Electrophilic Hydrosilylation of Electron-Rich Alkenes Derived from Enamines

The low-electron count, air-stable, platinum complexes [Pt(ItBu')(ItBu)][BArF] (C1) (ItBu=1,3-di-tert-butylimidazol-2-ylidene), [Pt(SiPh)3(ItBuiPr)2][BArF] (C2) (ItBuiPr=1-tert-butyl-3-iso-propylimidazol-2-ylidene), [Pt(SiPh)3(ItBuMe)2][BArF] (C3), [Pt(GePh3)(ItBuiPr)2][BArF] (C4), [Pt(GePh)3(ItBuMe)2][BArF] (C5) and [Pt(GeEt)3(ItBuMe)2][BArF] (C6) (ItBuMe=1-tert-butyl-3-methylimidazol-2-ylidene) are efficient catalysts (particularly the germyl derivatives) in both the silylative dehydrocoupling and hydrosilylation of electron rich alkenes derived from enamines. The steric hindrance exerted by the NHC ligand plays an important role in the selectivity of the reaction. Thus, bulky ligands are selective towards the silylative dehydrocoupling process whereas less sterically hindered promote the selective hydrosilylation reaction. The latter is, in addition, regioselective towards the β-carbon atom of both internal and terminal enamines, leading to β-aminosilanes. Moreover, the syn stereochemistry of the amino and silyl groups implies an anti Si-H bond addition across the double bond. All these facts point to a mechanistic picture that, according to experimental and computational studies, involves a non-classical hydrosilylation process through an outer-sphere mechanism in which a formal nucleophilic addition of the enamine to the silicon atom of a platinum σ-SiH complex is the key step. This is in sharp contrast with the classical Chalk-Harrod mechanism prevalent in platinum chemistry.

Bioinspired Functionalization of Carbonyl Compounds Enabled by Metal Chelated Bifunctional Ligands

In Nature, enzymatic reactions proceed through exceptionally ordered transition states giving rise to extraordinary levels of stereoselection. In those reactions, the active site of the enzyme plays crucial roles - through one position, it holds the substrate in the proximity to the reaction epicentre that facilitates both the reactivity and stereoselectivity of the chemical process. Inspired by this natural phenomenon, synthetic chemists have designed bifunctional ligands that not only coordinate to a metal centre but also preassociate with an organic substrate, for example aldehyde and ketone, and exerts stereodirecting influence to accelerate the attack of the incoming reacting partner from a particular enantiotopic face. The chief goal of the current review is to give an overview of the recently developed approaches enabled by privileged bio-inspired bifunctional ligands that not only bind to the metal catalyst but also activates carbonyl substrates via organocatalysis, thereby easing in the new bond forming step. As carbonyl α-functionalizations are dominated by enamine and enolate chemistry, the current review primarily focusses on enamine- and enolate-metal catalysis by bifunctional ligands. Thus, developments based on traditional cooperative catalysis occurring through two directly coupled but independent catalytic cycles of an organocatalyst and a metal catalyst are not covered.

Kinetic Study on the Base-Catalyzed Imine-Enamine Tautomerism of a Chiral Biologically Active Isoxazoline Derivative by HPLC on Amylose Tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase

Isoxazoline is a nitrogen- and oxygen-containing five-membered heterocyclic scaffold with diverse biological profiles such as antimicrobial, fungicidal, anticancer, antiviral, analgesic and anti-inflammatory activity. Accordingly, the use of this peculiar structural framework in drug discovery is a successful strategy for the development of new drug candidates. Here, a chiral saccharin/isoxazoline hybrid was considered to investigate the tendency of the imine moiety of the heterocyclic ring to tautomerize to the enamine form in the presence of a basic catalyst. The pseudo-first-order rate constants for the base-catalyzed tautomerization process were measured in different solvents and at different temperatures by off-column kinetic experiments based on the amylose (3,5-dimethylphenylcarbamate)-type chiral stationary phase. The kinetic results obtained in this study may be a useful aid in the perspective of designing experimental conditions to control the stereointegrity of these types of pharmacologically active compounds and drive their synthesis toward the preferred, imine or enamine, tautomer.

Synthesis and crystal structures of three Schiff bases derived from 3-formyl-acetyl-acetone and benzyl-, tert-butyl- and (S)-methyl-benzyl-amine

Treatment of 3-formyl-acetyl-acetone with the amines benzyl-amine, tert-butyl-amine and (S)-methyl-benzyl-amine led to the formation of the corresponding Schiff bases 3-[(benzyl-amino)-methyl-idene]pentane-2,4-dione, C13H15NO2 (1), 3-[(tert-butyl-amino)-methyl-idene]pentan-2,4-dione, C10H17NO2 (2) and 3-{[(S)-benz-yl(meth-yl)amino]-methyl-idene}pentane-2,4-dione, C14H17NO2 (3). The mol-ecules of all three compounds exist as enamine tautomers that contain a nearly planar amino-methyl-ene-pentane-2,4-dione core with a strong intra-molecular N-H⋯O hydrogen bridge. The R group attached to the enamine N atom has no significant influence on the bond lengths and angles of the amino-methyl-ene-pentane-2,4-dione core. The supra-molecular structures in 1-3 are mainly based on weak C-H⋯O hydrogen bonds.

Recent Advances in Ynenamine Chemistry

Enamines are formed by reacting a carbonyl compound with an amine under dehydration conditions. A vast array of transformations has been achieved via preformed enamine chemistry. Recently, by introducing conjugating double bonds to the enamine functionality, dienamines, and trienamines have propelled the discovery of several previously unattainable remote-site functionalization reactions of carbonyl compounds. Comparatively, alkyne-conjugating enamine analogues have recently shown high potential in multifunctionalization reactions while remaining underexplored. In this account, we systematically summarized and discussed recent advances in synthetic transformations based on ynenamine-containing compounds.

Isoxazolyl-Derived 1,4-Dihydroazolo[5,1-c][1,2,4]Triazines: Synthesis and Photochemical Properties

New fluorescent dyes containing an assembled 1,4-dihydroazolo[5,1-c][1,2,4]triazine (DAT) core and an isoxazole ring were synthesized through a reaction between diazopyrazole or diazoimidazoles and isoxazolyl-derived enamines in mild conditions. The photophysical characteristics (maxima absorption and emission, Stokes shifts, fluorescent quantum yields, and fluorescence lifetimes) of the new fluorophores were obtained. The prepared DATs demonstrated emission maxima ranging within 433-487 nm, quantum yields within 6.1-33.3%, and a large Stokes shift. The photophysical characteristics of representative DAT examples were studied in ten different solvents. Specific (hydrogen bonds) and non-specific (dipole-dipole) intermolecular and intramolecular interactions were analyzed using XRD data and spectral experiments. Solvatochromism was analyzed using Lippert-Mataga and Dimroth-Reichardt plots, revealing the relationship between the DAT structure and the nature of solute-solvent interactions. The significant advantages of DATs are the fluorescence of their powders (QY up to 98.7%). DAT-NMe₂10 expressed bright aggregation-induced emission (AIE) behavior in DMSO and THF as the water content increased. The numerous possible variations of the structures of the heterocycles included in the DATs, as well as substituents, create excellent prospects for adjusting their photophysical and physicochemical properties.

Regio- and Stereoselective Hydroelementation of SF5-Alkynes and Further Functionalizations

Herein is described a fully regio- and stereoselective hydroelementation reaction of SF5-alkynes with N, O and S-nucleophiles and further functionalization of the corresponding Z-(hetero)vinyl-SF5 intermediates, a suitable platform to access α-SF5 ketones and esters, β-SF5 amines and alcohols under mild reaction conditions.  Experimental and computational comparative studies between SF5- and CF3-alkynes have been performed to highlight and explain the difference of reactivity and selectivity observed between these two fluorinated motifs.

Structural and Mechanistic Basis for the Inactivation of Human Ornithine Aminotransferase by (3S,4S)-3-Amino-4-fluorocyclopentenecarboxylic Acid

Ornithine aminotransferase (OAT) is overexpressed in hepatocellular carcinoma (HCC), and we previously showed that inactivation of OAT inhibits the growth of HCC. Recently, we found that (3S,4S)-3-amino-4-fluorocyclopentenecarboxylic acid (5) was a potent inactivator of γ-aminobutyric acid aminotransferase (GABA-AT), proceeding by an enamine mechanism. Here we describe our investigations into the activity and mechanism of 5 as an inactivator of human OAT. We have found that 5 exhibits 10-fold less inactivation efficiency (kinact/KI) against hOAT than GABA-AT. A comprehensive mechanistic study was carried out to understand its inactivation mechanism with hOAT. pKa and electrostatic potential calculations were performed to further support the notion that the α,β-unsaturated alkene of 5 is critical for enhancing acidity and nucleophilicity of the corresponding intermediates and ultimately responsible for the improved inactivation efficiency of 5 over the corresponding saturated analogue (4). Intact protein mass spectrometry and the crystal structure complex with hOAT provide evidence to conclude that 5 mainly inactivates hOAT through noncovalent interactions, and that, unlike with GABA-AT, covalent binding with hOAT is a minor component of the total inhibition which is unique relative to other monofluoro-substituted derivatives. Furthermore, based on the results of transient-state measurements and free energy calculations, it is suggested that the α,β-unsaturated carboxylate group of PLP-bound 5 may be directly involved in the inactivation cascade by forming an enolate intermediate. Overall, compound 5 exhibits unusual structural conversions which are catalyzed by specific residues within hOAT, ultimately leading to an enamine mechanism-based inactivation of hOAT through noncovalent interactions and covalent modification.

Second-Generation Enamine-Type Schiff Bases as 2-Amino Acid-Derived Antifungals against Fusarium oxysporum: Microwave-Assisted Synthesis, In Vitro Activity, 3D-QSAR, and In Vivo Effect

In this manuscript, the synthesis of enamine-type Schiff bases 1−48 derived from the amino acids L-Ala, L-Tyr, and L-Phe was carried out. Their in vitro activity and in vivo protective effect against Fusarium oxysporum were also evaluated through mycelial growth inhibition and disease severity reduction under greenhouse conditions. The in vitro activity of test compounds 1−48 showed half-maximal inhibitory concentrations (IC50) at different levels below the 40 mM range. Deep analysis of the IC50 variations indicated that the size of the substituent on the acetylacetone derivatives and the electronic character on the cyclohexane-3-one fragment influenced the antifungal effect. 3D-QSAR models based on atoms (atom-based approach) were built to establish the structure−activity relationship of the test Schiff bases, showing a good correlation and predictive consistency (R2 > 0.70 and Q2 > 0.60). The respective contour analysis also provided information about the structural requirements for potentiating their antifungal activity. In particular, the amino acid-related fragment and the alkyl ester residue can favor hydrophobic interactions. In contrast, the nitrogen atoms and enamine substituent are favorable regions as H-donating and electron-withdrawing moieties. The most active compounds (40 and 41) protected cape gooseberry plants against F. oxysporum infection (disease severity index < 2), involving adequate physiological parameters (stomatal conductance > 150 mmol/m2s) after 45 days of inoculation. These promising results will allow the design of novel Schiff base-inspired antifungals using 2-amino acids as precursors.

Quantification of the Electrophilicities of Diazoalkanes: Kinetics and Mechanism of Azo Couplings with Enamines and Sulfonium Ylides

Kinetics and mechanism of the reactions of methyl diazoacetate, dimethyl diazomalonate, 4-nitrophenyldiazomethane, and diphenyldiazomethane with sulfonium ylides and enamines were investigated by UV-Vis and NMR spectroscopy. Ordinary alkenes undergo 1,3-dipolar cycloadditions with these diazo compounds. In contrast, sulfonium ylides and enamines attack at the terminal nitrogen of the diazo alkanes to give zwitterions, which undergo various subsequent reactions. As only one new bond is formed in the rate-determining step of these reactions, the correlation lg k₂ (20 °C)=s_N (N+E) could be used to determine the one-bond electrophilicities E of the diazo compounds from the measured second-order rate constants and the known reactivity indices N and s_N of the sulfonium ylides and enamines. The resulting electrophilicity parameters (-21<E<-18), which are 11-14 orders of magnitude smaller than that of the benzenediazonium ion, are used to define the scope of one-bond nucleophiles which may react with these diazoalkanes.

Design of Bifunctional Amino Tf-Amide Organocatalysts and Application in Various Asymmetric Transformations

In this personal account, we describe our recent developments on the four types of amino Tf-amide catalysts in asymmetric transformations. Firstly, axially chiral biaryl-based secondary-amino Tf-amide catalyzed various stereoselective reactions via enamine intermediates. Secondly, pyrrolidine-based secondary-amino aliphatic Tf-amide catalyzed asymmetric direct Mannich reaction. Thirdly, chiral primary-amino aliphatic Tf-amide catalyzed asymmetric direct aldol reaction and conjugate addition. Finally, modified chiral amino aromatic Tf-amide catalyzed asymmetric transformations. These four different strategies are illustrated by using various organocatalyzed asymmetric transformations.

Trifluoroethanol as a Unique Additive for the Chemoselective Electrooxidation of Enamines to Access Unsymmetrically Substituted NH-Pyrroles

An electrochemical method for the synthesis of unsymmetrically substituted NH-pyrroles is described. The synthetic strategy comprises a challenging heterocoupling between two structurally diverse enamines via sequential chemoselective oxidation, addition, and cyclization processes. A series of aryl- and alkyl-substituted enamines were effectively cross-coupled from an equimolar mixture to synthesize various unsymmetrical pyrrole derivatives up to 84 % yield. The desired cross-coupling was achieved by tuning the oxidation potential of the enamines by utilizing a "magic effect" of the additive trifluoroethanol (TFE). Additionally, extensive computational studies reveal the unique role of TFE in promoting the heterocoupling process by regulating the activation energies of the reaction steps through H-bonding and C-H⋅⋅⋅π interactions. Importantly, the developed electrochemical protocol was found to be equally efficient for the homocoupling of enamines to form symmetric pyrroles up to 92 % yield.

Synthesis of Azulene Derivatives from 2H-Cyclohepta[b]furan-2-ones as Starting Materials: Their Reactivity and Properties

A variety of synthetic methods have been developed for azulene derivatives due to their potential applications in pharmaceuticals and organic materials. Particularly, 2H-cyclohepta[b]furan-2-one and its derivatives have been frequently used as promising precursors for the synthesis of azulenes. In this review, we describe the development of the synthesis of azulenes by the reaction of 2H-cyclohepta[b]furan-2-ones with olefins, active methylenes, enamines, and silyl enol ethers as well as their reactivity and properties.

Interfacial versus Bulk Properties of Hole-Transporting Materials for Perovskite Solar Cells: Isomeric Triphenylamine-Based Enamines versus Spiro-OMeTAD

Here, we report on three new triphenylamine-based enamines synthesized by condensation of an appropriate primary amine with 2,2-diphenylacetaldehyde and characterized by experimental techniques and density functional theory (DFT) computations. Experimental results allow highlighting attractive properties including solid-state ionization potential in the range of 5.33-5.69 eV in solid-state and hole mobilities exceeding 10^-3 cm²/V·s, which are higher than those in spiro-OMeTAD at the same electric fields. DFT-based analysis points to the presence of several conformers close in energy at room temperature. The newly synthesized hole-transporting materials (HTMs) were used in perovskite solar cells and exhibited performances comparable to that of spiro-OMeTAD. The device containing one newly synthesized hole-transporting enamine was characterized by a power conversion efficiency of 18.4%. Our analysis indicates that the perovskite-HTM interface dominates the properties of perovskite solar cells. PL measurements indicate smaller efficiency for perovskite-to-new HTM hole transfer as compared to spiro-OMeTAD. Nevertheless, the comparable power conversion efficiencies and simple synthesis of the new compounds make them attractive candidates for utilization in perovskite solar cells.

Rid Enhances the 6-Hydroxypseudooxynicotine Dehydrogenase Reaction in Nicotine Degradation by Agrobacterium tumefaciens S33

Agrobacterium tumefaciens S33 degrades nicotine through a hybrid of the pyridine and pyrrolidine pathways. The oxidation of 6-hydroxypseudooxynicotine to 6-hydroxy-3-succinoyl-semialdehyde-pyridine by 6-hydroxypseudooxynicotine dehydrogenase (Pno) is an important step in the breakdown of the N-heterocycle in this pathway. Although Pno has been characterized, the reaction is not fully understood; what is known is that it starts at a high speed followed by a rapid drop in the reaction rate, leading to the formation of a very small amount of product. In this study, we speculated that an unstable imine intermediate that is toxic with regard to the metabolism is produced in the reaction. We found that a Rid protein (designated Rid-NC) encoded by a gene in the nicotine-degrading gene cluster enhanced the reaction. Rid is a widely distributed family of small proteins with various functions, and some subfamilies have deaminase activity to eliminate the toxicity of the reactive intermediate, imine. Biochemical analyses showed that Rid-NC relieved the toxicity of the presumed imine intermediate produced in the Pno reaction and that, in the presence of Rid-NC, Pno maintained a high level of activity and the amount of the reaction product was increase by at least 5-fold. Disruption of the rid-NC gene led to slower growth of strain S33 on nicotine. The mechanism of Rid-NC-mediated detoxification of the imine intermediate was discussed. A phylogenetic analysis indicated that Rid-NC belongs to the rarely studied Rid6 subfamily. These results further our understanding of the biochemical mechanism of nicotine degradation and provide new insights into the function of the Rid6 subfamily proteins.IMPORTANCE Rid is a family of proteins that participate in metabolite damage repair and is widely distributed in different organisms. In this study, we found that Rid-NC, which belongs to the Rid6 subfamily, promoted the 6-hydroxypseudooxynicotine dehydrogenase (Pno) reaction in the hybrid of the pyridine and pyrrolidine pathways for nicotine degradation by Agrobacterium tumefaciens S33. Rid-NC hydrolyzed the presumed reactive imine intermediate produced in the reaction to remove its toxicity on Pno. The finding furthers our understanding of the metabolic process of the toxic N-heterocyclic aromatic compounds in microorganisms. This study demonstrated that the Rid family of proteins also functions in the metabolism of N-heterocyclic aromatic alkaloids, in addition to the amino acid metabolism, and that Rid6-subfamily proteins also have deaminase activity, similar to the RidA subfamily. The ability of reactive imines to damage a non-pyridoxal-5'-phosphate-dependent enzyme was reported. This study provides new insights into the function of the Rid family of proteins.

Configurational and Constitutional Dynamics of Enamine Molecular Switches

Dual configurational and constitutional dynamics in systems based on enamine molecular switches has been systematically studied. pH-responsive moieties, such as 2-pyridyl and 2-quinolinyl units, were required on the "stator" part, also providing enamine stability through intramolecular hydrogen-bonding (IMHB) effects. Upon protonation or deprotonation, forward and backward switching could be rapidly achieved. Extension of the stator π-system in the 2-quinolinyl derivative provided a higher E-isomeric equilibrium ratio under neutral conditions, pointing to a means to achieve quantitative forward/backward isomerization processes. The "rotor" part of the enamine switches exhibited constitutional exchange ability with primary amines. Interestingly, considerably higher exchange rates were observed with amines containing ester groups, indicating potential stabilization of the transition state through IMHB. Acids, particularly BiIII , were found to efficiently catalyze the constitutional dynamic processes. In contrast, the enamine and the formed dynamic enamine system showed excellent stability under basic conditions. This coupled configurational and constitutional dynamics expands the scope of dynamic C-C and C-N bonds and potentiates further studies and applications in the fields of molecular machinery and systems chemistry.

Unexpected synthesis and crystal structure of N-{2-[2-(2-acetyl-ethen-yl)phen-oxy]eth-yl}-N-ethenyl-4-methyl-benzene-sulfonamide

The title compound, C21H23NO4S, obtained by alkaline treatment of 1,5-bis-(1-phen-oxy)-3-aza-pentane at moderate heating, is a N-tosyl-ated secondary vinyl-amine. An intra-molecular S=O⋯H-C hydrogen bond generates a 13-membered ring. The benzalacetone moiety adopts a trans conformation with respect to the C=C double bond, which is slightly longer than usual due to the conjugation with a neighbouring acetyl group. Theoretical predictions of potential biological activities were performed, suggesting that the title compound can inhibit gluconate 2-de-hydrogenase (85% probability), as well as to act as a mucomembranous protector (73%).

Synthesis, Antiproliferative and Cytotoxic Activities, DNA Binding Features and Molecular Docking Study of Novel Enamine Derivatives

Novel enamine derivatives were synthesized from the reaction of lactone and chalcones and their antiproliferative and cytotoxic activities against six cancer cell lines (e. g., HeLa, HT29, A549, MCF7, PC3 and Hep3B) and one normal cell lines (e. g., FL) were investigated along with their mode of interactions with CT-DNA. Most of the enamine derivatives with IC₅₀ values of 86-168 μM demonstrated much stronger antiproliferative activity than the starting molecules against the cancer cells. While, among the enamine derivatives, four compounds displayed higher cytotoxic potency than the control drugs (5-fluorouracil and cisplatin) against the Hep3B cell lines, these compounds did not exhibit any significant toxicity against normal cells, FL. The UV/VIS spectral data suggest that eight compounds cause hypochromism with a slight bathochromic shift (∼6 nm), indicating that they bind to the DNA by way of an intercalative or minor groove binding mode. The binding constants of the compounds are in the range of 0.1×103 M^-1 -2.3×104 M^-1 . The antiproliferative activity of studied enamine derivatives could possibly be due to their DNA binding as well as their cytotoxic properties. In addition to these assays, the chalcones and enamine derivatives were investigated by molecular docking to calculate the synergistic effect of antiproliferative activities against six human cancer cell lines.

Synthesis and antiproliferative activity of novel (+)- usnic acid analogues

Twenty one novel (+)- usnic acid-based analogues belonging to three classes such as enamines, imines, and pyrazoles were synthesized. All the synthesized compounds were characterized by their spectral data (¹H NMR, ¹³C NMR, IR, and HRMS). The synthesized compounds were evaluated for their antiproliferative activity against a panel of four human cancer cell lines including HeLa (cervix), MDA-MB-231 (breast), A549 (lung), and MiaPaca (pancreas) by employing SRB cell proliferation assay. Screening results indicated that all synthesized compounds showed enhanced activity than the parent compound. Most significantly, compounds 2e and 4a showed potent antiproliferative activity against all the cancer cell lines tested. Compounds 2e and 4a arrested the cell cycle in G2/M phase and induced apoptosis in HeLa cells. In view of significant antiproliferative activity, compounds 2e and 4a can be considered as lead molecules for further development.[Formula: see text].

Streptavidin-Hosted Organocatalytic Aldol Addition

In this report, the streptavidin-biotin technology was applied to enable organocatalytic aldol addition. By attaching pyrrolidine to the valeric motif of biotin and introducing it to streptavidin (Sav), a protein-based organocatalytic system was created, and the aldol addition of acetone with p-nitrobenzaldehyde was tested. The conversion of substrate to product can be as high as 93%. Although the observed enantioselectivity was only moderate (33:67 er), further protein engineering efforts can be included to improve the selectivity. These results have proven the concept that Sav can be used to host stereoselective aldol addition.

Differentiation between Enamines and Tautomerizable Imines Oxidation Reaction Mechanism using Electron-Vibration-Vibration Two Dimensional Infrared Spectroscopy

Intermediates lie at the center of chemical reaction mechanisms. However, detecting intermediates in an organic reaction and understanding its role in reaction mechanisms remains a big challenge. In this paper, we used the theoretical calculations to explore the potential of the electron-vibration-vibration two-dimensional infrared (EVV-2DIR) spectroscopy in detecting the intermediates in the oxidation reactions of enamines and tautomerizable imines with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). We show that while it is difficult to identify the intermediates from their infrared and Raman signals, the simulated EVV-2DIR spectra of these intermediates have well resolved spectral features, which are absent in the signals of reactants and products. These characteristic spectral signatures can, therefore, be used to reveal the reaction mechanism as well as monitor the reaction progress. Our work suggests the potential strength of EVV-2DIR technique in studying the molecular mechanism of organic reactions in general.

Regioselective Fluorination of 7-Oxo-1,2,4-benzotriazines Using Selectfluor

7-Oxo-1,2,4-benzotriazines (benzo[1,2,4]triazin-7-ones) are reversible thioredoxin reductase inhibitors that exhibit very strong correlations to pleurotin. In this article, we provide the first synthesis of fluorinated derivatives. Fluorination using Selectfluor of benzo[1,2,4]triazin-7-ones occurs regioselectively and in high yield at the enamine-activated position. This electron N-lone pair activation overrides the activation/deactivation effects of some other substituents. The reaction time was significantly reduced with the use of microwave irradiation at 120 °C and 7 bar. The cytotoxicity and cyclic voltammetry measurements for 8-fluoro-1,3-diphenylbenzo[e][1,2,4]triazin-7(1H)-one (2) are presented and compared with its synthetic precursor, 1,3-diphenylbenzo[e][1,2,4]triazin-7(1H)-one (1a).

Efficient and Stable Perovskite Solar Cells Using Low-Cost Aniline-Based Enamine Hole-Transporting Materials

Metal-halide perovskites offer great potential to realize low-cost and flexible next-generation solar cells. Low-temperature-processed organic hole-transporting layers play an important role in advancing device efficiencies and stabilities. Inexpensive and stable hole-transporting materials (HTMs) are highly desirable toward the scaling up of perovskite solar cells (PSCs). Here, a new group of aniline-based enamine HTMs obtained via a one-step synthesis procedure is reported, without using a transition metal catalyst, from very common and inexpensive aniline precursors. This results in a material cost reduction to less than 1/5 of that for the archetypal spiro-OMeTAD. PSCs using an enamine V1091 HTM exhibit a champion power conversion efficiency of over 20%. Importantly, the unsealed devices with V1091 retain 96% of their original efficiency after storage in ambient air, with a relative humidity of 45% for over 800 h, while the devices fabricated using spiro-OMeTAD dropped down to 42% of their original efficiency after aging. Additionally, these materials can be processed via both solution and vacuum processes, which is believed to open up new possibilities for interlayers used in large-area all perovskite tandem cells, as well as many other optoelectronic device applications.

13 C Kinetic Isotope Effects as a Quantitative Probe To Distinguish between Enol and Enamine Mechanisms in Aminocatalysis

A combination of experimental ¹³ C kinetic isotope effects (KIEs) and high-level density functional theory (DFT) calculations is used to distinguish between "enamine" and "enol" mechanisms in the Michael addition of acetone to trans-β-nitrostyrene catalyzed by Jacobsen's primary amine thiourea catalyst. In light of the recent findings that the widely used ¹⁸ O-incorporation probe for these mechanisms is flawed, the results described in this communication demonstrate an alternative probe to distinguish between these pathways. A key advantage of this probe is that quantitative mechanistic information is obtained without modifying experimental conditions. This approach is expected to find application in resolving mechanistic debates, while providing valuable information about the key transition state of organocatalyzed reactions involving the α-functionalization of carbonyls.

Ti-Catalyzed Hydroamination for the Synthesis of Amine-Containing π-Conjugated Materials

A series of conjugated enamines were prepared by Ti catalyzed anti-Markovnikov hydroamination. The synthetic route is efficient with yields of up to 94 % and the 100 % atom efficiency of the reaction means that these products are easily isolated and purified. Due to the extended conjugated system, the enamine tautomers were observed exclusively in both solid and solution phases, as determined by X-ray crystallography and NMR spectroscopy. These new conjugated molecules, with N incorporated into the backbone, show interesting photophysical properties including photo-luminescent quantum yields of up to 0.26. Notably, through the incorporation of B to give a donor-acceptor π-conjugated system, a redshift of approximately 100 nm is observed for the emission maximum along with the anticipated solvatochromic shifts.

Design and reactions of a carbon Lewis base/boron Lewis acid frustrated Lewis pair

The conjugated dienamine 4 selectively adds Piers' borane [HB(C₆F₅)₂] to give the enamine/borane system 5, which features a boratirane structure by internal enamine carbon Lewis base to boron Lewis acid interaction. Compound 5 behaves as a C/B frustrated Lewis pair and undergoes typical addition reactions to benzaldehyde, several nitriles and to sulfur dioxide.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

New Organocatalytic Asymmetric Synthesis of Highly Substituted Chiral 2-Oxospiro-[indole-3,4'- (1',4'-dihydropyridine)] Derivatives

Herein, we report our preliminary results concerning the first promising asymmetric synthesis of highly functionalized 2-oxospiro-[indole-3,4′-(1′,4′-dihydropyridine)] via the reaction of an enamine with isatylidene malononitrile derivatives in the presence of a chiral base organocatalyst. The moderate, but promising, enantioselectivity observed (30%–58% ee (enantiomeric excess)) opens the door to a new area of research for the asymmetric construction of these appealing spirooxindole skeletons, whose enantioselective syntheses are still very limited.

Synthesis of Cyclic α-Aminophosphonates through Copper Catalyzed Enamine Activation

A copper-catalyzed tandem cyclization triggered addition strategy that relies on electrophilic enamine activation has been used to synthesize various cyclic α-aminophosphonates derivatives in good to excellent yields. Both five and six membered rings can be generated under mild conditions with high regioselectivity. A mechanism based on copper catalyzed enamine activation is proposed.

Synthesis of Oxazolidinone and Tosyl Enamines by Tertiary Amine Catalysis

A procedure for the synthesis of oxazolidinone and tosyl enamines is reported. Alkynoyl oxazolidinones and tosyl imides undergo reaction to form enamines in the presence of catalytic amounts of tertiary amines. The data suggest that an amide anion is formed during the reaction, which undergoes conjugate addition to form the final product.

Diversification of function in the haloacid dehalogenase enzyme superfamily: The role of the cap domain in hydrolytic phosphoruscarbon bond cleavage

Phosphonatase functions in the 2-aminoethylphosphonate (AEP) degradation pathway of bacteria, catalyzing the hydrolysis of the C-P bond in phosphonoacetaldehyde (Pald) via formation of a bi-covalent Lys53ethylenamine/Asp12 aspartylphosphate intermediate. Because phosphonatase is a member of the haloacid dehalogenase superfamily, a family predominantly comprised of phosphatases, the question arises as to how this new catalytic activity evolved. The source of general acid-base catalysis for Schiff-base formation and aspartylphosphate hydrolysis was probed using pH-rate profile analysis of active-site mutants and X-ray crystallographic analysis of modified forms of the enzyme. The 2.9 A X-ray crystal structure of the mutant Lys53Arg complexed with Mg2+ and phosphate shows that the equilibrium between the open and the closed conformation is disrupted, favoring the open conformation. Thus, proton dissociation from the cap domain Lys53 is required for cap domain-core domain closure. The likely recipient of the Lys53 proton is a water-His56 pair that serves to relay the proton to the carbonyl oxygen of the phosphonoacetaldehyde (Pald) substrate upon addition of the Lys53. The pH-rate profile analysis of active-site mutants was carried out to test this proposal. The proximal core domain residues Cys22 and Tyr128 were ruled out, and the role of cap domain His56 was supported by the results. The X-ray crystallographic structure of wild-type phosphonatase reduced with NaBH4 in the presence of Pald was determined at 2.4A resolution to reveal N epsilon-ethyl-Lys53 juxtaposed with a sulfate ligand bound in the phosphate site. The position of the C2 of the N-ethyl group in this structure is consistent with the hypothesis that the cap domain N epsilon-ethylenamine-Lys53 functions as a general base in the hydrolysis of the aspartylphosphate bi-covalent enzyme intermediate. Because the enzyme residues proposed to play a key role in P-C bond cleavage are localized on the cap domain, this domain appears to have evolved to support the diversification of the HAD phosphatase core domain for catalysis of hydrolytic P-C bond cleavage.