Thermal Ring Opening Research Articles

High dielectric multiwalled carbon nanotube-polybenzoxazine nanocomposites for printed circuit board applications

The present work describes the development of polybenzoxazine (PBZ) nanocomposite with high dielectric constant using varying weight percentages of (0.5, 1.0, and 1.5 wt. %) benzoxazine functionalized MWCNT (CNT-BS) and benzoxazine through thermal ring opening polymerization. The dielectric constants are increased with increasing weight percentages of incorporation of CNT-BS, whereas dielectric losses are in the reverse trend. Data from Cyclic Voltammogram and impedance studies confirm the conducting behavior of CNT-BS/PBZ nanocomposites. The increase in the weight ratio of CNT-BS enhances the values of Tg and thermal stability. The uniform distribution of functionalized CNT-BS was ascertained from transmission electron microscope.

Ring opening of 2-aza-3-borabicyclo[2.2.0]hex-5-ene, the Dewar form of 1,2-dihydro-1,2-azaborine: stepwise versus concerted mechanisms

The ring opening of the Dewar form of 1,2-dihydro-1,2-azaborine, 2-aza-3-borabicyclo[2.2.0]hex-5-ene (3) is investigated by theoretical methods by using multiconfiguration SCF (CASSCF) and coupled cluster theory [CCSD(T)] with basis sets up to polarised quadruple-zeta quality. The title compound was previously reported to form photochemically in cryogenic noble gas matrices from 1,2-dihydro-1,2-azaborine (4). Four reaction paths for the thermal ring opening of 3 to 4 could be identified. These are the conventional disrotatory and conrotatory electrocyclic ring-opening pathways where the BN unit is only a bystander. Two more favourable paths are stepwise and involve 1,3-boron–carbon interactions. The lowest energy barrier for the isomerisation reaction, 22 kcal mol−1, should be high enough for an experimental observation in solution. However, in solution the dimerisation of 3 is computed to have a very low barrier (3 kcal mol−1), and thus 3 is expected to be a short-lived reactive intermediate.

Generation and metathesis of azomethine imines in reaction of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes with het(aryl)methylidenemalononitriles

A new metathesis reaction of azomethine imines is found. Catalytic or thermal diaziridine ring opening of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes leads to azomethine imines reacting further with het(aryl)methylidenemalononitriles to give in situ new azomethine imines inaccessible by common synthetic methods. New azomethine imines are detected as pyrazolines formed via a 1,4-H shift and trapped by the [3+2] cycloaddition with various dipolarophiles to yield 1,5-diazabicyclo[3.3.0]octane derivatives bearing pharmacophoric heterocycles, e.g. furan, nitrofuran, thiophene, and indole. The best results are achieved in the Et2O·BF3-catalyzed reactions in ionic liquids.

ChemInform Abstract: Rhodium‐Catalyzed Ring Opening of Benzocyclobutenols with Site‐Selectivity Complementary to Thermal Ring Opening.

AbstractA Rh‐catalyzed ring opening reaction, showing a site‐selectivity contrasting that of the corresponding thermal process, leads to the development of a novel alkyne insertion reaction providing dihydronaphthalene derivatives.

Cationic 3H-indolium dyes by ring-opening of benzo[1,3]oxazine

The reaction of 2-methylbenzo[1,3]oxazine with (hetero)aromatic aldehydes under acid catalysis afforded directly polymethine dyes, formed by a thermal ring-opening of the oxazine cycle followed by condensation with the aldehydes. These dyes have the structure of a cationic thermally stable coloured open form of a photochromic benzo[1,3]oxazine and do not undergo ring-closure to afford the closed form again, even under basic treatment. All of the dyes showed reversible acidochromic properties.

1,5-(1,7)-Biradicals and Nitrenes Formed by Ring Opening of Hetarylnitrenes

Several aromatic and heteroaromatic nitrenes and carbenes undergo photochemical and sometimes also thermal ring opening. Depending on benz-annelation, the ring-opened species may have the character of either nitrenes (for α-annelation) or 1,5-(1,7-)-biradicals (for β-annelation). Both types have been observed, and they are clearly distinguished by their characteristic electron spin resonance spectra. In addition, ring opening of hetarylnitrenes to nitrile ylides can be observed whenever there is a meta-relationship between a ring nitrogen atom and the nitrene (or carbene) centre. The factors governing the two types of ring opening have been investigated. The nitrenes and carbenes are generated by either low temperature Ar matrix photolysis or flash vacuum thermolysis of azides, tetrazoles, triazoles, or diazo compounds with matrix isolation of the products.

Identification and analytical properties of new synthetic cannabimimetics bearing 2,2,3,3-tetramethylcyclopropanecarbonyl moiety

By means of gas chromatography with mass spectrometry detector (GC–MS), liquid chromatography–mass spectrometry (LC–MS) and nuclear magnetic resonance spectroscopy (NMR), structure of a series from a novel class of synthetic cannabimimetics bearing 2,2,3,3-tetramethylcyclopropanecarbonyl moiety was established. It was found that this fragment could undergo thermal ring-opening into isomeric structures. The title compounds under action of hydrochloric acid can transform into new compounds which structure is discussed in the paper. The compounds identified could be referred to a new class of ‘designer drugs’ and are in illegal turnover in Russia and Belarus since the summer of 2011. Analytical data obtained in the paper will make possible reliable identification of such new ‘designer drugs’ during forensic examination.

Biodegradable polyphosphazenes containing antibiotics: synthesis, characterization, and hydrolytic release behavior

Novel polyphosphazenes containing the fluoroquinolone antibiotic substituents, ciprofloxacin or norfloxacin, were synthesized and characterized. Nano/microfibers of several selected polymers were prepared by an electrospinning technique. The sensitivity to hydrolysis, pH behavior, and antibiotic release profile of all the polymers as films and the selected polymers as nano/microfibers were investigated. The hydrolysis release behavior was further studied by an in vitro antibacterial test against E. coli. Protection and deprotection reactions of ciprofloxacin and norfloxacin were carried out before and after polymer synthesis to prevent cross-linking of multi-functional reagents with polymers. No more than 25 mol% of antibiotics can be introduced to poly(dichlorophosphazene) which was synthesized by a thermal ring opening polymerization of hexachlorocyclotriphosphazene. Various amino acid ethyl esters (glycine, alanine, and phenylalanine) were linked to the polymers as co-substituents to increase the solubility as well as to control the rate of antibiotic release. Depending on the polymer compositions, 5–23% weight loss and 4–30% antibiotic release were observed in a six week hydrolysis study at 37 °C. The corresponding nano/microfibers showed a much faster degradation and antibiotics release due to a substantially larger surface area. The in vitro antibacterial tests showed an antibacterial effect as long as ciprofloxacin or norfloxacin was released from the polymers. Meanwhile, all the polymers gave a near-neutral hydrolysis environment with the pH ranging from 5.9 to 6.8.

Rhodium-Catalyzed Ring Opening of Benzocyclobutenols with Site-Selectivity Complementary to Thermal Ring Opening

A rhodium catalyst induced ring opening of benzocyclobutenols with selective cleavage of the C(sp(2))-C(sp(3)) bond adjacent to the hydroxyl group. The site-selectivity markedly contrasted with that of their thermal ring-opening reaction. The rhodium-catalyzed ring opening led to the development of a new alkyne insertion reaction constructing a dihydronaphthalene framework.

Structures of 4-Iminopyrido[1,2-a]pyrimidines, Pyrido[1,2-a]pyrimidin-4-ones, Pyridopyrimidinium Olates, and Thiazolo[3,2-a]pyrimidine Analogues

The Structure-Correlation Principle of Bürgi and Dunitz is invoked in an analysis of the structures of 2-chloro-8-methyl-4-(2-(4-picolinyl)imino-4H-pyrido[1,2-a]pyrimidine 8, 7-chloro-5-(2-thiazolyl)imino-5H-thiazolo[3,2-a]pyrimidine 9, 2-methylamino-4H-pyrido[1,2-a]pyrimidin-4-one 10, 7-methylthio-5H-thiazolo[3,2-a]pyrimidin-5-one 11, 2,3-dihydro-7-methylthio-5H-thiazolo[3,2-a]pyrimidin-5-one 12, and 1-methyl-2-[(o-tert-butylphenyl)imino]-1,2-dihydropyrido[1,2-a]pyrimidin-1-ium-4-olate 13, which have been determined by X-ray crystallography. The most notable structural peculiarities are the long ‘amidine’ and ‘amide’ C–N bonds (1.40–1.50 Å) and the tilting of the ‘amidine’ C=N and ‘amide’ C=O groups towards a ring nitrogen atom (NCX = 114–118°). Also the ‘amidine’ C=N (1.28 Å) and ‘amide’ C=O bonds (1.22–1.24 Å) are long, i.e. in the normal range for resonance-stabilized amidines and amides in spite of the lack of such resonance in these compounds. These features mimic the transition states for ring opening to ketenes. The long amidine and amide C–N bonds and acute NCX angles are in accord with the observed thermal ring opening to ketenimines and ketenes, respectively.

SBA-15 filled polybenzoxazine nanocomposites for low-k dielectric applications

Low dielectric polybenzoxazine nanocomposites (SBA-15/PBZ) containing mesoporous silica (SBA-15) were successfully synthesized, using benzoxazine functionalized SBA-15 (BZ/SBA-15) and benzoxazine monomer, through thermal ring opening polymerization. SBA-15 was functionalized with benzoxazine functional silane via grafting method. The benzoxazine functionality in BZ/SBA-15 was confirmed by FT-IR, 13C CPMAS, 29Si CPMAS NMR, XPS, TGA and nitrogen porosimetry analysis. By increasing the amount of BZ/SBA-15, the SBA-15/PBZ nanocomposites possess higher Tg and good thermal stability. 7.5 Wt % of BZ/SBA-15 reinforced polybenzoxazine nanocomposite possesses an ultra-low dielectric constant of 1.75 at 1 MHz.

A [4+4] annulation strategy for the synthesis of eight-membered carbocycles based on intramolecular cycloadditions of conjugated enynes

A [4+4] annulation strategy for the synthesis of eight-membered carbocycles is reported that proceeds via a cascade involving two pericyclic processes. In the first step, the [4+2] cycloaddition of a conjugated enyne with an electron-deficient cyclobutene generates a strained six-membered cyclic allene that isomerizes to the corresponding 1,3-cyclohexadiene. In the second step, this bicyclo[4.2.0]octa-2,4-diene intermediate undergoes thermal or acid-promoted 6-electron electrocyclic ring opening to furnish a 2,4,6-cyclooctatrienone. The latter transformation represents the first example of the promotion of 6-electron electrocyclic ring opening reactions by acid.

Synthesis and Properties of Diethyl Tetramethyl Disiloxane Bisbenzocyclobutene and Its Polymer

Hydrosilylation reaction of 4-vinylbenzocyclobutene(4-VBCB) with 1,1,3,3-tetramethyl-1,3-dihydro disiloxane(TMHS) was employed to afford the monomer of diethyl tetramethyl disiloxane-bisbenzocyclobutene(DES-bis-BCB).The structures of DES-bis-BCB and the prepolymer prepared from DES-bis-BCB by thermal isomerization ring opening polymerization reaction were characterized and analyzed.The surface morphology and thermal stability of the films were investigated by atomic force microscopy(AFM) and thermogravimetric analysis(TGA),respectively.The results showed that the root-mean square(RMS) roughness was 0.53 nm,and the starting mass loss temperature was about 390 ℃.It indicated that DES-bis-BCB exhibited good film-forming property and excellent thermal stability.

Steady state and transient kinetics in crystalline solids: the photochemistry of nanocrystalline 1,1,3-triphenyl-3-hydroxy-2-indanone

Electronic excitation of crystalline 1,3,3-triphenyl-1-hydroxy-2-indanone results in the exclusive formation of hydroxy-benzocyclobutane while irradiation in solution leads to formation of the isomeric photoenols, which subsequently tautomerize. Using nanocrystals suspended in water we were able to use transmission pump–probe methods to detect the reactive intermediates involved in the solid state. Transients assigned to the 1,4-biradical (triplet enol) formed by adiabatic photodecarbonylation were detected. It was found that product formation occurs from analogous transients, both in solution and in the solid state. While enols revert to ketones in time scales that range from a few hundred nanoseconds to tens of microseconds, benzocyclobutanol remains kinetically trapped in the crystal lattice, but undergoes a thermal ring opening when dissolved.

1,3‐Dipolar Cycloaddition of Azomethine Ylides to Aldehydes: Synthesis of anti α‐Amino‐β‐Hydroxy Esters

AbstractA 1,3‐dipolar cycloaddition reaction of azomethine ylides to aldehydes is described. The azomethine ylides, generated by thermal electrocyclic ring opening of aziridines, adds to aldehydes in good yields with moderate to good selectivities to furnish oxazolidines. The oxazolidines were subsequently hydrolyzed to the corresponding amino alcohols, giving the anti diastereomer as the major product.

ChemInform Abstract: Cheletropic Reactions in Diphosphirane Series.

Abstract In the diphosphirane serie, whereas the fragmentation is a side reaction during the photochemical, thermal or cationic ring opening, the oxidation of the phosphorus lone pairs leads quantitatively to the corresponding phosphaalkenes, and the unstable phosphinidene.

ChemInform Abstract: Effect of Electron-Withdrawing Groups on the Thermal Ring Opening of 3H-Pyrazoles to Diazoalkenes.

We report that 3H-pyrazoles bearing and electron-withdrawing or conjugated group at C-3 undergo ring opening to diazoalkenes

ChemInform Abstract: Cycloaddition of Benzothiete and Electron-Deficient Nitriles.

The o-quinoid 8π electron system 2, generated by thermal ring opening of benzothiete (1), enters regio-specific [8π + 2π] cycloaddition reactions with electron-deficient nitriles 3a-d, yielding the 4H-1,3-benzothiazines 4a-d. A competitive dimerization of 1 leads to 1,5-dibenzo[b,f]dithiocin (5). Depending on the nitrile further competitive or subsequent reactions (2 + 3b 7b, 2 + 3d 4d 8d) can occur. The cycloadducts 10e and 11e gained from 3e anticipate a primary cleavage of 3e to methylisothiocyanate 9e which reacts at the CN double bond as well as at the CS double bond.

ChemInform Abstract: A Highly Stereoselective Synthesis of Substituted Butadienes and Conjugated Polyenes.

A highly stereoselective synthesis of 1,3-butadienes and its higher enyl analogs has been developed through thermal dehydrative ring opening of cyclopropyl carbinols in dimethylsulphoxide.

ChemInform Abstract: Kinetic Study of Thermolysis of Diarylhomonaphthoquinones. endo/exo Substituent and Solvent Effects.

The kinetics of thermal cyclopropane ring-opening of a series of m- and p-substituted endo/exo diphenylbromohomonaphthoquinones 1a–i and the unsubstituted diphenylchlorohomonaphthoquinone 1j have been investigated and compared with biphenyl-2,2′-diylhalogenohomonaphthoquinones 2a,b. The first-order rate constants k/s−1 for 1a–i at 100 °C in toluene increased with the electron-donating ability of the substituents. The kinetic substituent effects were much more pronounced for the exo family than for the endo one and revealed the crucial role of the resonance contribution of diaryl groups; log (k/ko)exo = −1.99σ+ + 0.086 and log (k/ko)endo = −0.784σ+ + 0.002, respectively. The compounds 2a,b thermolyzed very quickly as compared with the corresponding diphenylhalogenohomonaphthoquinones 1e,j. The kinetic solvent effects on the thermolysis of representative compound 1e were so minute that the rates tended to slightly increase with the solvent polarity but decrease with the solvent basicity. These kinetic results were interpreted in terms of a concerted disrotatory ring opening of the incorporated cyclopropane ring.