Pyridyl Groups Research Articles

Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand.

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N2NN' (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-U and 1-Th are structurally very similar, they display disparate reactivities with TMS3SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N2NN')UCl2 (1-U) and 1 equiv of TMS3SiK in THF unexpectedly formed [Cl(N2NN')U]2O (2-U) featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (N2NN')ThCl2 (1-Th) and 1 equiv of TMS3SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azide complex 3-Th by reaction with NaN3. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.

Ligand-Directed Metalation of a Gold Pyrazolate Cluster.

Solid "[AuL]" (HL = 3-[pyrid-2-yl]-5-tertbutyl-1H-pyrazole) can be crystallized as cyclic [Au3(μ-L)3] and [Au4(μ-L)4] clusters from different solvents. The crystalline tetramer contains a square Au4 core with an HT:TH:TH:HT arrangement of ligand substituents, which preorganizes the cluster to chelate to additional metal ions via its pendant pyridyl groups. The addition of 0.5 equiv of AgBF4 to [AuL] yields [Ag2Au4(μ3-L)4][BF4]2, where two edges of the Au4 square are spanned by Ag+ ions via metallophilic Ag···Au contacts. Treatment of [AuL] with [Cu(NCMe)4]PF6 affords the metalloligand helicate [Cu2Au2(μ-L)4][PF6]2, via oxidation of the copper and partial fragmentation of the cluster.

8(meso)-Pyridyl-BODIPYs: Effects of 2,6-Substitution with Electron-Withdrawing Nitro, Chloro, and Methoxycarbonyl Groups.

The introduction of electron-withdrawing groups on 8(meso)-pyridyl-BODIPYs tends to increase the fluorescence quantum yields of this type of compound due to the decrease in electronic charge density on the BODIPY core. A new series of 8(meso)-pyridyl-BODIPYs bearing a 2-, 3-, or 4-pyridyl group was synthesized and functionalized with nitro and chlorine groups at the 2,6-positions. The 2,6-methoxycarbonyl-8-pyridyl-BODIPYs analogs were also synthesized by condensation of 2,4-dimethyl-3-methoxycarbonyl-pyrrole with 2-, 3-, or 4-formylpyridine followed by oxidation and boron complexation. The structures and spectroscopic properties of the new series of 8(meso)-pyridyl-BODIPYs were investigated both experimentally and computationally. The BODIPYs bearing 2,6-methoxycarbonyl groups showed enhanced relative fluorescence quantum yields in polar organic solvents due to their electron-withdrawing effect. However, the introduction of a single nitro group significantly quenched the fluorescence of the BODIPYs and caused hypsochromic shifts in the absorption and emission bands. The introduction of a chloro substituent partially restored the fluorescence of the mono-nitro-BODIPYs and induced significant bathochromic shifts.

Design, synthesis, and anticancer activity of novel 4,6-dimorpholinyl-1,3,5-triazine compounds

Abstract A series of novel 4,6-dimorpholinyl-1,3,5-triazine derivatives 6a–6r were obtained through N-substitution and Claisen-Schmidt condensation. 1H NMR, 13C NMR, and mass spectrometry were used to characterize the molecular structures of the derivatives. The in vitro antiproliferation activity of derivatives was evaluated using the MTT assay against SW620 (human colon cancer cells), A549 (human nonsmall cell lung cancer cells), HeLa (human cervical cancer cells), and MCF-7 (human breast cancer cells). Compound 6o bearing a pyridyl group exhibited good cytotoxicity against four cancer cells, with IC50 values of 8.71, 9.55, 15.67, and 21.77 μM, sequentially. In addition, compound 6a showed some selectivity against SW620.

A Dicyanomethyl Radical Conjugated with a Pyridylamino Group: Combining Radical-based Dynamic Covalent Chemistry and Coordination Chemistry.

In this work, we aimed to develop a dicyanomethyl radical that undergoes both reversible C-C bond formation/dissociation and metal-ligand coordination reactions to combine dynamic covalent chemistry (DCC) based on organic radicals with coordination chemistry. We have previously reported a dicyanomethyl radical conjugated with a triphenylamine (1⋅) that exhibits a monomer/dimer equilibrium between the σ-bonded dimer (12 ). We designed and synthesized a novel dicyanomethyl radical with a pyridyl group as a coordination point (2⋅) by replacing the phenyl group of 1⋅ with a 3-pyridyl group. We showed that 2⋅ is also in an equilibrium with the σ-bonded dimer (22 ) in solution and has suitable thermodynamic parameters for application in DCC. 22 coordinates to PdCl2 in a 2 : 2 ratio to selectively form a metallamacrocycle (22 )2 (PdCl2 )2 , and its structure was clarified by single crystal X-ray analysis. Variable-temperature NMR, ESR, and electronic absorption measurements revealed that (22 )2 (PdCl2 )2 also undergoes the reversible C-C bond formation/dissociation reaction. Ligand-exchange experiment showed that 22 was liberated from (22 )2 (PdCl2 )2 by the addition of another ligand with a higher affinity for PdII . This work demonstrated that DCC based on dicyanomethyl radicals works orthogonally to metal-ligand coordination reactions.

A Dicyanomethyl Radical Conjugated with a Pyridylamino Group: Combining Radical‐based Dynamic Covalent Chemistry and Coordination Chemistry

AbstractIn this work, we aimed to develop a dicyanomethyl radical that undergoes both reversible C−C bond formation/dissociation and metal‐ligand coordination reactions to combine dynamic covalent chemistry (DCC) based on organic radicals with coordination chemistry. We have previously reported a dicyanomethyl radical conjugated with a triphenylamine (1⋅) that exhibits a monomer/dimer equilibrium between the σ‐bonded dimer (12). We designed and synthesized a novel dicyanomethyl radical with a pyridyl group as a coordination point (2⋅) by replacing the phenyl group of 1⋅ with a 3‐pyridyl group. We showed that 2⋅ is also in an equilibrium with the σ‐bonded dimer (22) in solution and has suitable thermodynamic parameters for application in DCC. 22 coordinates to PdCl2 in a 2 : 2 ratio to selectively form a metallamacrocycle (22)2(PdCl2)2, and its structure was clarified by single crystal X‐ray analysis. Variable‐temperature NMR, ESR, and electronic absorption measurements revealed that (22)2(PdCl2)2 also undergoes the reversible C−C bond formation/dissociation reaction. Ligand‐exchange experiment showed that 22 was liberated from (22)2(PdCl2)2 by the addition of another ligand with a higher affinity for PdII. This work demonstrated that DCC based on dicyanomethyl radicals works orthogonally to metal‐ligand coordination reactions.

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction.

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)3Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO2 reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO2 to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and dπ(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

Sulfones Bearing Perfluorinated Pyridine Group: Synthesis and Photocatalytic Reaction with α‐(Trifluoromethyl)styrenes

AbstractA photocatalytic protocol for the reductive coupling of tetrafluoropyridyl‐substituted sulfones with α‐(trifluoromethyl)styrenes in the presence of zinc and sodium iodide is reported. Sulfones were obtained by oxidation of the corresponding sulfides catalyzed by molybdenum or ruthenium salts. A combination of sulfonyl fragment and electron depleting fluorinated pyridyl group is a key feature determined properties of these sulfones. Their propensity to undergo single electron reduction and to form electron donor‐acceptor complexes with N‐arylamines were evaluated. Plausible pathways of fragmentation of radical anion generated after single electron reduction of sulfones were analyzed, and the pathway leading to the formation of the alkyl radical and the sulfinate anion was found preferable.

Molecular and silica-supported metal complexes as new catalysts for hydrosilylation

Two disiloxanes with pyridyl or isocyanide functional groups, as well as silica materials carrying pyridyl or thiol functionalities were used as ligands for Pt(IV) and Fe(III) ions. The molecular and silica-supported metal complexes were tested in a model hydrosilylation reaction between 1,3-divinyl tetramethyldisiloxane and 1,1,3,3-tetramethyldisiloxane, conducted in (pseudo)-heterogeneous conditions. The new hydrosilylation catalysts afforded 75-100 % conversion and 87-93 % selectivity, based on NMR data, while some of them presented recyclability; they were also successful in silicones crosslinking at room temperature. In particular, the iron-isocyanide disiloxane complex is proposed as a promising, cheap, unsophisticated and stable catalyst, capable to act in heterogeneous conditions, without the need of dry solvent or inert atmosphere. Silica-supported Pt or Pt/Pd catalysts, obtained with commercial or recovered noble metals precursors, presented high conversion and increased regioselectivity in the hydrosilylation reaction, being reusable, and are efficient in other hydrosilylation reactions, useful in organic and silicon chemistry.

Water-Soluble Gd(III)-Porphyrin Complexes Capable of Both Photosensitization and Relaxation Enhancement.

With the aim of developing more stable Gd(III)-porphyrin complexes, two types of ligands 1 and 2 with carboxylic acid anchors were synthesized. Due to the N-substituted pyridyl cation attached to the porphyrin core, these porphyrin ligands were highly water-soluble and formed the corresponding Gd(III) chelates, Gd-1 and Gd-2. Gd-1 was sufficiently stable in neutral buffer, presumably due to the preferred conformation of the carboxylate-terminated anchors connected to nitrogen in the meta position of the pyridyl group helping to stabilize Gd(III) complexation by the porphyrin center. 1H NMRD (nuclear magnetic relaxation dispersion) measurements on Gd-1 revealed high longitudinal water proton relaxivity (r1 = 21.2 mM-1 s-1 at 60 MHz and 25 °C), which originates from slow rotational motion resulting from aggregation in aqueous solution. Under visible light irradiation, Gd-1 showed extensive photoinduced DNA cleavage in line with efficient photoinduced singlet oxygen generation. Cell-based assays revealed no significant dark cytotoxicity of Gd-1, while it showed sufficient photocytotoxicity on cancer cell lines under visible light irradiation. These results indicate the potential of this Gd(III)-porphyrin complex (Gd-1) as a core for the development of bifunctional systems acting as an efficient photodynamic therapy photosensitizer (PDT-PS) with magnetic resonance imaging (MRI) detection capabilities.

Deep blue thermally activated delayed fluorescent emitters based on methyl-modified acridin-9(10H)-one acceptor

Developing deep blue thermally activated delayed fluorescence (TADF) materials with high efficiencies and good color purities is still a formidable challenge. Here, two novel deep blue TADF emitters, namely MePhCz-AD and MePhCz-AD-Py, were synthesized by incorporating two 3,6-diphenyl-9H-carbazole donors onto the methyl-modified acridin-9(10H)-one (AD) acceptor, respectively. The high rigidity of AD acceptor effectively suppressed non-radiative transition and vibration relaxation of excited states, which is favorable for a good emitting quantum yield and narrow spectral width. The presence of methyl groups at 2,7-sites of AD ring slightly reduced the acceptor strength and contributed to the deep blue emission color. Incorporation of phenyls at 3,6-sites of carbazole ring delocalized the distribution of the highest occupied molecular orbital (HOMO), promoting the radiative decay process. As a result, these two emitters showed deep blue TADF with fast radiative decay rates kR > 107 s−1. In addition, with the introduction of pyridyl (Py) group at 10-site of AD ring instead of phenyl, both the spin-orbital coupling (SOC) constant between high-lying triplet states and the lowest singlet excited state (S1) and thus the reverse intersystem crossing rate constant (kRISC) were dramatically promoted for MePhCz-AD-Py. The organic light-emitting diode with MePhCz-AD-Py as doped emitter exhibited a maximum external quantum efficiency (EQEmax) of 15.4% with a relatively narrow FWHM of 60 nm and the deep-blue color coordinates of (0.15, 0.15).

Porphyrin-silica gel hybrids as effective and selective copper(II) adsorbents from industrial wastewater

Porphyrins are an important class of ligands with a tremendous ability to capture metal ions closely related to the rich coordination chemistry of porphyrins. Herein we use this characteristic to develop silica gel grafted derivatives for water remediation applications. Therefore, two porphyrin derivatives, one with three and the other with four mercaptopyridyl units were grafted on silica gel functionalized with 3-aminopropyltriethoxysilane. The new adsorbents Si3PyS and Si4PyS were characterized using a suitable set of techniques confirming the covalent attachment of the porphyrins to the silica surface. Additionally, microscopy and N2 adsorption analysis confirmed the structural integrity and preservation of the mesoporous structure of Si during surface modification. The results show that both hybrid materials exhibit good chemical and thermal stability and an outstanding Cu2+ removal capability, with a chemical adsorption capacity of 176.32 mg g–1 and 184.16 mg g–1, respectively. These materials have also been used in real water and industrial wastewater samples with minimal interference in their adsorption capabilities. Density Functional Theory calculations were performed to confirm the good performance of the hybrid materials Si3PyS and Si4PyS towards metal ions. The functionalization of silica surface with porphyrin-based ligands bearing additional binding motifs drastically improves the adsorption capability of the new hybrids towards metal ions. The presence of pyridyl units brings a meaningful advantage, since both porphyrin core and appended pyridyl groups are able of binding Cu2+ ions with high affinity, contributing to the enhancement of the chelating features of the adsorbents prepared when compared with other ligands supported in silica-based materials.

Encapsulated-Type Iridium(III) Phosphorescent Emitters with a Hexadentate Ligand of Three Different Bidentate Units

The preparation and photophysical properties of new phosphorescent iridium(III) emitters are reported. The metal center is encapsulated by a hexadentate ligand made up of three different bidentate fragments: orthometalated 2-phenylpyridine, orthometalated 2-benzylpyridine, and metalated 2-ethylpyridine. Their formation involves activation reactions of C(sp2)–H and C(sp3)–H bonds assisted by pyridyl groups.

Photoluminescent iridium(III) complexes with phenylvinylpyridine-base cyclometalating ligands: Synthesis, photophysical properties, and photocyclization

Photoresponsive materials have recently garnered significant research interest and have been designed using various molecules. A viable approach to develop photoresponsive materials is by integrating photoisomeric structures into the organic ligands of phosphorescent transition metal complexes. In this study, we designed two photoresponsive cyclometalating ligands, 2-phenyl-3-vinylpyridine (HL1) and trans-4-methyl-2-phenyl-3-styrylpyridine (HL2), via the replacement of a phenyl group with a pyridyl group in vinylbiphenyl. Upon photoirradiation under an argon atmosphere, both ligands underwent photocyclization and demonstrated hypochromism and bathochromic shifts in absorption. Notably, HL2 underwent additional oxidative dehydrogenation upon photoirradiation in the presence of oxygen, resulting in an elongated conjugation structure that exhibited significant fluorescence intensity increase at 363 nm. Additionally, three photoresponsive iridium(III) complexes cyclometalated with HL1 or HL2 were synthesized and characterized. The photocyclization process of the complexes was much faster than that of the free ligands because the phenylpyridine moiety became coplanar upon cyclometalation. Upon photoirradiation under an argon atmosphere, these complexes exhibited significant luminescence enhancement by about 10–20 folds because of photocyclization resulting in a rigid closed structure that limited nonradiative decay of the excited complexes. These molecules present a unique strategy to construct photocyclization complexes and provide important insights into the development of photoresponsive materials.

Concise synthesis of low-cost fullerene derivatives as electron transport materials for efficient air-processed invert perovskite solar cells

Due to its excellent charge extraction ability, fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) is widely used as electron transport materials (ETM) in invert perovskite solar cells. However, the complicated synthetic routes and low productivity of PCBM limiting its commercial application. Moreover, the insufficient defect passivation ability of PCBM is contributed to inferior device performance because it lacks hetero-atoms/groups with lone pair electrons, it is highly desirable for exploration of new fullerene-based ETM with excellent photoelectric properties. Therefore, three new fullerene malonate derivatives were synthesized by simple two-step reaction with a high yield, and then developed as electron transport materials in invert perovskite solar cells which fabricated in air condition. The constituent thiophene and pyridyl group of the fullerene-based ETM can heighten the chemical interaction between under-coordinated Pb2+ and lone pair electrons of N, S atom by electrostatic interaction. Hence, the air-processed unencapsulated device with new fullerene-based electron transport materials (C60-bis(pyridin-2-ylmethyl) malonate (C60-PMME)) can obtain a enhanced power conversion efficiency (PCE) of 18.38%, which is significantly higher than the PCBM-based devices (16.64%). Additionally, the C60-PMME-based devices exhibit significantly more outstanding long-term stability than PCBM-based devices, owing to the strong hydrophobic properties of these new fullerene-based ETM. This study shows the promising potentials of these new low-cost fullerene derivatives as ETM to replace commercially used fullerene derivatives PCBM.

Structure and Stability of Metalloporphyrin Networks on Au(111)

We use density functional theory (DFT) calculations to analyze the properties of FeTPyP self-assembled monolayers (SAMs) and FeTPyP+Fec metal–organic coordination networks (MOCNs) on Au(111). Through a rigorous step-by-step approach, we determine the relative importance of the multiple competing interactions that determine the optimum structure and stability of both networks. For FeTPyP/Au(111) SAMs, we see a complex energy landscape, with molecules preferring to stay at molecule–molecule distances dop ∼ 13.7 and 14.3 Å. Molecule–surface interactions are more prominent in less dense packing (dop ∼ 14.3 Å), and intermolecular interactions favor the compact structure (dop ∼ 13.7 Å). In the case of the FeTPyP–Fec/Au(111) MOCN, the strong Fec–Npy bond between the peripheral iron atom Fec and nitrogen atoms of neighboring pyridyl groups (Npy) is what causes the network development and that the preferred molecule–molecule distance remains at dop ∼ 13.7 Å. Our findings demonstrate the necessity of theoretical treatments that take into account all relevant interactions in order to adequately describe the primary characteristics of surface-supported SAMs and MOCNs.

Ligand Tailoring and Functional Group Fusion Strategy for Developing Photochromic Compounds: A Case Study for Pyridinedicarboxylic Acid

Hybrid photochromic materials (HPMs) can produce novel photoresponsive properties, thanks to the synergistic effect of each component. One of the widely explored ways toward a novel HPM system is the utilization of the phototriggered electron transfer (ET) mechanism. In this work, the photochromic functionality of pyridinedicarboxylic acid with the chemical formula of H2-L1·H2O (1, H2-L1 = 2,6-pyridinedicarboxylic acid) and related metal–organic complexes (MOCs), [Zn(HL1)2]·2.5H2O (2), [Pb(L2)] (3, H2-L2 = 2,4-pyridinedicarboxylic acid), [In(L2)(HL2)]·2H2O (4), and [Eu3(L2)4(OH)(H2O)2]·11H2O (5), has been first proved via the ligand tailoring and functional group fusion strategy. The photogenerated radicals derived from ET were experimentally characterized. Different from the previously investigated HPMs based on electron-deficient pyridiniums and rigid polydentate N-ligands as electron acceptors (EAs), this work exploits a new category of ligands for developing HPMs and investigating the underlying photoresponsive functionalities. Considering the large varieties of carboxylate and pyridine moieties, this study offers a general strategy for developing new HPMs and exploring the potential photoresponsive functionality via integrating the carboxylic groups as electron donors (EDs) and pyridyl groups as EAs in the single ligand.

Effect of pyridyl position on constructing acid-base dual-responsive fluorophores with ESIPT and AIE characteristics

Exceedingly large Stokes shift solid-state fluorescent materials with acid/base dual responses are promising for potential practical applications. However, their realization is extremely challenging because of the difficulty in designing molecules possessing these functionalities simultaneously. Herein, three pyridyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) isomers (o-Py-HBT, m-Py-HBT, and p-Py-HBT) were rationally designed with a pyridyl group located at ortho, meso, and para positions relative to the phenolic hydroxyl (-OH), respectively. These compounds are both excited-state intramolecular proton transfer (ESIPT)- and aggregation-induced emission (AIE)-active. The intrinsic –OH and introduced pyridyl moiety were exploited as base- and acid-responsive units, respectively. Among the three HBT isomers, m-Py-HBT exhibited the most significant emission shift upon acidic stimuli. Theoretical calculations confirmed that the prominent intramolecular charge transfer of the protonated keto-tautomer of m-Py-HBT accounts for the trifluoroacetic acid-induced bathochromic shift of ∼33 nm. In addition, it was concluded that intramolecular H-bonding strength and –OH acidity jointly affected the base response. Finally, m-Py-HBT was successfully applied for the fabrication of trichromatic fluorescent patterns upon acid or base stimulation, exhibiting great application potential for encrypted pattern production. The exploration of structure-property correlations provides experimental and theoretical guidance for designing and developing multi-stimuli responsive solid-state molecular switches.

Phenothiazine-, Dihydroacridine-, and Acridone-Based Boron Difluoride Complexes: Synthesis and Structure-Property Relationships.

Boron difluoride complexes of phenothiazine, 9,10-dihydroacridine, and acridone derivatives with a pyridyl group were synthesized. Their structures, dynamic conformational behaviors, and photophysical properties, including solid-state fluorescence, were disclosed. The effects of the oxidation state of the sulfur atom in the phenothiazine derivatives on their properties were also clarified.

Mechanistic Insight into Palladium/Brønsted Acid Catalyzed Methoxycarbonylation and Hydromethoxylation of Internal Alkene: A Computational Study.

Density functional theory (DFT) calculations were performed to study the palladium/Brønsted acid-catalyzed methoxycarbonylation and hydromethoxylation reactions of internal alkene. The calculated results show that the pyridyl group (N atom) in bidentate phosphine ligand with built-in base (L1) plays a crucial role in controlling the selectivity. With the help of the pyridyl group, the methanolysis steps in the methoxycarbonylation reaction and the hydromethoxylation reaction become easy, and both the linear ester methyl 3,4-dimethylpentanoate (P1) and the hydromethoxylation product 2-methoxy-2,3-dimethylbutane (P2) could be obtained. In contrast, the possibility of leading to branched ester P1' was ruled out according to our calculations. The steric effect could account for the observed selectivity. In the presence of the DPEphos ligand (L2) that does not bear the pyridyl group, the methanolysis step in the methoxycarbonylation reaction becomes the rate-determining step with a high overall energy barrier. Neither linear nor branched methoxycarbonylation product could be generated. The palladium/Brønsted acid co-catalyzed hydromethoxylation also become difficult without the assistance of the pyridyl group in the presence of the L2 ligand. Instead, TsOH-catalyzed hydromethoxylation reaction could take place to generate the ether product P2.