One-Pot Synthesis of Orange-Red Fluorescent Dimeric 2H-Pyrrolo[2,3-c]isoquinoline-2,5(3H)-diones from Benzamides and Maleimides via Ru(II)-Catalyzed Sequential C–C/C–N/C–C Bond Formation

Ru(II)-Catalyzed C-H Alkylation of N-Benzyltriflamide with Maleimides: Synthesis of o-Succinimide Substituted Benzaldehydes

A ruthenium-catalyzed N-benzyltriflamide assisted C-H alkylation with maleimide followed by hydrolysis of in situ generated imine has been developed for the first time. This synthetic method results in the efficient synthesis of o-succinimide derivatives of benzaldehydes. This reaction involves less expensive and mild reaction conditions and shows excellent site selectivity and good functional group compatibility.

Rh(III)-Catalyzed Controlled Ortho-Amidation of Arylamides with Dioxazolones Using Weakly Coordinating Native Primary Amide as the Directing Group

Herein, we report a controlled introduction of an amide unit at the ortho-position of an electron-deficient arylamide system without affording any cyclized products using user-friendly dioxazolone as an amidating reagent in the presence of a Rh(III)-catalyst. This is the first report where native primary amide has been utilized as a weakly coordinating group for site-selective C-N bond formation reaction. The developed protocol works under external auxiliary-free conditions with a wide substrate scope.

Pd-Catalyzed Sequential Electrophilic Cyclization/Selective C-H Annulation Cascade: Synthesis of Isoxazole-Phthalimide-Fused Poly-Heterocyclics

Harnessing the organo-palladium intermediates generated from electrophilic cyclizations for tandem C-C bond construction is a challenging task but constitutes an excellent tool for constructing complex motifs from simpler substrates. We realize herein such a cyclative annulation of alkynyl-oxime ethers with maleimides for the facile construction of isoxazole-phthalimide hybrid motifs through Pd(II) catalysis. This protocol features excellent regio-selectivity in C-H selection, a broad substrate scope, good functional group tolerance, and scalability. Necessary KIE & labeling studies give insight into the reaction mechanism.

Two distinct protocols for the synthesis of unsymmetrical 3,4-disubstituted maleimides based on transition-metal catalysts

Two tandem catalytic systems are described for the synthesis of novel 3,4-disubstituted maleimides using the same Ugi adducts as starting materials. 4-Aryl-3-pyrrolyl- and 4-aryl-3-indolyl-maleimides were successfully obtained via a Pd(OAc)2/PPh3 based protocol. In contrast, maleimide-fused pyrrolo and indolo[1,2-a]quinolines were obtained in a complementary methodology using CuI/L-proline. These strategies involved a combination of benzylic amine oxidation, trans-amidation, intramolecular Knoevenagel condensation, and N-arylation reactions. Computational investigations provide further insights into this reaction sequence.

Ru(II)-Catalyzed Weakly Coordinating Carbonyl-Assisted Dialkynylation of (Hetero)Aryl Ketones

Functionalized aryl(heteroaryl) ketones are present in many natural products as key structural components and serve as basic synthetic building blocks for various organic transformation reactions. Therefore, the development of an effective and sustainable route for making these classes of compounds remains challenging yet highly desirable. Herein, we report a simple and efficient catalytic system for dialkynylation of aromatic/heteroaromatic ketones via a double C-H bond activation in the presence of less expensive ruthenium(II)-salt as a catalyst using the weakly and native carbonyl group as the desired directing group. The developed protocol is highly compatible, tolerant, and sustainable toward various functional groups. The synthetic utility of the developed protocol has been demonstrated through the scale-up synthesis and functional group transformation. Control experiments support the involvement of the base-assisted internal electrophilic substitution (BIES) reaction pathway.

Selective Construction of Spiro or Fused Heterocyclic Scaffolds via One-pot Cascade Reactions of 1-Arylpyrazolidinones with Maleimides

Presented herein is a controllable selective construction of spiro or fused heterocyclic scaffolds through the one-pot cascade reactions of 1-phenylpyrazolidinones with maleimides. To be specific, succinimide spiro pyrazolo[1,2-a]pyrazolones were effectively formed via [4 + 1] spiroannulation of 1-phenylpyrazolidinones with maleimides through simultaneous C(sp²)-H bond activation/functionalization and intramolecular cyclization along with the traceless fusion of the pyrazolidinonyl unit into the final product. In this reaction, air acts as a cost-effective and environmentally sustainable oxidant to assist the regeneration of the Rh(III) catalyst. Alternatively, succinimide-fused pyrazolidinonylcinnolines were formed from the same starting materials through an initial [4 + 1] spiroannulation followed by base-promoted skeleton rearrangement of the in situ formed spiro product without isolation. Notable features of these protocols include easily tunable selectivity, broad substrate scope, cost-effective and sustainable oxidant, excellent atom economy, and facile scalability.

Ru(II)-catalyzed external auxiliary-free primary amide-directed inverse Sonogashira reaction on (hetero)arylamides

Herein, we report ruthenium(II)-catalyzed weakly coordinating primary amide-assisted ortho-di-alkynylation of (hetero)arylamides via double C-H bond activation in the presence of bromo-alkynes as coupling partners. The attractive features of the developed strategy lie in the usage of an inexpensive ruthenium(II)-salt, external auxiliary-free directing group and simple reaction conditions, along with a broad substrate scope, high reaction yields and scale-up synthesis.

Recent advances in transition-metal-catalyzed directed C-H alkenylation with maleimides

Transition-metal-catalyzed directed C-H alkenylation with maleimides has attracted much attention in recent years, as maleimide core moieties are present in various natural products and pharmaceuticals. In addition, these derivatives can be readily modified into biologically important compounds including succinimides, pyrrolidines and γ-lactams. The efficient chelation-assisted inert C-H bond activation strategy provides straightforward access to a wide array of structurally diverse molecules containing maleimide units. This review describes the major progress and mechanistic investigations on Heck-type reaction/cyclization of maleimides with organic molecules until early 2022.

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.

Ru(II)-Catalyzed Controlled Cross-Dehydrogenative Coupling of Benzamides with Activated Olefins via Weakly Coordinating Primary Amides

Ru(II)-catalyzed regioselective ortho-alkenylation of primary benzamides with activated olefins has been realized over the competitive cyclized products. This reaction overall proceeds via a cross-dehydrogenative coupling (CDC) reaction using a simple and weakly coordinating primary amide group in the presence of an inexpensive Ru(II) salt and allows the controlled introduction of olefin motifs at the ortho-position of benzamides. The key to the success of this strategy depends on fine-tuning the reaction conditions. The developed protocol has demonstrated excellent regio/diastereoselectivity and a good functional group tolerance with wide substrate scope and obviates the requirement of external auxiliaries as well as the costly metal catalyst. Detailed mechanistic studies indicate the involvement of the base-assisted internal electrophilic-type substitution (BIES) step in the reaction mechanism.

Room-Temperature Synthesis of Isoindolone Spirosuccinimides: Merger of Visible-Light Photocatalysis and Cobalt-Catalyzed C-H Activation

A room-temperature C-H bond functionalization of benzamides has been developed by merging a photocatalyst with a cobalt catalyst for the synthesis of isoindolone spirosuccinimides. The reaction proceeds in aerobic conditions and does not require any sacrificial external oxidants such as Ag(I) or Mn(III) salts. Visible light activates the photocatalyst, and it acts as an electron-transfer reagent and helps in the fundamental organometallic steps by modulating the oxidation state of the cobalt complex. This C-H bond functionalization and spirocyclization showed wide substrate scope and good functional group tolerance. A possible reaction mechanism was proposed from the experimental outcome, showing that C-H bond activation is irreversible and not the rate-determining step.

Pd(II)-Catalyzed One-Pot Multiple C-C Bond Formation: En Route Synthesis of Succinimide-Fused Unsymmetrical 9,10-Dihydrophenanthrenes from Aryl Iodides and Maleimides

An expeditious approach has been developed for the synthesis of succinimide-fused unsymmetrical 9,10-dihydrophenanthrenes from simple aryl iodides and maleimides. The developed transformation, overall proceeding with high regioselectivity via a cascade approach through palladium(II)-catalyzed Micheal-type addition/C-H activation/intramolecular cross-dehydrogenative coupling (ICDC)/C-H activation, allows formation of four fundamental carbon-carbon bonds in one-pot fashion. The reactions tolerate broad functional groups and satisfy the parameters of atom and step economy. Detailed mechanistic studies were carried out to support the proposed synthetic pathway.