Streamlined Ruthenium(II) Catalysis for One-Pot 2-fold Unsymmetrical C–H Olefination of (Hetero)Arenes

Recent advances in carboxyl-directed dimerizations and cascade annulations via C-H activations

C-H functionalization provides an efficient route to construct complex organic molecules. The introduction of directing groups enhances the site-selectivity of the reaction. Carboxyl as a directing group can be easily transformed into other functional groups afterwards. Due to its good reactivity, it can undergo cascade annulation reactions to build valuable heterocycle skeletons in one pot. Moreover, carboxyl can easily be removed via decarboxylation, which allows it to serve as a unique traceless directing group in C-H functionalization. These characteristics make carboxyl a promising directing group, which is superior to nitrogen-containing compounds with strong coordination ability to a certain extent. This feature article reviews the applications of carboxyl as a classical directing group and a unique traceless-directing group in cascade annulation reactions to access diverse carbocycles and heterocycles.

Additive-Controlled Divergent Synthesis of Fluorenone-4-carboxylic Acids and Diphenic Anhydrides via Rhodium-Catalyzed Oxidative Dimeric Cyclization of Aromatic Acids

A rhodium-catalyzed one-pot access to valuable polycyclic frameworks of fluorenone-4-carboxylic acids and diphenic anhydrides via the oxidative dimeric cyclization of aromatic acids has been developed. This transformation proceeded via carboxyl-assisted 2-fold C-H activation followed by intramolecular Friedel-Crafts or dehydration reactions. The silver salt additive plays a vital role in the chemoselectivity of the products. Diphenic anhydride 3l exhibits a maximum fluorescence quantum yield of up to 59%.

Sequential ortho-/meta-C-H functionalizations of N-tosyl-benzamides for the synthesis of polyfunctionalized arenes

Selective one-pot sequential ortho-/meta-C-H functionalizations provided highly desirable polyfunctionalized arenes. Starting from readily available carboxylic acid derivatives, the concomitant formation of C-O and C-halogen bonds was achieved under mild reaction conditions (12 examples, up to 75% yield). The utility of the products was illustrated with post-functionalization reactions and Metiglinid synthesis.

Enaminone-directed ruthenium(II)-catalyzed C-H activation and annulation of arenes with diazonaphthoquinones for polycyclic benzocoumarins

The weakly coordinating enaminone functionality has been leveraged for a C-H bond activation strategy under ruthenium catalysis and employed in the regioselective annulative coupling of arenes with diazonaphthoquinones, offering polycyclic benzocoumarins in very high yields. The enaminone motif plays a dual role and the protocol operates through a Ru(II)/Ru(IV) catalytic pathway which is amenable to the diversification of various pharmacophore-coupled substrates.

Ruthenium(II)-Catalyzed Highly Chemo- and Regioselective Oxidative C6 Alkenylation of Indole-7-carboxamides

We disclosed the first efficient method for highly chemo- and regioselective C6 alkenylation of indole-7-carboxamides using inexpensive Ru(II) catalyst through chelation assisted C-H bond activation. Electronically diverse indole-7-carboxamides and alkenes react efficiently to produce a wide range of C6 alkenyl indole derivatives. Further the C6 alkenyl indole-7-carboxamides modified to their derivatives through simple chemical transformations. The observed regioselectivity and kinetics has been evidenced by deuterium incorporation and intermolecular competitive studies. In addition, for mechanistic insights, the intermediates were analyzed by HRMS.

Ruthenium(II)-catalyzed C-H activation and (4+2) annulation of aromatic hydroxamic acid esters with allylic amides

A (4+2) annulation under Ru(II)-catalysis is reported using aromatic hydroxamic acid esters as the oxidizing directing group and allylic amides as unactivated olefin coupling partners, delivering a wide variety of aminomethyl isoquinolinones in good to excellent yields. This annulation is distinctive as allylic congeners typically result in allylation and not the annulation. Late-stage derivatization of a bioactive synthetic bile acid has been showcased.

Ru-Catalyzed (E)-Specific ortho-C-H Alkenylation of Arenecarboxylic Acids by Coupling with Alkenyl Bromides

In the presence of [p-cymene)RuCl₂]₂, (E)-configured alkenyl bromides couple with aromatic carboxylates to form ortho-vinylbenzoic acids. This C-H vinylation proceeds in high yields without any activating phosphine ligands and has an excellent functional group tolerance. Starting from commonly available (E/Z )-mixtures of alkenyl bromides, (E)-configured vinyl arenes or dienes are formed exclusively. Mechanistic studies show that this selectivity is achieved because the (E)-configured alkenyl bromides undergo a smooth coupling, whereas the (Z)-isomers are rapidly eliminated with the formation of alkynes.

Regioselective C-H Alkenylation and Unsymmetrical Bis-olefination of Heteroarene Carboxylic Acids with Ruthenium Catalysis in Water

An efficient weak carboxylate-assisted oxidative cross-dehydrogenative C-H/C-H coupling (CDC) of heteroarenes with readily available olefins has been devised employing water as green solvent under ruthenium(II) catalysis. The reaction is operationally simple, accommodates a large variety of heteroaromatic carboxylic acids as well as olefins, and facilitates a diverse array of high-value olefin-tethered heteroarenes in high yields (up to 87%). The potential of this ortho-C-H bond activation strategy has also been exploited toward tunable synthesis of densely functionalized heteroarenes through challenging unsymmetrical bis-olefination process in a one-pot sequential fashion. Mechanistic investigation demonstrates a reversible ruthenation process and C-H metalation step might not be involved in the rate-determining step.

Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air

A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot sequential strategy is also disclosed to access Heck-type products that are apparently difficult to make directly from arene carboxylic acids.

Rhodium(III)-catalysed decarboxylative C-H functionalization of isoxazoles with alkenes and sulfoxonium ylides

Decarboxylative C-H functionalization of isoxazoles with electron-deficient alkenes and sulfoxonium ylides at the C5 position was achieved in the presence of rhodium(iii) catalysts to give the corresponding alkenylation and acylmethylation products, respectively.

Ruthenium(II)-catalyzed amide directed spiroannulation with naphthoquinones: access to spiro-isoindolinone frameworks

A mild ruthenium(ii)-catalyzed spiroannulation between benzamides and naphthoquinones is developed for the succinct synthesis of biologically relevant spiro-isoindolinone scaffolds. A base promoted transannulation of spirocyclic products en route to valuable benzo[b]phenanthridinetriones in good yields has also been accomplished.

Rh-Catalyzed Annulation of Benzoic Acids, Formaldehyde, and Malonates via ortho-Hydroarylation to Indanones

A three-component reaction from readily available low-cost materials of benzoic acids, formaldehyde, and malonates for the preparation of indanones by rhodium catalysis is reported. The annulation is initiated by an ortho-hydroarylation of benzoic acids, and a Lewis acid is not required. The solvent has a significant influence to the reaction, and 2-substituted or nonsubstituted indanones are obtained by the change of solvent.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Sulfoximine-Assisted Unsymmetrical Twofold C-H Functionalization of Arenes

An unprecedented ruthenium (Ru)-catalyzed twofold unsymmetrical annulation of 3-O/N-allyl benzoic acid derivatives with isocyanates for the construction of dihydro-furan/indole-fused phthalimide scaffolds is discussed. This double-unsymmetrical functionalization of both o,o'-C-H bonds of arene moiety is explicitly viable under the influence of methylphenyl sulfoximine directing group constructing three different [C-C/C-C(O)/N-C(O)] bonds under a single catalytic system. A broad scope with all six-carbon-substituted arene motifs, control experiments, and gram-scale synthesis make the synthetic model viable and significant.

Access to 5H-benzo[a]carbazol-6-ols and benzo[6,7]cyclohepta[1,2-b]indol-6-ols via rhodium-catalyzed C–H activation/carbenoid insertion/aldol-type cyclization

The rhodium-catalyzed mono-ortho C–H activation/carbenoid insertion/aldol-type cyclization of 3-aldehyde-2-phenyl-1H-indoles with diazo compounds has been developed.

Recent Advancements in Transition-Metal-Catalyzed One-Pot Twofold Unsymmetrical Difunctionalization of Arenes

Transition-metal-catalyzed direct C-H bond activation reactions have been embraced as a powerful synthetic tool to access diverse functionalized arenes. However, site-selective incorporation of multiple distinct functionalities in an arene has always been a formidable challenge. Recent efforts from the synthetic community have disclosed a few dynamic synthetic approaches to fabricate multifunctionalized arenes in one-pot using a single catalytic system. These reports manifested the immense potential of such approaches to expedite contemporary organic synthesis towards building molecular complexity. In this minireview, we have illustrated the recent progress in this area, highlighting the contribution from several synthetic chemists including our group.