Showing papers on "Isomerization published in 2018"

Rhodium-Catalyzed, Remote Terminal Hydroarylation of Activated Olefins through a Long-Range Deconjugative Isomerization

Abstract: The Rh-catalyzed, remote terminal hydroarylation of active olefins at the C7-position of indoles and the ortho-position of indolines and anilines with the appropriate choice of a N-PtBu2 directing group through long-range deconjugative isomerization has been reported. This transformation not only overcomes the conjugate rule of Michael acceptors but also controls the positional selectivity of indoles, representing a significant advancement in both alkene isomerization and the C–H alkylation of indoles.

Asymmetric remote C-H borylation of internal alkenes via alkene isomerization.

Abstract: Recent years have witnessed the growing interest in the remote functionalization of alkenes for it offers a strategy to activate the challenging C-H bonds distant from the initiation point via alkene isomerization/functionalization. However, the catalytic enantioselective isomerization/functionalization with one single transition metal catalyst remains rare. Here we report a highly regio- and enantioselective cobalt-catalyzed remote C-H bond borylation of internal alkenes via sequential alkene isomerization/hydroboration. A chiral ligand featured twisted pincer, anionic, and non-rigid characters is designed and used for this transformation. This methodology, which is operationally simple using low catalyst loading without additional activator, shows excellent enantioselectivity and can be used to convert various internal alkenes with regio- and stereoisomers to valuable chiral secondary organoboronates with good functional group tolerance.

Switching the Proton Conduction in Nanoporous, Crystalline Materials by Light.

Abstract: Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.

Contra-Thermodynamic, Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two-Dimensional Chemical Space

Abstract: Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z-α-substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp2 )-B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E-α-substituted styrenyl BPins to generate the corresponding Z-isomers with high fidelity. Preliminary validation of this contra-thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z-configured trisubstituted alkenes, as well as in a concise synthesis of the anti-tumor agent Combretastatin A4.

Wavelength‐Controlled Dynamic Metathesis: A Light‐Driven Exchange Reaction between Disulfide and Diselenide Bonds

Abstract: Wavelength-controlled dynamic processes are mostly based on light-triggered isomerization or the cleavage/formation of molecular connections. Control over dynamic metathesis reactions by different light wavelengths, which would be useful in controllable dynamic chemistry, has rarely been studied. Taking advantage of the different bond energies of disulfide and diselenide bonds, we have developed a wavelength-driven exchange reaction between disulfides and diselenides, which underwent metathesis under UV light to produce Se-S bonds. When irradiated with visible light, the Se-S bonds were reversed back to those of the original reactants. The conversion of the exchange depends on the wavelength of the incident light. This light-driven metathesis chemistry was also applied to tune the mechanical properties of polymer materials. The visible-light-induced reverse reaction was compatible with reductant-catalyzed disulfide/diselenide metathesis, and could be utilized to develop a dissipative system with light as the energy input.

Cobalt-Catalyzed Regioselective Olefin Isomerization Under Kinetic Control

Abstract: Olefin isomerization is a significant transformation in organic synthesis, which provides a convenient synthetic route for internal olefins and remote functionalization processes. The selectivity of an olefin isomerization process is often thermodynamically controlled. Thus, to achieve selectivity under kinetic control is very challenging. Herein, we report a novel cobalt-catalyzed regioselective olefin isomerization reaction. By taking the advantage of fine-tunable NNP-pincer ligand structures, this catalytic system features high kinetic control of regioselectivity. This mild catalytic system enables the isomerization of 1,1-disubstituted olefins bearing a wide range of functional groups in excellent yields and regioselectivity. The synthetic utility of this transformation was highlighted by the highly selective preparation of a key intermediate for the total synthesis of minfiensine. Moreover, a new strategy was developed to realize the selective monoisomerization of 1-alkenes to 2-alkenes dictated by i...

A Chromium Hydroxide/MIL-101(Cr) MOF Composite Catalyst and Its Use for the Selective Isomerization of Glucose to Fructose

Abstract: A metal–organic framework (MOF)‐based catalyst, chromium hydroxide/MIL‐101(Cr), was prepared by a one‐pot synthesis method. The combination of chromium hydroxide particles on and within Lewis acidic MIL‐101 accomplishes highly selective conversion of glucose to fructose in the presence of ethanol, matching the performance of optimized Sn‐containing Lewis acidic zeolites. Differently from zeolites, NMR spectroscopy studies with isotopically labeled molecules demonstrate that isomerization of glucose to fructose on this catalyst, proceeds predominantly via a proton transfer mechanism.

Thermally-induced reversible structural isomerization in colloidal semiconductor CdS magic-size clusters.

Abstract: Structural isomerism of colloidal semiconductor nanocrystals has been largely unexplored. Here, we report one pair of structural isomers identified for colloidal nanocrystals which exhibit thermally-induced reversible transformations behaving like molecular isomerization. The two isomers are CdS magic-size clusters with sharp absorption peaks at 311 and 322 nm. They have identical cluster masses, but slightly different structures. Furthermore, their interconversions follow first-order unimolecular reaction kinetics. We anticipate that such isomeric kinetics are applicable to a variety of small-size functional nanomaterials, and that the methodology developed for our kinetic study will be helpful to investigate and exploit solid–solid transformations in other semiconductor nanocrystals. The findings on structural isomerism should stimulate attention toward advanced design and synthesis of functional nanomaterials enabled by structural transformations. Few structural isomers of colloids, with identical masses but different structures, have been identified. Here, the authors observe an interesting example of structural isomerism in a pair of semiconductor magic-size clusters, which reversibly transform between one another with first-order unimolecular reaction kinetics.

A proline switch explains kinetic heterogeneity in a coupled folding and binding reaction

Abstract: The interactions of intrinsically disordered proteins (IDPs) with their molecular targets are essential for the regulation of many cellular processes. IDPs can perform their functions while disordered, and they may fold to structured conformations on binding. Here we show that the cis/trans isomerization of peptidyl-prolyl bonds can have a pronounced effect on the interactions of IDPs. By single-molecule spectroscopy, we identify a conserved proline residue in NCBD (the nuclear-coactivator binding domain of CBP) whose cis/trans isomerization in the unbound state modulates the association and dissociation rates with its binding partner, ACTR. As a result, NCBD switches on a time scale of tens of seconds between two populations that differ in their affinities to ACTR by about an order of magnitude. Molecular dynamics simulations indicate as a cause reduced packing of the complex for the cis isomer. Peptidyl-prolyl cis/trans isomerization may be an important previously unidentified mechanism for regulating IDP interactions.

Palladium-Catalyzed Selective Generation of CO from Formic Acid for Carbonylation of Alkenes

Abstract: A general and selective palladium-catalyzed alkoxycarbonylation of all kinds of alkenes with formic acid (HCOOH, FA) is described. Terminal, di-, tri-, and tetra-substituted including functionalized olefins are converted into linear esters with high yields and regioselectivity. Key-to-success is the use of specific palladium catalysts containing ligands with built-in base, e.g., L5. Comparison experiments demonstrate that the active catalyst system not only facilitates isomerization and carbonylation of alkenes but also promotes the selective decomposition of HCOOH to CO under mild conditions.

Multicatalytic Stereoselective Synthesis of Highly Substituted Alkenes by Sequential Isomerization/Cross-Coupling Reactions.

Abstract: Starting from readily available alkenyl methyl ethers, the stereoselective preparation of highly substituted alkenes by two complementary multicatalytic sequential isomerization/cross-coupling sequences is described. Both elementary steps of these sequences are challenging processes when considered independently. A cationic iridium catalyst was identified for the stereoselective isomerization of allyl methyl ethers and was found to be compatible with a nickel catalyst for the subsequent cross-coupling of the in situ generated methyl vinyl ethers with various Grignard reagents. The method is compatible with sensitive functional groups and a multitude of olefinic substitution patterns to deliver products with high control of the newly generated C═C bond. A highly enantioselective variant of this [Ir/Ni] sequence has been established using a chiral iridium precatalyst. A complementary [Pd/Ni] catalytic sequence has been optimized for alkenyl methyl ethers with a remote C═C bond. The final alkenes were isolat...

Proton-Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans.

Abstract: Spiropyrans undergo Cspiro–O bond breaking to their ring-open protonated E-merocyanine form upon protonation and irradiation via an intermediate protonated Z-merocyanine isomer. We show that the extent of acid-induced ring opening is controlled by matching both the concentration and strength of the acid used and with strong acids full ring opening to the Z-merocyanine isomer occurs spontaneously allowing its characterization by 1H NMR spectroscopy as well as UV/vis spectroscopy, and reversible switching between Z/E-isomerization by irradiation with UV and visible light. Under sufficiently acidic conditions, both E- and Z-isomers are thermally stable. Judicious choice of acid such that its pKa lies between that of the E- and Z-merocyanine forms enables thermally stable switching between spiropyran and E-merocyanine forms and hence pH gating between thermally irreversible and reversible photochromic switching.

Tuning the Thermal Isomerization of Phenylazoindole Photoswitches from Days to Nanoseconds.

Abstract: The growing interest in light-driven molecular switches and optical oscillators led to the development of molecules that are able to interconvert from a stable to a metastable configuration upon photochemical triggering and to return to the thermodynamically stable form as soon as the light stimulus is removed. Controlling a wide range of back-isomerization lifetimes in the dark is a crucial goal for potential application of these compounds such as molecular machines. We herein present a novel class of easily synthesizable azo photoswitches based on the arylazoindole core. Most notably, minimal modifications of the core, such as methylation, dramatically change the Z-to-E thermal isomerization rate from days (Me in position 1) to the nanosecond range (Me in position 2). Moreover, fine-tuning of the Z-to-E lifetimes can be achieved choosing a proper dimethyl sulfoxide-water (or buffered water) solvent mixture. The photochemical and thermal mechanisms have been elucidated by a thorough computational and spectroscopic analysis. This allowed to detect three different pathways of thermal isomerization and to identify the hydrazone tautomer of the phenylazoindole as the major actor in the fast Z-E thermal isomerization of the NH-substituted switch in protic media.

Dominant Role of Entropy in Stabilizing Sugar Isomerization Transition States within Hydrophobic Zeolite Pores.

Abstract: Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than within hydrophilic micropores; however, relative contri...

Pushing the Limits on Metal-Organic Frameworks as a Catalyst Support: NU-1000 Supported Tungsten Catalysts for o-Xylene Isomerization and Disproportionation.

Abstract: Acid-catalyzed skeletal C–C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Bronsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal–organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WOx clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000...

Catalytic conversion of glucose to 5-hydroxymethyfural over Fe/β zeolites with extra-framework isolated Fe species in a biphasic reaction system

Abstract: A series of Fe/β zeolite catalysts with various Fe/Al ratios were prepared by liquid ion-exchanged method and were characterized by N2 adsorption, XRD, UV–Vis DRS, EPR, NH3-TPD, and FTIR of pyridine adsorption techniques. The catalysts were used to catalyze the transformation of glucose into 5-hydorxymethyfural (HMF) in a biphasic reaction system under mild reaction conditions. The Fe/β catalysts with Lewis acidic Fe sites is able to effectively improve the HMF yield compared to H-β zeolite under identical reaction conditions, which is ascribed to the enhanced isomerization activity of glucose-to-fructose by Lewis acidic Fe species. Moreover, the Fe/Al ratio had a remarkable impact on the conversion of glucose and the yield of HMF, indicating that the synergetic effect of Lewis and Bronsted acid sites is critical to achieving high HMF yield. Under optimal reaction conditions, Fe/β-0.06 catalyst afforded 61% HMF yield at glucose conversion of 95% after the reaction of 90 min at 120 °C, which is one of the most effective catalysts for the conversion of glucose into HMF in a biphasic medium up to now. Furthermore, extra-framework isolated Fe species bound to the framework Al sites, are most likely the active sites for the isomerization of glucose-to-fructose in the conversion of glucose. The catalysts were very stable and could be reused at least five runs without significant loss in the catalytic activity.

Palladium-catalysed anti-Markovnikov selective oxidative amination.

Abstract: In recent years, the synthesis of amines and other nitrogen-containing motifs has been a major area of research in organic chemistry because they are widely represented in biologically active molecules. Current strategies rely on a multistep approach and require one reactant to be activated prior to the carbon-nitrogen bond formation. This leads to a reaction inefficiency and functional group intolerance. As such, a general approach to the synthesis of nitrogen-containing compounds from readily available and benign starting materials is highly desirable. Here we present a palladium-catalysed oxidative amination reaction in which the addition of the nitrogen occurs at the less-substituted carbon of a double bond, in what is known as anti-Markovnikov selectivity. Alkenes are shown to react with imides in the presence of a palladate catalyst to generate the terminal imide through trans-aminopalladation. Subsequently, olefin isomerization occurs to afford the thermodynamically favoured products. Both the scope of the transformation and mechanistic investigations are reported.

Rhodium-Catalyzed Enantioconvergent Isomerization of Homoallylic and Bishomoallylic Secondary Alcohols

Abstract: We present herein an unprecedented enantioselective isomerization of homoallylic and bishomoallylic secondary alcohols, catalyzed by a commercially available rhodium-complex and a base. This catalytic redox-neutral process provides an effective access to chiral ketones in high efficiency and enantioselectivity, without the use of any stoichiometric reagent or generation of any waste. For the reaction of homoallylic alcohols, this system achieved not only a stereoconvergent access to chiral ketones bearing a β-stereocenter (up to 95%, 86% ee) but also a concomitant oxidative kinetic resolution of the alcohol substrates (S > 20). In the case of bishomoallylic alcohols, an intriguing ligand-induced divergent reactivity was observed. A terminal-to-internal alkene isomerization promoted by Rh/L7 followed by redox isomerization using Rh/BINAP system produced chiral ketones bearing a γ-stereocenter with high yield and enantioselectivity. Mechanistic studies provided strong support for the redox-isomerization pathway with chain walking of the key alkyl-Rh intermediate.

Lewis Acid Triggered Vinylcyclopropane–Cyclopentene Rearrangement

Abstract: We report a mild Lewis acid induced isomerization of donor–acceptor cyclopropanes, containing an alkenyl moiety and diverse electron-withdrawing group(s) at the adjacent positions, into substituted cyclopentenes. We have found that 1,1,2-trisubstituted cyclopent-3-enes were exclusively obtained in yield of 51–99% when cyclopropanes with a 2-substituted alkenyl group as a donor underwent isomerization. For cyclopropanes bearing a trisubstituted alkenyl group either the corresponding cyclopent-3-enes or isomeric cyclopent-2-enes having two acceptor groups at the C(1) atom were formed, with the reaction selectivity being determined by the applied Lewis acid. We have shown that the reactivity of the donor–acceptor cyclopropane increases with the increase of the electron-donating character of (hetero)aromatic group attached to the alkenyl moiety. The synthetic utility of the developed methodology was also demonstrated through the synthesis of polysubstituted cyclopentane and piperidine derivatives.

How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5

Abstract: Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C4–C8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy ...

Catalytic Asymmetric Dearomatization of Indolyl Dihydropyridines through an Enamine Isomerization/Spirocyclization/Transfer Hydrogenation Sequence

Abstract: A highly efficient synthesis of enantioenriched spiroindolines by catalytic asymmetric dearomatization of indolyl dihydropyridines through a chiral phosphoric acid catalyzed enamine isomerization/spirocyclization/transfer hydrogenation sequence has been developed. This reaction proceeds under mild reaction conditions, affording novel spiroindolines in good yields (up to 88 %) with excellent enantioselectivity (up to 97 % ee). DFT calculations provide insights into the reaction mechanism as well as the origin of stereochemistry.

Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam

Abstract: This contribution studies the steam-assisted dealkylation of 4-n-propylphenol (4-n-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (e.g., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (i) analyzing products distribution under varying temperature and contact time conditions, (ii) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (iii) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed...

VUV Photoionization Study of the Formation of the Simplest Polycyclic Aromatic Hydrocarbon: Naphthalene (C10H8)

Abstract: The formation of the simplest polycyclic aromatic hydrocarbon (PAH), naphthalene (C10H8), was explored in a high-temperature chemical reactor under combustion-like conditions in the phenyl (C6H5)–vinylacetylene (C4H4) system. The products were probed utilizing tunable vacuum ultraviolet light by scanning the photoionization efficiency (PIE) curve at a mass-to-charge m/z = 128 (C10H8+) of molecules entrained in a molecular beam. The data fitting with PIE reference curves of naphthalene, 4-phenylvinylacetylene (C6H5CCC2H3), and trans-1-phenylvinylacetylene (C6H5CHCHCCH) indicates that the isomers were generated with branching ratios of 43.5±9.0 : 6.5±1.0 : 50.0±10.0%. Kinetics simulations agree nicely with the experimental findings with naphthalene synthesized via the hydrogen abstraction–vinylacetylene addition (HAVA) pathway and through hydrogen-assisted isomerization of phenylvinylacetylenes. The HAVA route to naphthalene at elevated temperatures represents an alternative pathway to the hydrogen abstract...

Evidence for a Single Electron Shift in a Lewis Acid-Base Reaction.

Abstract: The Lewis acid–base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B(C6F5)3) to give the conventional B–Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV–vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid–base product. The ...