Electron-rich Alkynes Research Articles

Increasing the Structural Span of Alkyne Metathesis

A new generation of alkyne metathesis catalysts, which are distinguished by high activity and an exquisite functional group tolerance, allows the scope of this transformation to be extended beyond its traditional range. They accept substrates that were previously found problematic or unreactive, such as propargyl alcohol derivatives, electron-deficient and electron-rich acetylenes of various types, and even terminal alkynes. Moreover, post-metathetic transformations other than semi-reduction increase the structural portfolio, as witnessed by the synthesis of a annulated phenol derivative via ring-closing alkyne metathesis (RCAM) followed by a transannular gold-catalyzed Conia-ene reaction. Further examples encompass a post-metathetic transannular ketone-alkyne cyclization with formation of a trisubstituted furan, a ruthenium-catalyzed redox isomerization, and a Meyer-Schuster rearrangement/oxa-Michael cascade. These reaction modes fueled model studies toward salicylate macrolides, furanocembranolides, and the cytotoxic macrolides acutiphycin and enigmazole A; moreover, they served as the key design elements of concise total syntheses of dehydrocurvularin (27) and the antibiotic agent A26771B (36).

Copper-Catalyzed Carboarylation of Alkynes via Vinyl Cations

Copper-catalyzed arylation of electron rich alkynes reveals stabilized trisubstituted vinyl cation equivalents that react with pendant arene nucleophiles to form all carbon tetrasubstituted alkenes. The new process streamlines the synthesis of important medicinally relevant molecules.

Alkyne Hydroamination and Trimerization with Titanium Bis(phenolate)pyridine Complexes: Evidence for Low-Valent Titanium Intermediates and Synthesis of an Ethylene Adduct of Titanium(II)

A class of titanium precatalysts of the type (ONO)TiX_2 (ONO = pyridine-2,6-bis(4,6-di-tert-butylphenolate); X = Bn, NMe_2) has been synthesized and crystallographically characterized. The (ONO)TiX_2 (X = Bn, NMe_2, X_2 = NPh) complexes are highly active precatalysts for the hydroamination of internal alkynes with primary arylamines and some alkylamines. A class of titanium imido/ligand adducts, (ONO)Ti(L)(NR) (L = HNMe_2, py; R = Ph, ^tBu), have also been synthesized and characterized and provide structural analogues to intermediates on the purported catalytic cycle. Furthermore, these complexes exhibit unusual redox behavior. (ONO)TiBn_2 (1) promotes the cyclotrimerization of electron-rich alkynes, likely via a catalytically active Ti^(II) species that is generated in situ from 1. Depending on reaction conditions, these Ti^(II) species are proposed to be generated through Ti benzylidene or imido intermediates. A formally Ti^(II) complex, (ONO)Ti^(II)(η^2-C_2H_4)(HNMe_2) (7), has been prepared and structurally characterized.

Generation of Rhodium(I) Carbenes from Ynamides and Their Reactions with Alkynes and Alkenes

Rh(I) carbenes were conveniently generated from readily available ynamides. These metal carbene intermediates could undergo metathesis with electron-rich or neutral alkynes to afford 2-oxopyrrolidines or be trapped by tethered alkenes to yield 3-azabicyclo[3.1.0]hexanes, a common skeleton in numerous bioactive pharmaceuticals. Although the scope of the former is limited, the latter reaction tolerates various substituted alkenes.

Gold-Catalyzed Benzannulation of Electronically Rich/Rich and Deficient/Deficient Oxoalkynes with Alkynes

Two different strategies have been developed to perform the gold-catalyzed benzannulation of both electron-rich pyridine containing oxoalkynes with electron-rich alkynes as well as electron-deficient pyridine containing oxoalkynes with electron-deficient­ alkynes. The benzannulation between electron-rich oxoalkynes with electron-rich alkynes proceeds in the presence of highly electron-deficient gold catalysts, whereas using catalytic amount of alcohol benzannulation was performed efficiently between electron-deficient oxoalkynes with electron-deficient alkynes.

Expanding the Chemical Structure Space of Opto-Electronic Molecular Materials: Unprecedented Push–Pull Chromophores by Reaction of a Donor-Substituted Tetracyanofulvene with Electron-Rich Alkynes

The reaction of a 3,5-bis(N,N-dimethylanilino)-substituted 2,4,6,6-tetracyanopentafulvene (TCPF) with mono- and bis(N,N-dimethylanilino)acetylene provides facile access to push-pull chromophores with diverse new scaffolds. The starting TCPF reacts with bis(N,N-dimethylanilino)acetylene in a formal [2+2] cycloaddition at the exocyclic double bond, followed by retroelectrocyclization, to yield an ethenylene-extended push-pull pentafulvene. The transformation with 4-ethynyl-N,N-dimethylaniline also yields a similar extended pentafulvene as well as two other products that required X-ray analysis for their structure elucidation. One features an 8,8-dicyanoheptafulvene core formed by formal [2+2] cycloaddition, followed by ring opening via fragmentation. The second is a chiral cyclobutenylated tetrahydropentalene, resulting from a cascade of formal [6+2] and [2+2] cycloadditions. All new nonplanar push-pull chromophores display amphoteric redox behavior with both strong electron-donating and -accepting potency. Notably, the N,N-dimethylanilino-substituted extended pentafulvenes show remarkably low oxidation potentials (0.27/0.28 V vs Fc/Fc(+) reference) that are lower than those for N,N-dimethylaniline itself. The push-pull-substituted extended pentafulvenes feature intense electronic absorption bands, extending over the entire visible spectral range into the near infrared, and low highest occupied molecular orbital-lowest unoccupied molecular orbital gaps. These properties, together with high thermal stability and good solubility, suggest the potential use of the new chromophores as advanced materials in molecular electronics devices.

イオン認識能を有する機能性高分子のクリック合成

The rapid growth of the click chemistry concept produced a wide variety of functional polymers. In this research field, polymeric ion sensors, which can recognize specific metal cations or anions, have mainly been synthesized by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). However, due to several disadvantages suggested for the CuAAC, the development of new types of click chemistry reactions has recently started. Among them, it was found that the high-yielding addition reactions between electron-rich alkynes and cyano-based acceptors offer promising application possibilities as polymeric chemosensors. The donor-acceptor type products feature intense charge-transfer bands in the visible region. The recognition of some specific metal cations or anions resulted in well-defined visible colorimetric changes in the optical spectra.

Polymeric ion sensors with multiple detection modes achieved by a new type of click chemistry reaction

The rapid growth of the click chemistry concept enables the production of a wide variety of functional polymers. Among the new generation of click chemistry reactions, the highly efficient addition reactions between electron-rich alkynes and cyano-based acceptors, referred to as alkyne-acceptor click chemistry, have found promising application possibilities as polymeric chemosensors. The donor-acceptor chromophores, formed by this click chemistry reaction, feature intense charge-transfer (CT) bands in the visible region, but they are hardly fluorescent. Importantly, the chromophores possess two different nitrogen atoms, namely the aniline nitrogen and cyano nitrogen. The recognition of some specific metal cations by different nitrogen atoms in the polymers led to different modes of changes in the absorption spectra. For example, the hard acid of Fe(3+) ion was recognized by the aniline nitrogen, resulting in a decrease in the CT bands. On the other hand, the soft acid of the Ag(+) ion was captured by the cyano nitrogen, leading to a bathochromic shift in the CT bands. Some specific anions, such as CN(-), F(-), and I(-) ions, were also recognized by a chemodocimetric detection mode, discoloring the original solutions. When the CT bands decreased upon the addition of analytes, the polymers were found to serve as turn-on fluorescent sensors. In this perspective, the detailed detection modes of the new polymeric chemosensors are fully described.

6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties and reactivity behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The electron paramagnetic resonance (EPR) spectra of the radical anion of a tetrakis(silylalkynyl) DCF, generated by Na metal reduction, show delocalization of both the charge and unpaired electron to the nitrogens of the cyano moieties and also, notably, to the silicon atoms of the four alkynyl moieties. By contrast, in the radical anion of the previously reported tetraphenyl DCF, coupling to the four phenyl rings is strongly attenuated. The data provide physical evidence for the different conjugation between the DCF core and the substituents in both systems. We also report the preparation of new fulvene-based push-pull chromophores via formal [2 + 2] cycloaddition-retroelectrocyclization reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated DCFs show opposite regioselectivity of the cycloaddition, which can be explained by the differences in electronic communication between substituents and the DCF core as revealed in the EPR spectra of the radical anions.

Cycloaddition reactions of polyenic donor–π-acceptor systems with an electron-rich alkyne: access to new chromophores with second-order optical nonlinearities

The formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between 4-ethynyl-N,N-dimethylaniline and polyenic Donor-π-Acceptor (D-π-A) systems takes place to yield compounds bearing two donors and one acceptor. Structural, linear and second-order nonlinear optical (NLO) properties of the new molecules reveal the stronger polarization of these systems when compared to analogous merocyanines lacking the dimethylaminophenyl (DMA) ring.

Multi-coloration of polyurethane derivatives through click postfunctionalization, electrochemical oxidation, and Ag+ ion complexation

A colorless polyurethane derivative bearing electron-rich alkynes substituted by the dialkylaniline donors at both sides was prepared by polyaddition between a diol monomer and tolylene-2,4-diisocyanate. Click chemistry-type postfunctionalization of this polyurethane was accomplished by a [2 + 2] cycloaddition followed by retro-cyclization with the electron-accepting tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), yielding new orange or green colored polyurethane derivatives with the intramolecular charge-transfer (CT) band centered at 476 and 698 nm in CH2Cl2, respectively. These polymers were comprehensively characterized by GPC, 1H NMR and IR spectroscopies, and elemental analysis. Despite its sufficient thermal stability, the precursor polyurethane was readily oxidized in air under mild heating conditions. Inspired by this result, the electrochemical oxidation of the precursor polyurethane was performed. Application of potentials higher than the first oxidation potential of 0.25 V (vs. Ag/AgCl) reasonably produced the blue colored aromatic amine-based cation radical. Although the cyclic voltammogram of the polyurethane films was irreversible, the blue color was almost persistent under an inert atmosphere. The colors of the postfunctionalized polyurethanes were further changed by Ag+ ion complexation with cyano groups, which modulated the strengths of the intramolecular donor–acceptor interactions. Thus, the solution color of the TCNE-adducted polyurethane changed to magenta and that of the TCNQ-adducted polyurethane to a yellowish-green upon the Ag+ ion addition. The plots in the 1976L*a*b* color space revealed the production processes of each color.

N,N′-Dicyanoquinone Diimide-Derived Donor–Acceptor Chromophores: Conformational Analysis and Optoelectronic Properties

A formal [2 + 2] cycloaddition-cycloreversion (CA-CR) between N,N'-dicyanoquinone diimides (DCNQIs) and electron-rich alkynes was explored, providing a new class of π-conjugated donor-acceptor chromophores. These DCNQI adducts exist in the solid state as single diastereoisomers, but as two interconverting diastereoisomers in solution. Solid- and solution-state evidence for intramolecular charge transfer (CT) was obtained; additionally, the DCNQI adducts exhibit positive solvatochromism and significant solution-state third-order polarizabilities.

ChemInform Abstract: Homo‐ and Heterocoupling of Terminal Alkynes Using Catalytic CuCl2 and DBU.

Homocoupling of terminal alkynes has been efficiently achieved using catalytic amounts of CuCl2 and DBU. This methodology could be extended to couple two different terminal alkynes together by taking one of the alkyne partners, preferably the electron rich alkyne, in five fold excess than the other.

Origin of Second-Order Nonlinear Optical Effects of Nonpoled Donor–Acceptor Chromophores on Surface

Thin films of alkyl conjugated donor–acceptor (D–A) molecules, prepared by a high-yielding addition reaction between electron-rich alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), were investigated by ultraviolet–visible–near infrared (UV–vis–NIR) spectroscopy, in-plane and out-of-plane X-ray diffraction (XRD), and second-harmonic generation (SHG) measurements. The variable self-assembling ability of D–A chromophores produced different molecular orientations in the thin films. The TCNQ-adducted D–A molecule formed a centrosymmetric bilayer structure based on the strong interactions between the chromophores, while the film of the TCNE-adducted counter molecule was composed of interdigitated alkyl layers and randomly oriented chromophores due to their weak interactions. The interfacial interactions between a substrate and a TCNE-adducted molecular layer were expected to induce a noncentrosymmetric chromophore orientation in the thin film, which produced an effective SHG coefficient (deff) of ca. 21 pm V-1 without electric poling.

ChemInform Abstract: Adapting Semiconducting Polymer Doping Techniques to Create New Types of Click Postfunctionalization

AbstractReview: 77 refs.

A convenient synthesis of new chromophoric tetracyanobutadiene-scaffolded peptides via a dipolar [2+2] cycloaddition–cycloreversion reaction

Herein, we report a novel approach for the synthesis of π-conjugated peptide-based donor–acceptor (D-π-A) chromophores, by reacting electron-rich alkynes with tetracyanoethylene. The desired tetracyanobutadiene-scaffolded peptides were obtained in good yields with various optical properties, λ max: 321–492 nm, ε: 21,000–65,000 mol −1 dm 3 cm −1 depending on the substitution pattern of the cyanobutadiene scaffold.

Visualizing the Product of a Formal Cycloaddition of 7,7,8,8‐Tetracyano‐p‐quinodimethane (TCNQ) to an Acetylene‐Appended Porphyrin by Scanning Tunneling Microscopy on Au(111)

Click it: The applicability of a formal [2+2] cycloaddition between electron-rich alkynes and electron-deficient TCNQ on an atomically clean Au(111) surface was demonstrated by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). At low coverage, monomeric and self-assembled dimeric species of the initial compounds as well as of the reaction product, a TCNQ-conjugated porphyrin, could be visualized.

Palladium-Catalyzed C−CN Activation for Intramolecular Cyanoesterification of Alkynes

Conditions for the C-CN activation and intramolecular cyanoesterification of alkynes to provide butenolides in good to excellent yields are presented. Pd catalysts, high temperatures/short reaction times (microwave irradiation), and Lewis basic solvents minimized competitive decarbonylation. Less sterically encumbered, electron-rich alkynes underwent cyanoesterification with greater ease compared to sterically encumbered, electron-deficient alkynes. The results led to the hypothesis that migratory insertion of the alkyne, rather than C-CN activation, might be the product-determining step.

Application of Alkyne‐TCNQ Addition Reaction to Polymerization

The polymerization using a high-yielding addition reaction between electron-rich alkynes and 7,7,8,8-tetracyanoquinodimethane (TCNQ) derivatives is described. The bifunctional monomer containing two TCNQ moieties and the counter comonomer bearing two dialkylaniline (DAA)-substituted alkynes are reacted in 1,2-dichloroethane under mild heating conditions. At the high monomer concentrations, high molecular weight linear polymers are obtained, while the reaction at the low monomer concentrations produces a significant amount of the cyclic compounds. A clear relationship between the monomer concentration and the cyclic compound amount is demonstrated. The obtained polymers feature a sufficient thermal stability with the decomposition temperature exceeding 300 °C as well as strong charge-transfer (CT) bands and redox activities ascribed to the produced donor-acceptor moieties. These features are also used to optimize the polymerization conditions and to estimate the chemical structures.

Creation of persistent charge-transfer interactions in TCNQ polyester

A 7,7,8,8-tetracyanoquinodimethane polyester was converted into a strongly colored polyester with low-energy charge-transfer (CT) bands by treatment with electron-rich alkyne molecules under mild conditions. The post-functionalization was monitored by its ultraviolet-visible-near infrared (IR) spectra, which suggested a quantitative addition reaction without any side reactions. This result was further supported by 1H and 13C nuclear magnetic resonance, IR and elemental analyses. The functionalized polymer retained a high thermal stability with a decomposition onset temperature of ∼200 °C and ∼63% residual soot at 500 °C. The CT chromophores in the polymer showed a clear positive solvatochromism and redox behaviors in both anodic and cathodic directions. The HOMO and LUMO levels estimated from the first oxidation potential (Eox,1) and the first reduction potential (Ered,1), respectively, were markedly elevated by this post-functionalization. The extent of HOMO elevation (1.45 eV) was more significant than that of LUMO elevation (0.44 eV). A clean post-functionalization of 7,7,8,8-tetracyanoquinodimethane polyester by a high-yielding addition reaction with electron-rich alkyne molecules provided highly colored polymer products with very stable charge-transfer bands in the visible to near-infrared region. Electrochemistry revealed elevated HOMO and LUMO levels after the post-functionalization.