Retro Reaction Research Articles

Intramolecular Cyclization and a Retro-Ene Reaction Enable the Rapid Fragmentation of a Vitamin B1-Derived Breslow Intermediate.

In solution, analogues of the Breslow intermediate formed during catalysis by benzoylformate decarboxylase (BFDC) undergo rapid, irreversible fragmentation. The ability of BFDC to prevent this reaction and preserve its cofactor is a striking example of an enzyme 'steering' a reactive intermediate towards a productive pathway. To understand how BFDC suppresses the off-pathway reactivity of this Breslow intermediate, a clear mechanistic understanding of the fragmentation reaction is required. Here, DFT calculations reveal an unexpected mechanism for the solution-phase fragmentation that involves an intramolecular cyclization and a subsequent retro-ene reaction to release the final products. Free energy profiles demonstrate that this pathway is significantly more facile than the previously proposed mechanism that invoked Breslow intermediate enolates as intermediates. Additional computations have been performed to understand why related Breslow intermediates do not undergo analogous fragmentation reactions. Calculations performed with two closely related Breslow intermediates suggest that subtle differences in the relative values of ΔG≠ for protonation and fragmentation dictate whether a given intermediate will fragment or not. These differences and the fragmentation mechanism unveiled in this work may have ramifications for the mechanism of BFDC and other thiamin-dependent enzymes and could provide general lessons related to the control of reactive intermediates by enzymes.

Pyrolysis mechanistic study on sulfated polysaccharide from marine algal biomass with density functional theory method

Sulfated polysaccharide (SPS) derived from marine algae represents a promising renewable carbon biomass source. Through pyrolysis, SPS can be converted into C5 platform chemicals, including furfural (FF) and 5-methylfurura (5-MF). Understanding the fundamental reaction mechanisms during pyrolysis is essential for the advancing pyrolysis techniques. This work presents a detailed reaction mechanism that reveal new pathways leading to the most important products and intermediates. Simulations focusing on the initial steps in SPS pyrolysis showed that the unimolecular degradation of SPS is dominated by SO3 elimination, with a barrier height of 157.2 kJ/mol; ring-opening and glycosidic bond fission were identified as important competitive reactions, both of which have a barrier height of 207.6 kJ/mol and 260.1 kJ/mol, respectively. The 4,5-unsaturated acyclic rhamnose derived from the glycosidic bond fission of acyclic reducing end group can transform into 5-MF with high selectivity. In addition, the acyclic reducing end group is readily dehydrated to form a 2,3-unsaturated acyclic reducing end group, a reactive unsaturated intermediate. Its decomposition involves an 8-membered ring transition state that requires a low barrier height and results in the formation of a C5 fragment, which can be cyclized directly and dehydrated to form 5-MF. The majority of CO2 is mostly formed by the synergistic breakage of C-Ccarboxyl bonds and glycosidic bonds, with a barrier height of 197.9 kJ/mol. Moreover, a mechanism is uncovered for the formation of formic acid (FA) from a 4,5-unsaturated no-reducing end group. The major pyrolysis channel is initiated by a retro Diels-Alder reaction and followed by a retro-ene reaction to produce a 3,4-unsaturated non-reducing end group, which further decomposes into FA and a new 4,5-unsaturated non-reducing end group.

Experimental and modeling study on ignition kinetics of ethyl methyl carbonate

Ethyl methyl carbonate (EMC) is an essential electrolyte of lithium-ion batteries (LIBs), and the spontaneous ignition of electrolyte is considered to be an important inducement for the thermal runaway (TR) combustion of LIBs. To better understand its ignition chemistry and combustion kinetics, the ignition delay times (IDTs) of ethyl methyl carbonate were measured by using a Rapid Compression Machine (RCM) over a wide range of temperatures (810–980 K), pressures (20 and 40 bar), equivalence ratios (φ = 0.5, 1.0, and 2.0), and a fuel concentration of 2 %, yielding results that are in substantial variance with those calculated with detailed kinetics in the literature over the entire range of conditions investigated. An improved chemical kinetic model was developed to simulate the ignition and combustion characteristics of EMC under intermediate-temperature region by employing the updated kinetic parameters. Results show that the overall performance of the present kinetics model is substantially improved with respected to the prediction of ignition delay times, intermediate species profiles, as well as laminar flame speeds across different temperature, pressure, and equivalence ratio conditions. Furthermore, sensitivity and pathway analyses show that the ignition chemistry of EMC is mainly controlled by retro-ene reaction and H abstraction reactions.

N-Unsubstituted 2- and 3-thiophenimines.

N-Unsubstituted 2-thiophenemethanimine and 3-thiophenemethanimine are the simplest derivatives of a family of imines, the thienyl imines. These two thienyl aldimines and the C-methyl derivatives were prepared in a gas solid reaction by dehydrocyanation of the corresponding α-aminonitriles or in the gas phase by a retro-ene reaction from N-allyl derivatives, then characterized by IR and NMR spectroscopy at low temperature and used in transimination reactions. From several angles, these compounds in the free or complexed state have been compared to the corresponding recently synthesized furanimines and to other N-unsubstituted imines, with the aim of studying the specificity of each of them.

Exploring pyrolysis of the aromatics in shale oil by experimental study and kinetic modelling

In this work, the main aromatics in shale oil from retorting Huadian oil shale and the functional groups during the pyrolysis of shale oil were studied experimentally. The aromatics in shale oil are mainly monocyclic aromatic hydrocarbons (MAHs). Alkylbenzenes are the most abundant in MAHs of shale oil. Then, nonylbenzene was selected as representative to explore the pyrolysis mechanism through theoretical calculation and kinetic modelling due to the activity of its alkyl side-chain and the abundance in alkylbenzenes of shale oil. The bond dissociation energies (BDE) of the C–C and C–H bonds were calculated using the CBS-QB3 quantum chemistry method to study its thermostability. Finally, the detailed pyrolysis kinetic mechanism of nonylbenzene was written by the GRI-Mech3.0 mechanism. A large number of the kinetic parameters were computed by the group-additivity method. In addition, the kinetic parameters of these two non-negligible retroene reactions of nonylbenzene were calculated by the CBS-QB3 method to develop the kinetic model. In summary, the pyrolysis mechanism can improve the understanding of shale oil reactivity due to that it could be chosen as a model compound for novel surrogate shale oil model and has a significant guidance on the development of the kinetic model of alkylbenzenes.

Steam cracking of methyl ester: A modeling study on the influence of the hydrocarbon backbone

Biodiesel can be used to produce clean platform chemicals for the chemical industry, as an alternative to or blend with conventional steam cracking feedstocks. Research on the decomposition chemistry of biodiesel is crucial for understanding their combustion, aging and potential as source for base chemical production. Three methyl esters are studied with varying hydrocarbon chain length and a different degrees of saturation to evaluate the impact of these structural differences on the decomposition chemistry and the decomposition product composition. A new kinetic model is developed with the automatic kinetic model generation tool Genesys for the pyrolysis of methyl butanoate, methyl decanoate and methyl dec-4-enoate. New ab initio calculations are carried out for the pyrolysis of methyl butanoate and for retro-ene reactions in unsaturated hydrocarbons and methyl esters, because of their importance in the secondary decomposition chemistry. The first-principles based kinetic model is validated using in-house experimental data gathered in a tubular reactor and against literature experimental data gathered in a jet-stirred reactor for methyl decanoate pyrolysis. A rate of production analysis reveals the main channels towards the pyrolysis products. The presence of the double bond in the hydrocarbon chain leads to a higher production of aromatic species, at the cost of a lower ethylene and propylene production. The high selectivity to aromatics is caused by the fast formation of 1,3-cyclopentadiene from the unsaturated, resonantly stabilized radicals formed during the decomposition.

Ultrasound-Responsive Hydrogels for On-Demand Protein Release.

The development of tunable, ultrasound-responsive hydrogels that can deliver protein payload on-demand when exposed to focused ultrasound is described in this study. Reversible Diels-Alder linkers, which undergo a retro reaction when stimulated with ultrasound, were used to cross-link chitosan hydrogels with entrapped FITC-BSA as a model protein therapeutic payload. Two Diels-Alder linkage compositions with large differences in the reverse reaction energy barriers were compared to explore the influence of linker composition on ultrasound response. Selected physicochemical properties of the hydrogel construct, its basic degradation kinetics, and its cytocompatibility were measured with respect to Diels-Alder linkage composition. Focused ultrasound initiated the retro Diels-Alder reaction, controlling the release of the entrapped payload while also allowing for real-time visualization of the ongoing process. Additionally, increasing the focused ultrasound amplitude and time correlated with an increased rate of protein release, indicating stimuli responsive control.

Thermal decomposition of 1-hexene by flash pyrolysis: A study of initial decomposition mechanism

Thermal decomposition of 1-hexene at temperatures 295-1410 K was conducted using a flash pyrolysis micro-reactor coupled to laser-based vacuum ultraviolet photoionization time-of-flight mass spectrometer (VUV-PI-TOFMS). The decomposition mechanism of 1-hexene was developed with the help of theoretical calculation performed at the MRCI/cc-pvtz//CASSCF/6–31+G(d,p) level. The γ-scission and diradical retro-ene reactions were determined as the main initial decomposition reactions in the temperature range 990-1240 K. Two diradical retro-ene reaction channels, 1,5-diradical and 1,6-diradical reactions, were proposed in order to interpret the appearance of the C4H8 species. The 1,5-diradical retro-ene reaction involved a 1,5-diradical intermediate that subsequently decomposed via CC β-scissions to the C2, C3 and C4 products. The 1,6-diradical retro-ene reaction proceeded via a 1,6-diradical intermediate and CC β-scissions to produce the C2 and C4 species. The proposed diradical retro-ene mechanism was evidenced indirectly by the early product distribution of 1-hexene pyrolysis in a flow reactor at 1173 K determined by synchrotron radiation VUV-PI-TOFMS. It was verified in the flash pyrolysis of 1-heptene as well.

Flash vacuum pyrolysis of acetylenic esters: A mechanistic study

Products from flash vacuum pyrolysis of eleven acetylenic esters are reported in this study. The mechanism of their pyrolysis is proposed based on the products isolated. These reveal a combination of two main mechanistic approaches. Esters with β-hydrogens favor a retro-ene reaction via a six-membered transition state, the absence of such β-hydrogen leads to a carbene intermediate followed by insertion and cyclization.

Efficient Organocatalytic Dehydrogenation of Ammonia Borane

Dehydrogenation of ammonia borane by sterically encumbered pyridones as organocatalysts is reported. With 6‐tert‐butyl‐2‐thiopyridone as the catalyst, a turnover frequency (TOF) of 88 h−1 was achieved. Experimental mechanistic investigations, substantiated by DLPNO‐CCSD(T) computations, indicate a mechanistic scenario that commences with the protonation of a B−H bond by the mercaptopyridine form of the catalyst. The reactive intermediate formed by this initial protonation was observed by NMR spectroscopy and the molecular structure of a surrogate determined by SCXRD. An intramolecular proton transfer in this intermediate from the NH3 group to the pyridine ring with concomitant breaking of the S−B bond regenerates the thiopyridone and closes the catalytic cycle. This step can be described as an inorganic retro‐ene reaction.

A DFT investigation of the retro-ene reactions of β-hydroxyacetylenes: concerted or stepwise mechanism

The retro-ene reactions of β-hydroxyacetylene and its 10 derivatives have been investigated theoretically at the B3LYP/6-31+G(d) level. The reactions are found to proceed through asynchronous concerted transition states. The thermodynamic data reveal that the reactions are exergonic with increase in the standard entropy. However, due to high values of the activation Gibbs free energy (∆G#) and negative values of the activation entropy (∆S#), reactions occur at elevated temperatures. The substituent groups in general do not have pronounced effect on the activation enthalpies, except the phenyl group in which case, it is lowered. The concerted versus stepwise mechanism has been investigated by complete active space-self consistent field (CASSCF) calculations also.

Tailoring the Hard–Soft Interface with Dynamic Diels–Alder Linkages in Polyurethanes: Toward Superior Mechanical Properties and Healability at Mild Temperature

The current thermoremendable polyurethanes (PUs) based on reversible Diels–Alder (DA) chemistry featuring both good mechanical properties and efficient macrodamage healability are up to now built with the DA bonds located in the hard domains with restricted mobility, which hinders the DA reversibility. Thus, the healing process in these materials requires predissociation of DA bonds via the retro reaction at high temperatures (110–180 °C) followed by DA re-formation, which may compromise the material performance integrity. Here, we show a design of thermoset PUs containing dynamic DA entities engineered at the interface between the hard and soft domains that combine high modulus and toughness with mild-temperature-triggered molecular mobility, which is a prerequisite for efficient healing capability. This concept of molecular design enables PUs to have great mechanical properties (Young’s modulus ∼80–225 MPa, ultimate tensile strength ∼16–30 MPa, and toughness ∼26–96 MJ m–3) and simultaneously remarkable ...

The growing relevance of biological ene reactions.

The ene reaction involves the addition of an 'ene' to an 'enophile.' The retro-ene reaction is the reverse of the ene reaction. In recent years various biological molecules have been found to form covalent intermediates (ene-adducts) that might be the result of an ene reactions. Such adducts have been characterized or implicated for dihydropyridines and pyridininum cofactors derived from vitamin B3, such as the reduced and oxidized forms of nicotinamide adenine dinucleotide (NADH/NAD); flavin cofactors derived from vitamin B2, such as flavin adenine dinucleotide, FAD, and flavin mononucleotide, FMN; vitamin C; the oxime intermediate of nitric oxide synthase; tyrosine; and other biomolecules. Given the ubiquitous nature of these cofactors, it might be speculated that the formation of ene-adducts is a more common principle in biochemistry.

Rhodium-Catalyzed Tandem Addition-Cyclization-Rearrangement of Alkynylhydrazones with Organoboronic Acids.

Transition metal-mediated catalysis routinely enables substrates of multiple π-systems to be efficiently coupled with various carbon nucleophiles along with simultaneous ring formation. This transformation, however, remains unexplored in connection with pericyclic processes. Reported here is a protocol for cycloalkene synthesis based on the merger of rhodium catalysis and a retro-ene reaction. The approach allows alkyne-tethered hydrazones and organoboronic acids to undergo a cascade of addition-cyclization-rearrangement reactions to provide cycloalkene products. The process is initiated by the rhodium-catalyzed addition-cyclization and completed with the allylic diazene rearrangement. The reaction can also be rendered asymmetric by using chiral diene ligands for the rhodium catalyst, whereby enantioselective addition to the C═N bond establishes the C-N stereocenter whose chirality is transferred to an allylic C-H center via suprafacial rearrangement.

Flash vacuum pyrolysis of sulfamoyl azides and chlorides: Facile gas-phase generation of transient N-sulfonylamines

Two sulfamoyl azides, H2NS(O)2N3 and MeN(H)S(O)2N3, have been isolated as neat substances and characterized. The solid-state structure of the parent molecule H2NS(O)2N3 has been established with X-ray crystallography. The thermal decomposition of both azides has been studied by combining flash vacuum pyrolysis (FVP, 400 °C) and matrix-isolation IR spectroscopy. In addition to the complete dissociation fragments for H2NS(O)2N3 (SO2, N2, and H2) and MeN(H)S(O)2N3 (SO2, N2, and CH2NH), the retro-ene decomposition products HN3 and N-sulfonylamines HNSO2 and MeNSO2 form, respectively. Alternatively, quantitative yield of both N-sulfonylamines in the gas phase occurs when sulfamoyl chlorides H2NS(O)2Cl and MeN(H)S(O)2Cl are used as the FVP precursors (400 °C). Consistent with these experimental observations, computational studies on the potential energy profiles for the decomposition of the two azides at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level conclusively suggest that the N2-elimination is energetically more favorable than the HN3-elimination through either retro-ene reaction or 1,2-elimination. In contrast, the facile HCl-elimination from sulfamoyl chlorides provides a general method for the gas-phase generation of N-sulfonylamines, which were known as transient reactive intermediates in synthetic chemistry.

New Approach to Accomplish the Covalent Functionalization of Boron Nitride Nanosheets: Cycloaddition Reactions

We investigated the occurrence of cycloadditions on dimensional boron nitride. Our results indicated that the [2 + 2] cycloaddition of benzynes, ethyne, and maleic anhydride is allowed both from thermodynamical and kinetic stand points. In fact, our results indicated that the reaction energy associated with the formation of the [2 + 2] cycloaddition product is more exothermic than that computed for graphene. The activation energy determined for the [2 + 2] path is 15.9 kcal/mol, suggesting the feasibility of the process. Furthermore, we found that the activation energy for the retro reaction is 30.6 kcal/mol. This value is large enough to guarantee the stability of the functional groups but not too high to impede their detachment upon heating. In addition to these findings, we found that cycloadditions can be used to reduce the band gap of boron nitride nanosheets. In light of these results, we propose the use of cycloaddition reactions to attain the temperature-controlled functionalization of boron nitri...

Chemistry of Alkylaromatics Reconsidered

To investigate upgrading crude oil, alkylaromatic compounds are often chosen as model compounds to better understand their reactivity. In recent kinetic models of this chemistry, the main reaction consuming the alkylaromatic is a four-membered ring “retro-ene” reaction. Here, the transition state of that reaction is discovered to be inconsistent with six-membered ring retroene reactions reported in the literature, leading to inaccurate conclusions. A new detailed kinetic model is constructed using Reaction Mechanism Generator (RMG), and thermodynamic parameters of key compounds and radicals are identified to limit model accuracy. Thermochemistry for key species in the chemistry of hexylbenzene, including hexylbenzene, alkylbenzenes, alkylbenzene radicals, aliphatic radicals, and styrene, was calculated using the CBS-QB3 quantum chemistry method to improve the accuracy of the hexylbenzene pyrolysis model. The kinetics of a key beta scission reaction were also calculated. The results of these calculations h...

Stereoselective allylic reduction via one-pot palladium-catalyzed allylic sulfonation and sulfinyl retro-ene reactions

A tandem approach for the stereoselective allylic reduction has been developed based on a strategy combining the palladium-catalyzed S-allylation and the sulfinyl retro-ene reactions.

Rapid identification of limonoids from Cipadessa baccifera and Xylocarpus granatum using ESI-Q-ToF-MS/MS and their structure-fragmentation study

Limonoids found to be chemotaxonomic markers from the plants of the Meliaceae and Rutaceae families. In the present study, rapid identification of limonoids from Cipadessa baccifera and Xylocarpus granatum were achieved using fast and simple electrospray ionization quadruple time-of-flight mass spectrometry (ESI-Q-ToF-MS/MS) in positive-ion mode. Although the structures of these compounds were found to be similar, Collision Induced Dissociation (CID) mass spectrometric analysis of these protonated/sodiated molecules indicated different fragmentation patterns by which the structures were confirmed. The fragment ions were formed due to the loss of neutral components like H2O, CO2, methanol, as well as McLafferty rearrangement and Retro-ene reaction. Furthermore, MS/MS spectra revealed different fragmentation pathways for different classes of limonoids which further aided dereplication.

Acetic Anhydride Mediated Retro-Ene Reaction via a [4.4.3]Propel­lane Skeleton Intermediate Containing a Quaternary Ammonium Linkage

A novel retro-ene reaction via a [4.4.3]propellane intermediate containing a quaternary ammonium linkage was developed. The feature of this acetic anhydride mediated rearrangement includes the elimination of an acetoxy group at the C14 position and subsequent intramolecular nucleophilic addition of a nitrogen functional group to form an isolable ammonium salt intermediate. We clarified the reaction mechanism utilizing the deuterated derivative.