Browsing by Author "Türkmen, Yunus Emre"

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Open Access
Access to symmetrical and unsymmetrical cyclobutanes via template directed [2+2] photodimerization reactions of cinnamic acids
(Georg Thieme Verlag, 2023-08-16) Yağcı, Bilge Banu; Munir, Badar; Zorlu, Y.; Türkmen, Yunus Emre
In this work, we have developed a general and broadly applicable template-directed photochemical [2+2]-cycloaddition reaction which provides access to a wide range of symmetrical and unsymmetrical cyclobutane products. The use of 1,8-dihydroxynaphthalene as a covalent template paved the way for successful and highly selective photochemical homodimerization and heterodimerization reactions in the solid state between cinnamic acid derivatives. Notably, the method works equally well with aryl- and heteroaryl-containing substrates leading to the formation of β-truxinic acid analogues as single diastereomers and in high yields (up to 99%).
Open Access
Alcohols and phenols as hydrogen bonding catalysts
(Taylor & Francis, 2017) Türkmen, Yunus Emre; Harned, A.
Hydrogen-bonding catalysis has been a rapidly growing field of organocatalysis and has found numerous applications in a broad range of organic transformations. In this chapter, applications of hydrogen bonding catalysts based on alcohols and phenols will be covered with emphasis on catalyst design and mechanistic investigations. The first part of this chapter describes the development of alcohol-based hydrogen-bonding catalysts. Chiral hydrogen bond donors such as TADDOL and BAMOL derivatives have been shown to be highly effective catalysts in a variety of enantioselective transformations including Diels–Alder, hetero-Diels–Alder, Mukaiyama aldol, and nitroso aldol reactions. A photoswitchable hydrogen-bonding catalyst system that was used in a Morita–Baylis–Hillman reaction was also described in this section. Efforts to enhance the activities of alcohol-based hydrogen bond donors resulted in the development of fluorinated alcohols as effective organocatalysts. The second part of this chapter describes the utilization of such fluorinated alcohol catalysts in ring-opening polymerization of lactides, carbon dioxide fixation of epoxides, and epoxidation of alkenes. Finally, hydrogen-bonding catalysts based on phenols are reviewed in the third section of this chapter. These include dual hydrogen bond donors such as PHANOL, diarylacetylene diol, and BINOL derivatives along with examples from enantioselective catalysis.
Open Access
Catalytic aza-Nazarov cyclization reactions to access α-methylene-γ-lactam heterocycles
(Beilstein-Institut Zur Forderung der Chemischen Wissenschaften, 2023-07-17) Yağcı, Bilge Banu; Dönmez, Selin Özge; Şahin, O.; Türkmen, Yunus Emre
We have developed a catalytic aza-Nazarov reaction of N-acyliminium salts generated in situ from the reaction of a variety of cyclic and acyclic imines with α,β-unsaturated acyl chlorides to afford substituted α-methylene-γ-lactam heterocycles. The reactions proceed effectively in the presence of catalytic (20 mol %) amounts of AgOTf as an anion exchange agent or hydrogen-bond donors such as squaramides and thioureas as anion-binding organocatalysts. The aza-Nazarov cyclization of 3,4-dihydroisoquinolines with α,β-unsaturated acyl chlorides gives tricyclic lactam products 7 in up to 79% yield with full diastereocontrol (dr = >99:1). The use of acyclic imines in a similar catalytic aza-Nazarov reaction with 20 mol % of AgOTf results in the formation of α-methylene-γ-lactam heterocycles 19 in up to 76% yield and with good to high diastereoselectivities (4.3:1 to 16:1). We have demonstrated the scalability of the reaction with a gram-scale example. The relative stereochemistry of the α-methylene-γ-lactam products 19 has been determined via the single-crystal X-ray analysis of lactam 19l. In order to shed light on the details of the reaction mechanism, we have performed carefully designed mechanistic studies which consist of experiments on the effect of β-silicon stabilization, the alkene geometry of the α,β-unsaturated acyl chloride reactants, and adventitious water on the success of the catalytic aza-Nazarov reaction.
Open Access
Development of one-pot benzylic amination reactions of azine N-oxides
(Elsevier, 2018) Liman, Menekşe; Türkmen, Yunus Emre
An efficient one-pot synthetic methodology has been developed for the benzylic amination reactions of methyl-substituted azine N-oxides that operate under mild conditions. The reaction was found to tolerate quinoline and isoquinoline N-oxides with electron donating and withdrawing substituents as the electrophilic reaction partners as well as a broad range of nucleophilic primary, secondary and aromatic amines, affording the benzylic amination products in up to 82% yield.
Open Access
Intramolecular through-space charge transfer between benzofuran and ynone groups on a naphthalene spacer
(Royal Society of Chemistry, 2023-12-07) Çalıkyılmaz, Eylül; Karaoğlu, Gözde; Demir, M.; Şahin, O.; Ulgut, Burak; Akdağ, A.; Türkmen, Yunus Emre
Open Access
Investigation of the hydrogen bond donating ability of 1,8-naphthalenediol by NMR spectroscopy and its use as a hydrogen bonding catalyst
(TUBITAK, 2018) Türkmen, Yunus Emre
The hydrogen bond donating ability of 1,8-naphthalenediol was investigated via a series of 1H, 13C, and 31P NMR experiments. Complexation studies using triphenylphosphine oxide and cyclohexanone as hydrogen bond acceptors revealed that 1,8-naphthalenediol is a more effective hydrogen bond donor compared to 1-naphthol and 8-methoxy-1-naphthol. Afterwards, its effectiveness as a hydrogen bonding catalyst was demonstrated in the Friedel-Crafts-type addition reaction of indole to trans-β-nitrostyrene.
Open Access
Na-promoted bimetallic hydroxide nanoparticles for aerobic c-h activation catalyst design principles and insights into reaction mechanism
(American Chemical Society, 2024-10-25) Erdivan, Beyzanur; Çalıkyılmaz, Eylül; Bilgin, Suay; Erdali, Ayşe Dilay; Gül, Damla Nur; Ercan, Kerem Emre; Türkmen, Yunus Emre; Özensoy, Emrah
A precious metal-free bimetallic Fe x Mn1-x (OH) y hydroxide catalyst was developed that is capable of catalyzing aerobic C-H oxidation reactions at low temperatures, without the need for an initiator, relying sustainably on molecular oxygen. Through a systematic synthetic effort, we scanned a wide nanoparticle synthesis parameter space to lay out a detailed set of catalyst design principles unraveling how the Fe/Mn cation ratio, NaOH(aq) concentration used in the synthesis, catalyst washing procedures, extent of residual Na+ promoters on the catalyst surface, reaction temperature, and catalyst loading influence catalytic C-H activation performance as a function of the electronic, surface chemical, and crystal structure of Fe x Mn1-x (OH) y bimetallic hydroxide nanostructures. Our comprehensive XRD, XPS, BET, ICP-MS, 1H NMR, and XANES structural/product characterization results as well as mechanistic kinetic isotope effect (KIE) studies provided the following valuable insights into the molecular level origins of the catalytic performance of the bimetallic Fe x Mn1-x (OH) y hydroxide nanostructures: (i) catalytic reactivity is due to the coexistence and synergistic operation of Fe3+ and Mn3+ cationic sites (with minor contributions from Fe2+ and Mn2+ sites) on the catalyst surface, where in the absence of one of these synergistic sites (i.e., in the presence of monometallic hydroxides), catalytic activity almost entirely vanishes, (ii) residual Na+ species on the catalyst surface act as efficient electronic promoters by increasing the electron density on the Fe3+ and Mn3+ cationic sites, which in turn, presumably enhance the electrophilic adsorption of organic reactants and strengthen the interaction between molecular oxygen and the catalyst surface, (iii) in the fluorene oxidation reaction the step dictating the reaction rate likely involved the breaking of a C-H bond (k H /k D = 2.4), (iv) reactivity patterns of a variety of alkylarene substrates indicate that the C-H bond cleavage follows a stepwise PT-ET (proton transfer-electron transfer) pathway.
Open Access
Polarization‐enhanced hydrogen bonding in 1,8‐dihydroxynaphthalene: conformational analysis, binding studies and hydrogen bonding catalysis
(Wiley, 2020-11-16) Mammadova, Flora; Hamarat, B.; Ahmadli, Dilgam; Şahin, O.; Bozkaya, U.; Türkmen, Yunus Emre
In this article, the presence and effects of polarization‐enhanced hydrogen bonding in 1,8‐dihydroxynaphthalene (1,8‐DHN) were investigated in detail through a series of experimental and computational studies. First, the conformation of 1,8‐DHN, and its ability to make intra‐ and intermolecular hydrogen bonds were investigated in solid state by X‐ray crystallography, in solution by NMR spectroscopy, and computationally by density functional theory. Second, equilibrium binding constants, which were determined by 31P‐NMR titration studies, demonstrated stronger complexation of Ph3PO with 1,8‐DHN compared to mono‐naphthol derivatives 8‐methoxy‐1‐naphthol and 1‐naphthol. In the final section, 1,8‐DHN was observed to be an effective catalyst for the Friedel‐Crafts‐type addition reaction of indoles to β‐nitrostyrenes, and a rationale for this catalytic activity was provided via computational studies. All the findings described in this work support the enhanced hydrogen bond donating ability of 1,8‐DHN due to polarization caused by the six‐membered intramolecular hydrogen bond present in its structure.
Open Access
Precious metal-Free LaMnO3 perovskite catalyst with an optimized nanostructure for aerobic C–H bond activation reactions: alkylarene oxidation and naphthol dimerization
(American Chemical Society, 2021-02-03) Şahin, Yeşim; Sika-Nartey, Abel Tetteh; Ercan, Kerem Emre; Koçak, Yusuf; Senol, Sinem; Özensoy, Emrah; Türkmen, Yunus Emre
In this article, we describe the development of a new aerobic C–H oxidation methodology catalyzed by a precious metal-free LaMnO3 perovskite catalyst. Molecular oxygen is used as the sole oxidant in this approach, obviating the need for other expensive and/or environmentally hazardous stoichiometric oxidants. The electronic and structural properties of the LaMnO3 catalysts were systematically optimized, and a reductive pretreatment protocol was proved to be essential for acquiring the observed high catalytic activities. It is demonstrated that this newly developed method was extremely effective for the oxidation of alkylarenes to ketones as well as for the oxidative dimerization of 2-naphthol to 1,1-binaphthyl-2,2-diol (BINOL), a particularly important scaffold for asymmetric catalysis. Detailed spectroscopic and mechanistic studies provided valuable insights into the structural aspects of the active catalyst and the reaction mechanism.
Open Access
Synthesis of acenaphthylene-fused heteroarenes and polyoxygenated benzo[j]fluoranthenes via a pd-catalyzed Suzuki-Miyaura/C-H arylation cascade
(Beilstein-Institut, 2024-12-23) Yence, Merve; Ahmadli, Dilgam; Sürmeli, Damla; Karacaoğlu, Umut Mert; Pal, Sujit; Türkmen, Yunus Emre
Acenaphthylene-fused heteroarenes with a variety of five- and six-membered heterocycles such as thiophene, furan, benzofuran, pyrazole, pyridine and pyrimidine were synthesized via an efficient Pd-catalyzed reaction cascade in good to high yields (45-90%). This cascade involves an initial Suzuki-Miyaura cross-coupling reaction between 1,8-dihalonaphthalenes and heteroarylboronic acids or esters, followed by an intramolecular C-H arylation under the same conditions to yield the final heterocyclic fluoranthene analogues. The method was further employed to access polyoxygenated benzo[j]fluoranthenes, which are all structurally relevant to benzo[j]fluoranthene-based fungal natural products. The effectiveness of our strategy was demonstrated via a concise, four-step synthesis of the tetramethoxybenzo[j]fluoranthene derivative 18 , which represents a formal total synthesis of the fungal natural product bulgarein.
Open Access
Template-directed photochemical homodimerization and heterodimerization reactions of cinnamic acids
(American Chemical Society, 2021-09-01) Yağcı, Bilge Banu; Zorlu, Y.; Türkmen, Yunus Emre
We developed a general method for the selective photochemical homo- and heterodimerization of cinnamic acid derivatives with the use of commercially available 1,8-dihydroxynaphthalene as a covalent template. A variety of symmetrical and unsymmetrical β-truxinic acids were obtained in high yields and as single diastereomers. The use of a template not only provides the alignment of the two olefins with suitable proximity (<4.2 Å) but also allows the heterodimerization of two different cinnamic acids, leading to unsymmetrical β-truxinic acid products.
Embargo
Template-directed selective photodimerization reactions of 5-arylpenta-2,4-dienoic acids
(American Chemical Society, 2024-07-10) Munir, Badar; Yağcı, Bilge Banu; Zorlu, Yunus; Türkmen, Yunus Emre
We developed an efficient method that enables selective photodimerization of 5-arylpenta-2,4-dienoic acids (i.e., vinylogous cinnamic acids). The use of 1,8-dihydroxynaphthalene as a template ensures proximity of the two reacting olefins so that irradiation of template-bound dienoic acids gives mono [2 + 2] cycloaddition products in good to excellent yields (up to 99%), as single regioisomers, and with high diastereoselectivities (dr = 3:1 to 13:1). The geometrical and stereochemical features of compounds 12a, 16a, and 22a were analyzed by X-ray crystallography.
Open Access
Two-dimensional bimetallic hydroxide nanostructures for catalyzing low-temperature aerobic C–H bond activation in alkylarene and alcohol partial oxidation
(American Chemical Society, 2022-12-08) Sika-Nartey, Abel Tetteh; Sahin, Yesim; Ercan, Kerem Emre; Kap, Zeynep; Kocak, Yusuf; Erdali, Ayşe Dilay; Erdivan, Beyzanur; Türkmen, Yunus Emre; Ozensoy, Emrah
Two-dimensional (2D) bimetallic NixMn1–x(OH)y layered double hydroxide (LDH) nanostructures were synthesized and optimized as a remarkably active catalytic platform for low-temperature aerobic C–H bond activation in alkylarenes and partial oxidation of alcohols using a wide substrate (i.e., reactant) and diverse solvent scope. The NixMn1–x(OH)y structure consists of nonprecious and earth-abundant metals that can effectively operate at low catalyst loadings, requiring only molecular oxygen as the stoichiometric oxidant. Structurally diverse alkylarenes as well as primary and secondary alcohols were shown to be competent substrates where oxidation products were obtained in excellent yields (93–99%). Comprehensive catalyst structural characterization via XRD, ATR-IR, TEM, EDX, XPS, BET, and TGA indicated that the ultimately optimized Ni0.6Mn0.4(OH)y-9S catalyst possessed not only particular rotational faults in its β-Ni0.6Mn0.4(OH)y domains but also distinct α/β-Ni0.6Mn0.4(OH)y interstratification disorders, in addition to a relatively high specific surface area of 125 m2/g, a 2D platelet morphology, and an average Mn oxidation state of +3.5, suggesting the presence of both Mn3+ and Mn4+ species in its structure working in a synergistic fashion with the Ni2+/x+ cations (the latter is justified by the lack of catalytic activity in the monometallic LDH catalysts Ni(OH)2 and Mn(OH)2). Kinetic isotope effect studies carried out in the fluorene oxidation reaction (kH/kD = 5.7) revealed that the rate-determining step of the catalytic oxidation reaction directly involved the scission of a C–H bond. Moreover, the optimized catalyst was demonstrated to be reusable through the application of a regeneration protocol, which can redeem the full initial activity of the carbon-poisoned spent catalyst in the fluorene oxidation reaction.