Transition Metal-Catalyzed Dual C-H Activation/Annulation Reactions Involving Internal Alkynes

Recently, transition metal-catalyzed ortho-C-H bond activation/annulations involving two internal alkyne molecules have been extensively used to synthesize highly substituted polycyclic aromatic scaffolds. Such reactions have emerged as a powerful atom and step-economical strategy for the assembly of multifunctional bioactive molecules. In this context, we focused on the recent achievements of dual C-H bond activation/annulations, as well as functionalization reactions involving diaryl/alkyl alkynes.

Access to CF3-benzofulvenes via palladium-catalyzed cascade arylation/Trost-Oppolzer cyclization/double-bond isomerization

Herein, we demonstrated a Pd-catalyzed cascade reaction that involves arylation, Trost-Oppolzer type Alder ene reaction, and double bond isomerization using the 4-(2-alkynylphenyl)-allylcarbonates and aryl boronic acids. This cascade process delivers a wide array of distinctive functionalized CF3-benzofulvenes in good yields with high stereoselectivity (E). A single palladium catalyst orchestrates the two individual reactions in a single operation. Trost-Oppolzer type Alder ene reaction is the key in this transformation, also called a rare acid-free iso-Nazarov type cyclization.

Boron Trifluoride-Mediated Domino Dehydration/Electrophilic Cyclization of Silylalkynols Leading to 2,3-Fused Tricyclic Benzofulvenes

A BF3-mediated domino dehydration/electrophilic cyclization of silylalkynols to form 2,3-fused tricyclic benzofulvenes was achieved. In the latter step, in situ generated BF3·OH2 enables the electrophilic activation of alkynes. The predominant Z-selectivity of the reaction is also discussed.

Regioselective Direct C-H Phosphorylation of Benzofulvenes

A practical and straightforward protocol to access site-selective C-3 phosphorylated benzofulvenes by means of cross-dehydrogenative coupling of two nucleophilic moieties, employing silver as a promoter, is described here. Remarkably, this protocol provides a broad, structurally diverse phosphorylated benzofulvene. Initial mechanistic studies shed light on its radical coupling nature, and late-stage transformations offer highly diversified benzofulvenes core.

Palladium-Catalyzed Ortho C-H Arylation of Unprotected Anilines: Chemo- and Regioselectivity Enabled by the Cooperating Ligand [2,2'-Bipyridin]-6(1H)-one

Metal-catalyzed C-H functionalizations on the aryl ring of anilines usually need cumbersome N-protection-deprotection strategies to ensure chemoselectivity. We describe here the Pd-catalyzed direct C-H arylation of unprotected anilines with no competition of the N-arylation product. The ligand [2,2'-bipyridin]-6(1H)-one drives the chemoselectivity by kinetic differentiation in the product-forming step, while playing a cooperating role in the C-H cleavage step. The latter is favored in an anionic intermediate where the NH moiety is deprotonated, driving the regioselectivity of the reaction toward ortho substitution.

Palladium-Catalyzed Aminomethylative Oppolzer-Type Cyclization of Enynes: Access to Aminomethylated Benzofulvenes

A novel palladium-catalyzed Oppolzer-type cyclization reaction aided by the aminomethyl cyclopalladated complex has been developed, which provides rapid access to functionalized benzofulvenes with excellent stereoselectivity. The corresponding products can undergo Diels-Alder reaction with maleimides, providing a series of complex polycyclic compounds with excellent regio- and stereoselectivities.

TfOH-promoted Classical Nazarov-type Cyclization of Benzofulvenes: Synthesis of Polycyclic 5H,10'H-spiro[benzo[k]phenanthridine-5,6'-dibenzopentalenes]

The reaction of o-benzofulvene with TfOH leads to intramolecular cyclization through novel C-C and C-N bond formation, resulting in the formation of 5H,10'H-spiro[benzo[k]phenanthridine-5,6'-dibenzopentalene]. This protocol provides a new molecular framework with reasonable to excellent yields and tolerates various electron-withdrawing/donating substituents. This method yields diastereoselectivity of up to >20:1. Furthermore, it is free of bases, oxidants, and metals and proceeds under mild reaction conditions, which are favorable for synthetic organic chemistry.

Transition Metal Catalyzed Free-Amine (-NH2 ) Directed C-H Bond Activation and Functionalization for Biaryl Frameworks

Transition-metal-catalyzed direct transformation of inert C-H bond has revolutionized the arsenal of main-stream organic synthesis, providing a new upfront to forge structurally enriched and biologically relevant scaffolds in a step- and atom-economical way. Past decades have accounted for the major developments in this realm, proclaiming excellent site-selectivity by exploiting a variety of coordinating directing groups (DGs). Consideration of versatile, abundant, sp³ -hybridized free-amine (-NH₂ ) functionality for the same purpose has always been a formidable task owing to its innate reactivity. In recent years, free-amine functionality has emerged as a potent DG for a wide range of C-C and C-heteroatom bonds formations and annulation cascades. In this review article, we have discussed the advancements of free-amine directed C-H activation/functionalization reactions towards biaryl frameworks made within a decade (2012 to 2021).

Selective Synthesis of Dihydrophenanthridine and Phenanthridine Derivatives from the Cascade Reactions of o-Arylanilines with Alkynoates through C-H/N-H/C-C Bond Cleavage

In this paper, an unprecedented selective synthesis of dihydrophenanthridine and phenanthridine derivatives through the cascade reactions of 2-arylanilines with alkynoates is presented. Mechanistic studies showed that the formation of the dihydrophenanthridine scaffold involves an initial C(sp²)-H alkenylation of 2-arylaniline with alkynoate followed by an intramolecular aza-Michael addition. When this reaction is carried out at elevated temperature, the in situ formed substituted dihydrophenanthridine readily undergoes a retro-Mannich-type reaction to give the corresponding phenanthridine through C-C bond cleavage. Compared with literature methods, this novel protocol has advantages such as easily obtainable substrates with a free amino group, pharmaceutically privileged products, cheap catalysts, and conveniently controllable selectivity.

Synthesis of Benzofulvenes via Cp*Co(III)-Catalyzed C-H Activation and Carbocyclization of Aromatic Ketones with Internal Alkynes

A highly efficient and practical synthesis of benzofulvenes was developed via ketone-directed Cp*Co(III)-catalyzed sequential C-H activation, addition, cyclization, and dehydration cascade processes between simple aromatic ketones and alkynes. The reaction tolerates a variety of functional groups, and various benzofulvenes were efficiently synthesized in 42-92% yields.