Thermal Rearrangement Research Articles

Synthesis of fused tricyclic systems by thermal Cope rearrangement of furan-substituted vinyl cyclopropanes.

A novel method for the stereoselective construction of hexahydroazuleno[4,5-b]furans from simple precursors has been developed. The route involves the use of our recently developed Brønsted acid catalysed cyclisation reaction of acyclic ynenones to prepare fused 1-furanyl-2-alkenylcyclopropanes that undergo highly stereoselective thermal Cope rearrangement to produce fused tricyclic products. Substrates possessing an E-alkene undergo smooth Cope rearrangement at 40 °C, whereas the corresponding Z-isomers do not react at this temperature. Computational studies have been performed to explain the difference in behaviour of the E- and Z-isomers in the Cope rearrangement reaction. The hexahydroazuleno[4,5-b]furans produced by Cope rearrangement have potential as advanced intermediates for the synthesis of members of the guaianolide family of natural products.

A bonding evolution analysis for the thermal Claisen rearrangement: an experimental and theoretical exercise for testing the electron density flow.

A comprehensive theoretical investigation of the thermal Claisen rearrangement of allyl vinyl ether (AVE) to allylacetaldehyde has been carried out. We present the use of the electron localization function (ELF) to monitor the bonding evolution aspects in the course of this thermal rearrangement and the results are compared with a photo-impulsive process where instantaneous vibration frequencies are monitored [I. Iwakura, Phys. Chem. Chem. Phys., 2011, 12, 5546-5555]. Our results reveal an asynchronous electron density rearrangement inasmuch that the breaking of the C3-O bond and the formation of C1-C5 do not take place simultaneously. We also demonstrate how the bonding evolution brings about the natural appearance of the curly arrows representing the electronic flow in molecular rearrangements. This holds the key to gaining an unprecedented insight into the mapping of the electron density flow while the bonds change throughout the reaction progress.

Quantitative syntheses of permethylated closo-1,10-R2C2B8Me8 (R = H, Me) carboranes. Egg-shaped hydrocarbons on the Frontier between inorganic and organic chemistry.

Electrophilic methylation of the closo-1,10-R2C2B8H8 (1) (R = H or Me) dicarbaboranes at higher temperatures or thermal rearrangement of the 1,6-R2C2B8Me8 (3) compounds at 400–500 °C generated the B-permethylated derivatives closo-1,10-R2C2B8Me8 (2) in quantitative (>95%) yields. The compounds exhibit extreme air stability as a consequence of a rigid, egg shaped hydrocarbon structures incorporating inner 1,10-C2B8 carborane core.

Formation of enhanced regenerated grating in few-mode fiber by CO_2 laser pretreatment

In this work, we experimentally investigated the effect of CO2 laser pretreatment on regenerated grating formulated in few-mode fibers (FMFs) (two-mode and four-mode step-index fibers). In the preparation, the pristine FMFs were first treated with direct CO2 laser annealing followed by a slow cooling procedure to minimize the frozen-in stresses and the thermal stresses in the fibers before the grating inscription. After the thermal regeneration process, the produced regenerated gratings (RGs) were then subject to a thermal durability test at 1050°C for 10 h. In the comparison against the results from the RGs formulated in non-treated FMFs, the finding indicates that RGs in treated fibers have a better regeneration ratio and thermal resilience. It is believed that the thermal stress relaxation and structural rearrangement in the fiber glass are the major factors that lead to the lower grating recovery during the state of thermal regeneration process and higher degradation in the grating strength in a longer exposure period. The elimination and reduction of stresses in the fibers by the laser pretreatment in the earliest stage enables the grating to be formed in a more thermal stable condition, hence, RGs with prolonged lifespan can be produced.

One-pot synthesis of trans-β-lactams from ferrocenylketene generated by thermal Wolff rearrangement

ABSTRACTA series of β-lactams containing the ferrocene moiety were synthesized through the Staudinger reaction between ferrocenylketene generated by the thermal Wolff rearrangement of the corresponding diazo ketone and various imines. The stereochemical outcome has been investigated and the trans-products were isolated as the main products, opposite to the reported results by Bonini and coworkers. The absolute configuration of (±)-trans-1,4-diphenyl-3-ferrocenylazetidin-2-one (3c) was determined by X-ray analysis. The stereoselectivity is discussed from the viewpoint of the reaction mechanism.

Eco-Friendly Halogen-Free Flame Retardant Cardanol Polyphosphazene Polybenzoxazine Networks

We report on the preparation of hexa-functional cardanol (renewable phenolic compound) benzoxazine with a phosphazene core (CPN) for use as a greener eco-friendly halogen-free flame retardant reactive additive for the formation of sustainable polyphosphazene polybenzoxazine networks for flame resistant applications. The structure and purity of the monomer was confirmed by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H-, 13C-, 31P NMR) and gel permeation chromatography studies. The CPN monomer showed good compatibility with benzoxazine monomer (CPN0) as suggested by the cocuring studies. The thermal properties of the copolymer can be directly tuned by altering the composition of the monomer blend. The occurrence of phosphazene–phosphazane thermal rearrangement is also suggested for the thermal behavior (thermogravimetry analysis) at higher loading of CPN in the monomer feed ratio. An improvement in mechanical properties of the copolymer with increase in glass transition temperature was c...

Inherently flame retardant vinyl bearing hyperbranched polysiloxanes having improved thermal stability-Ceramization and analysis of associated thermal properties

This study describes the synthesis and characterization of novel hyperbranched silicone polymers having high thermal stability and inherent flame retardancy achieved purely by molecular architecture, without the incorporation of any specific additive or filler. A series of hyper-branched terpolymers (MeDPhDViT) containing vinyl, phenyl and methyl moieties were synthesized by the acid catalysed copolymerization of vinyltriethoxysilane (VTES), diphenyldimethoxysilane (DPDMS) and dimethyldiethoxysilane (DMDES). Curing of these polymers is effected through condensation of the hydroxyl groups triggered by dibutyltindiluarate (DBTDL) catalyst. Glass to rubber transition temperature (Tg)of the cured polymers vary systematically from −110 °C to +50 °C with the increase in the content of vinyl T (ViT) units in the polymer backbone. The terpolymer with maximum ViT-unit content shows an onset of thermal decomposition above 540 °C and leaves 86% char residue at 900 °C, attributed to the retardation of depolymerisation by ViT unit. The limiting oxygen index (LOI) of the terpolymers increases from 26 to 37% with increase in ViT unit content alone, from 13.4 to 53.3 mol% and the terpolymer with 53.3 mol % ViT unit rates as V-0, the best in the UL-94 flame test. The ceramic residue from the cured elastomer comprises of free carbon and silica/Si-O-C ceramics. A probable mechanism has been proposed for the rearrangement of vinyl groups to carbonaceous ceramics under high temperature environment. The vinyl groups promote the formation of a carbonized, in particular graphitized, layer and SiOC phases during pyrolysis, as evidenced by TG-MS, pyrolysis GC-MS, Raman spectroscopy and FESEM analysis. Comparative study with a non-phenylated silicone elastomer confirms the mechanism of formation of carbon rich residue by the thermal rearrangement of vinyl groups.

Exploiting the Continuous in situ Generation of Mesyl Azide for Use in a Telescoped Process

The hazardous diazo transfer reagent mesyl azide has been safely generated and used in situ for continuous diazo transfer as part of an integrated synthetic process with an embedded safety quench. Diazo transfer to β‐keto esters and a β‐ketosulfone was successful. In‐line phase separation, by means of a continuous liquid–liquid separator, enabled direct telescoping with a thermal Wolff rearrangement.

Rearrangements of 1,2,4-Oxadiazole: “One Ring to Rule Them All”

1,2,4-Oxadiazoles are heterocycles characterized by low aromaticity and the presence of a weak O–N bond and are widely studied due to their tendency to rearrange into more stable heterocyclic compounds. This review covers literature from the last fifteen years, highlighting the general features of 1,2,4-oxadiazoles and their applications. Regarding the reactivity, the development of classical reactions (thermal and photochemical rearrangements) is presented in terms of synthetic utility and mechanistic insight. Among the relevant rearrangement reactions, the Boulton–Katritzky Rearrangement (BKR), Migration – Nucleophilic Attack – Cyclization (MNAC), and Addition of the Nucleophile, Ring Opening, and Ring Closure (ANRORC) reactions are discussed, together with recent noteworthy syntheses and applications of the 1,2,4-oxadiazole ring.

Synthesis of Allobetulin Using Phenylthiourea

A new method for preparing allobetulin via thermal rearrangement of betulin in the presence of phenylthiourea without using acidic reagents was found.

Electrical and Structural Characterization of Ba3Mo1-xNb1+xO8.5-x/2: The Relationship between Mixed Coordination, Polyhedral Distortion and the Ionic Conductivity of Ba3MoNbO8.5.

The electrical and structural properties of the series Ba3Mo1-xNb1+xO8.5-x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba3Mo1-xNb1+xO8.5-x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O4 and (Mo/Nb)O6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10-6 S cm-1 for x = 0.0 to 1.1 × 10-7 S cm-1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba3MoNbO8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MoNbO8.5 system.

Sorption, diffusion, and permeability of humid gases and aging of thermally rearranged (TR) polymer membranes from a novel ortho-hydroxypolyimide

We studied in this work the properties of a new membrane (TR-PBO) obtained by solid state thermal rearrangement at 450°C of a recently developed polyimide precursor, (mHAB-6FDA), which was synthesized by reaction of (3,3-diamino-4,4-dihydroxybiphenyl, mHAB) with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The mHAB monomer is an isomer of the commercial 3,3′-dihydroxybenzidine (pHAB), used to form the more popular polyimide precursor pHAB-6FDA. TR-PBO membranes obtained from mHAB-6FDA showed excellent CO2 permeability (720 Barrer) and good CO2/CH4 ideal selectivity of 23. We found out that the thermal rearrangement enhances the solubility and diffusion coefficients of CO2 at 10bar by factors as high as 1.3 and 5, respectively. Larger enhancements, however, were observed in the case of CH4, causing the diffusivity selectivity to decrease by a factor of 2.6 and the solubility selectivity to decrease by a factor of ~1.5 upon rearrangement. The pure gas solubility was modeled with the Dual Sorption Mode model and the NELF model. The two models were then used to predict the mixed gas behavior in terms of solubility-selectivity, highlighting the effects of competition that are consistent with those observed in other glassy polymers.We also performed moisture sorption tests and gas permeability measurements in the presence of humidity. It was observed that the thermal rearrangement increases the membrane hydrophobicity and, consistently, the CO2 and CH4 permeability of mTR-PBO membranes is much more stable in the presence of humidity than that of the precursor polyimide membranes. Finally, the effect of aging on the membrane performance was analyzed. A 30% decrease in the CO2 permeability of TR polymer membranes (around 50µm thick) was observed after 6 months, while the selectivity increased by 20%. These results indicate that, even after 6 months, the performance of the TR polymer membrane was outstanding and close to Robeson's upper bound.

Iminocyclohexadienylidenes: Carbenes or Diradicals? The Hetero-Wolff Rearrangement of Benzotriazoles to Cyanocyclopentadienes and 1H-Benzo[b]azirines.

The thermal rearrangements of benzotriazole 1 to fulvenimine 4 and 1H-benzazirine 7 are investigated at DFT and CASPT2 levels of theory. Ring opening of benzotriazole 1 to 2-diazo-cyclohexadienimine 2 followed by N2 elimination affords Z- and E-2-iminocyclohexadienylidenes 3, which have triplet ground states (3A″). The open-shell singlet (OSS) (1A″) and closed-shell singlet (CSS) (1A') of 3 lie ∼15 and 40 kcal/mol higher in free energy, respectively. The OSS 3 (1A″) is best described as a 1,3-diradical, whereas the CSS (1A') has the character of a carbene. A hetero-Wolff rearrangement of OSS 3 yields fulvenimine 4, which is a precursor of cyanocyclopentadiene 5, with a calculated activation barrier of 38 kcal/mol at the CASPT2(8,8) level, whereby there is a surface crossing from the OSS to the CSS near the transition state. The barrier for cyclization to 1H-benzo[b]azirine 7 is only ∼13 kcal/mol. Therefore, reaction paths involving the singlet iminocyclohexadienylidene diradicals 3 will necessarily cause equilibration with 1H-benzazirine 7 prior to ring contraction to iminofulvene 4 and cyanocyclopentadiene 5, in agreement with experimental observations based on 13C labeling. The thermolysis of 1-acetylbenzotriazole 7 leads to the analogous N-acetyl-diazocyclohexadienimines 8, N-acetyliminocyclohexadienylidene diradicals 9, and N-acetylfulvenimine 10. The E-N-acetyliminocyclohexadienylidene E9 ring closes to the N-acetyl-1H-benzazirine 11 prior to ring contraction to N-acetylfulvenimine 10, and the Z-N-acetyl-2-diazocyclohexadienimine Z8 ring closes to 2-methylbenzoxazole 12. 1H-benzazirines are predicted to be spectroscopically observable species.

QM/MM nonadiabatic dynamics simulations on photoinduced Wolff rearrangements of 1,2,3-thiadiazole.

The photoinduced rearrangement reaction mechanism of 1,2,3-thiadiazole remains experimentally elusive. Two possible mechanisms have been proposed to date. The first is a stepwise mechanism via a thiocarbene intermediate; the second is an excited-state concerted rearrangement mechanism. Herein we have adopted both the electronic structure calculations and nonadiabatic dynamics simulations to study the photoinduced rearrangement reactions of 1,2,3-thiadiazole in the S2, S1, and S0 states in solution. On the basis of QM(CASPT2)/MM [quantum mechanics(complete active space self-consistent field second-order perturbation theory)/molecular mechanics] calculations, we have found that (1) the thiocarbene intermediate is not stable; thus, the stepwise mechanism should be unfavorable; (2) the excited-state decay from the S2 via S1 to S0 state is ultrafast and completed within ca. 200 fs; therefore, both the S2 and S1 states should not have a long enough time for the excited-state rearrangements. Instead, we have computationally proposed a modified photoinduced rearrangement mechanism. Upon irradiation, the S2 state is first populated (114.0 kcal/mol), followed by an ultrafast S2 → S1 → S0 excited-state decay along the S-N bond fission, which eventually leads to a very "hot" intermediate with the S-N bond broken (18.3 kcal/mol). Then, thermal rearrangements to thioketene, thiirene, and ethynethiol occur in a concerted asynchronous way. This mechanistic scenario has been verified by full-dimensional trajectory-based nonadiabatic dynamics simulations at the QM(CASPT2)/MM level. Finally, our present computational work provides experimentally interesting mechanistic insights into the photoinduced rearrangement reactions of cyclic and acyclic diazo compounds.

Highly Selective Synthesis of Dihydrobenzo[d]isoxazoles and Dihydrobenzo[d]oxazoles from Oximes and Arynes via in Situ Generation of Nitrones.

An efficient method for the synthesis of dihydrobenzo[d]isoxazoles and dihydrobenzo[d]oxazoles bearing a quaternary carbon center has been developed. The reaction involves generation of a ketonitrone intermediate in situ from a ketoxime and an aryne. A novel thermal rearrangement of the dihydrobenzo[d]isoxazole products to the corresponding dihydrobenzo[d]oxazoles has been observed. These transformations tolerate a variety of functional groups and offer a rapid and efficient way to diverse dihydrobenzo[d]isoxazoles and dihydrobenzo[d]oxazoles under mild transition-metal-free conditions.

Polybenzoxazole (PBO)-based gas separation membranes thermally derived from blends of Ortho-functional polyimide and polyamide precursors

Abstract This study investigates the physical properties and gas separation performance of a series of PBO-based membranes derived from thermal conversion of blends containing ortho -OH polyamide (PHA) and polyimide (API) precursors. Homogeneous films were obtained in varying PHA/API molar ratios before and after thermal conversion, indicating good compatibility between the two precursors and resulting PBOs. This study reveals that PBO conversion efficiency, fractional free volume and gas transport properties of the resulting PBO-based blend films show an unexpected non-linear dependence on the blend composition (i.e., PHA/API ratio), due to an interplay between API thermal rearrangement (TR) process and PHA cyclodehydration process. Particularly, the B11 blend membrane, containing API and PHA in 1:1 M ratio, showed an optimal combination of gas separation performance and mechanical robustness, while the B31 blend (PHA:API = 3:1) showed little degradation of separation performance relative to both constituent polymers. This is possibly because of the localization of the excess PHA fraction that discourages the formation of inter-connected diffusive transport pathways. In summary, physical blending of API and PHA precursors at an optimal composition is demonstrated to be a simple yet instrumental approach to mitigate the mechanical deterioration issue of API-TR process while greatly improving the separation performance of the PBOs derived from the PHA cyclodehydration process.

Thermally rearranged mixed matrix membranes for CO2 separation: An aging study

In this work, the aging behavior of a thermally rearranged polybenzoxazole-co-imide (TR-PBOI) mixed matrix membrane loaded with 0.5wt.% of oxidized multi-wall carbon nanotubes (MWCNT) was evaluated and then compared to a pure TR polymeric membrane prepared from the same precursor. To the best of authors knowledge, this is the first report of a mixed matrix membrane being prepared through the dispersion of MWCNTs within a thermally rearranged polymer matrix for CO2 separation. Microporous structures were created in both membranes when thermally rearranged at 375°C, facilitating fast mass transfer ideal for membrane gas separation. The TR mixed matrix membrane with oxidized CNTs demonstrated improved separation properties with regard to both permeability and selectivity compared to the pure TR polymeric membrane due to a greater degree of thermal rearrangement (11.3%) than what was exhibited by the TR membrane (6.7%). Moreover, the high CO2 solubility typical of TR polymers coupled with diffusivity enhancements improved the CO2/N2 selectivity. The addition of oxidized CNTs to the TR-PBOI polymer did not significantly influence the aging behavior of the mixed matrix membrane. Both pure TR-PBOI and mixed matrix membranes exhibited an increase in CO2 selectivity due to physical aging. The improved separation properties in conjunction with an unchanged membrane stability over time suggested that the addition of CNTs to pure TR membranes could be an excellent approach toward improving the performance of thermally rearranged membranes applied toward gas separation.

Me3SiCF2Br-Self-Assisted Domino Reaction: Catalytic Synthesis of α,α-Difluorocyclopentanones from Methylvinylketones.

A complete self-assistance strategy based on Me3SiCF2Br was used in the domino conversion of methylvinylketones into α,α-difluorinated cyclopentanones. Initiated by 5 mol % of nBu4NBr, the multistep reaction occurred in one pot under mild conditions via the in situ formation of silyl dienol ethers, difluorocyclopropanation, thermal vinylcyclopropane-cyclopentene rearrangement, and desilylation. The process takes advantage of the multitasking capability of Me3SiCF2Br as the difluorocarbene source, a silicon-based transfer agent, and a bromine anion releaser.

A Tethered Ru-S Complex with an Axial Chiral Thiolate Ligand for Cooperative Si-H Bond Activation: Application to Enantioselective Imine Reduction.

An axial chiral version of the 2,6-dimesitylphenyl group attached to sulfur is reported. Its multistep preparation starts from (S)-binol, and the thiol group is established by a racemization-free thermal Newman-Kwart rearrangement. The new chiral thiolate ligand decorated with one mesityl group is used in the synthesis of a tethered ruthenium chloride complex. Its spectroscopic characterization revealed solvent-dependent epimerization at the ruthenium center. The major diastereomer is crystallographically characterized. Chloride abstraction with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF4 ) yields the corresponding coordinatively unsaturated ruthenium complex with the Ru-S bond exposed. Si-H bond activation at this Ru-S bond proceeds in syn fashion but with moderate facial selectivity (d.r.=70:30), generating diastereomeric chiral-at-ruthenium hydrosilane adducts. Their application to catalytic imine hydrosilylation led to promising enantioinduction (40 % ee), thereby providing proof of concept for asymmetric catalysis involving cooperative Si-H bond activation.

Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles.

Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N2 elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5-aryltetrazoles can result in double N2 elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.