Methylene pyrolysis

Experiments with monoethyl and monocarbomethoxy di- -xyljlene (4) gave similar results. These experiments do not, however, shed any light on whether the mpture of the methylene—methylene bonds in the dimer upon pyrolysis is simultaneous or sequential. 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

Chlorinated by-products of ethylene oxychlorination typically include 1,1,2-trichloroethane chloral [75-87-6] (trichloroacetaldehyde) trichloroethylene [7901-6]-, 1,1-dichloroethane cis- and /n j -l,2-dichloroethylenes [156-59-2 and 156-60-5]-, 1,1-dichloroethylene [75-35-4] (vinyhdene chloride) 2-chloroethanol [107-07-3]-, ethyl chloride vinyl chloride mono-, di-, tri-, and tetrachloromethanes (methyl chloride [74-87-3], methylene chloride [75-09-2], chloroform, and carbon tetrachloride [56-23-5])-, and higher boiling compounds. The production of these compounds should be minimized to lower raw material costs, lessen the task of EDC purification, prevent fouling in the pyrolysis reactor, and minimize by-product handling and disposal. Of particular concern is chloral, because it polymerizes in the presence of strong acids. Chloral must be removed to prevent the formation of soflds which can foul and clog operating lines and controls (78). 

IsoxazoIine, 5-isocyanato-3-phenyI-pyrolysis, 6, 39 2-Isoxazoline, 4-methylene-synthesis, 6, 91... 

Hexafluorabenzene may also add to methylene Lnphenylphosphorane to form a new pentafluorophenyl-bearing yhde Treatment of tins ylide with an acid fluonde or acid anhydride followed by pyrolysis (shown in equation 58) forms the corresponding pentafluorophenylacetylene [66] (equation 58). 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

Py-GC/MS of Laropal K80 results in a number of intense oligomeric fragments, while smaller fragments are minor pyrolysis products [71]. Cyclohexene, cyclohexanol, cyclohexanone, methyl-cyclohexanone and methylene-cyclohexanone can be attributed to secondary pyrolysis products of the monomeric units (Figure 12.7). The most intense... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

The first stable derivative of methylene cyclopropene was the quinocyclopropene 108 reported in 1964 by Kende98 it was prepared from the cyclopropenium cation 106 which underwent pyrolysis and bromination with NBS to the p-hydroxy-phenyl substituted cation 107, which gave quinocyclopropene 108 by deprotonation ... 

Diazomethane when heated with copper powder gives nitrogen and an insoluble polymethylene, indicating that one of its reactions is the decomposition into methylene radicals. The methylene radical can also be formed in the gas phase and detected by a mirror experiment.81 The pyrolysis of ketene in the gas phase gives carbon monoxide and methylene radical. The methylene radical both reacts with itself to give ethylene and removes tellurium mirrors, forming tellurform-aldehyde.82 Thus the methylene diradical(P) behaves as expected. 

Methylene-1,4-pentadiene (83), prepared by flash vacuum pyrolysis of 1,5-diacetoxy-3-(acetoxymethyl)pentane, dimerizes at 95 °C in benzene to give predominantly one isomer of 1,4,4-trivinylcyclohexene (84) as the major product (equation 44)63. 

The procedure described is a modification of that developed by Diels and Criegee. Bicyclo[2.1.0]pentane has been prepared by the pyrolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene, the pyrolysis of N-phenyl-2-oxo-3-azabicyclo[2.2.1]heptane, and the addition of methylene to cyclobutene. ... 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

Methylbenzofuran-3-carbaldehydes undergo ready condensation with Meldrum s acid (isopropylidene malonate) to afford arylmethylene derivatives 83. These on flash vacuum pyrolysis at 500-600 C give 3-dibenzofuranols 84 (Scheme 21). The arylmethylene derivative, e.g., 85, presumably undergoes conversion to a methylene ketene (86, Scheme 22) on pyrolysis, which undergoes a [1,5-H] shift and subsequent cyclization and tautomerization, yielding the dibenzofuranol 87. The derived methyl ether 88 has been converted by mild acetylation with acetyl chloride and aluminum chloride and subsequent boron trichloride-induced demethylation to the natural product ruscodibenzofuran (8). A limitation is imposed on this method because 3-acetyl-2-methyldibenzofurans fail to condense with Meldrum s acid so that l-methyl-3-dibenzofuranols are not available by this method. ... 

Flash vacuum pyrolysis (FVP) represents a special method for inducing decomposition of thietane compounds. Block et al. succeeded for the first time in the preparation of methylene sulfine in the gas phase, applying this method to thietane 1-oxide. The reaction starting at 600°C was followed by mass and microwave spectroscopy. 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

Methylene-2-oxetanone, or diketene (37), does not decarboxylate on heating but cleaves to form ketene at 250 °C. A thermal rearrangement of 3-aryl-4-benzal-2-oxetanones (38) to form naphthalene derivatives appears to involve this type of pyrolysis, followed by a [ 2 + 4] cycloaddition (78JOC1146, 72LA(765)15>. 

Difluorocyclopentadiene was prepared via a pyrolysis reaction the difluoro-methylene centre was installed (Eq. 119) using a DAST fluorination [318]. A fluorobutenolide building block was prepared by a Wadsworth-Emmons reaction of isopropylidene glyceraldehyde removal of the ketal protecting group led to the formation (Eq. 120) of the unsaturated lactone in acceptable overall yield... 

In another series of related experiments Errede and coworkers prepared o-quinodimethane itself by the flash pyrolysis of o-methylbenzyltrimethylam-monium hydroxide 23 [72, 73], The conditions of this experiment were such that the o-quinodimethane was quenched soon after it was formed by cooling to - 78 °C. The product trapped out under these conditions was an approximately 25 75 mixture of 1,2,5,6-dibenzocyclooctadiene 20 and the spiro o-quinodimethane dimer 24 (Fig. 14). Dimer 24 can readily be seen to be the result of the Diels-Alder reaction of one o-quinodimethane bis-exo-methylene diene unit across one of the exo-methylene groups of another o-quinodimethane. The spirodimer... 

Kassel67 proposed an activation energy of 44 kcal. for the reaction on the basis of an uncertain mechanism for CH4 pyrolysis, and Bawn3 reported that ethane was not formed when methylene was (presumably) produced in the presence of methane at 300°C. by reaction of Na vapor with CH2Br2. 

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