Palladium-Catalyzed Site-Selective [3+2] Annulation via Benzylic and meta C-H Bond Activation

Palladium-Catalyzed Selective Benzylic C-H Alkylation of Aromatic Sulfonamides with Maleimides

An efficient method for the selective benzylic C-H alkylation of sulfonamides using maleimides has been developed. The reaction proceeds via the benzylic C(sp3)-H bond activation of sulfonamide in the presence of a Pd(II) catalyst without requiring any oxidant, additive, or external ligand. This methodology is highly compatible with a wide variety of substituted maleimides. A plausible reaction mechanism is also proposed to account for this alkylation reaction.

Computational Insights into Palladium-Catalyzed Site-Selective Anilide and Benzamide-Type [3+2] Annulation via Double C-H Bond Activation

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

Ligand-Enabled Palladium-Catalyzed [3 + 2] Annulation of Aryl Iodides with Maleimides via C(sp3)-H Activation

Palladium-catalyzed intermolecular [3 + 2] annulation reactions via C-H activation represent a powerful and charming tool for assembling cyclopentanes. Herein, we have developed a strategy for the palladium-catalyzed intermolecular alkene-relayed annulation reaction of aryl iodides and maleimides via C(sp3)-H activation for the construction of polycyclic structures. In contrast to directed-group-enabled intermolecular maleimide-relayed [3 + 2] annulation reactions, this protocol stands out for its utilization of aryl iodides as substrates. Notably, monoprotected amino acids played a crucial role as ligands in this reaction, which is rarely observed in C-H activation reactions initiated with organohalides.

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

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.

Palladium-Catalyzed (3 + 2) Annulation of Aromatic Acids by C(sp3)-H Olefination and Decarboxylative Cross-Coupling Reaction

A palladium-catalyzed (3 + 2) annulation of 2-methylbenzoic acid with maleimide using Ac-Leu-OH as a powerful ligand has been reported. Through a site-selective γ-C(sp3)-H olefination reaction and a sequential decarboxylative cross-coupling reaction, a five-membered cyclic ring was obtained as the final product. This novel reaction features great site selectivity and reactivity to generate various cyclic products in moderate to good yields.

Palladium-Catalyzed Enantioselective Directed C(sp3)-H Functionalization Using C5-Substituted 8-Aminoquinoline Auxiliaries

8-Aminoquinoline (AQ) has proven to be a highly effective bidentate directing group for palladium-catalyzed C-H functionalization reactions. However, enantiocontrol of AQ-directed C(sp3)-H functionalization reactions has been challenging. Herein, a new protocol is presented for the Pd-catalyzed enantioselective arylation of unactivated β C(sp3)-H bonds of alkyl carboxamides with aryl iodides using a C5-iodinated 8-aminoquinolines (IQ) auxiliary in conjugation with a BINOL ligand. Additionally, a C5-aryl substituted 8-aminoquinoline auxiliary can facilitate enantioselective alkenylation and alkynylation of benzylic C(sp3)-H bonds of 3-arylpropanamides with the corresponding bromide reagents under similar conditions.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Palladium-Catalyzed [3 + 2] Annulation of ortho-Substituted Iodoarenes with Maleimides via a Consecutive Double Heck-type Strategy

Herein, we report an efficient [3 + 2] annulation of ortho-substituted iodoarenes with maleimides via a palladium-catalyzed consecutive double Heck-type strategy, leading to fused tricyclic frameworks of pharmaceutical relevance. The protocol ensued through consecutive inter- and intramolecular Heck couplings effectively. This approach was compatible with a large variety of substrates and functional groups, and it was remarkably tolerated with unprotected maleimide.

Relay Copper-Catalyzed Synthesis of Imidazo[1,5-a]pyridine Scaffolds from Phenylalanine and Halohydrocarbon

An efficient and straightforward strategy to synthesize imidazo[1,5-a]pyridine compounds from phenylalanine and halohydrocarbon has been successfully developed. The protocol features a relay copper-catalyzed reaction involving intermolecular C-O coupling and intramolecular C-N cyclization, providing an approach to access a diverse range of imidazo[1,5-a]pyridine derivatives with unique aza quaternary carbon centers.

Transition-Metal-Catalyzed Dehydrogenative (3 + 2) Annulation of Aromatic Compounds: Synthesis of Indenes and Indanes via Dual Functionalization of Benzylic and ortho C-H Bonds

Intermolecular (3 + 2) annulation emerges as a potent approach for constructing 5-membered carbocycles through the fusion of two distinct components. This synopsis encapsulates recent strides in the realm of transition-metal-catalyzed dehydrogenative (3 + 2) annulation of aromatic hydrocarbons, achieved through the dual functionalization of benzylic and ortho C-H bonds. Encompassing three pivotal strategies, namely, (i) C-H bond activation, (ii) benzylic oxidation, and (iii) π-coordination activation, this review offers an overview of the field's recent developments.

Carbene Formation or Reduction of the Diazo Functional Group? An Unexpected Solvent-Dependent Reactivity of Cyclic Diazo Imides

The control of the reactivity of diazo compounds is commonly achieved by the choice of a suitable catalyst, e.g. via stabilization of singlet carbenes or radical intermediates. Herein, we report on the light-promoted reactivity of cyclic diazo imides with thiols, where the choice of solvent results in two fundamentally different reaction pathways. In dichloromethane (DCM), a carbene is formed initially and engages in a cascade C-H functionalization/thiolation reaction to deliver indane-fused pyrrolidines in good to excellent yields. When switching to acetonitrile solvent, the carbene pathway is shut down and an unusual reduction of the diazo compound occurs under otherwise identical reaction conditions, where the aryl thiol acts as reductant. A combined set of experimental and computational studies was carried out to obtain mechanistic understanding and to support that indane formation proceeds via the insertion of a triplet carbene, while the reduction of diazo imides proceeds via an electron transfer process.

Theoretical Investigations of Palladium-Catalyzed [3+2] Annulation via Benzylic and meta C-H Bond Activation

The palladium-catalyzed reaction of aromatic amides with maleimides results in the formation of a double C-H bond activation product, which occurs at both the benzylic and meta positions. Computational chemistry studies suggest that the first C-H bond activation unfolds via a six-membered palladacycle, maleimide insertion, protonation of the Pd-N bond, and then activation of the meta C-H bond. The process concludes with reductive elimination, producing an annulation product. The energy decomposition analysis (EDA) showed that the deformation energy favors the ortho C-H bond activation process. However, this route is non-productive. The interaction energy controls the site where the maleimide is inserted into the Pd-C(sp3 ) bond, which determines its site selectivity. The energetic span model indicates that the meta C-H bond activation step is the one that determines the turnover frequency. Regarding the directing group, it has been concluded that the strong Pd-S bonding and the destabilizing effect of the deformation energy allow the 2-thiomethylphenyl to function effectively as a directing group.

Palladium-Catalyzed Aerobic Oxidative Spirocyclization of Alkyl Amides with Maleimides via β-C(sp3)-H Activation

An efficient method for the synthesis of bicyclic spirodiamine molecules via β-C(sp3)-H bond activation of aliphatic amides, followed by cyclization with maleimides, has been developed. The reaction proceeds through an amide-directed β-C(sp3)-H bond activation of alkyl amides and subsequent cyclization with maleimides. The methodology is highly compatible with a wide variety of maleimides. Amides derived from biologically active aliphatic and fatty acids were also found to be highly compatible with the protocol. A palladacycle was synthesized and found to be the active intermediate in this reaction. A plausible reaction mechanism was also proposed to account for this spirocyclization.

Access to 3-Aminomethylated Maleimides via a Phosphine-Catalyzed Aza-Morita-Baylis-Hillman Type Coupling

A designed method for the preparation of 3-aminomethylated maleimides via Morita-Baylis-Hillman (MBH) reaction was developed. This phosphine-catalyzed coupling adopted maleimides and 1,3,5-triazinanes as the substrate, giving a series of 3-aminomethylated maleimide derivatives with a double bond retained on the maleimide ring in 41-90% yield. Acylation, isomerization, and Michael addition of the obtained products demonstrated the synthetic application of the present protocol. The results of control experiments indicated that phosphorus ylide formation and elimination take place during the reaction pathway.

Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides through Dual C-H Activation

A palladium-catalyzed [3 + 2] annulation of substituted aromatic amides with maleimides providing tricyclic heterocyclic molecules in good to moderate yields through weak carbonyl chelation is reported. The reaction proceeds via a dual C-H bond activation where the first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position to afford a five-membered cyclic ring. An external ligand Ac-Gly-OH has been used to succeed in this protocol. A plausible reaction mechanism has been proposed for the [3 + 2] annulation reaction.

Radical annulation using a radical reagent as a two-carbon unit

Radical annulation has emerged as one of the most efficient and straightforward methods for synthesizing cyclic/polycyclic compounds as the core structures can be constructed through a single procedure comprising multiple bond-forming steps. Particularly, radical annulation using a radical reagent as a cyclization partner and two-carbon unit greatly expands the diversity of cyclic skeletons and the functionality of radical reagents. We herein have highlighted the representative processes reported in the past decade for radical annulation using a radical reagent as a two-carbon unit, including [2 + 2 + 2], [3 + 2], [4 + 2], and [5 + 2] modes, with an emphasis on their reaction mechanisms. These studies not only pave the way toward annulation but also provide insight into the exploration of a new reaction mode for radical chemistry.

Palladium-Catalyzed C-H Functionalization of Aryl Acetamides and Benzoquinones: Synthesis of Substituted Aryl Quinones

An efficient synthesis of aryl-substituted quinones via Pd(II)-catalyzed C-H functionalization of less expensive and abundant benzoquinones with aryl acetamides is demonstrated. An auxiliary ligand N,N-bidentate-directing group 8-aminoquinoline plays a crucial role in the success of the reaction. A broad range of substituted phenyl acetamides including commercially available drug molecules were examined and also found to be highly compatible with quinones. The aryl-substituted quinones were also easily converted into aryl-substituted hydroquinone derivatives. A plausible reaction mechanism was proposed to account for the selective distal C-H bond functionalization.

Cobalt-catalyzed C(sp3)-H bond functionalization to access indole derivatives

Herein, we develop an efficient method of cobalt-catalyzed C(sp3)-H bond functionalization to synthesize indole derivatives. The highlight of this protocol is accomplished by the sequential C-H activation. This “cobalt/ organic...

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp³)−H bonds using transition‐metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp³)−H bonds, and the selection among very similar C−H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C−C and C−heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal‐catalyzed C(sp³)−H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Rhodium(iii)-catalyzed [5+1] annulation of 2-alkenylphenols with maleimides: access to highly functionalized spirocyclic skeletons

A new edition of [5+1] annulation reaction of maleimides with 2-alkenylphenols has been discovered under a Rh(iii)-catalytic system. The process leads to an efficient synthesis of valued spirocyclic scaffolds bearing an oxygen-containing spiro carbon in a single step and shows a broad substrate scope with good functional group tolerance. The synthetic utility has been exemplified by synthesizing highly functionalized 2,2-disubstituted-2H-chromene skeletons and a gram-scale synthesis with a low catalyst loading.