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Curt Wentrup - One of the best experts on this subject based on the ideXlab platform.
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Flash Vacuum Pyrolysis
2018Co-Authors: Curt WentrupAbstract:Flash Vacuum Pyrolysis (FVP) allows the generation, direct observation, and in many cases isolation of reactive intermediates and unusual molecules such as carbenes, nitrenes, free radicals, and cumulenes. Detailed characterization of unstable species is achieved in conjunction with matrix isolation or gas‐phase spectroscopic techniques. In addition, FVP has many important applications in organic synthesis, where reactions are often initiated by extrusion of small molecules (eg, N2, CO, CO2, or SO2), or by elimination (eg, HCl, CH3COOH, HNCO, or isobutene), or by (retro)pericyclic reaction. These reactions are chemoselective, stereospecific, and usually proceed via the lowest activation energy paths to the products, which are often formed in high yields. Photochemistry is often complementary to Pyrolysis, and the two methods in combination are powerful tools in investigations of chemical reactions.
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Flash Vacuum Pyrolysis techniques and reactions
Angewandte Chemie, 2017Co-Authors: Curt WentrupAbstract:Flash Vacuum Pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding Pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser Pyrolysis, pulsed Pyrolysis with supersonic jet expansion, very low pressure Pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and Pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.
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Flash Vacuum Pyrolysis of azides triazoles and tetrazoles
Chemical Reviews, 2017Co-Authors: Curt WentrupAbstract: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...
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Sulfonyl isocyanides RSO2NC
2017Co-Authors: Curt Wentrup, Horst BriehlAbstract:Flash Vacuum Pyrolysis (FVP) of 5-azido-1-aryltetrazoles results in triple N2 elimination and formation of aryl isocyanides RNC, which rearrange in part to aroylnitriles RCN under the reaction conditions. Similar FVP of 5-azido-1-arenesulfonyltetrazoles generates a compound absorbing in the IR spectrum (77 K) at 2090 cm-1 and assigned the structure of arenesulfonyl isocyanide, ArSO2NC 11. FVP at temperatures above 600 oC results in progressively more nitrile ArSO2CN 12. Compound 11 also disappears on warming above -80 o
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rearrangements of 4 quinolylcarbene 3 quinolylcarbene and 2 quinolylcarbene to 1 naphthylnitrene and cyanoindenes by falling solid Flash Vacuum Pyrolysis
Journal of Organic Chemistry, 2016Co-Authors: Nigel Aylward, Jurgen Becker, David Kvaskoff, Curt WentrupAbstract:The relationship between 4-quinolylcarbene 17, 3-quinolylcarbene 21, 2-quinolylcarbene 25, and 1-naphthylnitrene 35 has been explored experimentally and computationally. The diazomethylquinolines generated from (5-tetrazolyl)quinolines or 1,2,3-triazolo[1,5-a]quinoline by conventional Flash Vacuum Pyrolysis (FVP) were observed by IR spectroscopy. The carbenes were generated by falling solid Flash Vacuum Pyrolysis (FS-FVP). 4-Quinolylcarbene 17 was found to rearrange to 3-quinolylcarbene 21 and then to 2-quinolylcarbene 25, and finally via 1-naphthylnitrene 35 to 1-cyanoindene 36, which then isomerizes to 3- and 2-cyanoindenes 12 and 13. The thermal rearrangement of 2-quinolylcarbene to 1-naphthylnitrene was verified by ESR spectroscopy. The reaction mechanism has been elucidated with the help of calculations of the structures and energies of the quinolylcarbenes and 1-naphthylnitrene and the intervening aza-benzobicyclo[4.1.0]heptatrienes, aza-benzocycloheptatetraenes, and aza-benzocycloheptatrienylidenes and the transition states connecting them at the B3LYP/6-31G* level. The nonobserved 1,2-hydrogen shifts in aza-benzocycloheptatetraenes/aza-benzocycloheptatrienylidenes are found to have very high activation barriers.
Hamish Mcnab - One of the best experts on this subject based on the ideXlab platform.
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new synthetic routes to pyrrolo 1 2 a and 1 2 c imidazol 5 ones by Flash Vacuum Pyrolysis
ChemInform, 2010Co-Authors: Hamish Mcnab, C ThornleyAbstract:1-Substituted and 1,3-disubstituted pyrrolo[1,2-c]imidazol-5-ones 25–27 have been made in fair to excellent yield by Flash Vacuum Pyrolysis (FVP) of the Meldrum’s acid precursors 11–13. FVP of the appropriate propenoate precursor 23, 22 and 18–20 gives pyrrolo[1,2-c]imidazol-5-one 2, pyrrolo[1,2-a]imidazol-5-one 3 and their 6-methyl derivatives 28–30 respectively in 32–90% yield. The mechanism of the propenoate pyrolyses involves rate determining E to Z isomerisation of the alkene followed by elimination of an alcohol and electrocyclisation to the fused ring products.
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formation of dibenzofurans by Flash Vacuum Pyrolysis of aryl 2 allyloxy benzoates and related reactions
Organic and Biomolecular Chemistry, 2010Co-Authors: Michael Black, J I G Cadogan, Hamish McnabAbstract:Flash Vacuum Pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl 2-(allylthio)benzoates 6 at 650 °C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or S-allyl) bond, followed by ipso attack at the ester group, loss of CO2 and cyclisation of the resulting aryl radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted dibenzofurans 19b–f (73–90%) and dibenzothiophenes 20b–c (90–94%). Little selectivity is shown in the cyclisation of m-substituted substrates and competing interactions of the radical with the substituent—and ipso-attack—complicate the pyrolyses of o-substituted substrates. FVP of related radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas 2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of 4-methylphenyl 2-(allylamino)benzoate 42.
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methylene meldrum s acid derivatives of indoxyl and their cyclization reactions under Flash Vacuum Pyrolysis conditions
Synthesis, 2010Co-Authors: Alexander P Gaywood, Hamish McnabAbstract:Pure indoxyl can be obtained in 75% yield by Flash Vacuum Pyrolysis (FVP) of 2'-azidoacetophenone at 650 °C. Reaction of indoxyl with methoxymethylene Meldrum's acid takes place at the 1-position, and FVP of the resulting derivative provides 1-hydroxy-9H-pyrrolo[1,2-a]indol-9-one (54%). FVP of the isomeric 2-methylene compound gives pyrano[3,2-b]indol-2(5H)-one (42%).
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Synthesis of Azapyrrolo[3,2,1-jk]carbazoles, Azaindolo[3,2,1-jk]carbazoles, and Carbazole-1-carbonitriles by Gas-Phase Cyclization of Aryl Radicals
Synthesis, 2010Co-Authors: Lynne A. Crawford, Hamish Mcnab, Andrew R. Mount, Jeanne Verhille, Stuart I. WhartonAbstract:Flash Vacuum Pyrolysis of N-(2-nitroheteroaryl)indoles or -carbazoles at 875 °C gave aza analogues of strained pyrrolo[3,2,1-jk]carbazole (50-55%) and indolo[3,2,1-jk]carbazole (55-85%) ring systems, respectively, through generation of aryl radicals and cyclization. The corresponding reactions of N-(2-nitroheteroaryl)indazoles and -benzimidazoles at 850 °C, on the other hand, gave carbazole-1-carbonitrile derivatives (56-64%) by a mechanism involving radical ring opening and hydrogen atom rearrangement.
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synthetic routes to pyrrolizine 1 5 dione derivatives by Flash Vacuum Pyrolysis of amidomethylene derivatives of meldrum s acid
Organic and Biomolecular Chemistry, 2009Co-Authors: Hamish Mcnab, Mark Morrow, Simon Parsons, David A Shannon, Kirsti WithellAbstract:Methoxymethylene Meldrum's acid 1 reacts with 5- and 6-membered lactams in refluxing acetonitrile to give the N-substituted products 9–15. If the reactions are continued for extended times, the Meldrum's acid derivatives decompose to provide enamidoesters e.g.22–24. Flash Vacuum Pyrolysis of the 5-membered ring products 9–13 provides reasonable yields of the fused pyrrolones 31–35. The constitution of the products is supported by X-ray crystal structures of 10, 12, 19, 32 and 34.
Alan R Aitken - One of the best experts on this subject based on the ideXlab platform.
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Flash Vacuum Pyrolysis of benzylidene halides benzotrihalides and aryl halides over magnesium
Journal of Analytical and Applied Pyrolysis, 2017Co-Authors: Alan R Aitken, Philip K G Hodgson, Adebayo O OyewaleAbstract:Abstract Flash Vacuum Pyrolysis at 600 °C through glass wool coated with freshly sublimed magnesium is examined as a preparative method for dehalogenative coupling in organic synthesis. Substituted benzylidene chlorides give predominantly the corresponding stilbenes and in some cases these are readily isolated in pure form. With an ortho -halogen substituent, additional cyclisation gives phenanthrenes but the method is not compatible with the presence of several reactive groups. An ortho- methoxy substituent leads to unexpected formation of mono- and dimethyl products. With 1,4-bis(dihalomethyl)benzenes, halogenated polymers are deposited directly from the gas phase via generation of halogenated p -xylylenes. The 1,2- and 1,3-isomers lead respectively to benzocyclobutadiene, isolated as a dimer, and to pyrene. The 1,4-bis(trihalomethyl)benzenes give more highly halogenated polymers directly from the gas phase via halogenated p -xylylenes. While halobenzenes generally give the corresponding benzenes and biphenyls, 1,2-dihalobenzenes additionally produce triphenylene in preparatively useful yield by a process not involving intermediacy of free benzyne.
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syntheses of combretastatins d 1 d 2 and d 4 via ring contraction by Flash Vacuum Pyrolysis
Journal of Organic Chemistry, 2017Co-Authors: Marina Harras, Wolfgang Milius, Alan R Aitken, Rainer SchobertAbstract:We report the syntheses of combretastatins D-2 and D-4 as well as a formal synthesis of combretastatin D-1 by a conceptually new route harnessing a ring-contracting Flash Vacuum pyrolytic extrusion of sulfur dioxide from the respective 16-membered sulfone precursors. Via Flash Vacuum Pyrolysis, even metaparacyclophanes as small and strained as the hitherto unknown oxa[1.5]metaparacyclophane could be prepared as a side product en route to combretastatin D-2 by synchronous extrusion of SO2 and CO2.
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Syntheses of Combretastatins D‑1, D‑2, and D‑4 via Ring Contraction by Flash Vacuum Pyrolysis
2016Co-Authors: Marina Harras, Wolfgang Milius, Alan R Aitken, Rainer SchobertAbstract:We report the syntheses of combretastatins D-2 and D-4 as well as a formal synthesis of combretastatin D-1 by a conceptually new route harnessing a ring-contracting Flash Vacuum pyrolytic extrusion of sulfur dioxide from the respective 16-membered sulfone precursors. Via Flash Vacuum Pyrolysis, even metaparacyclophanes as small and strained as the hitherto unknown oxa[1.5]metaparacyclophane could be prepared as a side product en route to combretastatin D-2 by synchronous extrusion of SO2 and CO2
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recent advances in the synthesis of heterocyclic compounds using Flash Vacuum Pyrolysis
Advances in Heterocyclic Chemistry, 2015Co-Authors: Alan R Aitken, Youcef BoubaloutaAbstract:Abstract Applications of Flash Vacuum Pyrolysis to the synthesis of heterocyclic compounds published between 2000 and 2015 are described in six major sections with 150 literature references. The emphasis is on methods suitable for preparation of stable heterocyclic products in multi-gram quantities. Consideration of the relatively few reports of synthesis of three- and four-membered rings is followed by the largest single section on the synthesis of five-membered rings, mainly those containing one or two nitrogen atoms. A further major section discusses synthesis of six-membered ring products, again predominantly containing nitrogen, before the final shorter sections on seven-membered, eight-membered, and larger rings.
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cascade synthesis of new tetracyclic heteroaromatic thieno 2 3 b pyridine containing ring systems
ChemInform, 2010Co-Authors: Alan R Aitken, Alasdair N GarnettAbstract:Flash Vacuum Pyrolysis of appropriate stabilised ylides, prepared in a few steps from 2-(methylthio)nicotinic acid, give products containing previously unknown naphtho-, phenanthro-, benzothieno- and benzofuro-fused thieno[2,3-b]pyridine ring systems.
Yoshito Tobe - One of the best experts on this subject based on the ideXlab platform.
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formation of naphthodithiophene isomers by Flash Vacuum Pyrolysis of 1 6 di 2 thienyl and 1 6 di 3 thienyl 1 5 hexadien 3 ynes
Comptes Rendus Chimie, 2009Co-Authors: Rui Umeda, Motohiro Sonoda, Hiroshi Fukuda, Koji Miki, S Abdur M Rahman, Yoshito TobeAbstract:Abstract To develop a synthetic method of thiophene-based polycyclic aromatic compounds, we investigated the tandem cyclization of 1,6-dithienyl-1,5-hexadien-3-ynes under Flash Vacuum Pyrolysis (FVP) conditions at high temperatures (850 or 1050 °C). As a result, several isomeric naphthodithiophene derivatives were obtained as mixtures, from which a few isomers were isolated. The structural assignments of the products were performed on the basis of combination of the experimental and calculated 1 H NMR chemical shifts of either purified products or mixtures of them. Plausible mechanisms for the formation of the products are proposed.
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Flash Vacuum Pyrolysis of 1 6 diphenyl 1 5 hexadien 3 ynes tandem diaryldienyne cyclizations to form chrysene
Tetrahedron Letters, 2002Co-Authors: Motohiro Sonoda, Kayo Itahashi, Yoshito TobeAbstract:Flash Vacuum Pyrolysis of 1,6-diphenyl-1,5-hexadien-3-yne at 1000°C and its bromo derivative at 800°C yielded chrysene as the major product through tandem diaryldienyne cyclizations.
Stuart I. Wharton - One of the best experts on this subject based on the ideXlab platform.
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Synthesis of Azapyrrolo[3,2,1-jk]carbazoles, Azaindolo[3,2,1-jk]carbazoles, and Carbazole-1-carbonitriles by Gas-Phase Cyclization of Aryl Radicals
Synthesis, 2010Co-Authors: Lynne A. Crawford, Hamish Mcnab, Andrew R. Mount, Jeanne Verhille, Stuart I. WhartonAbstract:Flash Vacuum Pyrolysis of N-(2-nitroheteroaryl)indoles or -carbazoles at 875 °C gave aza analogues of strained pyrrolo[3,2,1-jk]carbazole (50-55%) and indolo[3,2,1-jk]carbazole (55-85%) ring systems, respectively, through generation of aryl radicals and cyclization. The corresponding reactions of N-(2-nitroheteroaryl)indazoles and -benzimidazoles at 850 °C, on the other hand, gave carbazole-1-carbonitrile derivatives (56-64%) by a mechanism involving radical ring opening and hydrogen atom rearrangement.