Sonogashira-type Cross-coupling Reaction Research Articles

Palladium-Catalyzed Cross-Coupling Reaction of Bis(cyclopentadienyl)diaryltitaniums with Terminal Alkynes

Organotitanium compounds find application in diverse reactions, including carbon–carbon bond formation and oxidation. While titanium (IV) compounds have been used in various applications, the potential of bis(cyclopentadienyl)diaryltitanium in cross-coupling reactions remains unexplored. This study focuses on Sonogashira-type cross-coupling reactions involving terminal alkynes and organotitanium compounds. Diaryltitanocenes were synthesized using titanocene dichloride with lithium intermediates derived from aryl iodide. Under open-flask conditions, reactions of diphenyltitanocenes with ethynylbenzene in the presence of 20 mol% Pd(OAc)2 in DMF produced coupling products in a remarkable 99% yield. Various diaryltitanocenes and alkynes under standard conditions yielded corresponding cross-coupling products with moderate to good yields. Notably, the Sonogashira-type alkynylation proceeds under mild conditions, including open-flask conditions, and without the need for a base. Furthermore, this cross-coupling is atom-economical and involves the active participation of both aryl groups of the diaryltitanocene. Remarkably, this study presents the first example of a Sonogashira-type cross-coupling using titanium compounds as pseudo-halides.

Aryl Radical Enabled, Copper-Catalyzed Sonogashira-Type Cross-Coupling of Alkynes with Alkyl Iodides.

Despite the success of Sonogashira coupling for the synthesis of arylalkynes and conjugated enynes, the engagement of unactivated alkyl halides in such reactions remains historically challenging. We report herein a strategy that merges Cu-catalyzed alkyne transfer with the aryl radical activation of carbon-halide bonds to enable a general approach for the coupling of alkyl iodides with terminal alkynes. This unprecedented Sonogashira-type cross-coupling reaction tolerates a broad range of functional groups and has been applied to the late-stage cross-coupling of densely functionalized pharmaceutical agents as well as the synthesis of positron emission tomography tracers.

Synthesis of Cy5-Labelled C5-Alkynyl-modified cytidine triphosphates via Sonogashira coupling for DNA labelling

New applications of palladium-catalyzed Sonogashira-type cross-coupling reaction between C5-halogenated 2′-deoxycytidine-5′-monophosphate and novel cyanine dyes with a terminal alkyne group have been developed. The present methodology allows to synthesize of fluorescently labeled C5-nucleoside triphosphates with different acetylene linkers between the fluorophore and pyrimidine base in good to excellent yields under mild reaction conditions. Modified 2′-deoxycytidine-5′-triphosphates were shown to be good substrates for DNA polymerases and were incorporated into the DNA by polymerase chain reaction.

Synthesis and Bond Activation Chemistry of Palladium(II) Pincer Complexes with a Weakly Coordinating Side Arm

A series of Pd(II) aryl complexes of a PNO-type pincer ligand bearing a weakly coordinated benzofuran side arm has been prepared. Crystallographic studies showed very long Pd–O distances of more than 2.3 Å, which is significantly longer than the Pd–O distances in structurally similar PNO-Pd complexes with an exocylic oxygen donor. Crystallographic and 19F NMR solution studies of complexes containing electron-donating (OMe) and electron-withdrawing (CF3) substituents in the position para to the benzofuran oxygen atom revealed the dependence of the Pd–O interactions on the nature of the aromatic group at the Pd center. The ability to influence these interactions by changing the electron density at the metal was demonstrated in the stoichiometric Sonogashira-type cross-coupling reactions between the Pd complexes and phenylacetylene, which proceed via a reversible aromatization/dearomatization of the pincer ligand. Complexes with the electron-poor CF3 group showed higher reactivity in comparison to their electron-neutral or -rich analogues. DFT studies of these systems provided further mechanistic insight into the origin of the observed reactivity patterns.

Synthesis of 3-alkyl-6-methyl-1,2,4,5-tetrazines via a Sonogashira-type cross-coupling reaction.

1,2,4,5-Tetrazines have become extremely useful tools in chemical biology. However, the synthesis of some challenging substrates such as asymmetrically disubstituted alkyltetrazines is still a limitation for the widespread use of this class of compounds. Herein we describe an efficient route to these compounds based on the Sonogashira coupling of 3-bromo-6-methyl-1,2,4,5-tetrazine and 3-bromo-6-phenyl-1,2,4,5-tetrazine with terminal alkynes. The preparation of the starting reagents has also been optimized. The alkynyl products have been used as intermediates for the synthesis of dialkyl-tetrazines through a sequence of hydrogenation and re-oxidation with unprecedented yields. The synthetic applicability of this new approach is showcased through the preparation of several unnatural amino acids bearing alkynyl- and alkyl-1,2,4,5-tetrazine fragments.

Toward the synthesis of thiadiazole-based therapeutic agents: synthesis, spectroscopic study, X-ray analysis, and cross-coupling reactions of the key intermediate 3,5-diiodo-1,2,4-thiadiazole

The 1,2,4-thiadiazole moiety is an important component of several biologically active compounds, and varying substituents on this aromatic ring is one of the possible methods to develop novel thiadiazole-based drugs for medicine. A key building block to this end, namely 3,5-diiodo-1,2,4-thiadiazole (1), has been synthesized and characterized in this work for the first time. 1 has exhibited high selectivity for the replacement of iodine atom at position C5 (carbon next to sulfur) in Sonogashira-type cross-coupling reactions with phenylacetylene. Therefore, 3-iodo-5-(phenylethynyl)-1,2,4-thiadiazole (4) or 3,5-bis(phenylethynyl)-1,2,4-thiadiazole (5) could be synthesized selectively depending on reaction conditions. All three novel molecules have been characterized by NMR, IR, Raman, mass, and UV spectroscopies, and their solid phase structures have been determined by single-crystal X-ray diffraction. 1 is expected to be a key starting material for producing thiadiazole-based therapeutic agents using cross-coupling reactions.

Aggregation-Induced Fluorescence of Carbazole and o-Carborane Based Organic Fluorophore.

Carbazole based fluorophores 9-butyl-3,6-bis-(phenylethynyl)-9H-carbazole (1) and 9-butyl-3,6-bis-(2-phenyl-o-carborane)-9H-carbazole (2) were synthesized via Sonogashira type cross-coupling reaction and followed by insertion with decaborane. Compound 1 exhibited far more intense fluorescence than 2 in THF solution, while in solid state 2 exerted stronger fluorescence than 1. The fluorescence quenching behavior of 2 in THF solution could be attributed to the intramolecular charge transfer of donor-acceptor system in 2, which was confirmed by electrochemical experiments and DFT calculations. The fluorescence enhancement of 2 in solid state can be ascribed to aggregational induced packing which was evidenced by crystallographic study. In addition, compound 2 showed typical aggregation induced emission (AIE) behavior.

A carrier-free and recyclable protocol for the cross-coupling of terminal alkynes with arylboronic acids in H2O/TBAB

A new and recyclable protocol was developed for Pd(OAc)2-catalyzed the cross-coupling reaction of terminal alkynes with arylboronic acids using environmentally friendly H2O/TBAB as reaction medium. A series of cross-coupling products containing internal acetylenic bond can be obtained with good selectivity and yield. The Pd(OAc)2/H2O/TBAB system was stable in the Sonogashira-type cross-coupling reaction and could be used at least three cycles without considerable decrease in catalytic performance. The results suggest that the unsupported and recyclable systems can be extended to the other realm of CC bond formation in synthetic organic chemistry.

Enantioselective palladium/copper-catalyzed C-C σ-bond activation synergized with Sonogashira-type C(sp3)-C(sp) cross-coupling alkynylation.

The Sonogashira-type cross-coupling reaction is one of the most significant alkynylation transformations in organic chemistry. However, highly enantioselective alkynylation via the Sonogashira-type cross-coupling reaction is rather limited, mainly due to the difficulties in matching the stereoselective induction of chiral ligands with the combinational behavior of Pd/Cu-based bimetallic catalysts. We herein report novel enantioselective palladium/copper-catalyzed alkyl alkynylation of cyclobutanones with terminal alkynes via tandem C-C bond activation/Sonogashira-type cross coupling reaction, in which a novel chiral TADDOL-derived phosphoramidite ligand bearing fluorine and silicon-based bulky groups simplified as TFSi-Phos is found to be an efficient ligand for both C(sp2)-C(sp3) bond cleavage and new C(sp3)-C(sp) bond formation. A wide range of chiral alkynylated indanones bearing an all-carbon quaternary stereocenter are obtained efficiently with up to 97.5 : 2.5 er.

C-C Forming Reactions with Palladium and Platinum: Lessons Learned in Our Group

This short paper details our experiences with Pd- and Pt-catalyzed cross-coupling and cyclization reactions in investigations covering the years from 1997 until now. Pd-catalysed Suzuki-Miyaura-, Heck- and Sonogashira-type cross coupling reactions in conjunction with other transformations in one pot, especially with the Wittig reaction, are highlighted. This is followed by a description of a combination of Heck- and cyclization reactions. Finally, Pt-catalyzed diene-yne cyclization reactions are discussed.

Synthesis and anti-mitotic activity of 2,4- or 2,6-disubstituted- and 2,4,6-trisubstituted-2H-pyrazolo[4,3-c]pyridines

An efficient synthetic route for the synthesis of 2H-pyrazolo[4,3-c]pyridines, primarily varying by the substituents at the 2-, 4- and 6-positions, is described here. A Sonogashira-type cross-coupling reaction was employed to yield 3-alkynyl-1H-pyrazole-4-carbaldehydes, ethanones and propanones from the corresponding 1H-pyrazol-3-yl trifluoromethanesulfonates. Subsequent treatment of the coupling products with dry ammonia afforded a versatile library of 2H-pyrazolo[4,3-c]pyridines, which were then evaluated for their cytotoxicity against K562 and MCF-7 cancer cell lines. The most potent of these compounds displayed low micromolar GI50 values in both cell lines. Active compounds induced dose-dependent cell-cycle arrest in mitosis, as shown by flow cytometric analysis of DNA content and phosphorylation of histone H3 at serine-10. Moreover, biochemical assays revealed increased activities of caspases-3/7 in treated cells, specific fragmentation of PARP-1, and phosphorylation of Bcl-2, collectively confirming apoptosis as the mechanism of cell death.

Sonogashira-type cross-coupling reactions catalyzed by copper complexes of pincer N-heterocyclic carbenes

Copper complexes of N-heterocyclic carbenes (NHC) have recently received great attention in catalysis, however, the application scope has been limited to neutral complexes bearing monodentate ligands. Herein, we report the synthesis and full characterization of a cationic Cu-pincer bis(NHC) complex with bulky tert-butyl wingtips that serves as catalyst for the coupling of aryl iodides and phenylacetylene, alongside its analogs with small alkyl substituents. Unlike other copper catalysts that require an inert atmosphere to prevent alkyne homocoupling, a competing side reaction to Sonogashira reaction, the Cu-pincer bis(NHC) complexes provide good to excellent cross-coupling yields in air. Interestingly, the reaction under argon affords substantially reduced cross-coupling, indicative of the oxygen involvement and its facilitating effect in the mechanism. In our controlled studies, the air-assisted cross-coupling reaction came to a complete stop when using a radical trap, suggesting the presence of a radical in the mechanism. Examining the reaction of Cu-pincer bis(NHC) complexes with oxygen via UV–vis spectroscopy reveals a dark green chromophore formation at −40 °C with strong absorbance (ε > 400 M−1cm−1) at 460 nm and 500 nm, likely due to a high-valent copper-oxygen adduct.

ChemInform Abstract: Experimental and Mechanistic Exploration of Zn‐Catalyzed Sonogashira‐Type Cross‐Coupling Reactions.

One of the most straightforward methods for the synthesis of disubstituted alkynes is the transition metal catalyzed Sonogashira-type cross-coupling reactions. Herein we report a detailed study of our recent research on the first Zn-catalyzed protocol for C(sp2)–C(sp) cross-coupling reaction of aryl iodides with terminal alkynes. A wide range of functional groups were tolerated in the reaction and both functionally and structurally diverse diaryl acetylenes were prepared efficiently using this protocol. The reaction mechanism for the Zn-catalyzed Sonogashira-type coupling reaction was investigated by means of density functional theory (DFT) methods on a model system. The calculations were performed using hybrid Becke Perdew Functional (BPV86) in conjugation with LANL-2DZ basis set and found that the reaction pathway involves oxidative addition and reductive elimination mechanism.

Palladium-free Sonogashira-type cross-coupling reaction of bromoisoxazolines or N-alkoxyimidoyl bromides and alkynes

A Cu(I)-catalysed Sonogashira-type cross coupling reaction with aliphatic or aromatic bromoisoxazolines or N-alkoxyimidoyl bromides and alkynes is reported. The protocol we developed employs catalytic amount of copper(I), non-toxic ligand bathophenanthroline and is tolerant to a wide range of functional groups and is therefore particulary adapted in the context of drug discovery.

Functionalized α,β-ynones: efficient ligand for Cu catalyzed Sonogashira-type cross-coupling reaction

1,3-Yones as σ-, π-electron donating ligands significantly accelerated the cross-coupling reactions of aryl iodides with terminal alkynes, in which as low as 0.1 to 1 mol% of CuI were efficiently activated.

Synthesis of rigid ethynyl-bridged polytopic picolinate ligands for MOF applications

Segmented homopolytopic ligands that consist of a rigid central arylene platform, ethynylene spacers, and terminal chelating picolinate subunits have been synthesized in good yields in a two-step procedure involving a Sonogashira-type cross coupling reaction between the ester methyl 5-bromopyridine-2-carboxylate and several arylacetylenes, followed by hydrolysis of the resulting methyl picolinates. A similar strategy has been employed for the preparation of heteroditopic ligands containing picolinate and a second non-chelating pyridine or benzoate unit. The compounds are potential candidates for organic linkers in metal–organic frameworks (MOFs).

Palladium-catalyzed cross-coupling reactions of triarylbismuthanes with terminal alkynes under aerobic conditions

A simple Pd-catalyzed Sonogashira-type cross-coupling reaction using of triarylbismuthanes is described. The reaction of triarylbismuthanes with terminal alkynes in the presence of 5 mol% of Pd(OAc)2, 6 equiv. AgF and 6 equiv. K3PO4 at room temperature afforded the coupling products in good to excellent yield. The reaction proceeded effectively under aerobic conditions, and all three aryl groups on bismuth could be transferred to the coupling products. The reaction was sensitive to the electronic nature of the triarylbismuthanes: substrates bearing an electron-withdrawing group on the aromatic ring showed higher reactivity than those having an electron-donating group.

ChemInform Abstract: Cu(I)‐Catalyzed Macrocyclic Sonogashira‐Type Cross‐Coupling.

A macrocyclic Cu-catalyzed Sonogashira-type cross-coupling reaction has been developed that employs an operationally simple CuCl/phen/Cs2CO3 catalyst system. Macrocyclizations can be performed at relatively high concentrations without the need for slow addition techniques and form macrocycles with various ring sizes and functional groups. The optimized protocol was employed in the synthesis of (S)-zearalane, demonstrating applicability toward the synthesis of a macrocycle with known biological activity.

Cu(I)-catalyzed macrocyclic Sonogashira-type cross-coupling.

A macrocyclic Cu-catalyzed Sonogashira-type cross-coupling reaction has been developed that employs an operationally simple CuCl/phen/Cs2CO3 catalyst system. Macrocyclizations can be performed at relatively high concentrations without the need for slow addition techniques and form macrocycles with various ring sizes and functional groups. The optimized protocol was employed in the synthesis of (S)-zearalane, demonstrating applicability toward the synthesis of a macrocycle with known biological activity.

ChemInform Abstract: Choline Chloride/CuCl as an Effective Homogeneous Catalyst for Palladium‐Free Sonogashira Cross‐Coupling Reactions.

Abstract Choline chloride–CuCl (ChCl–CuCl) has been employed as an efficient catalytic system in the palladium-free Sonogashira-type cross-coupling reactions of phenylacetylene with a variety of aryl halides. Choline chloride, as an effective ligand and a quaternary ammonium salt, demonstrated an efficient stabilizing effect on the Cu(I) species during the reaction. This homogeneous catalytic system is attractive in view of its ease of application and low cost.