Aryl Sulfoxides: A Traceless Directing Group for Catalytic C-H Activation of Arenes

The transition metal-catalyzed C-H activation of arenes directed by sulfoxides represents a compelling strategy in organic synthesis, owing to its exceptional regioselectivity and high efficiency. This innovative approach stands out for its traceless character, enabling the direct functionalization of arenes, before the easy removal or conversion of the key sulfinyl moiety. Beyond their utility as a directing group, sulfoxides have proved particularly valuable to mediate as chiral auxiliaries, presenting exciting prospects for the synthesis of stereo-enriched compounds upon C-H functionalization. The versatility demonstrated by the method paves the way to different structures with potential applications ranging from medicinal chemistry to organic electronics.

Palladium-Catalyzed Intermolecular Dehydrogenative C-2 Pentenylation of Indoles with 2-Methyl-2-butene

Five-carbon (C5) units are the fundamental building blocks that constitute a multitude of natural products. Herein we report an unprecedented unusual C5 functionalization of indole regioselectively at the C-2-position enabled by a (2-pyridyl)sulfonyl-directing palladium-catalyzed dehydrogenative strategy with a bulk chemical 2-methyl-2-butene as a C5 source. Compared to typical C5 functionalization using pentenyl alcohols, carbonates, borates, or halides as the C5 source, the protocol not only has a low cost advantage but also is of atom and step economy.

Efficient approach to 1,1'-bisindoles via copper(I)-catalyzed double domino reaction

A highly efficient copper(I)-catalyzed approach for the synthesis of 1,1'-bisindoles that is based on the formation of four bonds in one step has been developed. The unprecedented three component reaction between one molecule of a 1,2-bis(2-bromoaryl)hydrazine and two molecules of a 1,3-diketone employing 10 mol% CuI as a catalyst and Cs2CO3 as a base in DMSO at 100 °C for 24 h delivers substituted 1,1'-bisindoles with yields up to 92%. The new method proceeds as a double domino condensation/Ullmann type C-C coupling. It allows an efficient and practical access to substituted 1,1'-bisindoles in one step from easily available starting materials.

Synthesis of 5,6-Dihydrophenanthridines via Palladium-Catalyzed Intramolecular Dehydrogenative Coupling of Two Aryl C−H Bonds

5,6-Dihydrophenanthridines are common aza heterocycle frameworks of natural products and pharmaceuticals. Herein, we reported the first palladium-catalyzed intramolecular C−H/C−H dehydrogenative coupling reaction of two simple arenes to generate 5,6-dihydrophenanthridines. The approach features a broad substrate scope and good tolerance of functional groups, offering an efficient alternative synthesis route for important 5,6-dihydrophenanthridine compounds.

Pd(II)/Lewis acid catalyzed regioselective olefination of indole with dioxygen

Transition metal ion catalyzed indole olefination through C-H activation is a convenient protocol to synthesize versatile bioactive vinylindole compounds; however, in most cases, stoichiometric amounts of oxidants were necessary to accomplish the catalytic cycle. The present study describes a Pd(II)/LA (LA: Lewis acid) catalyzed indole olefination with dioxygen as the sole oxidant. The olefination reaction with electron-rich olefins proceeded smoothly through the pyrrolyl N-carboxamide group directed remote C-H activation at the C3 position of the indole with the Pd(II)/LA catalyst, whereas Pd(II) alone was a very sluggish catalyst under identical conditions. For the electron-deficient olefins, the directing N-carboxamide group was not essential for olefination with this Pd(II)/LA catalyst, demonstrating a different olefination pathway from that of electron-rich olefins. Remarkably, ¹H NMR kinetics disclosed that olefination proceeded much faster with electron-rich olefins than with electron-deficient ones.

Synthesis of Aporphine Analogues via Palladium-Catalyzed Intramolecular Aryl-Aryl Dehydrogenative Coupling

Reported herein is an intramolecular dehydrogenative coupling of two inert aryl C-H bonds for the synthesis of aporphine analogues. The process represents a novel tool for the preparation of aporphines via palladiun-catalyzed C-H bond activation. The present reaction is compatible with various functional groups, and the coupling products have been further applied for the synthesis of natural products aporphine and zenkerine.

An entry to 2-(cyclobut-1-en-1-yl)-1H-indoles through a cyclobutenylation/deprotection cascade

A transition-metal-free strategy for the synthesis of 2-(cyclobut-1-en-1-yl)-1H-indoles under mild conditions is described herein. A series of substituted 2-(cyclobut-1-en-1-yl)-1H-indoles are accessed by a one-pot cyclobutenylation/deprotection cascade from N-Boc protected indoles. Preliminary experimental and density functional theory calculations suggest that a Boc-group transfer is involved in the underlying mechanism.

Study on Palladium(II)-Catalyzed Mono-1-alkenylation of 9H-Carbazoles

A general and efficient method is reported for the direct mono-1-alkenylation of 9H-carbazole molecules with divalent palladium as a catalyst and an N-(2-pyridyl)sulfanyl directing group. This method also provides an efficient synthetic route for the synthesis of cross-dialkenylated carbazoles.

Regiocontrol in the oxidative Heck reaction of indole by ligand-enabled switch of the regioselectivity-determining step

Efficient control of regioselectivity is a key concern in transition-metal-catalyzed direct C-H functionalization reactions. Various strategies for regiocontrol have been established by tuning the selectivity of the C-H activation step as a common mode. Herein, we present our study on an alternative mode of regiocontrol, in which the selectivity of the C-H activation step is no longer a key concern. We found that, in a reaction where the C-H activation step exhibits a different regio-preference from the subsequent functionalization step, a ligand-enabled switch of the regioselectivity-determining step could provide efficient regiocontrol. This mode has been exemplified by the Pd(ii)-catalyzed aerobic oxidative Heck reaction of indoles, in which a ligand-controlled C3-/C2-selectivity was achieved for the first time by the development of sulfoxide-2-hydroxypyridine (SOHP) ligands.

Sterically Controlled C-H Olefination of Heteroarenes

The regioselective functionalization of heteroarenes is a highly attractive synthetic target due to the prevalence of multiply substituted heteroarenes in nature and bioactive compounds. Some substitution patterns remain challenging: While highly efficient methods for the C2-selective olefination of 3-substituted five-membered heteroarenes have been reported, analogous methods to access the 5-olefinated products have remained limited by poor regioselectivities and/or the requirement to use an excess of the valuable heteroarene starting material. Herein we report a sterically controlled C-H olefination using heteroarenes as the limiting reagent. The method enables the highly C5-selective olefination of a wide range of heteroarenes and is shown to be useful in the context of late-stage functionalization.

Palladium-catalysed 5-endo-trig allylic (hetero)arylation

A palladium-catalysed intramolecular allylic (hetero)arylation strategy for the synthesis of fused cyclopentenes incorporated with all-carbon quaternary and spiro centres is described. The method is straightforward, shows broad scope, proceeds in synthetically useful yields, and provides a rare means to construct complex cyclopentanoids. The reaction is believed to involve a kinetically unfavourable 5-endo-trig carbocyclisation of the tethered (π-allyl)palladium system. Further, this method was successfully applied as the key step in the total synthesis of diterpene natural products taiwaniaquinone H and dichroanone.

FeCl3 mediated dimerization of dihydropyrrolo[2,1-a]isoquinolines and chlorination of tetrasubstituted pyrroles

We have developed a mild and direct dimerization of dihydropyrrolo[2,1-a]isoquinolines through FeCl₃-mediated oxidative homocoupling. A series of dimeric dihydropyrroloisoquinoline derivatives were obtained in acceptable to excellent yields (up to >99%). Attempts to expand this dimerization system to the synthesis of dimeric pyrroles failed and chlorination products were obtained in most cases instead of dimeric pyrroles.

C-H Halogenation of Pyridyl Sulfides Avoiding the Sulfur Oxidation: A Direct Catalytic Access to Sulfanyl Polyhalides and Polyaromatics

Palladium-catalyzed oxidative C-H halogenation and acetoxylation reactions of S-unprotected sulfides, selectively directed by pyridinyl groups, allows the formation of C-X bonds (X = I, Br, Cl, OAc) by using simple halosuccinimide or phenyliodine diacetate (PIDA) oxidants. The undesired formation of sulfoxides and/or sulfones, which are usually observed under oxidative conditions, is fully obviated. Under the solvent-dependent conditions that we proposed, sulfide C-H functionalization is achieved in less than 1 h without any direct electrophilic halogenation at the pyridine moiety. N-Directed ortho-C-H activation of aryl also facilitates dibromination reactions which are hardly accessible with sulfone and sulfoxide counterparts because of their higher structural rigidity. This general method gives a straightforward access to polyhalide sulfides, without an organosulfur reduction step or protection-deprotection sequence. Polyhalide sulfides are valuable synthons that give a practical entry to new constrained polyaromatic and biphenyl sulfides, including synthetically challenging unsymmetrical examples.

Synthesis of a Double-Helical Naphthotetraindole Core via an Intramolecular Dehydrogenative Homocoupling Reaction

A novel naphthotetraindole (NTI) core was synthesized in two steps via the McMurry reaction and a [Cu]/[Ag]-promoted intramolecular dehydrogenative homocoupling reaction. NTI shows a unique X-shaped double-helical structure as determined by single-crystal X-ray diffraction analysis. The hole-transport mobility of the bare NTI core was measured to be 2.3 × 10^-5 cm² V^-1 s^-1, which is comparable with the commonly used Spiro-OMeTAD.

Access to 2-substituted 1-pyridin-3-yl-β-carboline derivatives by intramolecular radical cyclization-ring opening-SNAr substitution

A new method for the synthesis of 1-pyridin-3-yl-β-carboline derivatives involving an intramolecular radical reaction and S_NAr substitution, is described.

Cobalt-catalyzed cross-dehydrogenative coupling between N-(2-pyridyl) and free indoles for the synthesis of unsymmetrical 2,2′-biindoles

Cobalt-catalyzed cross-dehydrogenative coupling of N-(2-pyridyl) indoles with free indoles for the construction of unsymmetrical 2,2′-biindoles is developed.

Benzimidazolium Naphthoxide Betaine Is a Visible Light Promoted Organic Photoredox Catalyst

Benzimidazolium naphthoxide (^-ONap-BI⁺) was first synthesized and utilized as an unprecedented betaine photoredox catalyst. Photoexcited state of ^-ONap-BI⁺ generated by visible light irradiation catalyzes the reductive deiodination as well as desulfonylation reactions in which 1,3-dimethyl-2-phenylbenzimidazoline (Ph-BIH) cooperates with as an electron and hydrogen atom donor. Significant solvent effects on the reaction progress were discovered, and specific solvation toward imidazolium and naphthoxide moieties of ^-ONap-BI⁺ was proposed.

Rh(iii)-Catalyzed regio- and stereoselective bisindolylation of vinyl acetate: an efficient approach toward (E)-1,2-bis(2-indolyl)ethenes

An efficient Rh(iii)-catalyzed regio- and stereoselective cross-coupling between vinyl acetate andN-(2-pyridiyl)indoles providing a series of (E)-2,2′-bis(indolyl)ethenes has been developed.

Regioselective C5 alkenylation of 2-acylpyrroles via Pd(ii)-catalyzed C–H bond activation

A Pd(ii)-catalyzed regioselective C5 alkenylation of 2-acylpyrroles to synthesize various C5-alkenylated 2-acylpyrroles using a readily removable N-protecting group is developed.

Palladium-Catalyzed Dehydrogenative Coupling: An Efficient Synthetic Strategy for the Construction of the Quinoline Core

Palladium-catalyzed dehydrogenative coupling is an efficient synthetic strategy for the construction of quinoline scaffolds, a privileged structure and prevalent motif in many natural and biologically active products, in particular in marine alkaloids. Thus, quinolines and 1,2-dihydroquinolines can be selectively obtained in moderate-to-good yields via intramolecular C–H alkenylation reactions, by choosing the reaction conditions. This methodology provides a direct method for the construction of this type of quinoline through an efficient and atom economical procedure, and constitutes significant advance over the existing procedures that require preactivated reaction partners.

Substrate-Selective C-H Functionalization for the Preparation of Organosulfur Compounds from Crude Oil-Derived Components

The direct utilization of a natural feedstock in organic synthesis is an utmost challenge because the selective production of one product from a mixture of starting materials requires unprecedented substrate selectivity. In the present study, a simple and convenient procedure is evaluated for the substrate-selective alkenylation of a single component in a mixture of organosulfur compounds. Pd-catalyzed alkenylation of two-, three-, four-, and five-component mixtures of crude oil-derived sulfur species led to the exclusive C-H functionalization of only one compound. The observed remarkable substrate selectivity opens new opportunities for sustainable organic synthesis.

One-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H functionalization

A one-pot synthesis of fluorescent 2,4-dialkenylindoles by rhodium-catalyzed dual C–H bond alkenylation of indoles.

Transition metal-catalyzed site- and regio-divergent C–H bond functionalization

The regioselectivity of C–H functionalization reactions can be redirected to obtain regioisomeric products form the same starting materials.