Pyrolysis flash vacuum

TMs technique is not often encountered in synthetic organic chemistry, but it an prove invaluable in some circumstances. As with most of the topics in j is chapter, the exact type of apparatus used will depend on what is available in your department. One simple (schematic) set up is shown in g. 14.4. It is important to vaporize the substrate at the appropriate rate, ijid to make sure that the thermolysis temperature is correct If this formation is not available then some experimentation will inevitably have 13 be carried out A typical vaporization rate might be between 0.5 and l.Og jer h. 

Let us briefly touch on two more examples of FVP in the literature, although FVP has been used for many other useful transformations in the synthesis of organic compounds. 

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A2acyclobutenes have been used to generate l-a2abutadienes, which are intramolecularly as weU as intermolecularly cycli2ed to give tetrahydropyridines, eg, hexahydroquinoli2in-4-one [87842-80-6] (64,65). In the following, FVP = flash vacuum pyrolysis. 

Flash vacuum pyrolysis of 2-methoxycarbonylpyrrole (11) gives the ketene (12), characterized by IR absorption at 2110 cm. On warming to -100 to -90 °C the dimer (13) is formed (82CC360). Flash vacuum pyrolysis of indole-2-carboxylic acid (14) results in loss of water and the formation of a ketene (15) showing absorption at 2106 cm (82CC360). 

The intermediacy of N-arylbenzotriazoles in the formation of carbazoles from o-anilinobenzenediazonium salts has already been mentioned in Section 3.03.2.3. The parallel conversion of 1,4- and 1,5-diphenyl-l,2,3-triazoles to 3-phenylindole with minor amounts of the 2-isomer has been effected by flash vacuum pyrolysis (Scheme 106a) (75JCS Pl)l). Similar treatment of 1,3,5- or 3,4,5-triphenyl-1,2,4-triazole provides 1,3-diphenylisoindole (Scheme 106b) <75JCS(P1)12>. 

Although formally it could be classified with the ring transformations (Section 4.04.3.2.2), conversion of 2,4-diphenyl-l,3,4-oxadiazol-2-one (593) by flash vacuum pyrolysis at 500 °C into 3-phenylindazole (595) involves a C(3)—C(3a) ring closure of the diphenylnitrilimine (594) (79AG(E)721). 

Acetonitrile oxide was generated from 3,4-dimethylfuroxan oxide by flash vacuum pyrolysis and trapped at -40 °C where its and NMR spectra were examined. Warming to room temperature in the presence of propane produced 3,5-dimethyl-2-isoxazoline (Scheme 108) (79TL2443). The oxide could also be generated by photolysis of furoxan (68CC977). 

The conversion of small rings to smaller ones, without loss, is not common. 3-Chloroazetidine isomerizes reversibly to 2-chloromethylaziridine (Section 5.09.2.2.5). Flash vacuum pyrolysis can convert isoxazoles to azirines (Section 5.04.4.3). More common is the isomerization of medium-sized, i.e. five- or six-membered rings, e.g. certain succinimides (Scheme 23) (81JOC27) to azetidinediones, or bicyclic 1,2-dioxetanes to bis-oxiranes (Section 5.05.4.3.2). 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

Although by no means a preparative route to either 1- or 2-azirines, the elimination of nitrogen (by flash vacuum pyrolysis at 400 °C) from the regioisomeric 4,5-disubstituted IH-1,2,3-triazoles (376) leads to similarly regioisomeric 1-azirines (377) <73JCS(P1)550). 

A variety of acyclic and cyclic S-N compounds decompose at moderate temperatures (100-150°C) with the formal loss of a symmetrical NSN fragment, but this molecule has never been detected. The lowest energy isomer, linear NNS, is generated by flash vacuum pyrolysis of 5-phenyl-1,2,3,4-thiatriazole (Eq. 5.1). ... 

The mechanism of this reaction has not been thoroughly explored. Some work has been done in analysis of potential intermediates for the reaction, although these intermediates were generated using flash vacuum pyrolysis (FVP). Materials in this experiment were trapped and IR spectrum suggested the formation of a ketene prior to cyclization. 

Alternative techniques, such as flash vacuum pyrolysis vide infra-, 30 —> 33), have been applied to the Gould-Jacobs reaction. Use of microwave has in some cases provided an... 

The 5-substituted 1,3-dioxolan-4-one 23 is readily deprotonated at the 5 position and can be alkylated with a variety of alkyl halides. The resulting products 24 decompose upon flash vacuum pyrolysis (FVP) at 600°C with loss of acetone... 

Flash vacuum pyrolysis of alkenylbenzoxazine 190 gave 192 whose low yield was attributed to the competition between H-shift and intramolecular cycloaddition in the intermediate 191 (82TL4501) (Scheme 36). 

Cyclocondensation of 637 with ethanolamine gave the benzoxazoloquino-line 638a (83KG1664). Flash vacuum pyrolysis of 639 gave 638b (93JCS(CC)794) (Scheme 110). 

Flash-vacuum pyrolysis of 3-alkyl-or 3-aryl-2-azabicyclo[3.2.0]hepta-2,6-dienes 5a-c, prepared by the action of a Grignard reagent (RMgX) on the 3-methoxy derivative 5 (R = OMe), furnishes mixtures of the 2- and 7-substituted 3//-azepines 6 and 7, respectively.113... 

In contrast, flash-vacuum pyrolysis (FVP)155 or spray-vacuum pyrolysis (SVP)154 of the homologous phenethyl azidoformates yield, in every instance except for the 4-cyano and 4-nitro-derivatives, the thermally stable [1,3]oxazino[3,4-a]azepines 9 accompanied by lesser amounts of the oxazolidinones 10, formed by nitrene insertion at the benzylic carbon center. 

The first and to date only synthesis of the parent system 2 uses a flash-vacuum pyrolysis (FVP) of 7,8-diazapentacyclo[4.2.2.02-5.03 9.04 10]dec-7-ene (diazabasketene, 1). After condensation at -196 °C, the pyrolysis product is distilled in vacuum to give pure azocine in ca. 60% yield.12... 

Benzannulated azocines can be prepared starting from 4-phenyl-l,2.3-benzotriazine (16), flash-vacuum pyrolysis of which leads to 2-phenylbenzazete (17) (cf. Houben-Weyl. Vol. E16c, p 939), which is stable until about 40 °C and easily enters into cycloaddition reactions with dienes. With tetraphenylcyclopentadienone, a nonisolable adduct is formed which, by loss of carbon monoxide, gives an azabicyclo[4.2.0]octatriene derivative that isomerizes to the 1 -benzazocine 18.22... 

The combination of the flash vacuum pyrolysis (FVP) technique169 with mass spectrometry proved to be particularly useful in identification and characterization of both the fragmentation/rearrangement patterns, intermediates and/or final products formed (see Section IV.E.l). Usually, no structures are indicated in the mass spectra, although ionization and appearance potential can, in principle, provide structural information. 

In the case of flash vacuum pyrolysis, the reaction may be of some synthetic utility. For example, at 700 °C diphenyl sulphoxide produces diphenyl thiosulphonate in 40% yield, however, several other products are also formed. 

Phosphorus ylids are quite common (see 16-47) and keto-phosphorus ylids (RCOCH=PPh3) are also known. When these compounds are heating (flash vacuum pyrolysis, FVP) to great than >500°C, alkynes are formed. Simple alkynes can... 

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