Rh-Catalyzed Ortho C–H Alkynylation of Aromatic Aldehydes

Transient Directing Group-Assisted Palladium-Catalyzed C4-Alkynylation of Indoles

Pd-catalyzed C4-selective alkynylation of indoles was established by employing glycine as a transient directing group. This reaction exhibits high regioselectivity with the tolerance of a wide scope of functional groups to afford diverse alkynylated indoles in moderate to good yields. Moreover, the readily accessible scale-up synthesis and further decorations to achieve multifunctionalized indoles demonstrate the synthetic potential of this protocol.

Pd(II)-Catalyzed C4-Selective Alkynylation of Indoles by a Transient Directing Group

With alanine as a transient directing group, Pd-catalyzed regioselective alkynylation at the indole C4-position was successfully established in a good yield. The total synthesis of the PAF antagonist demonstrated the synthetic utility of this protocol. The regioselectivity was explicitly proven by the prepared C4-selective palladacycle intermediate in the catalytic process and the DFT calculation of the energy barriers of C4- and C2-site-selective C-H activation of indole.

Regiodivergent C-H alkynylation of 2-arylthiazoles switched by RuII and PdII catalysis

Two complementary regiodivergent C-H alkynylations of 2-arylthiazoles are reported. When RuII catalysis is employed, an aryl ortho-alkynylation process is favored. The alkynylated products are gained in good yields. With the use of PdII catalysis, a thiazole C5-alkynylation process is developed, allowing for the construction of C5-alkynylated products. This strategy not only expands the methods for the functionalization of 2-arylthiazoles, but also provides new opportunities for the rapid assembly of complex molecular structures, which may have great potential in organic synthesis, medicinal chemistry, and materials science.

The crucial role of silver(I)-salts as additives in C-H activation reactions: overall analysis of their versatility and applicability

Transition-metal catalyzed C-H activation reactions have been proven to be useful methodologies for the assembly of synthetically meaningful molecules. This approach bears intrinsic peculiarities that are important to be studied and comprehended in order to achieve its best performance. One example is the use of additives for the in situ generation of catalytically active species. This strategy varies according to the type of additive and the nature of the pre-catalyst that is being used. Thus, silver(I)-salts have proven to play an important role, due to the resulting high reactivity derived from the pre-catalysts of the main transition metals used so far. While being powerful and versatile, the use of silver-based additives can raise concerns, since superstoichiometric amounts of silver(I)-salts are typically required. Therefore, it is crucial to first understand the role of silver(I) salts as additives, in order to wisely overcome this barrier and shift towards silver-free systems.

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.

Catalytic C-H alkynylation of benzylamines and aldehydes with aldimine-directing groups generated in situ

Iridium-catalysed regioselective C-H alkynylation of unprotected primary benzylamines and aliphatic aldehydes has been achieved using in situ-installed aldimine directing groups. This protocol provides a straightforward route for the synthesis of the alkynylated primary benzylamine and aliphatic aldehyde derivatives, featuring good substrate compatibility and high regioselectivity.

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.

Cp*Rh(III)-Catalyzed Regioselective C(sp2)-H Mono- and Dialkynylation of Thioamides by Sulfur Coordination

An efficient Cp*Rh(III)-catalyzed regioselective C(sp²)-H mono- and dialkynylation of thioamides was described. This reaction was performed under mild conditions in high yields (up to 98%) with a broad substrate scope. Significantly, the versatility of this method was further demonstrated by controlled mono- and dialkynylation. Application of this protocol in the late stage functionalization of two drug molecules (Adapalene and Amoxapine) was also demonstrated.

Rhodium-catalysed ortho-alkynylation of nitroarenes

The ortho-alkynylation of nitro-(hetero)arenes takes place in the presence of a Rh(iii) catalyst to deliver a wide variety of alkynylated nitroarenes regioselectively. These interesting products could be further derivatized by selective reduction of the nitro group or palladium-catalysed couplings. Experimental and computational mechanistic studies demonstrate that the reaction proceeds via a turnover-limiting electrophilic C-H metalation ortho to the strongly electron-withdrawing nitro group.

Room-temperature C-H bond alkynylation by merging cobalt and photocatalysts

A new protocol is developed for the mono- and bis-ortho-C-H alkynylation of easily accessible benzamide derivatives using alkynyl bromides at room temperature by merging cobalt and photocatalysts. The diverse reactivity of various alkynyl bromides towards the C-H alkynylation and competing C-H/N-H bond annulation reactions has been demonstrated to give the corresponding products in good yields with excellent functional group tolerance.

Tetranuclear rollover cyclometalated organoplatinum-rhenium compounds; C-I oxidative addition and C-C reductive elimination using a rollover cycloplatinated dimer

The novel tetranuclear Pt(IV)-Re(VII) complex [Pt₂Me₄(OReO₃)₂(PMePh₂)₂(µ-bpy-2H)], 4, is synthesized through the reaction of silver perrhenate with a new rollover cycloplatinated(IV) complex [Pt₂Me₄I₂(PMePh₂)₂(µ-bpy-2H)], 3. In complex 4, while 2,2'-bipyridine (bpy) acts as a linker between two Pt metal centers, oxygen acts as a mono-bridging atom between Pt and Re centers through an unsupported Pt(IV)-O-Re(VII) bridge. The precursor rollover cycloplatinated(IV) complex 3 is prepared by the MeI oxidative addition reaction of the rollover cycloplatinated(II) complex [Pt₂Me₂(PMePh₂)₂(µ-bpy-2H)], 2. Complex 2 shows a metal-to-ligand charge-transfer band in the visible region, which was used to investigate the kinetics and mechanism of its double MeI oxidative addition reaction. Based on the experimental findings, the classical S_N2 mechanism was suggested for both steps and supported by computational studies. All complexes are fully characterized using multinuclear NMR spectroscopy and elemental analysis. Attempts to grow crystals of the rollover cycloplatinated(IV) dimer 3 yielded a new dimer rollover cyclometalated complex [Pt₂I₂(PMePh₂)₂(µ-bpy-2H)], 5, presumably from the C-C reductive elimination of ethane. The identity of complex 5 was confirmed by single crystal X-ray diffraction analysis.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Microwave-Assisted Palladium-Catalyzed Reductive Cyclization/Ring-Opening/Aromatization Cascade of Oxazolidines to Isoquinolines

An efficient palladium-catalyzed reaction of N-propargyl oxazolidines for the construction of 4-substituted isoquinolines under microwave irradiation is developed. This transformation proceeds through a sequential palladium-catalyzed reductive cyclization/ring-opening/aromatization cascade via C-O and C-N bond cleavages of the oxazolidine ring. The practical value of this method has also been explored by conducting a millimole-scale reaction, as well as by transforming the isoquinoline into a key intermediate for the synthesis of a lamellarin analogue.