Flash thermolysis

Oxiranes have been made by extrusion of nitrogen from oxadiazolenes (Scheme 81) (B-73MI50502), by flash thermolysis of dioxazolones (Scheme 82) (81JA5414), and (cc-lactones) by photolysis of l,2-dioxole-3,5-diones (Scheme 83) (73JOC2269). 

Flash thermolysis of compounds of the type (120), derivatives of Meldrum s acid , is a fairly general synthesis of ketenes (Scheme 103). Brown and coworkers (77AJC179) found that the spirooxirane (121) gave ketene, possibly via the expected carbonyloxirane (122) and probably by isomerization of carbenaoxirane (Scheme 104). 

Barrellene oxide flash thermolysis oxirenes in, 7, 125 Basic Violet 10 (C.I. 45170), 3, 879 Batrachotoxin, 4, 374 Baumgarten method cinnoline synthesis, 3, 44... 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Similarly, the use of a higher input power in retro-Diels-Alder reactions of anthracene derivatives has been reported to afford complete reaction in 3-5 min [36], This method is an alternative to the use of flash thermolysis. The use of graphite is a prerequisite for obtaining high temperatures in a short time. 

The 1,2,4,3-triazaphospholes are colorless or pale yellow distillable liquids or crystalline solids (76T2039,80ZN(B)1222>. They are not oxidized by air and are reluctant to react with sulfur. The flash thermolysis of two 1,2,4,3-triazaphospholes has been investigated <90PS(49/50)363>. 

The flash thermolysis of the dioxolene (124) gave diphenylketene among the products (Scheme 107) (70CC206), and it is conceivable that diphenyloxirene is the precursor of this product. 

Flash thermolysis of 4-substituted isoxazol-5(4//)-ones can be used to generate alkynes, isocyanides, aminoisocyanides and ketenimines (77C258, 76HCA2615). Decomposition of the oxime (538) at an oven temperature of 450 °C produces C02, benzonitrile and fulminic acid (539 equation 5) (79AG(E)467). This method thus offers a safe alternative to the synthesis of fulminic acid from the explosive metal fulminates. 

With a few exceptions these species dimerize readily [60, 119, 174-176], and most of the methods applicable to the synthesis of monomers involve thermolytic processes such as the flash thermolysis of 1,2,3-thiadiazoles [177,178], or of 5-silylated ketenedithioacetals [179]. 

Pyrrolizin-3-one (94a) is formed in 98% yield by flash thermolysis of 96, which was obtained from 2-formylpyrrole and Meldrum s acid. A possible ketene intermediate then reacts in an intramolecular acylation.66... 

Phenyl(trimethylsilyl)carbene (21) has been generated from phenyl(trimethylsilyl)diazo-methane (20) by gas-phase pyrolysis39,40 as well as by thermolysis97 or photolysis33,40,98,99 in solution, by flash thermolysis of the tosylhydrazone lithium salt 18040, and by pyrolysis... 

In recent work (2008) ultrafast photolysis of a potential diazo ketone precursor of p-biphenylmethyloxirene failed to detect the oxirene, the UV absorption of which could, however, have been hidden by another band [8], and in a combined experimental/computational (ab initio and molecular dynamics with DFT) study, flash thermolysis of a formal Diels-Alder adduct was interpreted as affording acetylmethyloxirene and benzene [9]. 

This section is concerned only with the thermal rearrangements of 1-trimethyl-siloxy, 1-alkoxy-, 1-phenylthio- and 1-trimethylsilylvinylcyclopropanes into cyclo-pentene derivatives, which occurred either on heating in the liquid phase (sealed tube) at about 300 °C for 30 min, or by passing through a conditioned hot tube at 300 °C with a contact time of 4 sec or by flash thermolysis at 600 °C for 10 m sec3). 

Ring opening of the oxaspiropentane 343 upon treatment with sodium phenylselenide (vide supra, Sect. 4.5, Eq. (34)) 59) and O-silylation produce the vinylcyclopropanol trimethylsilyl ether 344 which, on flash thermolysis at 670 °C, gave the siloxycyclo-pentene 345 as a 2 1 mixture of epimers at C(8). Then, allylation of the more substituted enolate arising from 345, opens a convenient way to the antitumor agent, aphidicolin 346 181>. 

The photochemical and thermolytic breakdown of cyclic sulfones offers a route to cyclopropanes, as shown in equation (4). The utility of the reactions is limited in that an aromatic group (57 R = Ar) is required for good yields to be obtained. Flash thermolysis of (57 R = Me) at 750 °C gives considerable quantities of acetaldehyde and 2-butene together with methylcyclopropane (79JCS(Pl)950). 

Thermolysis of thiete 1,1-dioxide (116), either in the vapor phase or in benzene solution, gives the oxathiole (117) in over 80% yield. The most likely mechanism involves a sulfene intermediate as shown in Scheme 38 (70CJC3704). Not surprisingly, the flash thermolysis of thietane 1,1-dioxide, the saturated analog of (116), gives only cyclopropane and propene. 

Primary enamines 115 have been obtained by flash thermolysis of adducts 116 at 600 °C in vacuo. A retro-Diels-Alder reaction occurs to give vinylamine (R = R1 = R2 = H). These primary enamines have been identified by their spectroscopic properties at — 80 °C. At temperatures > 80 °C isomerization to the imine occurs as well as self-condensation to various nitrogen heterocycles or acyclic azadienes. 

The cycloadducts formed on reaction of hexafluoro-2-butyne and 2-substituted furans can be hydrogenated selectively at the unsubstituted carbon double bond. On flash thermolysis at 400°C, these products undergo retro-Diels-Alder reaction to give 3,4-bis(trifluoromethyl)-substi-tuted furans [91JFC(54)249]. A thermally stable [4-1-2] cycloadduct is obtained on heating hexafluoro-2-butyne and 3,4-bis(trifluoromethyl)-furan the retro reaction occurs on photolysis (92JHC113) (Scheme 90). 

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