Pd-catalyzed Sonogashira Cross-coupling Reaction 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.

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.

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.

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.

Triptycene-Derived Photoresponsive Fluorescent Azo-Polymer as Chemosensor for Picric Acid Detection.

Two new triptycene-based azobenzene-functionalized polymers (TBAFPs) have been synthesized using the well-known Pd-catalyzed Sonogashira cross-coupling polycondensation reaction between 2,6-diethynyltriptycene and (meta or para) dibromo-azobenzenes. Enhancement of the fluorescent emission intensity was observed upon trans → cis isomerization of −N=N― linkage in TBAFPs. The cis-lifetime of TBAFP1 is rather long (greater than 2 days). The resulting materials were tested as a potential chemosensor for the detection of picric acid (PA)—a water pollutant as well as chemical constituent of explosives used in warfare. PA was found to interact strongly with TBAFPs, which led to significant quenching of the latter’s fluorescence emission intensities. The binding constants are in the order of 105 M–1. TBAFPs were also able to detect PA in nanomolar concentrations.

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.

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.

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.

Synthesis, Cytostatic and Antibacterial Evaluations of Novel 1,2,3-Triazolyl-tagged Pyrimidine and Furo[2,3-d]pyrimidine Derivatives

C-5 alkynylated and N-1 alkylated pyrimidine derivatives were synthesized by N-alkylation reaction of 5-iodouracil in the presence of NaH, as a base, followed by Pd-catalyzed Sonogashira cross-coupling reaction of N-alkyl-5-iodouracil derivatives (1 and 2) with corresponding terminal alkynes. Intramolecular in situ O- heteroannulation ring closure of N-1- alkyl-C-5- alkynylpyrimidine derivatives (3 and 5) generated novel 6-substituted furo[2, 3-d]pyrimidine derivatives (7 and 8). 1, 4-Disubstituted 1, 2, 3- triazole tethered 5- alkynylpyrimidines (14– 19) and 6-substituted furo[2, 3- d]pyrimidines (20–22) were successfully prepared by the copper(I)- catalyzed click reaction of 5-iodo-N- 1- propargylpyrimidine (2) using microwave irradiation, followed by Sonogashira cross- coupling reaction with corresponding terminal alkynes. In vitro antiproliferative activity of prepared compounds evaluated on human cancer cell lines cervix adenocarcinoma (HeLa), colon adenocarcinoma (CaCo- 2), chronic myeloid leukemia in blast crisis (K562), Burkitt lymphoma (Raji) revealed that pyrimidine (19) and furo[2, 3- d]pyrimidine (22) derivatives with 3, 5- difluorophenyl at pyrimidine and furo[2, 3- d]pyrimidine as well as p- (trifluoromethyl)phenyl at 1, 2, 3-triazole exhibited marked and selective inhibitory effects on the growth of K562 and Raji tumor cells. Antibacterial evaluations showed that pyrimidine derivative 14 substituted with p- tolylethynyl at C-5 of pyrimidine and benzyl at 1, 2, 3-triazole moiety was the most active of all evaluated compounds on the Gram positive bacterial strains Enterococcus faecalis. Further structure optimization of compounds 14, 19 and 22 is foreseen in order to obtain lead structural analogs with efficient and selective antitumoral and antibacterial activities.

Ferrocenyl aza-dipyrromethene and aza-BODIPY: Synthesis and properties

A series of ferrocenyl substituted tetraphenylazadipyrromethane and Aza-BODIPY were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the pyrrolic substituents on photophysical and electrochemical properties was explored. The results show substantial electronic communication between the ferrocene unit, and Aza-BODIPY core. The aza-dipyromethane and aza-BODIPY 3–6 are non-emissive in nature and their absorption maxima exhibit red shifted absorption compared to unsubstituted azabodipy. The computational calculation on Aza-BODIPY 5 and 6 was performed, which reveals that electron density transfers from ferrocene (donor) to azabodipy (acceptor) core and the photophysical properties are in good agreement with the computational data.

Regulating Circularly Polarized Luminescence Signals of Chiral Binaphthyl-Based Conjugated Polymers by Tuning Dihedral Angles of Binaphthyl Moieties

A series of chiral binaphthyl-based conjugated polymers enantiomers incorporating boron dipyrromethene (BODIPY) chromophore in the main chain backbone were designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. All of them can exhibit strong Cotton effects and circularly polarized luminescence (CPL) emission signals in THF solution. The CD absorption dissymmetry factors (gabs) and the luminescence dissymmetry factors (glum) can be regulated by tuning the dihedral angles of binaphthyl arising from different substitutions of BINOL hydroxyls. Interestingly, the chiral polymers can exhibit the gradual increase of both gabs and glum as the decrease of dihedral angles of the chiral binaphthyl moiety. This work can provide a new strategy for the development of CPL emission materials.

Symmetrical and unsymmetrical triphenylamine based diketopyrrolopyrroles and their use as donors for solution processed bulk heterojunction organic solar cells

Two small molecules DPP3 (D–π–A) and DPP4 (D–π–A–π–D) with triphenylamine (TPA) donors and diketopyrrolopyrrole (DPP) acceptors linked with ethyne linkers were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction.

Meso Alkynylated Tetraphenylethylene (TPE) and 2,3,3-Triphenylacrylonitrile (TPAN) Substituted BODIPYs.

The tetraphenylethylene (TPE) substituted BODIPY 2a, and 2,3,3-triphenylacrylonitrile (TPAN) substituted BODIPYs 2b and 2c were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical and electrochemical properties were investigated. The BODIPY 2a exhibits strong D-A interaction with poor fluorescence quantum yield. The BODIPYs 2b and 2c show red-shifted absorption and emission with higher fluorescence quantum yield compared to BODIPY 2a. The photonic properties of BODIPYs 2a-2c were compared with 4-ethynylbenzonitrile substituted BODIPY 3 and phenylacetylene substituted BODIPY 4. The results reveal that the electron donating group at the meso position of BODIPY blue shifts the absorption and emission with decreased fluorescence quantum yield, whereas the electron withdrawing group at the meso position of BODIPY red shifts the absorption and emission with enhanced quantum yields. The single crystal structures of BODIPYs 2a and 2b reflect the planar orientation of meso substituent and the BODIPY core, which leads to close π-π stacking. The extensive π-π stacking and strong donor-acceptor (D-A) interaction makes these BODIPYs AIE inactive. The experimental observations were supported by DFT calculation.

Ferrocenyl thiazoles: synthesis and properties

A set of ferrocenyl-substituted thiazoles 2 and 3 were designed and synthesized by Pd-catalyzed Sonogashira cross-coupling reaction. Their [2+2] cycloaddition–reteroelectrocyclization reaction with tetracyanoethene resulted in strong intramolecular charge transfer (ICT) chromophores 4 and 5. The electronic absorption spectra of 4 and 5 exhibit strong ICT bands in the near infrared (NIR) region. The electrochemical studies of ferrocenyl thiazoles 4 and 5 exhibit multistep redox waves. The computational studies show good agreement with the experimental results. The incorporation of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) unit results is improved thermal stability of ferrocenyl thiazoles.

β-Substituted triarylborane appended porphyrins: photophysical properties and anion sensing

β-Substituted triarylborane porphyrins were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction.