Showing papers on "Intramolecular force published in 2016"

Catalytic Ring-Opening of Cyclic Alcohols Enabled by PCET Activation of Strong O-H Bonds

Abstract: We report a new photocatalytic protocol for the redox-neutral isomerization of cyclic alcohols to linear ketones via C–C bond scission. Mechanistic studies demonstrate that key alkoxy radical intermediates in this reaction are generated via the direct homolytic activation of alcohol O–H bonds in an unusual intramolecular PCET process, wherein the electron travels to a proximal radical cation in concert with proton transfer to a weak Bronsted base. Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.

A Direct Mechanism of Ultrafast Intramolecular Singlet Fission in Pentacene Dimers

Abstract: Interest in materials that undergo singlet fission (SF) has been catalyzed by the potential to exceed the Shockley–Queisser limit of solar power conversion efficiency. In conventional materials, the mechanism of SF is an intermolecular process (xSF), which is mediated by charge transfer (CT) states and depends sensitively on crystal packing or molecular collisions. In contrast, recently reported covalently coupled pentacenes yield ∼2 triplets per photon absorbed in individual molecules: the hallmark of intramolecular singlet fission (iSF). However, the mechanism of iSF is unclear. Here, using multireference electronic structure calculations and transient absorption spectroscopy, we establish that iSF can occur via a direct coupling mechanism that is independent of CT states. We show that a near-degeneracy in electronic state energies induced by vibronic coupling to intramolecular modes of the covalent dimer allows for strong mixing between the correlated triplet pair state and the local excitonic state, d...

Reduced Intramolecular Twisting Improves the Performance of 3D Molecular Acceptors in Non-Fullerene Organic Solar Cells.

Abstract: A small-molecular acceptor, tetraphenylpyrazine-perylenediimide tetramer (TPPz-PDI4 ), which has a reduced extent of intramolecular twisting compared to two other small-molecular acceptors is designed. Benefiting from the lowest extent of intramolecular twisting, TPPz-PDI4 exhibits the highest aggregation tendency and electron mobility, and therefore achieves a highest power conversion efficiency of 7.1%.

Rhodium-catalyzed C–C coupling reactions via double C–H activation

Abstract: Various rhodium-catalyzed double C–H activations are reviewed. These powerful strategies have been developed to construct C–C bonds, which might be widely embedded in complex aza-fused heterocycles, polycyclic skeletons and heterocyclic scaffolds. In particular, rhodium(III) catalysis shows good selectivity and reactivity to functionalize the C–H bond, generating reactive organometallic intermediates in most of the coupling reactions. Generally, intermolecular, intramolecular and multi-component coupling reactions via double C–H activations with or without heteroatom-assisted chelation are discussed in this review.

Diastereoselective [3+2] Annulation of Aromatic/Vinylic Amides with Bicyclic Alkenes through Cobalt-Catalyzed C−H Activation and Intramolecular Nucleophilic Addition

Abstract: A highly diastereoselective method for the synthesis of dihydroepoxybenzofluorenone derivatives from aromatic/vinylic amides and bicyclic alkenes is described. This new transformation proceeds through cobalt-catalyzed C-H activation and intramolecular nucleophilic addition to the amide functional group. Transition-metal-catalyzed C-H activation reactions of secondary amides with alkenes usually lead to [4+2] or [4+1] annulation; to the best of our knowledge, this is the first time that a [3+2] cycloaddition is described in this context. The reaction proceeds under mild conditions and tolerates a wide range of functional groups. Mechanistic studies imply that the C-H bond cleavage may be the rate-limiting step.

Magnesium-catalysed nitrile hydroboration

Abstract: A β-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin). Stoichiometric reactivity studies indicate that catalytic turnover ensues through the generation of magnesium aldimido, aldimidoborate and borylamido intermediates, which are formed in a sequence of intramolecular nitrile insertion and inter- and intramolecular B–H metathesis events. Kinetic studies highlight variations in mechanism for the catalytic dihydroboration of alkyl nitriles, aryl nitriles bearing electron withdrawing (Ar(EWG)CN) and aryl nitriles bearing electron donating (Ar(EDG)CN) substitution patterns. Kinetic isotope effects (KIEs) for catalysis performed with DBpin indicate that B–H bond breaking and C–H bond forming reactions are involved in the rate determining processes during the dihydroboration of alkyl nitriles and Ar(EDG)CN substrates, which display divergent first and second order rate dependences on [HBpin] respectively. In contrast, the hydroboration of Ar(EWG)CN substrates provides no KIE and HBpin is not implicated in the rate determining process during catalysis. Irrespective of these differences, a common mechanism is proposed in which the rate determining steps are deduced to vary through the establishment of several pre-equilibria, the relative positions of which are determined by the respective stabilities of the dimeric and monomeric magnesium aldimide and magnesium aldimidoborate intermediates as a result of adjustments to the basicity of the nitrile substrate. More generally, these observations indicate that homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate previously unappreciated mechanistic diversity.

Rh-Catalyzed Intermolecular Reactions of α-Alkyl-α-Diazo Carbonyl Compounds with Selectivity over β-Hydride Migration

Abstract: ConspectusRh-carbenes derived from α-diazocarbonyl compounds have found broad utility across a remarkable range of reactivity, including cyclopropanation, cyclopropenation, C–H insertions, heteroatom–hydrogen insertions, and ylide forming reactions. However, in contrast to α-aryl or α-vinyl-α-diazocarbonyl compounds, the utility of α-alkyl-α-diazocarbonyl compounds had been moderated by the propensity of such compounds to undergo intramolecular β-hydride migration to give alkene products. Especially challenging had been intermolecular reactions involving α-alkyl-α-diazocarbonyl compounds.This Account discusses the historical context and prior limitations of Rh-catalyzed reactions involving α-alkyl-α-diazocarbonyl compounds. Early studies demonstrated that ligand and temperature effects could influence chemoselectivity over β-hydride migration. However, effects were modest and conflicting conclusions had been drawn about the influence of sterically demanding ligands on β-hydride migration. More recent adva...

Enantioselective Intramolecular C-H Insertion of Donor and Donor/Donor Carbenes by a Nondiazo Approach.

Abstract: The first enantioselective intramolecular C−H insertion and cyclopropanation reactions of donor- and donor/donor-carbenes by a nondiazo approach are reported. The reactions were conducted in a one-pot manner without slow addition and provided the desired dihydroindole, dihydrobenzofuran, tetrahydrofuran, and tetrahydropyrrole derivatives with up to 99 % ee and 100 % atom efficiency.

A novel non-fluorescent excited state intramolecular proton transfer phenomenon induced by intramolecular hydrogen bonds: an experimental and theoretical investigation

Abstract: Two molecules, 1-hydroxypyrene-2-carbaldehyde (HP) and 1-methoxypyrene-2-carbaldehyde (MP) were explored. We investigated their photophysical properties, using experimental transient absorption and theoretical density functional theory/time-dependent density functional theory (DFT/TDDFT). HP and MP have similar geometric conformations but exhibit entirely different photophysical properties upon excitation into the S1 state. In contrast to traditional excited state intramolecular proton transfer (ESIPT) in molecules that exhibit either single or dual fluorescence, HP has an unusual non-fluorescent property. Specifically, the ultrafast ESIPT process occurs in 158 fs and is followed by an intersystem crossing (ISC) component of 11.38 ps. In contrast to HP, MP undergoes only an 8 ps timescale process, which was attributed to interactions between solute and solvent. We concluded that this difference arises from intramolecular hydrogen bonds (IMHBs), which induce drastic structural alterntion upon excitation.

Excited-state intramolecular proton-transfer reaction demonstrating anti-Kasha behavior

Abstract: We report unusual photophysical properties observed on two newly designed 3-hydroxychromone derivatives exhibiting the excited-state intramolecular proton transfer (ESIPT) reaction. The efficiency of ESIPT reaction is greatly enhanced upon excitation with high energy quanta to Sn (n > 1) levels in low-polarity solvents. Based on detailed analyses of excitation and emission spectra as well as time-resolved emission kinetics we derive that conditions, in which this phenomenon contradicting Kasha's rule is observed, are quite different from that for observation of anti-Kasha emission.

Cobalt(III)-Catalyzed [5 + 1] Annulation for 2H-Chromenes Synthesis via Vinylic C–H Activation and Intramolecular Nucleophilic Addition

Abstract: A new cobalt-catalyzed phenolic OH-assisted C–H functionalization of 2-vinylphenols with allenes to give various 2H-chromenes is described. It is the first time that allenes are used as the coupling partners in the cobalt-catalyzed C–H activation reactions. In most cases, cobalt-catalyzed oxidative annulation of arenes with alkenes or alkynes via C–H activation gave [4 + 2] or [3 + 2] cyclization products, but the present catalytic reaction afforded an oxidative [5 + 1] cyclization products with the allenes acting as a one-carbon coupling partner. The catalytic reaction is proposed to proceed via the C–H activation of the vinyl group, allene insertion, and an unusual intramolecular regioselective phenoxide addition.

Ruthenium‐Catalyzed Hydroarylation and One‐Pot Twofold Unsymmetrical C−H Functionalization of Arenes

Abstract: A methyl phenyl sulfoximine (MPS) is used as a directing group in the ruthenium-catalyzed intramolecular hydroarylation of alkene-tethered benzoic acid derivatives to afford dihydrobenzofurans and indolines in good to excellent yields. A one-pot, unsymmetrical, twofold C-H functionalization involving intramolecular C-C and intermolecular C-C/C-N bond formations is successfully demonstrated by using a single set of catalytic reaction conditions, which is unprecedented thus far. A novel isoquinolone-bearing dihydrobenzofuran is constructed through an unsymmetrical twofold C-H functionalization.

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers.

Abstract: We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10(12) s(-1), respectively, in room temperature solutions.

Trapping σ-Alkyl-Palladium(II) Intermediates with Arynes Encompassing Intramolecular C-H Activation: Spirobiaryls through Pd-Catalyzed Cascade Reactions.

Abstract: A palladium-catalyzed cascade reaction based on the trapping of transient alkyl–PdII intermediates with arynes encompassing a C−H activation step has been developed. This synthetic pathway gives rise to hetero-spirocyclic scaffolds containing a biaryl motif, and opens up new synthetic strategies in the design of cascade reactions since it gathers several aspects of Pd chemistry, i.e., intra- and intermolecular carbopalladation of unsaturated species, C−H activation and C−C coupling processes.

Construction of spirocarbocycles via gold-catalyzed intramolecular dearomatization of naphthols

Abstract: A highly efficient, gold-catalyzed intramolecular dearomatization reaction of naphthols via 5-endo-dig cyclization is described. This facile and direct approach furnishes spirocarbocycles in excellent yields under mild conditions.

Estimating the energy of intramolecular hydrogen bonds from 1H NMR and QTAIM calculations

Abstract: The values of the downfield chemical shift of the bridge hydrogen atom were estimated for a series of compounds containing an intramolecular hydrogen bond O–H⋯O, O–H⋯N, O–H⋯Hal, N–H⋯O, N–H⋯N, C–H⋯O, C–H⋯N and C–H⋯Hal. Based on these values, the empirical estimation of the hydrogen bond energy was obtained by using known relationships. For the compounds containing an intramolecular hydrogen bond, the DFT B3LYP/6-311++G(d,p) method was used both for geometry optimization and for QTAIM calculations of the topological parameters (electron density ρBCP and the density of potential energy V in the critical point of the hydrogen bond). The calculated geometric and topological parameters of hydrogen bonds were also used to evaluate the energy of the hydrogen bond based on the equations from the literature. Comparison of calibrating energies from the 1H NMR data with the energies predicted by calculations showed that the most reliable are the linear dependence on the topological ρBCP and V parameters. However, the correct prediction of the hydrogen bond energy is determined by proper fitting of the linear regression coefficients. To obtain them, new linear relationships were found between the calculated ρBCP and V parameters and the hydrogen bond energies obtained from empirical 1H NMR data. These relationships allow the comparison of the energies of different types of hydrogen bonds for various molecules and biological ensembles.

Chemo-, Diastereo-, and Enantioselective Iridium-Catalyzed Allylic Intramolecular Dearomatization Reaction of Naphthol Derivatives.

Abstract: An iridium-catalyzed intramolecular asymmetric allylic dearomatization reaction of naphthol derivatives is described. Challenges confronted in this reaction include chemoselectivity between carbon and oxygen atoms as nucleophilic centers, diastereoselectivity when contiguous chiral centers are generated, and enantioselective control for constructing an all-carbon quaternary stereocenter. In the presence of an iridium catalyst generated from [{Ir(dbcot)Cl}2] (dbcot=dibenzocyclooctatetraene) and a new THQphos (tetrahydroquinolinedinaphthophosphoramidite) ligand, various spironaphthalenones were obtained with up to greater than 95:5 C/O selectivity, greater than 95:5 d.r., and 99 % ee, thus providing a general method for the dearomatization of naphthols.

Hydrogen bond cooperativity and anticooperativity within the water hexamer

Abstract: The hydrogen bond (HB), arguably the most important non-covalent interaction in chemistry, is getting renewed attention particularly in materials engineering. We address herein HB non-additive features by examining different structures of the water hexamer (cage, prism, book, bag and ring). To that end, we rely on the interacting quantum atoms (IQA) topological energy partition, an approach that has been successfully used to study similar effects in smaller water clusters (see Chem. – Eur. J., 19, 14304). Our IQA interaction energies, , are used to classify the strength of HBs in terms of the single/double character of the donor and acceptor H2O molecules involved in the interaction. The strongest hydrogen bonds on this new scale entail double donors and acceptors that show larger values of than those observed in homodromic cycles, paradigms of cooperative effects. Importantly, this means that besides the traditional HB anticooperativity ascribed to double acceptors and donors, the occurrence of these species is also related to HB strengthening. Overall, we hope that the results of this research will lead to a further understanding of the HB non-additivity in intramolecular and intermolecular interactions.

Catalytic Asymmetric Total Synthesis of (-)-Actinophyllic Acid.

Abstract: Described herein is a catalytic asymmetric total synthesis of (-)-actinophyllic acid, with the key step being a chiral phosphine-catalyzed [3 + 2] annulation between an imine and an allenoate to form a pyrroline intermediate in 99% yield and 94% ee. The synthesis also features CuI-catalyzed coupling between a ketoester and a 2-iodoindole to shape the tetrahydroazocine ring; intramolecular alkylative lactonization; SmI2-mediated intramolecular pinacol coupling between ketone and lactone subunits to assemble the complex skeleton of (-)-actinophyllic acid; and an unprecedented regioselective dehydroxylation.

Metal-Free sp2-C–H Borylation as a Common Reactivity Pattern of Frustrated 2-Aminophenylboranes

Abstract: C–H borylation is a powerful and atom-efficient method for converting affordable and abundant chemicals into versatile organic reagents used in the production of fine chemicals and functional materials. Herein we report a facile C–H borylation of aromatic and olefinic C–H bonds with 2-aminophenylboranes. Computational and experimental studies reveal that the metal-free C–H insertion proceeds via a frustrated Lewis pair mechanism involving heterolytic splitting of the C–H bond by cooperative action of the amine and boryl groups. The adapted geometry of the reactive B and N centers results in an unprecedentently low kinetic barrier for both insertion into the sp2-C–H bond and intramolecular protonation of the sp2-C–B bond in 2-ammoniophenyl(aryl)- or -(alkenyl)borates. This common reactivity pattern serves as a platform for various catalytic reactions such as C–H borylation and hydrogenation of alkynes. In particular, we demonstrate that simple 2-aminopyridinium salts efficiently catalyze the C–H borylation...

Iron‐Catalyzed Intramolecular C(sp2)−H Amination

Abstract: The nucleophilic iron complex Bu4N[Fe(CO)3(NO)] (TBA[Fe]) catalyzes the direct intramolecular C-H amination of α-azidobiaryls and (azidoaryl)alkenes into the corresponding carbazoles and indoles, respectively, under mild conditions and with low catalyst loadings. These features and the broad functional-group tolerance render this method a particularly attractive alternative to established noble-metal-based procedures.

Both Inter- and Intramolecular Coupling of O-H Groups Determine the Vibrational Response of the Water/Air Interface.

Abstract: Vibrational coupling is relevant not only for dissipation of excess energy after chemical reactions but also for elucidating molecular structure and dynamics It is particularly important for O–H stretch vibrational spectra of water, for which it is known that in bulk both intra- and intermolecular coupling alter the intensity and line shape of the spectra In contrast with bulk, the unified picture of the inter/intra-molecular coupling of O–H groups at the water–air interface has been lacking Here, combining sum-frequency generation experiments and simulation for isotopically diluted water and alcohols, we unveil effects of inter- and intramolecular coupling on the vibrational spectra of interfacial water Our results show that both inter- and intramolecular coupling contribute to the O–H stretch vibrational response of the neat H2O surface, with intramolecular coupling generating a double-peak feature, while the intermolecular coupling induces a significant red shift in the O–H stretch response

Benzazetidine synthesis via palladium-catalysed intramolecular C−H amination

Abstract: Hampered by a lack of practical syntheses, benzazetidines are one of the few rarely explored compounds in N-heterocyclic chemical space. An efficient synthesis of various benzazetidines by Pd-catalysed intramolecular C−H amination of N-benzyl picolinamides is now reported. Reagent-controlled reductive elimination of a Pd intermediate is a key aspect of this particular method.

An Oxidant-Free Strategy for Indole Synthesis via Intramolecular C–C Bond Construction under Visible Light Irradiation: Cross-Coupling Hydrogen Evolution Reaction

Abstract: We describe here an oxidant-free strategy to synthesize indoles, i.e., under visible-light irradiation (λ = 450 nm), catalytic amounts of an iridium(III) photosensitizer and cobaloxime catalyst transform various N-aryl enamines exclusively into indoles. Our methodology affords indoles in good to excellent yields under mild reaction conditions and produces H2 as the only byproduct. Spectroscopic and electrochemical studies verify that the designed system proceeds via visible-light-catalyzed oxidation of N-aryl enamines, followed by intramolecular radical addition to yield the indoles smoothly under ambient conditions.

Rhodium‐Catalyzed Synthesis of 4‐Bromo‐1,2‐dihydroisoquinolines: Access to Bromonium Ylides by the Intramolecular Reaction of a Benzyl Bromide and an α‐Imino Carbene

Abstract: Highly functionalized 4-bromo-1,2-dihydroisoquinolines were synthesized from readily available 4-(2-(bromomethyl)phenyl)-1-sulfonyl-1,2,3-triazoles. A bromonium ylide is proposed as the key intermediate, which can be formed by the intramolecular nucleophilic attack of the benzyl bromide on the α-imino rhodium carbene formed in the presence of the rhodium catalyst.

Planarized Intramolecular Charge Transfer: A Concept for Fluorophores with both Large Stokes Shifts and High Fluorescence Quantum Yields.

Abstract: Fluorophores were successfully used in several areas of chemistry and biochemistry. For many purposes, however, it is necessary that the fluorescence compound features a high fluorescence quantum yield as well as a large Stokes shift. The latter is, for example, achieved by the use of a twisted intramolecular charge-transfer (TICT) compound, which shows a twisted geometry in the excited state. However, the higher the twisting is, the lower becomes in general the fluorescence quantum yield as the resulting emission from the twisted state is forbidden. In order to escape this dilemma, we propose the model of planarized intramolecular charge-transfer (PLICT) states. These compounds are completely twisted in the ground states and planar in the excited states. By means of quantum chemical calculations (time-dependent (TD)-B3LYP and CC2) and experimental studies, we could demonstrate that 1-aminoindole and its derivatives form photoinduced PLICT states. They show both very large Stokes shifts (ν˜ =9000–13 500 cm−1, i.e., λ=100–150 nm) and high fluorescence quantum yields. These characteristics and their easy availability starting from the corresponding indoles, make them very attractive for the use as optical switches in various fields of chemistry as well as biological probes.