Pd-catalyzed Sonogashira Cross-coupling Research Articles

NIR-absorbing TCBD and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD bridged derivatives of 1,1′-bis(N,N-dimethylaniline)-substituted ferrocene

Symmetrical and unsymmetrical donor-acceptor (D–A) based chromophores were designed and synthesized by the Pd–catalyzed Sonogashira cross–coupling reaction of 1,1′-diiodoferrocene with (4-N,N-dimethylaminophenyl)ethyne followed by [2+2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ). Ferrocene was used as a central donor unit and 4-(N,N-dimethylamino)phenyl group as the end-capping donor. Herein, we report the cycloaddition-retrocyclization reaction of TCNE/TCNQ groups on the disubstituted ferrocene containing more than one triple bond to get disubstituted tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD products. The effect of the conjugation length and different π–acceptor linkers were investigated by photophysical, redox and computational studies. The redox study reveals multiple oxidation waves corresponding to donor moieties (ferrocene and 4-(N,N-dimethylamino)phenyl group) and reduction waves corresponding to acceptor units (TCBD / cyclohexa-2,5- diene-1,4-diylidene-expanded TCBD). The thermogravimetric analysis data shows that the compounds exhibit high thermal stability. The computational studies reveal HOMO-LUMO gaps consistent with the experimentally obtained data.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization.

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor–Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO2) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

A Cascade Sonogashira Cross-Coupling-Substitution-Elimination Reaction for the Synthesis of Linear Conjugated Dienynes.

The engagement in tandem of well-known organic reactions such as the Pd-catalyzed Sonogashira cross-coupling reaction, nucleophilic substitution and elimination reactions, enables the synthesis of otherwise difficult to obtain linear dienynes, in moderate to high yields. This retrosynthetic approach opens new ways to prepare highly conjugated alkenes and alkynes. Furthermore, ionic liquids are suitable solvents to perform the cascade reaction and recycle the metal catalysts.

2,1,3-Benzothiadiazole-based bis-silylated compounds: Synthesis and use in the preparation of highly fluorescent low-contend organic–inorganic hybrid materials

Herein, we report the synthesis of bis-silylated 2,1,3-benzothiadiazole derivatives and their use in the preparation of photoactive organic–inorganic hybrid materials with extremely low fluorophore/tetraethylorthosilicate molar ratios. The organic compounds were synthesized in good to excellent yields (55 %-94 %) using Pd-catalyzed Sonogashira cross-coupling as the key step. The bis-silylated benzothiadiazoles exhibited absorption in the UV-A region (∼376 nm), attributed to fully spin- and symmetry-allowed π-π* electronic transitions. An emission located in the blue-cyan region with a high quantum yield and large Stokes shift was observed, most likely due to a charge-transfer mechanism in the excited state. BTD-containing hybrid materials were obtained through a sol–gel process, for which the dye loading and aging time were studied. Optimal reproducibility of the photophysical results was achieved when the sol–gel reaction was conducted for 10 d at 60 °C. In the solid state, the obtained hybrid materials exhibited fluorescence emission in the blue-cyan-green regions, with relatively high quantum yields (∼ 30 %). The protocol allows for the preparation of analogous photoactive hybrid materials containing very low amounts of BTD, behaving as fluorescent fumed silica.

Phenothiazine and phenothiazine-5,5-dioxide-based push–pull derivatives: synthesis, photophysical, electrochemical and computational studies

Phenothiazine and phenothiazine-5,5-dioxide-based push–pull chromophores PTZ 1–6 were synthesized via Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclic ring-opening reactions in good yields.

High-performance, low-voltage organic field-effect transistors using thieno[3,2-b]thiophene and benzothiadiazole co-polymers

A series of new linear conjugated polymers incorporating different substituted thieno[3,2-b]thiophenes as donor, acetylene as π-bridge and benzothiadiazole as acceptor units were synthesizedviaPd-catalyzed Sonogashira cross-coupling polymerization.

Triptycene-based porous photoluminescent polymers with dual role: efficient capture of carbon dioxide and sensitive detection of picric acid

Design and synthesis of unique photoluminescent triptycene-based porous polymers (TBP-OH and TBP-NH2) bearing active functional groups is described herein. Pd catalyzed Sonogashira cross-coupling reaction was utilized to obtain these polymeric networks that are nanoporous and strongly fluorescent in THF. In solid state, these polymers demonstrated CO2 uptake up to 92 mg g−1 at 273 K/1bar and H2 up to 16 mg g−1 at 77 K/1bar which may be attributed to the presence of 3D robust triptycene and CO2-philic groups –OH and –NH2 in their polymeric framework. TBP-OH and TBP-NH2 also selectively capture CO2 over nitrogen and methane. CO2 capture by TBP-OH and TBP-NH2 is a physisorption process and hence reversible in nature. Suspensions of TBP-OH and TBP-NH2 in THF are strongly fluorescent and are also capable of detecting picric acid (an environmental pollutant and explosive) in trace amounts. The Stern–Volmer quenching constants (Ksv) for detection of picric acid (PA) are in the order of 105 M−1.

Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization.

A series of new naphthalimide and phenothiazine-based push–pull systems (NPI-PTZ1–5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics.

Design and synthesis of 1,8-naphthalimide functionalized benzothiadiazoles

Push-pull derivatives BTD2–BTD5 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling reaction followed by [2+2] cycloaddition–electrocyclic ring-opening reaction.

Theoretical investigation into the mechanism of copper-catalyzed Sonogashira coupling using trans-1,2-diamino cyclohexane ligand

The mechanism of copper-catalyzed Sonogashira coupling reaction employing trans-1,2-diamino cyclohexane ligand have been investigated with Density Functional Theory (DFT) method augmented with Conductor-like Polarizable Continuum Model (CPCM) solvation model. The cross-coupling reactions could be accelerated by employing chelating diamine ligands. Thus, we considered trans-1,2-diamino cyclohexane as the ligand for our study. These coupling reactions find its applicability in the synthesis of aryl acetylenes, the precursors for the various benzofuran derivatives which are present in many biologically important compounds. Considering various reaction pathways possible, it was found that diamine ligated copper (I) acetylide was the active state of the catalyst, which on further reaction with aryl halide undergoes a concerted oxidative addition – reductive elimination process giving the cross-coupled product aryl acetylene while regenerating the active catalytic species. Unlike the Pd-catalyzed Sonogashira cross-coupling, there occurs a concerted mechanism owing to the ease of bond formation between Csp2-Csp carbon atoms and instability of a Cu (III) metal center. This shows the mechanism of copper-catalyzed cross-couplings are quite different from that of Pd catalyzed reactions. The latter usually involves individual process involving oxidative addition and reductive elimination. The presences of various functional groups on the substrate molecules have a crucial role in determining the feasibility of the reaction. Henceforth, we have investigated the electronic effects of various functional groups in the substrate molecule on the activation barrier of the cross-coupling reaction.

Formation of Highly Efficient, Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor-Acceptor-Donor Conjugates.

The significance of multiple number of donor-acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor-acceptor-donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), (4-6) by the Pd-catalyzed Sonogashira cross-coupling reactions have been newly synthesized and characterized. Donor-acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, (1-3) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4-6 was found to be quantitatively quenched while for 1-3, this property varied between 73-65 % suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives (4-6) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (ΔGCS ) in these donor-acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc+ -DPP.- -TPA charge separated states (CSS) in the case of 4-6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4-6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc+ -DPP.- -TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4-6 as compared to Fc-DPP-control and simple DPP derived donor-acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in ΔGCS and ΔGCR and the associated electron coupling between different entities.

Excited-State Electron Transfer in 1,1,4,4-Tetracyanobuta-1,3-diene (TCBD)- and Cyclohexa-2,5-diene-1,4-diylidene-Expanded TCBD-Substituted BODIPY-Phenothiazine Donor-Acceptor Conjugates.

A new set of donor-acceptor (D-A) conjugates capable of undergoing ultrafast electron transfer were synthesized using 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-substituted phenothiazine, SM1-SM3, by a Pd-catalyzed Sonogashira cross-coupling reaction and a [2+2] cycloaddition-electrocyclic ring-opening reaction. The incorporation of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (abbreviated as DCNQ=dicyanodiquinodimethane) in BODIPY-substituted phenothiazine resulted in significant perturbation of the optical and electronic properties. The absorption spectrum of both SM2 and SM3 showed red shifted absorption as compared to SM1. Additionally, both SM2 and SM3 exhibited a distinct intramolecular charge-transfer (ICT) transition in the near-IR region more so for SM3. The electrochemical study revealed multi-redox processes due to the presence of redox-active phenothiazine, BODIPY, TCBD or DCNQ entities. Using data from spectral, electrochemical and computational studies, an energy-level diagram was established to witness excited-state electron-transfer events. Finally, evidence of electron transfer and their kinetic information was secured from studies involving a femtosecond transient absorption technique. The time constants for excited-state electron-transfer events in the case of SM2 and SM3 were less than 5 ps revealing ultrafast processes.

Solvent-Dependent Nanostructures Based on Active π-Aggregation Induced Emission Enhancement of New Carbazole Derivatives of Triphenylacrylonitrile.

In the present study, the carbazole and 2,3,3-triphenylacrylonitrile (TPAN) nanostructures (2-CTPAN and 2,2'-CTPAN) have been designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. CTPAN exhibit aggregation-induced emission enhancement (AIEE) behavior in water with high fluorescence quantum yield. Both the compounds show tunable self-assembly in water as well as in N,N-dimethylformamide (DMF) by extended π-π stacking interactions. CTPAN can be self-assembled into spherical particles in water and the structures of these self-assemblies have been investigated using X-ray diffraction. Interestingly, 2-CTPAN and 2,2'-CTPAN form organogels with a critical gelation concentration (CGC) of 11 and 15 mg mL-1 , respectively, in DMF and exhibit acicular and rod shaped morphology, respectively. The single-crystal structure of 2-CTPAN shows that the intermolecular C-H⋅⋅⋅π interactions lock the molecular conformation into a staircase-shaped supramolecular assembly. These AIEE active compounds reveal high water dispersibility, strong yellow fluorescence with high quantum yield, promising photostability and excellent biocompatibility, which make them potential bioimaging agents.

Synthesis of C4-alkynylisoxazoles via a Pd-catalyzed Sonogashira cross-coupling reaction.

A Pd-catalyzed Sonogashira cross-coupling reaction for the synthesis of C4-alkynylisoxazoles from 3,5-disubsitituted-4-iodoisoxazoles and terminal alkynes was described, which could afford the corresponding products with high yield (up to 98%). The results indicated that the steric effect from the group at the C3 position of the isoxazole had greater influence on the cross-coupling reaction than that from the group at the C5 position. In addition, the group at the C3 position of the isoxazole showed negligible electronic effects on the cross-coupling reaction. Furthermore, a gram-scale reaction of the Sonogashira coupling reaction was also investigated. Finally, a plausible mechanism for the Sonogashira coupling reaction was proposed.

Synthesis and photophysical properties of azuleno[1′,2′:4,5]pyrrolo[2,1-b]quinazoline-6,14-diones: Azulene analogs of tryptanthrin

Azulene analog of tryptanthrin, azuleno[1′,2′:4,5]pyrrolo[2,1-b]quinazoline-6,14-dione, was successfully prepared by the condensation reaction of azuleno[2,1-b]pyrrole-2,3-dione with isatoic anhydride in the presence of sodium hydride or diisopropylethylamine (DIPEA). Its 2-halo derivatives were also obtained in high yields by the condensation reaction with 5-haloisatoic anhydrides in the presence of DIPEA. Reactivity toward electrophilic reagents was revealed by halogenation with N-bromosuccinimide (NBS) or N-iodosuccinimide (NIS) to afford 12-halo derivatives in high yields. Among the halo derivatives, 2-iodo and 12-iodo derivatives were reactive enough to afford phenylethynyl derivatives under Pd-catalyzed Sonogashira cross-coupling conditions. Within the phenylethynyl derivatives, only 12-phenylethynyl derivative was transformed into its 1,1,4,4-tetracyanobutadiene (TCBD) derivative by the reaction with TCNE. Amphoteric redox properties of the novel azulene analogs of tryptanthrin were characterized by spectroscopic and voltammetric analyses.

1,8-Naphthalimide-Substituted BODIPY Dyads: Synthesis, Structure, Properties, and Live-Cell Imaging.

A set of 1,8-naphthalimide (NPI)-substituted 4,4-difluoroboradiaza-s-indacene (BODIPY) dyads 1 a-1 c were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction of ethynyl substituted NPI 1 with the meso-, β-, and α-halogenated BODIPYs a, b, and c, respectively. The BODIPY 1 c exhibits redshifted absorption, which suggests better electronic communication with substitution at the α-position of BODIPY compared with at the meso and β positions, which was further supported by time-dependent DFT calculations. The optical band gap follows the order 1 a>1 b>1 c. The single-crystal X-ray structures of dyads 1 a-1 c are reported, which reflect planar orientations of the BODIPY units with respect to the NPIs. The DFT-optimized structures show good correlation with the experimental data obtained from the single-crystal X-ray structures. The packing diagram of 1 a shows a sheet-like arrangement, 1 b forms a ladder-like structural motif, and 1 c forms a complex 3D structural arrangement. The dyads 1 a-1 c show low cytotoxicity (IC50 >100 μm). The confocal microscopy studies with HeLa and A375 cells (when treated with dyads 1 a-1 c) show that all the dyads easily enter the cell membrane and show significant multicolor intracellular fluorescence covering the entire visible range with clear emissions in blue, green, and red channels.

C3-Symmetric star shaped donor–acceptor truxenes: synthesis and photophysical, electrochemical and computational studies

This manuscript reports the design and synthesis of C3-symmetric star shaped donor and acceptor substituted truxenes 6, 7, 10 and 11 using Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition–retroelectrocyclization reactions.

Donor–acceptor phenothiazine functionalized BODIPYs

A set of unsymmetrical and symmetrical phenothiazine functionalized BODIPYs of type D–A, D–π–A and A–D–A, A–π–D–π–A were synthesized by condensation and Pd-catalyzed Sonogashira cross-coupling reactions. Their photophysical and electrochemical properties were investigated. The electronic absorption spectra shows that the acetylene linked phenothiazine functionalized BODIPYs 7a and 7b exhibit bathochromic shift as compared to directly linked phenothiazine functionalized BODIPYs 4a and 4b. The density functional theory (DFT) calculation show that the incorporation of acetylene linkage between phenothiazine and BODIPYs induces coplanarity and results lower HOMO–LUMO gap which leads to red shifted absorption. The unsymmetrical phenothiazine functionalized BODIPYs exhibits higher thermal stability as compared to symmetrical analogous and follow the order 7a > 4a > 4b > 7b.

Unsymmetrical and Symmetrical Push-Pull Phenothiazines.

A series of unsymmetrical and symmetrical push-pull phenothiazines (3-7) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction and subsequent [2 + 2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The effect of systematic variation of the number and nature of cyano-based acceptor TCNE and TCNQ units on the photophysical, electrochemical, and computational studies was investigated. The single-photon absorption on phenothiazines 3-7 reveals that substitution of 1,1,4,4-tetracyanobutadiene (TCBD) and a cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD unit results in strong intramolecular charge transfer and lowering of the LUMO energy level. The TCBD-linked and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD-linked phenothiazines 3-7 exhibit multiredox waves. The computational studies on phenothiazines 3-7 exhibit substantial stabilization of the LUMO with the increase in acceptor strength, which results in lowering of the HOMO-LUMO gap.