Sequentially Palladium-Catalyzed Processes in One-Pot Syntheses of Heterocycles

A Rational Design of Electrochemically and Photophysically Tunable Triarylamine Luminophores by Consecutive (Pseudo-)Four-Component Syntheses

The concatenation of Suzuki coupling and two-fold Buchwald-Hartwig amination in sequentially palladium-catalyzed consecutive multicomponent syntheses paves a concise, convergent route to diversely functionalized para-biaryl-substituted triarylamines (p-bTAAs) from simple, readily available starting materials. An extensive library of p-bTAAs permits comprehensive investigations of their electronic properties by absorption and emission spectroscopy, cyclic voltammetry, and quantum chemical calculations, which contribute to a deep understanding of their electronic structure. The synthesized p-bTAAs exhibit tunable fluorescence from blue to yellow upon photonic excitation with quantum yields up to 98 % in solution and 92 % in the solid state. Furthermore, a pronounced bathochromic shift of the emission maxima by increasing solvent polarity indicates positive emission solvatochromism. Aggregation-induced enhanced emission (AIEE) in dimethyl sulfoxide (DMSO)/water mixtures causes the formation of intensely blue fluorescent aggregates. Cyclic voltammetry shows reversible first and second oxidations of p-bTAAs at low potentials, which are tunable by variation of the introduced para substituents. 3D Hammett plots resulting from the correlation of oxidation potentials and emission maxima with electronic substituent parameters emphasize the rational design of tailored p-bTAAs with predictable electrochemical and photophysical properties.

Aroyl-S,N-Ketene Acetals: Luminous Renaissance of a Class of Heterocyclic Compounds

Aroyl-S,N-ketene acetals represent a peculiar class of heterocyclic merocyanines, compounds bearing pronounced and rather short dipoles with great push-pull characteristics that define their rich properties. They are accessible via a wide array of synthetic concepts and procedures, ranging from addition-elimination and condensation procedures up to rearrangement and metal-mediated reactions. With our work from 2020, aroyl-S,N-ketene acetals have been identified as powerful and promising dyes with pronounced and vastly tunable solid-state emission and aggregation-induced emission properties. One characteristic trademark of this class of dye molecules is the level of control that could be exerted, and which was thoroughly explored. Based on these results, the field was opened to extend the system to bi- and multichromophoric systems by the full toolkit of synthetic organic chemistry thus giving access to even more exciting properties and manifolded substance libraries capitalizing on the AIE properties. This review aims at outlining the reaction-based principles that allow for a swift and facile access to aroyl-S,N-ketene acetals, their methodical and structural evolution and the plethora of fluorescence and aggregation properties.

Masuda Borylation–Suzuki Coupling (MBSC) Sequence: A One-Pot Process to Access Complex (hetero)Biaryls

The direct formation of (hetero)biaryls from readily available (hetero)aryl halides under mild reaction conditions can be efficiently achieved through the Masuda borylation–Suzuki coupling (MBSC) sequence. The MBSC sequence catenates Pd-catalyzed Masuda borylation and Suzuki coupling into a one-pot process, giving access to diverse symmetrically and unsymmetrically substituted scaffolds. (Hetero)biaryls are ubiquitous structural motifs that appear in natural products, pharmaceutically relevant scaffolds, functional dyes, and several other structures. This review summarizes the development of the MBSC sequence and its improvements over the past two decades.

Concise Syntheses of Marine (Bis)indole Alkaloids Meridianin C, D, F, and G and Scalaridine A via One-Pot Masuda Borylation-Suzuki Coupling Sequence

N-Protected 3-iodoindoles were reacted with (di)azine halides in a sequentially Pd-catalyzed one-pot fashion, i.e., by Masuda borylation–Suzuki coupling (MBSC) sequence. This methodology was successfully applied to the concise syntheses of marine indole alkaloids meridianin C, D, F, and G, as well as to the bisindole alkaloid scalaridine A, which were obtained in moderate to excellent yield.

Rapid Sequentially Palladium Catalyzed Four-Component Synthesis of Novel Fluorescent Biaryl-Substituted Isoxazoles

A series of novel 3- and 5-biaryl-substituted isoxazoles was prepared by a rapid microwave-assisted four-component three-step synthesis: concatenating Sonogashira coupling, cyclocondensation, and Suzuki coupling in a one-pot fashion. The Pd-catalyst was successfully employed in the sense of a sequentially catalyzed process, i.e., without the addition of further catalyst loading. Biaryl-substituted isoxazoles with donor–acceptor decoration possess remarkable photophysical properties, such as high fluorescence quantum yields in solution up to ΦF = 0.86 and large Stokes shifts up to 10,000 cm−1. The experimental absorption and emission characteristics can be reproduced and rationalized by computations on the DFT (density functional theory) and TDDFT (time-dependent density functional theory) level of theory.

Concatenating Suzuki Arylation and Buchwald-Hartwig Amination by A Sequentially Pd-Catalyzed One-Pot Process-Consecutive Three-Component Synthesis of C,N-Diarylated Heterocycles

The concatenation of Suzuki coupling and Buchwald‐Hartwig amination in a consecutive multicomponent reaction opens a concise, modular and efficient one‐pot approach to diversely functionalized heterocycles, as exemplified for 3,10‐diaryl 10H‐phenothiazines, 3,9‐diaryl 9H‐carbazoles, and 1,5‐diaryl 1H‐indoles, in high yields starting from simple staring materials. Moreover, this one‐pot reaction is a sequentially palladium‐catalyzed process that does not require additional catalyst loading after the first coupling step.

A substituent-induced post-assembly modification cascade of a metallosupramolecular imine-type Co-complex

Here, we report a reaction cascade employing the substituent-induced post-assembly modification of a Co(iii) complex. Unexpectedly, we found that the (triisopropylsilyl)alkynyl moiety introduced to the Sonogashira reaction with the bromo-functionalized Co(iii) assembly plays a "Trojan horse" role, triggering a subsequent, second step of the cascade, i.e. Co(iii) to Co(ii) reduction. The reported substituent-activated Sonogashira-redox cascade reaction might set a new direction in the construction of specific chemical sensors.

Fluorescent Donor-Acceptor Psoralen Cruciforms by Consecutive Suzuki-Suzuki and Sonogashira-Sonogashira One-Pot Syntheses

Two novel donor-acceptor cruciform topologies are efficiently synthesized by site-selective Suzuki-Suzuki and Sonogashira-Sonogashira multicomponent reactions starting from a bromo-triflato-functionalized psoralen scaffold. In addition to tunability of photophysical properties, such as absorption and emission, many derivatives possess partially high relative fluorescence quantum yields in solution and fluoresce strongly in the solid state. Additionally, the promising compounds show solvatochromism and acidochromic effects. In addition, 8-p-anisyl-5-p-cyanophenyl-substituted psoralen exhibits aggregation-induced emission properties. Experimentally (applying the Lippert-Mataga model) and computationally (TD-DFT calculations), the pronounced charge transfer character of the longest wavelength absorption band was confirmed.

Diversity-oriented approach to functional thiophene dyes by Suzuki coupling-lithiation one-pot sequences

Functional thiophenes, e.g. for organic metal-free dye sensitized solar cells (DSSC), are accessible efficiently via a divergent and diversity-oriented synthetic strategy.

One-pot activation-alkynylation-cyclization synthesis of 1,5-diacyl-5-hydroxypyrazolines in a consecutive three-component fashion

A consecutive three-component activation–alkynylation–cyclization reaction of (hetero)aryl glyoxylic acids, oxalyl chloride, arylacetylenes, and hydrazides efficiently forms 1,5-diacyl-5-hydroxypyrazolines in moderate to good yields. The structures were unambiguously corroborated by comprehensive NMR spectroscopy and X-ray structure analyses of selected derivatives.

Novel meriolin derivatives as rapid apoptosis inducers

3-(Hetero)aryl substituted 7-azaindoles possessing multikinase inhibitor activity are readily accessed in a one-pot Masuda borylation-Suzuki coupling sequence. Several promising derivatives were identified as apoptosis inducers and, emphasizing the multikinase inhibition potential, as sphingosine kinase 2 inhibitors. Our measurements provide additional insights into the structure-activity relationship of meriolin derivatives, suggesting derivatives bearing a pyridine moiety with amino groups in 2-position as most active anticancer compounds and thus as highly promising candidates for future in vivo studies.

Application of Sequential Palladium Catalysis for the Discovery of Janus Kinase Inhibitors in the Benzo[ c]pyrrolo[2,3- h][1,6]naphthyridin-5-one (BPN) Series

The present account describes the discovery and development of a new benzo[ c]pyrrolo[2,3- h][1,6]naphthyridin-5-one (BPN) JAK inhibitory chemotype that has produced selective JAK inhibitors. Sequential palladium chemistry was optimized for the rapid access to a focused library of derivatives to explore the structure-activity relationships of the new scaffold. Several compounds from the series displayed potencies in the low nanomolar range against the four members of the JAK family with various selectivity profiles. Compound 20a, with an azetidine amide side chain, showed the best selectivity for JAK1 kinase vs JAK2, JAK3, and TYK2, with low nanomolar potency (IC₅₀ = 3.4 nM). On the other hand, BPNs 17b and 18 had good general activity against the JAK family with excellent kinome selectivity profiles. Many of the new BPNs inhibited JAK3-mediated STAT-5 phosphorylation, the production of inflammatory cytokines, and the proliferation of primary T cells. Moreover, BPN 17b showed very similar in vivo results to tofacitinib in a rheumatoid arthritis animal model.

Multi-component synthesis of fluorophores via catalytic generation of alkynoyl intermediates

This account summarizes recent developments of aggregation-induced emissive and emission solvatochromic fluorophores by multicomponent reactions. Key intermediates are catalytically generated alkynoyl derivatives that are directly transformed into luminophores in a one-pot fashion. The conciseness in combination with ready access to tailored chromophore libraries makes this synthetic methodological concept superior over classical multistep syntheses.

Substituted 1H-1,2,3-Triazol-4-yl-1H-pyrrolo[2,3-b]pyridines by De Novo One-Pot Ring-Forming Coupling/Cyclization/Desilylation Cu Alkyne/Azide Cycloaddition (AAC) Sequence

Substituted 1H-1,2,3-triazol-4-yl-pyrrolo[2,3-b]pyridines are efficiently prepared by a one-pot coupling/cyclization/desilylation Cu alkyne/azide cycloaddition (CuAAC) sequence in the sense of a consecutive three-component fashion. The key feature of this new de novo formation of azole and triazole anellation is the sequentially Pd/Cu-catalyzed process employing tri(iso-propyl)silyl (TIPS) butadiyne as a four-carbon building block. In addition, the sequence can be expanded in a four-component fashion also employing the in situ formation of the required azides.

De Novo Ring-Forming Consecutive Four-Component Syntheses of 4-Pyrazolyl-1,2,3-triazoles from (Triisopropylsilyl)butadiyne as a C4 Building Block

4-Pyrazolyl-1,2,3-triazoles can be readily synthesized in a one-pot fashion and moderate yield by employing a consecutive four-component process consisting of a sequentially Pd-Cu-catalyzed alkynylation-cyclocondensation-desilylation-CuAAC process. Most distinctly, (triisopropylsilyl)butadiyne is employed as a four-carbon building block in this one-pot de novo formation of both heterocyclic moieties.

Rapid access to unsymmetrical tolanes and alkynones by sequentially palladium-catalyzed one-pot processes

Alkynones and unsymmetrically substituted tolanes can be synthesized via consecutive three-component Kumada–(Negishi)–Sonogashira bisalkynylation sequences in good to excellent yields.