Electron-rich Alkynes Research Articles

Intermolecular 1,2-Aminoboration of Alkynes and the Critical Role of Electron-Rich Alkynes.

The intramolecular 1,2-aminoboration of alkynes by aminoboranes is rare and invariably requires a catalyst to proceed, while the intermolecular aminoboration of alkynes is yet entirely unknown. Through an exploration of the significance of electronics in alkynes for activating the B-N σ-bond of aminoboranes, we demonstrate in this work the first intermolecular 1,2-aminoboration of alkynes. These reactions employ a series of (amino)dihaloboranes and aminoboronic esters, mild reaction conditions, and no catalysts, yielding syn-addition alkene products with the incorporation of two crucial functionalities: amino and boryl. While highly electron-rich examples can afford the aminoborated products (Z)-2-borylethenamines, other alkynes, including unactivated and less electron-rich examples, do not lead to the corresponding aminoborated products due to the fundamental impediment that the reactions are significantly endergonic.

Design and synthesis of heterocycle-based push-pull NLOphores: A comprehensive study of their linear and non-linear optical properties

Push-pull chromophores, with strong intramolecular charge transfer (ICT), exhibit high ground-state polarization, driving strong NLO responses. The donor and acceptor abilities of the groups in the investigated compounds influence ICT, so the nonlinear response of these compounds was analyzed in relation to their strength. Two different families of NLOphores were synthesized through [2 + 2] cycloaddition-retroelectrocyclizations of heterocycle-based electron-rich alkynes with tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ). The λmax values of push-pull chromophores, particularly for charge transfer bands, fall within the range of 424–758 nm. The linear and nonlinear optical properties of these NLOphores were subsequently examined through a combination of experimental and theoretical methods. NLO measurements were conducted using a validated custom-made Z-scan device. The nonlinear absorption coefficients range from −1.44 × 10−4 cm.W−1 to −9.90 × 10−4 cm.W−1, while nonlinear refractive indices range from −1.59 × 10−7 cm2. W−1 to −2.26 × 10−6 cm2. W−1. As expected from their molecular structures, the TCNQ products displayed stronger nonlinear responses than the TCNE products.

Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly.

Pseudo-octahedral CrIIIN6 chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic CrIII building block capable of binding rare earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular assemblies, in which three peripherical CrIII sensitizers coordinated through a molecular wire to a central ErIII or YIII, have been prepared. Upon excitation of the CrIII spin-flip states, the downshifted Er(4I13/2 → 4I15/2) emission at 1550 nm was induced through intramolecular energy transfer. Time-resolved experiments at room temperature reveal a CrIII → ErIII energy transfer of 62-73% efficiencies with rate constants of about 8.5 × 105 s-1 despite the long donor-acceptor distance (circa 14 Å). This efficient directional intermetallic energy transfer can be rationalized using the Dexter formalism, which is promoted by a rigid linear electron-rich alkyne bridge that acts as a molecular wire connecting the CrIII and ErIII ions.

4+1] Heteroannulation to Form 1-Pyrrolines through Alder-Ene Reaction of in situ Generated Ynimines: Development and Application in Total Synthesis of Borrecapine.

We report in this paper a novel Cu-catalyzed synthesis of polysubstituted 1-pyrrolines. The reaction of β,γ-unsaturated oxime esters 4 with terminal alkynes 5 in the presence of a catalytic amount of Cu(OAc)2 and 2,2'-biquinoline affords the corresponding 1,6-enynimines, which undergo a highly stereoselective Alder-ene reaction to afford 1-pyrrolines with concomitant generation of a quaternary carbon and a 2-azadiene motif. It represents an unusual [4+1] heteroannulation reaction wherein terminal alkynes act as a one carbon donor and are 1,1-difunctionalized. Mechanistic studies suggest that the allylic hydrogen trans to the oxime ester main chain is selectively transferred to the electron-rich alkyne during the pericyclic reaction. The resulting 1-pyrrolines undergo facile acid-catalyzed regioselective Wagner-Meerwein rearrangement to provide 2H-pyrroles in excellent yields. The 2H-pyrroles are also accessible from 4 and 5 in a one-pot manner without isolation of the 1-pyrrolines. Leveraging this heteroannulation reaction as a key step, the first total synthesis of borrecapine, a 2,3,3,5-tetrasubstituted pyrrolidine, is accomplished.

Cycloaddition of Phenyltriazolinedione with Carbazole-Alkynes and Yne-Carbamates to Access Diazacyclobutenes.

Previously, we described the synthesis of stable, bicyclic examples of the rather rare diazacyclobutene (DCB) motif by means of a cycloaddition between triazolinediones and electron-rich thiolated alkynes. Here, we report the investigation of the cycloaddition of triazolinediones with related electron-rich yne-carbamates and carbazole-alkynes. Bicyclic DCBs arising from yne-carbamates were isolated in 8-65% yield, while those arising from carbazole-alkynes were isolated in 28-59% yield. Mechanistic studies and characterization of isolable byproducts shed light on the underlying issues leading to poor to moderate yields.

Perspectives on push-pull chromophores derived from click-type [2 + 2] cycloaddition-retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes.

Various push-pull chromophores can be synthesized in a single and atom-economical step through [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions involving diverse electron-rich alkynes and electron-deficient alkenes. In this review, a comprehensive investigation of the recent and noteworthy advancements in the research on push-pull chromophores prepared via the [2 + 2] CA-RE reaction is conducted. In particular, an overview of the physicochemical properties of the family of these compounds that have been investigated is provided to clarify their potential for future applications.

Heterodifunctionalization of Electron-Rich Alkynes Catalyzed by in Situ Generated Silylium Ions.

Silylium ions are versatile Lewis acids in organic synthesis. While they have been well-known for the activation of σ donors, catalysis initiated by the activation of π donors remains underdeveloped, particularly for alkynes. Herein, we demonstrate an example of silylium-catalyzed alkyne heterodifunctionalization. The silylium ion generated in situ from HNTf2 and the silyl reagent serve as superior catalysts in the efficient silylphosphination and silylcyanation of electron-rich alkynes with excellent regio- and stereoselectivity. The compatibility of this protocol with strongly coordinating ligands (Ph2P and CN) not only complements the metal-catalyzed systems but also expands the scope of silylium-catalyzed reactions.

Weaving electron-rich alkynes: a durable in situ skin for stabilizing zinc anodes

An electron-rich alkyne of thiophdiyne (e-TDYP) is engineered in situ onto a zinc anode to regulate the EDL properties. The synergy between the conjugated thiophene and alkyne groups of e-TDYP facilitates the desolvation and ion-capture.

Clicking together Alkynes and Tetracyanoethylene.

The [2+2] cycloaddition - retro-electrocyclization (CA-RE) reaction allows ready synthesis of redox-active donor-acceptor chromophores from an electron-rich alkyne and electron-poor olefins like tetracyanoethylene (TCNE). The detailed mechanism of the reaction has been subject of both computational and experimental studies. While several studies point towards a stepwise mechanism via a zwitterionic intermediate for the first step, the cycloaddition, the reaction follows neither simple second-order nor first-order kinetics. Recent studies have shown that the kinetics can be understood if an autocatalytic step is introduced in the mechanism, in which complex formation with the donor-substituted tetracyanobutadiene (TCBD) product possibly facilitates nucleophilic attack of the alkyne onto TCNE, generating the zwitterionic intermediate of the CA step. This Concept highlights the convenient use of the "click-like" CA-RE reaction to obtain elaborate donor-acceptor chromophores and the recent mechanistic results.

Direct Access to α,β-Alkynylamides via Pd-Catalyzed Carbonylation of Terminal Alkynes with Amines Using Chloroform as the CO Surrogate.

We describe the first-time use of a palladium-di(1-adamantyl)-n-butylphosphine catalytic system for the synthesis of alk-2-ynamides from terminal alkynes via aminocarbonylation reaction. This method avoids the direct use of toxic CO gas by utilizing chloroform as an in situ CO source. The aminocarbonylation of both aromatic and aliphatic electron-rich alkyne nucleophiles is reported for the first time. The mild condition provides alk-2-ynamides within 50 min under microwave irradiation at 80 °C.

Further Exploration of Aza-Cobalt-Cyclobutenes on CoIII(TIM) Complexes: Reactivity and Spectroelectrochemistry

The reaction between [CoIII(TIM)Cl2]PF6 (TIM is 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) and electron-rich terminal alkynes (HC2Y, Y = 4-N,N-bis(4-methoxyphenyl)aniline (TPA) or Fc) yielded a pair of constitutional isomers: 1-aza-2-cobalt-cyclobutene-containing trans-[Co(TIM′)((HC═C)Y)Cl]+ (1a (TPA) and 2a (Fc); TIM′ is the resultant derivative of TIM) and σ-coordinated trans-[Co(TIM)(C2Y)Cl]+ (1b (TPA) and 2b (Fc)). Additionally, the reactivity of the 1-aza-2-cobalt-cyclobutene species was explored based on trans-[Co(TIM′)((HC═C)Fc)Cl]PF6 (2a) because of its good synthetic yield. The reaction of 2a with AgOTf in MeCN resulted in trans-[Co(TIM′)((HC═C)Fc)(NCMe)](PF6)(OTf) (3), and the reaction with KCN in MeOH yielded trans-[Co(TIM′)((HC═C)Fc)CN]PF6 (4). Similarly, trans-[Co(TIM)(C2Ph)(CN)]PF6 (5) was prepared from the reaction between KCN and trans-[Co(TIM)(C2Ph)Cl]PF6. Molecular structures of 1b and 3–5 were established through single crystal X-ray diffraction. Electrochemical studies of 2–4 revealed that these complexes display an irreversible CoIII reduction as well as a reversible one electron ferrocenyl oxidation between 0.11 and 0.18 V (vs Fc+1/0). UV–vis spectroelectrochemical studies revealed a CoIII to Fc+ charge-transfer upon the one electron oxidation of 2a and 2b.

Switching Over of the Chemoselectivity: I2-DMSO-Enabled α,α-Dichlorination of Functionalized Methyl Ketones.

Precise control of the chemoselectivity of the halogenation of a substrate equipped with multiple nucleophilic sites is highly demanding and challenging. Most reported chlorinations of methyl ketones show poor compatibility or even exclusive selectivity toward electron-rich arene, olefin, and alkyne residues. This is attributed to the direct or in situ employment of electrophilic Cl2/Cl+ species. Here, we reported that, even bearing those competitive residues, methyl ketones can still undergo dichlorination to afford α,α-dichloroketones in a chemo-specific manner. Enabled by the I2-dimethyl sulfoxide catalytic system, in which hydrochloric acid only acts as a nucleophilic Cl- donor, this straightforward dichlorination reaction is safe and operator-friendly and has high atomic economy, giving access to structurally diverse α,α-dichloroketones in good yields and with good functional-group tolerance.

Elucidation of the molecular mechanisms of 1,2,3,5- and 1,2,4,5-tetrazines with strained and electron-rich alkynes

The mechanisms of the domino reactions between 4,6-diphenyl-1,2,3,5-tetrazine (TTZ 1) and 3,6-diphenyl-1,2,4,5-tetrazine (TTZ 2) with ynamine 3 and cyclooctyne 4, have been studied by DFT method. The domino reactions of TTZs with alkynes comprise two consecutive reactions, an aza Diels-Alder (ADA), and a retro Diels-Alder (rDA) reaction which leads to the resulting aromatic rings through loss of N2. Accordingly, the ADA step of the reaction of TTZ 1 with ynamine 3 proceeds through a stepwise mechanism, when the formation of the C4–C6 bond leads to zwitterionic intermediate IN1a in a regioselective manner. The second step consists of cyclization and spontaneously N2 loss. A stepwise ADA reaction between TTZ 2 and ynamine 3 results in zwitterionic intermediate IN1e and bicyclic intermediate IN2e. Subsequently, N2 removes with no significant energy barrier. The electron localization function (ELF) analysis shows that the ADA reactions of cyclooctyne 4 with TTZs 1 and 2 occur through a two-stage one-step mechanism, and a symmetric synchronous one-step mechanism, respectively. The global electron density transfer (GEDT) calculations indicated that the ADA reactions of TTZs with ynamine 3 and cyclooctyne 4 have strong and moderate polar natures, respectively.

Ultrafast synthesis of dialkyne-functionalized polythioether and post-polymerization modification via click chemistry

Herein, we prepare a polythioether with double “clickable” alkyne groups in each repeating unit via an extremely rapid polymerization method and combine it with Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” reaction to construct new topological polymers. A monomer having both electron-deficient and electron-rich dual alkyne functionalities was synthesized and subsequently reacted with 1,6-hexanedithiol (HDT) using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as a catalyst, and the reaction proceeded at room temperature for 1 min to give polythioether P1. The obtained polymer was then used as a polymer platform and reacted with a variety of azide compounds to modify the pendant alkynes via CuAAC reaction. The post-polymerization modification (PPM) studies have also been shown to be rapid as the reactions reached completion in 1 h. All synthesized polymers were analyzed using various spectroscopic methods and the results confirmed that quantitative “click” reaction efficiency was achieved in each reaction. It is believed the presented strategy not only provides a time-saving approach for both polymer synthesis and PPM, but also brings a new insight to impart the desired functionalities to a polymer chain with CuAAC “click” chemistry. Thus, a new type of polymer structure that can be prepared rapidly and undergoes fast PPM has been introduced to polymer chemistry with this study. • A Polythioether with clickable pendant units has been synthesized extremely rapidly in 1 min. • Post-polymerization modification (PPM) via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” reaction has been successfully carried out. • Quantitative “click” reaction efficiency has been achieved in all cases. • Time- and energy-saving polymerization and PPM method have been developed.

Rh(I)-Catalyzed Denitrogenative Transformations of 1,2,3-Thiadiazoles: Ligand-Controlled Product Selectivity and the Structure of the Key Organorhodium Intermediate Revealed

Ligand-controlled rhodium(I)-catalyzed denitrogenative transformations of a range of 4-vinyl-1,2,3-thiadiazoles possessing electron-donating substituents at the C5-position of the heterocycle have been demonstrated. With [Rh(COD)2]BF4, vinylic 1,2,3-thiadiazoles undergo an intramolecular transannulation reaction to afford substituted furans. In contrast, the [Rh(COD)DPPF]BF4 catalytic system inhibits the intramolecular reaction but promotes intermolecular transannulation with both electron-deficient and electron-rich terminal alkynes, providing access to densely functionalized thiophenes with unexpected regioselectivity. Experimental and computational mechanistic studies were performed to gain insights into the Rh(I)-catalyzed intramolecular transannulation of vinylic 1,2,3-thiadiazoles, with a focus on understanding the influence of the C5-substituent on reactivity and the role of the DPPF ligand. Importantly, our crystallographic data uncovered that the true structure of the organorhodium intermediate involved in Rh(I)-catalyzed denitrogenative reactions of 1,2,3-thiadiazoles is likely to be a four-membered cyclometalated Rh(III) complex.

Rhodium(III)-Catalyzed Atroposelective Synthesis of C–N Axially Chiral Naphthylamines and Variants via C–H Activation

Reported herein is the efficient and atroposelective construction of two categories of C-N atropisomers via rhodium(III)-catalyzed C-H activation of sulfoxonium ylides en route to [4+2] annulation with sterically hindered, electron-rich alkynes. This reaction proceeds with high regio- and enantioselectivity under redox-neutral conditions via a double-substrate activation strategy, providing a novel entry to C-N axially chiral 4-functionalized 1-naphthols.

Scalable Rhodaelectro-Catalyzed Expedient Access to Seven-Membered Azepino[3,2,1- hi ]indoles via [5 + 2] C–H/N–H Annulation

Scalable Rhodaelectro-Catalyzed Expedient Access to Seven-Membered Azepino[3,2,1- <i>hi</i> ]indoles via [5 + 2] C–H/N–H Annulation

The Varied Frustrated Lewis Pair Reactivity of the Germylene Phosphaketene (CH{(CMe)(2,6-i Pr2 C6 H3 N)}2 )GePCO.

The germylene species (CH{(CMe)(2,6‐iPr2C6H3N)}2)GePCO 1 is shown to react with the Lewis acids (E(C6F5)3 E=B, Al). Nonetheless, 1 participates in FLP chemistry with electron deficient alkynes or olefins, acting as an intramolecular FLP. In contrast, in the presence of B(C6F5)3 and an electron rich alkyne, 1 behaves as Ge‐based nucleophile to effect intermolecular FLP addition to the alkyne. This reactivity demonstrates that the reaction pathway is controlled by the nature of the electrophile and nucleophile generated in solution, as revealed by extensive DFT calculations.

Steric- and Electronic-Controlled Intramolecular [2 + 2]-Cycloaddition of Cyclohexadienone-Containing 1,7-Enynes.

Herein we have developed the silver-catalyzed electronic- and steric-controlled intramolecular formal [2 + 2]-cycloaddition of alkyne-tethered cyclohexadienones. Substrates with electron-rich alkynes and a less hindered quaternary carbon center afford tricyclic fused cyclobutenes through 1,7-enyne cyclization. In contrast, the formation of dihydrofurans was observed from electron-deficient alkynes via proton abstraction/C-O bond cleavage. The synthetic potential of this method was also broadened with a gram-scale reaction and various transformations on cyclobutene.

Deuterated N-difluoromethylthiophthalimide: A stable, scalable reagent for radical and electrophilic deuteriodifluoromethylthiolations

A new, stable and scalable reagent for deuteriodifluoromethylthiolation (deuterated N-difluoromethylthiophthalimide, PhthSCF2D) has been developed. This reagent can be applied for the photocatalytic radical deuteriodifluoromethylthiolation of various olefins and aldehydes (30 examples). Meanwhile, it can achieve the electrophilic deuteriodifluoromethylthiolation of a series of electrophilic substrates including electron-rich arenes, aryl/vinylboronicacids, alkynes, amines, thiols and β-ketoesters (22 examples). Some complex molecules can also be applied in both radical and electrophilic deuteriodifluoromethylthiolation using PhthSCF2D as the reagent.