Mechanism Studies of the Conversion of 13C‐Labeled n‐Butane on Zeolite H‐ZSM‐5 by Using 13C Magic Angle Spinning NMR Spectroscopy and GC–MS Analysis

By using ¹³C MAS NMR spectroscopy (MAS=magic angle spinning), the conversion of selectively ¹³C‐labeled n‐butane on zeolite H‐ZSM‐5 at 430–470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective ¹³C‐label in the n‐butane molecule, and 2) oligomerization–cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl‐substituted cyclopentenyl cations and condensed aromatic compounds. In situ ¹³C MAS NMR and complementary ex situ GC–MS data provided evidence for a monomolecular mechanism of the ¹³C‐label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to ¹³C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E_a=75 kJ mol^?1 for the scrambling and 71 kJ mol^?1 for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n‐butane as being the rate‐determining stage of the n‐butane conversion on zeolite H‐ZSM‐5.     

Coupling of Methanol and Carbon Monoxide over H‐ZSM‐5 to Form Aromatics

The conversion of methanol into aromatics over unmodified H‐ZSM‐5 zeolite is generally not high because the hydrogen transfer reaction results in alkane formation. Now circa 80 % aromatics selectivity for the coupling reaction of methanol and carbon monoxide over H‐ZSM‐5 is reported. Carbonyl compounds and methyl‐2‐cyclopenten‐1‐ones (MCPOs), which were detected in the products and catalysts, respectively, are considered as intermediates. The latter species can be synthesized from the former species and olefins. ¹³C isotope tracing and ¹³C liquid‐state NMR results confirmed that the carbon atoms of CO molecules were incorporated into MCPOs and aromatic rings. A new aromatization mechanism that involves the formation of the above intermediates and co‐occurs with a dramatically decreased hydrogen transfer reaction is proposed. A portion of the carbons in CO molecules are incorporated into aromatic, which is of great significance for industrial applications.     

Following Solid‐Acid‐Catalyzed Reactions by MAS NMR Spectroscopy in Liquid Phase—Zeolite‐Catalyzed Conversion of Cyclohexanol in Water

A microautoclave magic angle spinning NMR rotor is developed enabling in situ monitoring of solid–liquid–gas reactions at high temperatures and pressures. It is used in a kinetic and mechanistic study of the reactions of cyclohexanol on zeolite HBEA in 130 °C water. The ¹³C spectra show that dehydration of 1‐¹³C‐cyclohexanol occurs with significant migration of the hydroxy group in cyclohexanol and the double bond in cyclohexene with respect to the ¹³C label. A simplified kinetic model shows the E1‐type elimination fully accounts for the initial rates of 1‐¹³C‐cyclohexanol disappearance and the appearance of the differently labeled products, thus suggesting that the cyclohexyl cation undergoes a 1,2‐hydride shift competitive with rehydration and deprotonation. Concurrent with the dehydration, trace amounts of dicyclohexyl ether are observed, and in approaching equilibrium, a secondary product, cyclohexyl‐1‐cyclohexene is formed. Compared to phosphoric acid, HBEA is shown to be a more active catalyst exhibiting a dehydration rate that is 100‐fold faster per proton.     

Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations

Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with ¹H/¹³C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia.     

Direct Detection of Supramolecular Reaction Centers in the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 by 13C–27Al Solid‐State NMR Spectroscopy

Hydrocarbon‐pool chemistry is important in methanol to olefins (MTO) conversion on acidic zeolite catalysts. The hydrocarbon‐pool (HP) species, such as methylbenzenes and cyclic carbocations, confined in zeolite channels during the reaction are essential in determining the reaction pathway. Herein, we experimentally demonstrate the formation of supramolecular reaction centers composed of organic hydrocarbon species and the inorganic zeolite framework in H‐ZSM‐5 zeolite by advanced ¹³C–²⁷Al double‐resonance solid‐state NMR spectroscopy. Methylbenzenes and cyclic carbocations located near Brønsted acid/base sites form the supramolecular reaction centers in the zeolite channel. The internuclear spatial interaction/proximity between the ¹³C nuclei (associated with HP species) and the ²⁷Al nuclei (associated with Brønsted acid/base sites) determines the reactivity of the HP species. The closer the HP species are to the zeolite framework Al, the higher their reactivity in the MTO reaction.     

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C₁–n‐C₄ alkanes on Zn‐modified high‐silica zeolites ZSM‐5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C? H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n‐butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C₁–n‐C₄ alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.     

B‐MWW Zeolite: The Case Against Single‐Site Catalysis

Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3?x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using ¹¹B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO₃ units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.     

Controlling the Conformational Changes in Donor–Acceptor [4]‐Dendralenes through Intramolecular Charge‐Transfer Processes

The synthesis of two [4]‐dendralene compounds incorporating thiophene‐(p‐nitrophenyl) donor–acceptor units is presented. The dendralenes adopt two different conformers in solution and solid state and the transformation between the structures can be controlled by light and heat. The electron‐donating components of the dendralenes are represented by bromothienyl (in 13 ) and ethylenedioxythiophene(EDOT)‐thienyl (in 15 ) end‐groups. The most facile transformation involves the isomerisation of donor–acceptor conjugated systems ( a conformers) into structures in which only the thiophenes are conjugated ( b conformers), and this process is driven by ambient light. The structures of the two conformers of compound 13 are confirmed by single‐crystal X‐ray diffraction studies and the structural changes in both compounds have been monitored by ¹H NMR spectroscopy and absorption studies. The transformations were found to be first‐order processes with rate constants of k=0.0027 s^?1 and k=0.00022 s^?1 for 13 and 15 , respectively. Density functional theory calculations at the B3LYP/6‐31G* level give credence to the proposed mechanism for the a → b conversion, which involves photoinduced intramolecular charge transfer (ICT) as the key step. The EDOT derivative ( 15 ) can be polymerised by electrochemical oxidation and a combination of cyclic voltammetry and UV/Vis spectroelectrochemical experiments indicate that the a conformer can be trapped and stabilised in the solid state.     

Synthesis of [18,19,21,22‐13C4]‐Labeled Tautomycin as an NMR Probe of Protein Phosphatase Inhibitor

We have accomplished the synthesis of ¹³C‐labeled tautomycin at the C18, C19, C21, and C22 positions starting from 100 % [¹³C]triethylphosphonoacetate for the purpose of elucidating the dynamics and conformation of the C17–C26 moiety. NMR spectroscopy of ¹³C‐labeled tautomycin revealed strong binding with protein phosphatase type 1 and new features in the ¹³C NMR spectrum, such as the very small three‐bond coupling constants (²J).     

Extra‐Framework Aluminum‐Assisted Initial C−C Bond Formation in Methanol‐to‐Olefins Conversion on Zeolite H‐ZSM‐5

Surface methoxy species bound to an extra‐framework Al (SMS‐EFAL) was unambiguously identified by advanced ¹³C‐{²⁷Al} double‐resonance solid‐state NMR technique in the methanol‐to‐olefins reaction on H‐ZSM‐5 zeolite. The high reactivity of the SMS‐EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C?C bond formation in ethene was proposed and corroborated by density functional theory calculations.     

Tetraanionic Nitrogen‐Rich Tetrazole‐Based Energetic Salts

1,1,3,3‐Tetra(1H‐tetrazol‐5‐yl)propane‐based energetic salts were synthesized in a simple and straightforward manner. The structures of these new salts were determined by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, MS, and elemental analysis. All of these compounds showed good thermal stabilities above 180 °C, as confirmed by thermogravimetric–differential thermal analysis (TG–DTA) measurements. Moreover, these salts also exhibited high positive enthalpies of formation, high nitrogen content, good thermal stabilities, and moderate detonation properties.     

Acyclic Schiff base salts derived from 1,3‐diaminopropane and 1,3‐diamino‐2‐hydroxypropane

Two salts of acyclic Schiff base cationic ligands, namely N,N′‐bis(2‐nitrobenzyl)propane‐1,3‐diammonium dichloride monohydrate, C₁₇H₂₂N₄O₄²⁺·2Cl⁻·H₂O, (I), and 2‐hydroxy‐N,N′‐bis(2‐nitrobenzyl)propane‐1,3‐diammonium dichloride, C₁₇H₂₂N₄O₅²⁺·2Cl⁻, (II), were synthesized as precursors in order to obtain new acyclic and macrocyclic multidentate ligands and complexes. The cation conformations in compounds (I) and (II) are different in the solid state, although the cations are closely related chemically. Similarly, the hydrogen‐bonding networks involving ammonium cations, hydroxyl groups and chloride anions are also different. In the cation of compound (II), the hydroxyl group is disordered over two sets of sites, with occupancies of 0.785 (8) and 0.215 (8).     

A Highly Efficient Chemoselective Cyclocondensation of threo‐(1S,2S)‐2‐Amino‐1‐(4‐nitrophenyl)‐1,3‐propanediol with Ketones and Isomerization of the Condensates

A convenient procedure for highly efficient chemoselective cyclization of threo‐(1S,2S)‐2‐amino‐1‐(4‐nitrophenyl)propane‐1,3‐diol with some ketones was described. The structures of the condensates were elucidated on the basis of the IR, ¹H‐ and ¹³C‐NMR, and mass spectra. Ring‐ring tautomerism in 2‐aminopropane‐1,3‐diol chemistry is reported for the first time.     

Towards Eumelanin@Zeolite Hybrids: Pore‐Size‐Controlled 5,6‐Dihydroxyindole Polymerization

5,6‐Dihydroxyindole ( 1 ) and its N‐methyl derivative ( 2 ), key eumelanin building blocks, were inserted into zeolite L by sublimation at 175 °C for 5 days. At a 10 mg/300 mg indole/zeolite ratio, the resulting hybrids displayed a stable deep red coloration. CP/MAS ¹³C NMR and UV/Vis spectroscopy of the red species suggested the generation and accommodation of quinonoid biindole derivative(s) within the void space of the acidic zeolite channels. Removal of the zeolite matrix by treatment with HF gave a stable species that could be separated by HPLC and characterized by mass spectrometry as an oxygenated biindole derivative (or a mixture of isomers), suggesting addition of water to the original dimer and subsequent re‐oxidation. The characterization was corroborated by optimized molecular geometries and simulated UV spectra with density functional calculations. Loading 1 or 2 into the larger pores of SBA‐15 type mesoporous silica resulted in black eumelanin‐type polymers, confirming channel size dependence over the polymerization process.     

Efficient Synthesis of Molecular Precursors for Para‐Hydrogen‐Induced Polarization of Ethyl Acetate‐1‐13C and Beyond

A scalable and versatile methodology for production of vinylated carboxylic compounds with ¹³C isotopic label in C1 position is described. It allowed synthesis of vinyl acetate‐1‐¹³C, which is a precursor for preparation of ¹³C hyperpolarized ethyl acetate‐1‐¹³C, which provides a convenient vehicle for potential in vivo delivery of hyperpolarized acetate to probe metabolism in living organisms. Kinetics of vinyl acetate molecular hydrogenation and polarization transfer from para‐hydrogen to ¹³C via magnetic field cycling were investigated. Nascent proton nuclear spin polarization (%P_H) of ca. 3.3 % and carbon‐13 polarization (%P_13C) of ca. 1.8 % were achieved in ethyl acetate utilizing 50 % para‐hydrogen corresponding to ca. 50 % polarization transfer efficiency. The use of nearly 100% para‐hydrogen and the improvements of %P_H of para‐hydrogen‐nascent protons may enable production of ¹³C hyperpolarized contrast agents with %P_13C of 20–50 % in seconds using this chemistry.     

Synthesis of substituted 3‐phenyl‐6h‐pyrazolo[4,3‐d]isoxazoles from corresponding 4‐benzoyl‐5‐hydroxypyrazoles

Treatment of 1,(3)‐(di)substituted 4‐benzoyl‐5‐hydroxypyrazoles with phosphorus oxychloride affords the corresponding 4‐benzoyl‐5‐chloropyrazoles. Reaction of the latter with hydroxylamine leads to oximes, which can be cyclized to novel 3‐phenyl‐6H‐pyrazolo[4,3‐d]isoxazoles by treatment with sodium hydride in dimethyl formamide. Detailed nmr spectroscopic studies (¹H, ¹³C) with all obtained compounds are presented.     

Curing chemistry of phenylethynyl‐terminated imide oligomers: Synthesis of 13C‐labeled oligomers and solid‐state NMR studies

4‐(Phenylethynyl‐α,β‐¹³C)phthalic anhydride (PEPA) and ¹³C‐labeled phenylethynyl‐terminated imide (PETI) oligomers were synthesized, and solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy was used to determine the structure of cured oligomers. Solid‐state ¹³C NMR spectra were collected before and after thermal curing. Using solid‐state ¹³C NMR difference spectroscopy, several cure products were identified. The observed ¹³C NMR resonances were assigned to four different classes of cure products: aromatics, products from backbone addition (substituted stilbenes and tetraphenylethanes), polyenes, and cyclobutadiene cyclodimers. The effects of postcuring and oligomer chain length on the structure of the cured resins were examined. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3486–3497, 2000     

A Simple,Convenient, One‐Pot Synthesis of Dihydro‐azolopyrimidines,DFT Calculation,and NMR Determination by Using H‐Ferrierite Zeolite as Catalyst

The multicomponent reaction of acetophenone derivatives with heterocyclic amines and benzaldehyde derivatives in water in the presence of H‐ferrierite zeolite for short time 8–15 min afforded new series of [1,2,4]triazolo[1,5‐a]pyrimidines and pyrimido[1,2‐a]benzimidazole derivatives. The structure of the actual tutomeric product was established on the bases of spectral data [IR, NMR (¹H and ¹³C), and nuclear Overhauser effect] and density functional theory calculations.     

Synthesis and spectral properties of 2‐methylthio‐3H‐7‐[(o‐; m‐ and p‐substituted)phenoxy]‐4‐(p‐substituted‐phenyl)‐[1,5]benzodiazepines

A series of eleven new 2‐methylthio‐3H‐7‐[(o‐; m‐ and p‐substituted) phenoxy]‐4‐(p‐substituted‐phenyl)‐[1,5]benzodiazepines, which have potentially useful pharmacological activities, has been synthesized by condensing the 4‐[(o‐; m‐ and p‐R₁)phenoxy]‐1,2‐phenylendiamines with 3,3‐dimercapto‐1‐(p‐R₂‐phenyl)‐2‐propen‐1‐one. Afterward the lH‐[1,5]benzodiazepine‐2‐thiones obtained were treated with sodium hydride and methyl iodide. The structure of all products was corroborated by ir, ¹H nmr, ¹³C nmr and ms.     

New Insight into the Hydrocarbon‐Pool Chemistry of the Methanol‐to‐Olefins Conversion over Zeolite H‐ZSM‐5 from GC‐MS,Solid‐State NMR Spectroscopy,and DFT Calculations

Over zeolite H‐ZSM‐5, the aromatics‐based hydrocarbon‐pool mechanism of methanol‐to‐olefins (MTO) reaction was studied by GC‐MS, solid‐state NMR spectroscopy, and theoretical calculations. Isotopic‐labeling experimental results demonstrated that polymethylbenzenes (MBs) are intimately correlated with the formation of olefin products in the initial stage. More importantly, three types of cyclopentenyl cations (1,3‐dimethylcyclopentenyl, 1,2,3‐trimethylcyclopentenyl, and 1,3,4‐trimethylcyclopentenyl cations) and a pentamethylbenzenium ion were for the first time identified by solid‐state NMR spectroscopy and DFT calculations under both co‐feeding ([¹³C₆]benzene and methanol) conditions and typical MTO working (feeding [¹³C]methanol alone) conditions. The comparable reactivity of the MBs (from xylene to tetramethylbenzene) and the carbocations (trimethylcyclopentenyl and pentamethylbenzium ions) in the MTO reaction was revealed by ¹³C‐labeling experiments, evidencing that they work together through a paring mechanism to produce propene. The paring route in a full aromatics‐based catalytic cycle was also supported by theoretical DFT calculations.