Diazo compounds pyrolysis

Cyclohexadienylidenes, disubstituted at the 4-position are expected to be kinetically more stable than the parent carbene, however, the rearrangement to benzene derivatives is still very exothermic. The gas phase chemistry of 4,4-dimethyl-2,5-cyclohexadienylidene Is was investigated by Jones et al.100,101 The gas phase pyrolysis of the diazo compound 2s produces a mixture of p-xylene and toluene, and by crossover experiments it was demonstrated that the methyl group transfer occurs intermolecularly via free radicals. Thus, the pyrolysis of a mixture of the dimethyl and the diethyl derivative 2s and 2t... 

Carbenes are commonly generated by irradiation or pyrolysis of an appropriate diazo-compound (2). Apart from differences readily traced to the change in temperature, the chemical properties of the carbenes formed from photolysis and from thermolysis are usually quite similar. These observations... 

The reactivity of cage-annulated carbene (53) was found to depend markedly on the method of its formation." Pyrolysis of the corresponding tosylhydrazone sodium salt gave products of intramolecular CH insertion or H-abstraction. Photolysis of a diazirine precursor gave only azine products by reaction of the carbene with the precursor or diazo compound. Treatment of the m-dibromoalkane with BuLi gave products due to intermolecular insertion of the carbene into CH bonds of the solvent. 

Crandall et al. first showed that hexamethylbicyclopropylidene 39 rearranges at 400 °C in a flow pyrolysis system to yield a 10 1 mixture of two isomeric hexa-methylmethylenespiropentanes 98 and 99 (Scheme 18) [49,50] formed via tri-methylenemethane diradical intermediates. In close analogy to this result, the photolysis of the diazo compound 100 did not only give permethylbicyclopro-pylidene 24a,but a 45 55 mixture of 24a and 101, and upon flash vacuum pyrolysis at 400°C only 101 was formed (the yields were not reported) (Scheme 18) [98]. 

The present procedure uses sodium methoxide in methanol for generation of the tosylhydrazone salt. This procedure gives the highest reported yield and, unlike other procedures, also gives pure diazo compounds free from solvents. This vacuum pyrolysis method appears applicable to the formation of relatively volatile aryldiazomethanes from aromatic aldehydes. Table I gives yields of diazo compounds produced by this vacuum pyrolysis method. The yields have not been optimized. The relatively volatile diazo esters, ethyl a-... 

FORMATION OF DIAZO-COMPOUNDS BY VACUUM PYROLYSIS OF SODIUM SALTS OF TOSYLHYDRAZONES... 

The major limitation of the vacuum pyrolysis method appears to be thermal decomposition of less volatile diazo compounds during the pyrolysis. The vacuum pyrolysis method was unsuccessful for the preparation of 1-naphthyl diazomethane and 3,5-dichlorophenyldiazomethane. However, such diazo compounds could be prepared from the corresponding tosylhydrazone salts by pyrolysi s in ethylene glycol and extraction of the aryldiazomethane into... 

Phenyldiazomethane, 1, 834. A new method for preparation of this (and other aryl-diazomethanes) involves a vacuum pyrolysis of the sodium salt of benzaldehyde tosylhydrazone, a method introduced for carrying out the Bamford-Stevens reaction. The yield is 80%, the highest yield yet reported. Another advantage is that the reagent is obtained free from solvents. The pyrolysis can also be coitducted in ethylene glycol at 80° with extraction of the aryldiazomethane into hexane.1 Caution All diazo compounds arc highly toxic and potentially explosive. 

The reaction of dienamines and sulfonyl azides provides a method for the synthesis of vinyldiazomethanes.233,431 Likewise, the pyrolysis of a fluo-rinated 1-benzyltriazoline over a nickel surface yields difluorodiazometh-ane.145 The formation of diazo compounds from the reaction of azides with alkylidene phosphoranes apparently originates from an intermediate triazoline compound (Scheme 145)408,411 412,415,441... 

When the COR groups of bis(diazo) compound 144 are replaced by Ph or SiMe3 substituents, the derived carbene chemistry leads to different results69 (equation 34). The only identified product obtained from the flow pyrolysis of l,2-bis(diazobenzyl)-tetramethyldisilane was l,l,2,2-tetramethyl-3,4-diphenyl-l,2-disilacyclobutene, which rapidly reacts with air under insertion of an oxygen atom into the Si—Si bond. In contrast, the analogous 1,2-disilacyclobutene could not be directly isolated from the vacuum pyrolysis of l,2-bis[diazo(trimethylsilyl)methyl]-tetramethyldisilane. However,... 

R = R1 = Ph) in very high yield by photolysis, the pyrazole is formed in only 3 % yield from diphenylethyne.50) The formation of cyclopropenes from pyrazoles apparently occurs via the corresponding diazo-compound. Thus photolysis of (53, R = R1 = Ph) at between 330 and 410 nm has been shown to lead to the diazo-compound (56) which on further photolysis or pyrolysis is converted to (54)50). Indeed, photolysis of vinyldiazoalkanes provides a good route to cyclopropenes thus photolysis of E- (57) leads to 3-alkoxycarbonyl-3-cyanocyclopropene51), while (58) leads either to the corresponding cyclopropene or to reversal to the pyrazole5Z). 

Aliphatic diazocarbonyl and related diazo compounds can be decomposed by either photolysis or pyrolysis (or thermal decomposition), or by transition-metal-catalyzed decomposition generates (Scheme 2.51). For example, pyrolysis of diazoacetic ester at 425°C gives the carbethoxy carbene. 

The simplest way of generating a carbene is by photolysis or pyrolysis of a suitable diazo compound. This reaction gives a free carbene species and is therefore the method most often used. 

No gas-phase photolyses have yet been reported with these diazirines. Pyrolysis of the cycloheptyldiazirine yields a mixture of hydrocarbons, and the relative yields of these products are very close to the values obtained by the pyrolysis of diazocycloheptane. Photolysis of these diazirines in the liquid phase yields a hydrocarbon mixture similar to that obtained by pyrolysis, but with small increases in some of the minor products. The photoisomerization reaction leading to the normal diazo compound is readily observed for these two diazirines since the diazo compounds formed are relatively stable. In both cases, during photolysis of the (colorless) diazirines, a deep orange solution develops, and ultraviolet and infrared absorption spectra indicate the formation of the diazo compounds. Photolysis of cycloheptyldiazirine in the presence of acetic acid yields 47% of cycloheptylacetate, which indicates that the photoisomerization reaction under these conditions is a major primary reaction. Under the same conditions cyclooctyldiazirine yields 28% of the corresponding acetate. 

Phenyl(trimethylsilyl)carbene can be generated by gas-phase pyrolysis of phenyl(trimethyl-silyl)diazomethane (1) at 500 °C (see Houben-Weyl Vol. El9b, p 1427). An attempt to trap this carbene with 2,3-dimethylbuta-l,3-diene furnished cyclopropane 3 only in trace amounts besides products 2, 4, and 5. Cyclopropane 3 can be prepared independently and in better yield by photolyzing the diazo compound in the presence of the butadiene. Thermally induced ring expansion of 3 provides cyclopentene 4, a fact that explains the low yield found under pyrolysis conditions. 

The last heterocycloalkylidene to be mentioned here is the silacyclo-hexadienylidene 381, generated by vacuum pyrolysis of the diazo compound 380. Its intramolecular chemistry was disappointing no silabenzene was formed, but the dimer 382 was isolated (Eq. 120).445 A review on cyclo-alkenylidenes is available.437... 

Amino-2-nitrobenzofuran (60) undergoes an astonishing transformation in acetic acid, giving the diazo-compound (61). The benzofuranone (62) is converted into the cyclohexadienone (63) on treatment with methyl iodide and sodium methoxide. Flash vacuum pyrolysis of methylenephthalide... 

On the other hand, it is unlikely that methylene is formed from methane or other hydrocarbons in the pyrolysis of coke. In contrast to methylene, its analog silylene is a product of the pyrolysis of silane and disilane (8-17 Purnell and Walsh, 1966 Bowrey and Purnell, 1970). Laser-induced fluorescence was used to study the formation of silylene and its reactivity (see, e.g., Baggott et al., 1988 Jasinski and Chu, 1988), but silylene is not within the scope of this book as there are no diazo compounds involved in its chemistry. Literature in which the reactivity of silylene is compared with that of methylene is reviewed briefly in a publication of Skancke (1993, p. 640). 

Carbenes can be obtained by pyrolysis or photolysis of diazo compound. 

The rate of carbene formation by pyrolysis of diazo compounds shows unexpected substituent effects. The unimolecular rate constant for thermolysis of diphenyldiazomethane is increased by any single para substituent, be it electron-donating or electron-withdrawing. If both rings are substituted, one by an electron-donating and the other by an electron-withdrawing substituent, then this disubstituted compound decomposes faster than a compound monosubstituted with either substituent. However, symmetrically disubstituted diphenyldiazomethanes decompose more slowly than do the compounds monosubstituted with the same substituents. 

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