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.

Solid-state red-emissive (cyano)vinylene heteroaromatics via Pd-catalysed C-H homocoupling

Thiophene-based π-conjugated systems are important materials for organic electronics; thus, their synthesis is of topical interest. We report fluorescent thiophene/furan-based vinylene and cyanovinylene systems via Pd-catalysed homocoupling [Pd(OAc)₂, pivalic acid, KOAc, DMAc, 140 °C]. The methodology is versatile and allows the development of a variety of π-conjugated systems without the need for pre-functionalized building units. The reaction tolerates electron-rich, electron-deficient and large π-conjugated substrates. The developed compounds absorb in the visible region (400-515 nm) and emit green to orange fluorescence in the solution state (510-600 nm). Most importantly, the compounds exhibit strong aggregation-induced emission (AIE) in the NIR region (λ_em = 650 nm), with quantum yields reaching up to 10%. Steric hindrance imparted by vinylene/cyanovinylene units is responsible for the strong solid-state luminescence. DFT-optimized structures reveal an apparent twist of 20-40° in the molecular backbone of the compounds, supporting the AIE behaviour of the compounds.

Circular Discovery in Small Molecule and Conjugated Polymer Synthetic Methodology

Simple and efficient methods are a key consideration for small molecule and polymer syntheses. Direct arylation polymerization (DArP) is of increasing interest for preparing conjugated polymers as an effective approach compared to conventional cross-coupling polymerizations. As DArP sees broader utilization, advancements are needed to access materials with improved properties and different monomer structures and to improve the scalability of conjugated polymer synthesis. Presented herein are considerations for developing new methods of conjugated polymer synthesis from small molecule transformations, exploring how DArP has successfully used this approach, and presenting how emerging polymerization methodologies are developing similarly. While it is common to adapt small molecule methods to polymerizations, we demonstrate the ways in which information gained from studying polymerizations can inform and inspire greater advancements in small molecule transformations. This circular approach to organic synthetic method development underlines the value of collaboration between small molecule and polymer-based synthetic research groups.

A Perspective on Polythiophenes as Conformation Dependent Optical Reporters for Label-Free Bioanalytics

Poly(3-alkylthiophene) (PT)-based conjugated polyelectrolytes (CPEs) constitute an important class of responsive polymers with excellent optical properties. The electrostatic interactions between PTs and target analytes trigger complexation and concomitant conformational changes of the PT backbones that produce distinct optical responses. These conformation-induced optical responses of the PTs enable them to be utilized as reporters for detection of various analytes by employing simple UV-vis spectrophotometry or the naked eye. Numerous PTs with unique pendant groups have been synthesized to tailor their interactions with analytes such as nucleotides, ions, surfactants, proteins, and bacterial and viral pathogens. In this perspective, we discuss PT-target analyte complexation for bioanalytical applications and highlight recent advancements in point-of-care and field deployable assays. Subsequently, we highlight a few areas of critical importance for future applications of PTs as reporters, including (i) design and synthesis of specific PTs to advance the understanding of the mechanisms of interaction with target analytes, (ii) using arrays of PTs and linear discriminant analysis for selective and specific detection of target analytes, (iii) translation of conventional homogeneous solution-based assays into heterogeneous membrane-based assay formats, and finally (iv) the potential of using PT as an alternative to conjugated polymer nanoparticles and dots in bioimaging.

Statistical Copolymers of Thiophene-3-Carboxylates and Selenophene-3-Carboxylates; 77Se NMR as a Tool to Examine Copolymer Sequence in Selenophene-Based Conjugated Polymers

Herein, we demonstrate that homopolymerization and statistical copolymerization of 2-ethylhexyl thiophene-3-carboxylate and 2-ethylhexyl selenophene-3-carboxylate monomers is possible via Suzuki-Miyaura cross-coupling. A commercially available palladium catalyst ([1,3-bis(2,6-di-3-pentylphenyl)imidazol-2-ylidene](3-chloropyridyl)dichloropalladium(II) or PEPPSI-IPent) was employed...

Synthesis of Conjugated Polymers via Transition Metal Catalysed C-H Bond Activation

Transition metal catalysed C-H bond activation chemistry has emerged as an exciting and promising approach in organic synthesis. This allows us to synthesize a wider range of functional molecules and conjugated polymers in a more convenient and more atom economical way. The formation of C-C bonds in the construction of pi-conjugated systems, particularly for conjugated polymers, has benefited much from the advances in C-H bond activation chemistry. Compared to conventional transition-metal catalysed cross-coupling polymerization such as Suzuki and Stille cross-coupling, pre-functionalization of aromatic monomers, such as halogenation, borylation and stannylation, is no longer required for direct arylation polymerization (DArP), which involve C-H/C-X cross-coupling, and oxidative direct arylation polymerization (Ox-DArP), which involves C-H/C-H cross-coupling protocols driven by the activation of monomers' C(sp² )-H bonds. Furthermore, poly(annulation) via C-H bond activation chemistry leads to the formation of unique pi-conjugated moieties as part of the polymeric backbone. This review thus summarises advances to date in the synthesis of conjugated polymers utilizing transition metal catalysed C-H bond activation chemistry. A variety of conjugated polymers via DArP including poly(thiophene), thieno[3,4-c]pyrrole-4,6-dione)-containing, fluorenyl-containing, benzothiadiazole-containing and diketopyrrolopyrrole-containing copolymers, were summarized. Conjugated polymers obtained through Ox-DArP were outlined and compared. Furthermore, poly(annulation) using transition metal catalysed C-H bond activation chemistry was also reviewed. In the last part of this review, difficulties and perspective to make use of transition metal catalysed C-H activation polymerization to prepare conjugated polymers were discussed and commented.

Synthesis of π-Conjugated Polymers Containing Benzotriazole Units via Palladium-Catalyzed Direct C-H Cross-Coupling Polycondensation for OLEDs Applications

Four D-π-A conjugated polymers, namely P1-P4, which contain benzotriazole building blocks in their backbone as acceptor, are synthesized via palladium-catalyzed direct C-H cross-coupling polycondensation of 5,6-difluorobenzotriazole with different thiophene derivatives, including 3-octylthiophene, 2,2'-bithiophene, thieno[3,4-b][1,4]dioxine, and 4,4-dioctyl-4H-silolo-[3,2-b:4,5-b']dithiophene as donor units, respectively. Taking the polymer P1 as an example, the chemical structure of the polymer is demonstrated by 1H and 19F NMR spectra. The optical, electrochemical, and thermal properties of these polymers are assessed by UV-vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), and thermal gravimetric analysis (TGA), respectively. DFT simulations of all polymers are also performed to understand their physicochemical properties. Furthermore, P1 and P2, which have relatively higher molecular weights and better fluorescent quantum efficiency than those of P3 and P4, are utilized as lighting emitters for organic light-emitting diodes (OLEDs), affording promising green and red luminescence with 0.07% and 0.14% of maximum external quantum efficiency, respectively, based on a device with an architecture of ITO/PEDOT:PSS/PTAA/the polymer emitting layer/TPBi/LiF/Al.

Approaches for improving the sustainability of conjugated polymer synthesis using direct arylation polymerization (DArP)

Emerging strategies to enhance the sustainability of Direct Arylation Polymerization (DArP) are discussed, illustrating the great potential of this method.

Exploring the Utility of Buchwald Ligands for C-H Oxidative Direct Arylation Polymerizations

Oxidative C-H/C-H cross-coupling polymerizations provide an opportunity to synthesize conjugated polymers with an increased ease of monomer preparation, reduced environmental impact, and increased sustainability. Considering these attributes, it is necessary to expand the diversity of monomers that readily and efficiently participate in this coupling strategy to enable the development of conjugated polymers with a wide range of properties. Herein, the oxidative direct arylation polymerization toolbox is expanded to include 3,4-propylenedioxythiophene being synthesized via C-H/C-H cross-coupling methodologies. In conjunction with these efforts, the utilization of Buchwald ligands in C-H/C-H cross coupling polymerizations also is reported, and variations in the ligand structure provide insight into the role ligand choice has on C-H cross-coupling polymerizations. Specifically, it is determined that the phosphine functionality affects the rate-determining, concerted metalation-deprotonation step of the catalytic cycle, while bulky isopropyl substituents on the ligand's lower aryl ring promote reductive elimination. By balancing these steric effects on the ancillary ligands, polymers are synthesized to exhibit molecular weights above the effective conjugation length, with recovered yields >90%. In addition to expanding the scope of conjugated polymers accessible via oxidative direct arylation polymerization, these results provide the foundational understanding for utilizing Buchwald-type ligands in C-H-activated polymerizations.

Influence of an ester directing-group on defect formation in the synthesis of conjugated polymers via direct arylation polymerization (DArP) using sustainable solvents

We report the application of green solvents in DArP and the structure-dependent β-defect formation due to an ester directing group.

Dual-Catalytic Ag-Pd System for Direct Arylation Polymerization to Synthesize Poly(3-hexylthiophene)

Direct arylation polymerization (DArP) has gained interest in materials chemistry as a method to synthesize conjugated polymers with minimal use of harsh reagents and additional steps. Traditional DArP conditions do not readily yield ideal polymerization characteristics, including chain-growth and low dispersities. It would be of great utility to advance DArP methodology to become competitive with traditional conjugated polymerization techniques. We have developed conditions for a dual-catalytic Ag-Pd system for the synthesis of poly(3-hexylthiophene) (P3HT) that exhibits chain-growth kinetics, low dispersities, and catalyst chain association by Pd. Specifically, the presence of Ag-carboxylate additives plays a beneficial role in the polymerization as a C-H activating agent, while PEPPSI-iPr is used as the Pd source for C-C coupling. The addition of pyridine is necessary to inhibit Pd-mediated C-H activation in the interest of catalyst orthogonality, which can lower dispersities.

Synthesis of Conjugated Polymers Containing Octafluorobiphenylene Unit via Pd-Catalyzed Cross-Dehydrogenative-Coupling Reaction

Polycondensation via Pd-catalyzed cross-dehydrogenative-coupling reaction of 2,2',3,3',5,5',6,6'-octafluorobiphenyl with thiophene analogues was studied. The synthetic protocol, in which employment of prefunctionalized starting monomers was fully avoided, allowed straightforward access to an alternating π-conjugated polymer. The addition of K₂CO₃ to the catalytic system promotes the cross-coupling reaction and suppresses the undesired homocoupling reaction, producing the corresponding donor-acceptor type π-conjugated polymers with minor homocoupling defects. The reaction also proceeded using O₂ as the terminal oxidant, resulting in lower loading of the Ag oxidant. The obtained polymer was evaluated as an emitting material for an organic light-emitting diode.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Facile one-pot access to π-conjugated polymers via sequential bromination/direct arylation polycondensation

The synthesis of π-conjugated polymers starting from unfunctionalized aromatic monomers via sequential bromination/direct arylation polycondensation was investigated.

Synthesis of random poly(hexyl thiophene-3-carboxylate) copolymers via oxidative direct arylation polymerization (oxi-DArP)

Oxi-DArP is utilized for the simple, mild, and atom economical synthesis of two families of random P3HET copolymers without any preactivation of the monomers, demonstrating the compatibility of this emerging synthetic method with copolymerizations.

Oxidative Direct Arylation Polymerization Using Oxygen as the Sole Oxidant: Facile, Green Access to Bithiazole-Based Polymers

The most appealing oxidant, molecular oxygen, is employed for the first time as the sole oxidant in the transition metal-catalyzed oxidative direct arylation polymerization (C-H/C-H-type DArP), which eliminates by-product formation of stoichiometric metal salts except for water . Compared to conventional approaches, other than the avoidance of an end-capping procedure, the current protocol is remarkably advanced in the aspect of eco-friendliness, step- and cost-economy, and, of special significance, the purity of polymer products. As illustrative examples, six 5,5'-bithiazole-based polymers are synthesized using this new method, demonstrating higher number-average molecular weight (M_n up to 33 700) in better yields (up to 93 %) through only one step. The evolution of C-H/C-H-type DArP from heavy metal salts to O₂ alone as the oxidant could solve the problem of metal residues in polymers, which is considered harmful to the performance of devices.

C-H Arylation in the Synthesis of π-Conjugated Polymers

C-H activation reactions have allowed us to react traditionally chemically inert bonds in molecules to develop new methods for cross-coupling reactions. This type of reactivity can be applied to conjugated polymer materials in an effort to improve existing synthetic difficulties including harsh reaction conditions, multiple monomer functionalization steps, and organometallic reagent waste. In this Viewpoint, we highlight some of the encouraging advances in direct arylation polymerization (DArP) as well as ongoing challenges for future improvement and utility.