Electron-rich Aldehydes Research Articles

Carboxy-substituted D-π-A arylated chalcones: Synthesis, photophysical properties and preliminary evaluation as photosensitizers for DSSCs

Chalcones are versatile molecules with potential use in various fields and technologies. The judicious choice of chalcones’ building blocks allows the design of compounds with different properties and applications. In the present work, four highly conjugated carboxy-substituted chalcones with D-π-A architecture were synthesized aiming their evaluation as photosensitizers for dye-sensitized solar cells (DSSCs). In their architecture, electron-rich aldehydes were used as precursors of the donor units, and carboxylate or cyanoacrylate acceptor units were bonded to the acetophenone portion. The compounds presented intense UV–vis absorptions (ε in the order of 104 mol L−1 cm−1) centered between 322 and 429 nm, and blue to yellow fluorescence, with quantum yields ranging from less than 0.01 to 0.35. These chalcones were evaluated as photosensitizers for DSSCs, displaying short-circuit current density (ISC) ranging from 0.18 to 0.36 mA cm−2, open circuit voltage (Voc) from 0.52 to 0.56 V, fill factor (FF) between 54 and 67%, and power conversion efficiency from 0.05 to 0.13%. The two best performances were obtained with photosensitizers 6a and 7b which display the pairs dimethylaminophenyl/carboxylate and vanillin/cyanoacrylate as the donor and acceptor/anchoring units, respectively.

Olefination with Sulfonyl Halides and Esters: Synthesis of Unsaturated Sulfonyl Fluorides.

Methanedisulfonyl fluoride, CH2(SO2F)2, transforms aromatic aldehydes into β-arylethenesulfonyl fluorides, useful substrates for the SuFEx “click”-type transformations. The reaction mimics mechanism of the Horner–Wadsworth–Emmons olefination, which runs via addition of the carbanion, followed by cyclization–fragmentation of the four-membered ring intermediate. In the absence of base, electron-rich aldehydes follow an alternative pathway of the Knoevenagel condensation to provide unsaturated 1,1-disulfonyl fluorides. We demonstrate also trapping of elusive ethene-1,1-disulfonyl fluoride, CH2=C(SO2F)2, with 4-(dimethylamino)pyridine (DMAP) that forms zwitterionic adduct, characterized with X-ray studies.

Decatungstate Catalyzed Synthesis of Trifluoromethylthioesters from Aldehydes via a Radical Process.

Here we report a mild and general method for the trifluoromethylthiolation of aldehydes using N-trifluoromethylthiosaccharin as the CF3S radical source and sodium decatungstate (NaDT) as the photocatalyst. This reaction proceeds via hydrogen atom abstraction by photoactivated DT and features good functional groups and substrate tolerance. Generally, electron-rich aldehydes demonstrate better reactivity than electron-deficient ones and good selectivity is observed for the trifluoromethylthiolation of aldehydic C-H bonds over tertiary and benzylic C-H bonds. Preliminary mechanistic studies have shown that a free radical process is involved.

Dual Nickel/Photoredox-Catalyzed Deaminative Cross-Coupling of Sterically Hindered Primary Amines.

We report a method to activate α-3° amines for deaminative arylation via condensation with an electron-rich aldehyde and merge this reactivity with nickel metallaphotoredox to generate benzylic quaternary centers, a common motif in pharmaceuticals and natural products. The reaction is accelerated by added ammonium salts. Evidence is provided in support of two roles for the additive: inhibition of nickel black formation and acceleration of the overall reaction rate. We demonstrate a robust scope of amine and haloarene coupling partners and show an expedited synthesis of ALK2 inhibitors.

Triethylammonium Hydrogen Sulfate [Et3NH][HSO4]-Catalyzed Rapid and Efficient Multicomponent Synthesis of Pyrido[2,3-d]pyrimidine and Pyrazolo[3,4-b]pyridine Hybrids.

An operationally simple, one-pot multicomponent reaction has been developed for the assembly of pyrido[2,3-d]pyrimidine and pyrazolo[3,4-b]pyridine derivatives (4a–4am) in excellent yields (92–94%) with high purity. The reactions were easy to perform simply by mixing of electron-rich amino heterocycles (including aminouracils and aminopyrazoles), aldehyde, and acyl acetonitrile in the presence of [Et3NH][HSO4] under solvent-free conditions. The remarkable feature of the present approach is that the ionic liquid possesses dual solvent-catalytic engineering capability. Results of this study revealed that 1 mmol of the ionic liquid catalyst under solvent-free conditions at 60 °C is the best reaction parameter for the construction of fused pyridine and pyrimidine derivatives in excellent yields. The present methodology showed good results under gram-scale conditions, thereby indicating its applicability in industrial as well as academic settings in the near future.

A multi-component reaction of electron-rich arenes, potassium metabisulfite, aldehydes and aryldiazonium tetrafluoroborates

A multi-component reaction of electron-rich arenes, potassium metabisulfite, aldehydes and aryldiazonium tetrafluoroborates under mild conditions is reported, leading to a range of (arylsulfonyl)methylbenzenes in moderate to good yields.

Photoredox-Catalyzed Deaminative Alkylation via C-N Bond Activation of Primary Amines.

Primary amines are often cheap, naturally occurring, and chemically diverse starting materials. For these reasons, deaminative functionalization of amines has emerged as an important area of research. Recent advances in C-N activation transform simple α-1° and α-2° amines into alkylating reagents via Katritzky pyridinium salts. We report a complementary method that activates sterically encumbered α-3° primary amines through visible light photoredox catalysis. By condensing α-3° primary amines with electron-rich aryl aldehyde, we enable an oxidation and deprotonation event, which generates a key imidoyl radical intermediate. A subsequent β-scission event liberates alkyl radicals for coupling with electron-deficient olefins for the generation of unnatural γ-quaternary amino acids and other valuable synthetic targets.

Solution-Phase DNA-Compatible Pictet-Spengler Reaction Aided by Machine Learning Building Block Filtering.

SummaryThe application of machine learning toward DNA encoded library (DEL) technology is lacking despite obvious synergy between these two advancing technologies. Herein, a machine learning algorithm has been developed that predicts the conversion rate for the DNA-compatible reaction of a building block with a model DNA-conjugate. We exemplify the value of this technique with a challenging reaction, the Pictet-Spengler, where acidic conditions are normally required to achieve the desired cyclization between tryptophan and aldehydes to provide tryptolines. This is the first demonstration of using a machine learning algorithm to cull potential building blocks prior to their purchase and testing for DNA-encoded library synthesis. Importantly, this allows for a challenging reaction, with an otherwise very low building block pass rate in the test reaction, to still be used in DEL synthesis. Furthermore, because our protocol is solution phase it is directly applicable to standard plate-based DEL synthesis.

Asymmetric Formal Vinylogous Iminium Ion Activation for Vinyl-Substituted Heteroaryl and Aryl Aldehydes.

The vinyl group tethered to furfurals could be LUMO-lowered by forming formal vinylogous iminium ion intermediates catalyzed by a chiral secondary amine and underwent asymmetric [3 + 2] cycloaddition reactions with N-trifluoroethyl-substituted isatin imines, furnishing a variety of spirooxindoles incorporating a 3,2'-pyrrolidine motif with excellent stereoselectivity. In addition, this strategy has been successfully expanded to a number of vinyl-substituted electron-rich heteroaryl aldehydes and even some specific aryl aldehydes.

Synthesis and Characterization of Pheox- and Phebox-Aluminum Complexes: Application as Tunable Lewis Acid Catalysts in Organic Reactions.

Pheox- and Phebox-aluminum complexes were synthesized and subsequently characterized by spectroscopic analysis. These complexes acted as Lewis acid catalysts, and their catalytic activities were controlled by using the Pheox, Phebox, and heteroatom ligands. The Pheox-aluminum complex exhibited an opposite substrate selectivity to AlCl3 in a competitive hetero-Diels-Alder reaction between electron-rich and electron-deficient aldehydes.

A Highly Efficient NHC-Catalyzed Aerobic Oxidation of Aldehydes to Carboxylic Acids

An N-heterocyclic carbene (NHC) organocatalytic aerobic oxidation of aldehydes to the corresponding carboxylic acids is explored. Remarkably, this method allows for efficient conversion of different classes of aldehydes including highly challenging electron-rich aryl aldehydes, ortho-substituted aryl aldehydes, various heteroaromatic aldehydes and α,β-unsaturated aldehydes under mild reaction conditions. These substrates, under previously reported NHC-catalyzed methods, are typically unreactive or give poor yields, require high reaction temperatures and reaction times of several days.

Gold(I)-Catalyzed 1,3-O-Transposition of Ynones: Mechanism and Catalytic Acceleration with Electron-Rich Aldehydes

The gold-catalyzed 1,3-O-transposition of ynones occurs intermolecularly via a cyclic organo-gold acetal intermediate formed from the nucleophilic oxo attack of a second ynone, i.e. either starting material or product, on a gold-activated ynone. The combination of 1H NMR monitored kinetic data, analyzed using variable time normalization analysis (VTNA) and kinetic modeling, and density functional theory (DFT) was used to elucidate the mechanism. A significant acceleration of the reaction rate could be achieved by the addition of a substoichiometric amount of electron-rich aldehyde as a mediator, allowing the gold-catalyzed 1,3-O-transposition of terminal ynones to ynaldehydes. The mechanism is further supported by NMR characterization of the acetal intermediate and 18O labeling experiments. A model for predicting the reactivity from aldehyde frontier molecular orbital energies is also presented.

Multicomponent Catalytic Asymmetric Synthesis of trans-Aziridines.

A multicomponent trans-aziridination of aldehydes, amines, and diazo compounds with BOROX catalysts is developed. The optimal protocol is slightly different for aryl aldehydes than for aliphatic aldehydes. The key to the success with aryl aldehydes was allowing the catalyst, aldehyde, and amine to react for 20 min before addition of the diazo compound. A variety of 11 different electron-poor and electron-rich aryl aldehydes were screened to give trans-aziridines in 73-90% yield with 82-99% ee and trans/cis selectivities of 19:1 to >99:1. The optimal protocol for the trans-aziridination of aliphatic aldehydes did not require prereaction of the catalyst, aldehyde, and amine, and instead, the diazo compound could be added directly. The scope of the reaction is limited to unbranched aliphatic aldehydes and was tolerant of a number of functional groups including ethers, esters, epoxides, carbamates, and phthalimides. A total of 10 aliphatic aldehydes were examined and found to give trans-aziridines in 60-88% yield with 60-98% ee and trans/cis selectivities of 6:1 to >99:1. Alkenyl aldehydes did not react, but an alkynyl aldehyde gave a 71% yield and 95% ee of an aziridine that was found to be the cis- and not the trans-diastereomer. The aryl and aliphatic aldehydes both gave the trans-aziridines with the same absolute configuration with the same catalyst; however, in those cases where cis-aziridines were formed, the configuration was opposite for those formed from aryl versus aliphatic aldehydes.

Base-promoted three-component cascade approach to unsymmetrical bis(indolyl)methanes

Here we report a base-catalyzed reaction of two different indoles with aldehydes under heating to produce unsymmetrical bis(indolyl)methanes (BIMs), in which one of the indole ring must be N-substituted. Mixture of EtOH-H2O is used as solvent. The reaction did not give symmetrical BIMs of N-substituted indoles or N–H indoles. However, traces of latter were formed in few cases, especially when electron-rich aldehydes were used. Diversely substituted indoles and aldehydes were used for the reaction. The reaction proceeds via 3-indolylalcohol, which we confirmed through isolation. The method also gives good yield on multigram scale reaction.

Selective and eco-friendly procedures for the synthesis of benzimidazole derivatives. The role of the Er(OTf)3 catalyst in the reaction selectivity.

An improved and greener protocol for the synthesis of benzimidazole derivatives, starting from o-phenylenediamine, with different aldehydes is reported. Double-condensation products were selectively obtained when Er(OTf)3 was used as the catalyst in the presence of electron-rich aldehydes. Conversely, the formation of mono-condensation products was the preferred path in absence of this catalyst. One of the major advantages of these reactions was the formation of a single product, avoiding extensive isolation and purification of products, which is frequently associated with these reactions.Theoretical calculations helped to understand the different reactivity established for these reactions. Thus, we found that the charge density on the oxygen of the carbonyl group has a significant impact on the reaction pathway. For instance, electron-rich aldehydes better coordinate to the catalyst, which favours the addition of the amine group to the carbonyl group, therefore facilitating the formation of double-condensation products.Reactions with aliphatic or aromatic aldehydes were possible, without using organic solvents and in a one-pot procedure with short reaction time (2–5 min), affording single products in excellent yields (75–99%). This convenient and eco-friendly methodology offers numerous benefits with respect to other protocols reported for similar compounds.

Promoted Selective Synthesis of Benzylpyrazolocoumarins Using Silica Sodium Carbonate (SSC): A Novel Green Synthetic Technology Using SSC as a Highly Efficient and Environmentally Safe Catalyst

In this study, silica sodium carbonate (SSC) as an eco-friendly solid recyclable Brønsted base and highly efficient catalyst has been successfully applied to green and promoted selective synthesis of benzylpyrazolocoumarins in aqueous medium. Using this solid Brønsted catalyst has some advantages in the synthesis of benzylpyrazolocoumarins including obtaining excellent yields of products, short reaction times, green and safety protocol, easy to handle, and capability of catalyst to recycle. By this achievement some interesting potential biological active compounds were obtained via a green procedure. Methods: A green and highly efficient protocol for the catalytic synthesis of benzylpyrazolocoumarins by the use of SSC as an eco-friendly and recyclable Brønsted base is introduced. Results: Some potential biologically active benzylpyrazolocoumarins were efficiently synthesized via a green Domino reaction. To examine generality of the reaction, a wide range of both electron-rich and electron-poor aryl aldehydes were applied. The efficiency of catalyst on the reaction progress with other acidic and basic catalysts were studied and compared with each other. Also, the capability of the catalyst to reuse was evaluated that the results showed SSC can be reused upto five times without remarkable miss of activity. Conclusion: The SSC showed that could successfully catalyze alternative Knoevenagel condensations and Michael addition reactions. Furthermore, by this achievement some important potential biologically active products were efficiently synthesized in a green procedure. Keywords: Benzylpyrazolocoumarin, silica sodium carbonate, green catalyst, heterogeneous solid base, green procedure, aqueous medium.

Electronic Effects on Narcissistic Self-Sorting in Multicomponent Self-Assembly of Fe-Iminopyridine meso-Helicates.

Small changes in the electron donating ability of coordinating groups have substantial effects on the multicomponent self-assembly of Fe (II)-iminopyridine-based meso-helicate complexes. Both the nature of the internal diamine core and the terminal formylpyridine reactants control the rate of the assembly process, the thermodynamic favorability of the meso-helicate products, and the selective incorporation of different aldehyde termini into the assembly. Steric congestion at the coordinating ligands can prevent assembly altogether, and favorable incorporation of electron-rich aldehyde termini is observed, even though the rate of reaction is accelerated by the use of electron-poor aldehyde reactants. NMR and electrospray ionization mass spectrometry analyses were employed to determine the synergistic nature of narcissistic self-sorting in this system, which depends on both the rigidity of the central core and the electronic donor ability of the aldehyde terminus. These experiments illustrate that significant control of self-sorting and self-assembly is possible upon extremely small variations in ligand structure, rigidity, and donor ability.

Fast Knoevenagel Condensations Catalyzed by an Artificial Schiff-Base-Forming Enzyme.

The simple catalytic motifs utilized by enzymes created by computational design and directed evolution constitute a potentially valuable source of chemical promiscuity. Here we show that the artificial retro-aldolase RA95.5-8 is able to use a reactive lysine in a hydrophobic pocket to accelerate promiscuous Knoevenagel condensations of electron-rich aldehydes and activated methylene donors. Optimization of this activity by directed evolution afforded an efficient enzyme variant with a catalytic proficiency of 5 × 10(11) M(-1) and a >10(8)-fold catalytic advantage over simple primary and secondary amines. Divergent evolution of de novo enzymes in this way could be a promising strategy for creating tailored biocatalysts for many synthetically useful reactions.

ChemInform Abstract: Nickel‐Catalyzed Reactions Directed Toward the Formation of Heterocycles

AbstractReview: 69 refs.

Nickel-Catalyzed Reactions Directed toward the Formation of Heterocycles.

Heterocycles have garnered significant attention because they are important functional building blocks in various useful molecules, such as pharmaceuticals, agricultural chemicals, pesticides, and materials. Several studies have been conducted regarding the preparation of heterocyclic skeletons with an emphasis on selectivity and efficiency. Three strategies are typically employed to construct cyclic molecules, namely, cyclization, cycloaddition, and ring-size alterations. Although each method has certain advantages, cycloaddition may be superior from the viewpoint of divergence. Specifically, cycloadditions enable the construction of rings from several pieces. However, the construction of heterocycles via cycloadditions is more challenging than the construction of carbocycles. For heterocycle construction, simple pericyclic reactions rarely work smoothly because of the large HOMO-LUMO gap unless well-designed combinations, such as electron-rich dienes and aldehydes, are utilized. Thus, a different approach should be employed to prepare heterocycles via cycloadditions. To this end, the use of metallacycles containing heteroatoms is expected to serve as a promising solution. In this study, we focused on the preparation of heteroatom-containing nickelacycles. Because nickel possesses a relatively high redox potential and an affinity for heteroatoms, several methods were developed to synthesize heteronickelacycles from various starting materials. The prepared nickelacycles were demonstrated to be reasonable intermediates in cycloaddition reactions, which were used to prepare various heterocycles. In this Account, we introduce the following four methods to prepare heterocycles via heteronickelacycles. (1) Direct oxidative insertion of Ni(0) to α,β-unsaturated enone derivatives: treatment of 3-ethoxycarbonyl-4-phenyl-3-buten-2-one with Ni(0) afforded an oxa-nickelacycle, which reacted with alkynes to give pyrans. (2) Substitution of a part of a cyclic compound with low-valent nickel, accompanied by elimination of small molecules such as CO, CO2, and acetophenone: treatment of phthalic anhydride with Ni(0) in the presence of ZnCl2 afforded the oxanickelacycle, which was formed via decarbonylative insertion of Ni(0) and reacted with alkynes to give isocumarins. (3) Cyclization to a nickelacycle, accompanied by two C-C σ-bond activations: insertion of Ni(0) into an arylnitrile, followed by aryl cyanation of an alkyne, gave alkenylnickel as an intermediate. The alkenylnickel species subsequently underwent an intramolecular nucleophilic attack with an arylcarbonyl group to form a cyclized product with concomitant cleavage of the C-C σ-bond between the carbonyl and aryl groups. (4) Assembly of several components to form a heteroatom-containing nickelacycle via cycloaddition: a new [2 + 2 + 1] cyclization reaction was carried out using an α,β-unsaturated ester, isocyanate, and alkyne via a nickelacycle. On the basis of these four strategies, we developed new methods to prepare heterocyclic compounds using nickelacycles as the key active species.