strain energy of the suspected palladacycle generated via C–H bond palladation, direct functionalization of a C–H bond less than a three-bond distance from a catalyst center is highly challenging. In this short review, we summarize the advances on palladium-catalyzed bridging C–H activation, in which an inert proximal C–H bond palladation is promoted by the elementary step of migratory insertion of an alkene, an alkyne or a metal carbene intermediate.

1 Introduction

2 Palladium-Catalyzed Alkene Bridging C–H Activation

2.1 Intramolecular Reactions

2.2 Intermolecular Reactions

3 Palladium-Catalyzed Alkyne Bridging C–H Activation

3.1 Intermolecular Reactions

3.2 Intramolecular Reactions

4 Palladium-Catalyzed Carbene Bridging C–H Activation

5 Conclusion and Outlook

Sequential Cross-Coupling/Annulation of ortho-Vinyl Bromobenzenes with Aromatic Bromides for the Synthesis of Polycyclic Aromatic Compounds

A sequential cross-coupling/annulation of ortho-vinyl bromobenzenes with aromatic bromides was realized, providing a direct and modular approach to access polycyclic aromatic compounds. A vinyl-coordinated palladacycle was proposed as the key intermediate for this sequential process. Excellent chemoselectivity and regioselectivity were observed in this transformation. The practicability of this method is highlighted by its broad substrate scope, excellent functional group tolerance, and rich transformations associated with the obtained products.

Palladium-Catalyzed Intermolecular [4+1] Spiroannulation by C(sp3 )-H Activation and Naphthol Dearomatization

A novel palladium-catalyzed [4+1] spiroannulation was developed by using a C(sp³ )-H activation/naphthol dearomatization approach. This bimolecular domino reaction of two aryl halides was realized through a sequence of cyclometallation-facilitated C(sp³ )-H activation, biaryl cross-coupling, and naphthol dearomatization, thus rendering the rapid assembly of a new class of spirocyclic molecules in good yields with broad functional-group tolerance. Preliminary mechanistic studies indicate that C-H cleavage is likely involved in the rate-determining step, and a five-membered palladacycle was identified as the key intermediate for the intermolecular coupling.