Formation of substituted benzocyclobutenes through flash vacuum pyrolysis

Benzocyclobutenes carrying a substituent in the four membered ring are obtained in high yield from 2-methyl benzaldehydes through a reaction sequence involving a pyrolytic 1,4-elimination of HCl in the crucial step.     

Catalysis in flash vacuum pyrolysis

The temperature required for flash pyrolytic elimination of water from $\text{o}$-aminobenzyl alcohols and of carbon dioxide from dihydrobenzoxazinones to give azaxylylenes is considerably lowered by the presence of alumina and silica gel in the hot zone.     

Aryl-aryl bond formation by flash vacuum pyrolysis of benzannulated thiopyrans

In contrast to fully unsaturated 7-membered ring sulfur heterocycles (thiepines), some of which extrude sulfur and give the ring-contracted hydrocarbon even at room temperature in solution, benzannulated thiopyrans (6-membered sulfur heterocycles) require flash vacuum pyrolysis (FVP) conditions in the gas phase at temperatures in the range of 1000-1200 degrees C to promote the corresponding reaction. Thus, FVP of benzo[kl]thioxanthene (1) gives fluoranthene, and naphtho[2,1,8,7-klmn]thioxanthene (6) gives benzo[ghi]fluoranthene (7). FVP of thioxanthone (9) gives fluorenone (10), together with lesser amounts of dibenzo[b,d]thiophene (11), from competing decarbonylation.     

Exploratory flash vacuum pyrolysis of some cyclopentadienyliron complexes

At 500°C, flash vacuum pyrolysis of (η⁵-C₅H₅)₂Fe₂(CO)₄ provides a rapid, practical synthesis of the tetranuclear complex (η⁵-C₅H₅)₄Fe₄(μ₃-CO)₄: at higher temperatures, ferrocene is formed. Ferrocene itself undergoes little change under flash vacuum pyrolysis conditions, even at 725°C. Pivaloylferrocene is unchanged at 650°C but cracks to yield isobutene between 675 and 700°C; this reaction does not proceed by simple elimination since formylferrocene can be recovered unchanged under flash vacuum pyrolysis conditions which give substantial quantities of isobutene from pivaloylferrocene.     

Facile synthesis of reactive benzoazetinone by flash vacuum pyrolysis of isatoic anhydride

Pyrolysis of isatoic anhydride (3) at 550°C and ca. 1×10⁻² torr gave benzoazetinone (1, 80%) as the only product. The existence of 1 was confirmed by low-temperature NMR spectroscopy at −90°C. Above −20°C, 1 was converted to dimer (4, 50%), trimer (5, 22%), and anthranilic acid (6, 12%). Pyrolysis of 3 at 800°C and ca. 1×10⁻² torr gave 1-cyanocyclopentadiene (7) in 38% yield.     

Preparation of isobenzofurandiones by flash vacuum pyrolysis involving retro-Diels-Alder expulsion of ethylene and concomitant C-O cleavage of methoxy or ethylenedioxy groups

Flash vacuum pyrolysis (fvp) of a number of substrates, prepared by hydrogenating adducts derived from dimethoxy- or ethylenedioxy-substituted benzynes and furan, affords isobenzofurandiones through retro-Diels-Alder expulsion of ethylene and C-O bond cleavage of the methoxy or ethylenedioxy groups. The parent isobenzofuran-4,5-dione is reactive and undergoes two-fold conjugate addition of water to afford 3,4-dihydroxybenzene-1,2-dicarboxaldehyde.     

The method of flash vacuum pyrolysis,preparative applications and analytical studies

Flash vacuum pyrolysis has been used to prepare a variety of differently substituted derivatives of the benzocyclobutene ring system starting with simple precursors. An analytical gas flow reactor is described which simulates reaction conditions of flash vacuum pyrolysis experiments. This reactor allows to optimise reaction conditions and to obtain structure reactivity correlations for thermolytic gas phase reactions.     

Flash flow pyrolysis: mimicking flash vacuum pyrolysis in a high-temperature/high-pressure liquid-phase microreactor environment

Flash vacuum pyrolysis (FVP) is a gas-phase continuous-flow technique where a substrate is sublimed through a hot quartz tube under high vacuum at temperatures of 400-1100 °C. Thermal activation occurs mainly by molecule-wall collisions with contact times in the region of milliseconds. As a preparative method, FVP is used mainly to induce intramolecular high-temperature transformations leading to products that cannot easily be obtained by other methods. It is demonstrated herein that liquid-phase high-temperature/high-pressure (high-T/p) microreactor conditions (160-350 °C, 90-180 bar) employing near- or supercritical fluids as reaction media can mimic the results obtained using preparative gas-phase FVP protocols. The high-T/p liquid-phase "flash flow pyrolysis" (FFP) technique was applied to the thermolysis of Meldrum's acid derivatives, pyrrole-2,3-diones, and pyrrole-2-carboxylic esters, producing the expected target heterocycles in high yields with residence times between 10 s and 10 min. The exact control over flow rate (and thus residence time) using the liquid-phase FFP method allows a tuning of reaction selectivities not easily achievable using FVP. Since the solution-phase FFP method does not require the substrate to be volatile any more--a major limitation in classical FVP--the transformations become readily scalable, allowing higher productivities and space-time yields compared with gas-phase protocols. Differential scanning calorimetry measurements and extensive DFT calculations provided essential information on pyrolysis energy barriers and the involved reaction mechanisms. A correlation between computed activation energies and experimental gas-phase FVP (molecule-wall collisions) and liquid-phase FFP (molecule-molecule collisions) pyrolysis temperatures was derived.     

Gas-phase detection of HSOH: synthesis by flash vacuum pyrolysis of di-tert-butyl sulfoxide and rotational-torsional spectrum

Gas-phase oxadisulfane (HSOH), the missing link between the well-known molecules hydrogen peroxide (HOOH) and disulfane (HSSH), was synthesized by flash vacuum pyrolysis of di-tert-butyl sulfoxide. Using mass spectrometry, the pyrolysis conditions have been optimized towards formation of HSOH. Microwave spectroscopic investigation of the pyrolysis products allowed-assisted by high-level quantum-chemical calculations--the first measurement of the rotational-torsional spectrum of HSOH. In total, we have measured approximately 600 lines of the rotational-torsional spectrum in the frequency range from 64 GHz to 1.9 THz and assigned some 470 of these to the rotational-torsional spectrum of HSOH in its ground torsional state. Some 120 out of the 600 lines arise from the isotopomer H(34)SOH. The HSOH molecule displays strong c-type and somewhat weaker b-type transitions, indicating a nonplanar skew chain structure, similar to the analogous molecules HOOH and HSSH. The rotational constants (MHz) of the main isotopomer (A=202 069, B=15 282, C=14 840), determined by applying a least-squares analysis to the presently available data set, are in excellent agreement with those predicted by quantum-chemical calculations (A=202 136, B=15 279, C=14 840). Our theoretical treatment also derived the following barrier heights against internal rotation in HSOH (when in the cis and trans configurations) to be V(cis) approximately equal to 2216 cm(-1) and V(trans) approximately equal to 1579 cm(-1). The internal rotational motion results in detectable torsional splittings that are dependent on the angular momentum quantum numbers J and K(a).     

Synthesis of 1.6-diazabiphenylene by flash vacuum pyrolysis of 2,5,9,10-tetra-azaphenanthrene; NMR evidence for rehybridisation in biphenylenes

A mixed Ullmann reaction between 2- and 4-chloro-3-nitropyridines furnished 3,3′-dinitro-2,4′-bipyridyl and reduction to 2,5,9,10-tetra-azaphenanthrene followed by thermal extrusion of nitrogen gave 1,6-diazabiphenylene; different complexing abilities of the two nitrogen atoms of this compound were revealed by ¹H nmr using Eu(fod)₃ which are consistent with previous work on the rehybridisation in strained ring systems and the value of ¹J_CH in biphenylene.     

Synthesis of N,N-di(arylmethylidene)arylmethanediamines by flash vacuum pyrolysis of arylmethylazides

Flash vacuum pyrolysis of arylmethylazides 7a-d gave 2,4-diazapentadienes 5a-d in high yield (76-92%). The thermal cyclization of 5a-d gave cis-imidazolines 1a-d, further heating or Swern oxidation of 1a-d gave dehydrogenated products, imidazoles 2a-d.     

Gasification and catalytic conversion of biomass by flash pyrolysis

The influence of the water content, the gasification atmosphere and the action of inorganic catalysts on biomass pyrolysis is discussed. The pyrolysis conditions are characterized by a heating rate of a wood sample of 250–300°C/s and a residence time of the gas in the hot zone of the reactor of less than 1 s. Volumes, carbon contents, weights and energy balances at different temperatures (600–1000°C) are tabulated. The total volume of pyrolysis gas and the yields increase with increasing temperature and water content. This increase is especially due to the production of hydrogen. A large part of the gasified hydrogen comes from water. Water that has not been driven off from green wood is more gasifiable than water that is re-added or from the atmosphere. Three types of catalysts were tested: alumina, aluminosilicate material and nickel-supported catalyst. The results show that it is possible to control the distribution of gas species by controlling the water content of the biomass and selecting the catalyst on which the wood is pyrolysed. A nickel catalyst on mordenite seems to be very efficient in directing pyrolysis gas production towards synthesis gas.     

Isomerization of linear to angular [3]phenylene and PAHs under flash vacuum pyrolysis conditions

[structure: see text]. Flash vacuum pyrolysis (FVP) of linear [3]phenylene affords its angular counterpart and the same mixture of polycyclic aromatic hydrocarbon isomers as that observed on FVP of angular [3]phenylene. A mechanism, supported by a 13C labeling study, is proposed to explain these results.     

An empirical study of the effect of the variables in a flash vacuum pyrolysis (FVP) experiment

The effect of the variation of the experimental parameters on the conversion of precursor to products in a typical flash vacuum pyrolysis (FVP) experiment was investigated empirically. Temperature-conversion plots can be used to optimise FVP conditions and their mechanistic significance is exemplified. At a given temperature, the conversion can be increased by an increase in the background pressure, or by packing a section of the furnace tube with inert material (particularly when placed at the trap end of the furnace tube) or by employing a catalyst. Despite the prevailing view that only intramolecular reactions take place by FVP, it has been shown by a 'dual-FVP' cross-over experiment that the dimerisation of benzyl radicals occurs in the gas-phase, before the cold trap, under standard conditions. However, reduction in through-put rate, increase in furnace temperature and reduction in background pressure all reduce the amount of gas-phase coupling.     

Synthesis of spirocyclosiloxanes by flash vacuum pyrolysis of 2,7-dimethyl-2,3: 7,8-diepoxy-5-silaspiro[4.4]nonane and cyclosiloxanes

Flash vacuum pyrolysis (FVP) of 2,7-dimethyl-2,3 : 7,8-diepoxy-5-silaspiro[4.4]-nonane with cyclotetra- or cyclopenta-siloxanes (D₄ or D₅) leads to products which apparently result from transannular insertion of [OSiO] or an equivalent synthon inti SIO single bonds of D₄ or D₅: 2,4,6,8,10-pentasila-1,3,5,7,9,11-hexaoxaspiro[5.5]undecane (D₂QD₂) and 2,4,6,8,10,12-hexasila-1,3,5,7,9,11,13-heptaoxaspiro[5.7]tridecane (D₃QD₂) respectively. The scope of this reaction has been explored with substituted cyclotetrasiloxanes such as heptamethylcyclotetrasiloxane, vinylheptamethylcyclotetrasiloxane, phenylheptamethylcyclotetrasiloxane, and chloromethylheptamethylcyclotetrasiloxane. In all cases, substituted spirocyclosiloxane products (D₂QDD^x) were obtained. ²⁹Si NMR has proved particularly useful for the assignment of structure of these spirocyclosiloxanes.     

Improved technique of direct oxygen microdetermination by static flash combustion pyrolysis

Summary The direct oxygen determination by static flash combustion pyrolysis in an empty quartz tube has been improved in two respects. The working temperature of 1120° C given originally is lowered to 900° C by the use of platinized carbon. The 15-minutes period considered necessary for the pyrolysis of the sample and for the conversion of oxygen into carbon monoxide is shortened by half. The method has been successfully applied for the determination of oxygen in sulphur-containing compounds by addition of a liquid-nitrogen trap to the apparatus.

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Zusammenfassung Die direkte Mikrobestimmung des Sauerstoffes nach der statischen Schnellverbrennungspyrolyse im leeren Quarzrohr wurde in zweierlei Hinsicht verbessert. Eine Erniedrigung der Arbeitstemperatur von 1120° C auf 900° C wurde durch die Anwendung von platiniertem Gasruß ermöglicht. Außerdem wurde die als notwendig angegebene Zeit von 15 Minuten für die Pyrolyse der Probe und die Überführung des Sauerstoffes in Kohlenmonoxid auf die Hälfte verkürzt.Die Methode wurde auch erfolgreich auf die Bestimmung des Sauerstoffes in schwefelhaltigen Substanzen angewendet, nachdem die Apparatur mit einer Kühlfalle ausgestattet war.

     

One-pot synthesis of 7H-dibenzo[b,d]azepin-7-one by heterogeneous flash vacuum pyrolysis with MCM-41 catalysts

Homogeneous and heterogeneous flash vacuum pyrolysis (fvp) reactions of 2-(1H-1,2,3-benzotriazol-1-yl)phenylethanone (1) are reported. Heterogeneous reactions were carried out with Al-MCM-41 catalysts, mesoporous molecular sieves of the type M41S. In both cases, 7H-dibenzo[b,d]azepin-7-one (4) was the major product; however, in the catalytic reactions, yields and selectivity were very high. A mechanism for this reaction is also discussed.     

Acephenanthrylenes from flash vacuum thermolysis of diarylmethylidenecycloproparenes

Upon flash vacuum thermolysis at 750 °C fluorenylidenecyclopropa[b]naphthalene (1) undergoes opening of the three-membered ring and rearrangement to give a range of C₂₄H₁₄ polycyclic aromatic hydrocarbons. Dibenz[e.l]- and -[e.k]acephenanthrylene (7) and (12), respectively, have been identified while the plausible naphth[1,2-e]- and [2,3-e]acephenanthrylenes (9) and (14) were not detected. With diphenylmethylidenecyclopropa[b]naphthalene (2) cyclodehydrogenation and rearrangement also provide C₂₄H₁₄ polycycles; dibenz[e.k]acephenanthrylene (12) is identified and dibenz[a.e]aceanthrylene (15) is a proposed product.     

A new route to vinyltributyltin compounds by flash pyrolysis of alkyltributyltin acetates

Flash pyrolysis of alkyltributyltin acetates, at high temperatures (600–850° C) under a moderate vacuum provides a convenient route to vinyltin derivatives.