Gas-phase thermolysis

Gas-phase thermolysis of (69) and (70) at 600 C/12 Torr occurs to give the corresponding cis and trans alkenes respectively, suggesting a concerted [<, +<,2a] cycloreversion process (70JA1763). 

The benzo-fused derivative 8 of l,4-dimethyTl,4-dihydro-l,4-diazocine can be prepared by gas phase thermolysis of the cyclobutene derivative 7,31 a reaction that can be reversed photo-chemically. In contrast to the parent system, the heterocyclic ring of the benzannulated compound displays no aromatic character. 

Klunder A. J. H., Zwanenburg B. Gas Phase Thermolysis in Natural Product Synthesis in Gas Phase React. Org. Synth. 1997 107, Ed. Yannick V., Pb. Gordon and Breach, Amsterdam... 

Keywords retro-Diels-Alder reaction, gas phase thermolysis in natural product synthesis... 

One exception to the above general fragmentation pattern is the formation of the ring-rearranged sulfenate (249) in the gas-phase thermolysis of thietane oxide (247a) at elevated temperatures . Although the temperature of this thermolysis is considerably higher than those used in the other studies, it is difficult to account for the (not totally unprecedented ) difference in the results. 

However, bicyclic dichloride 95 under the same conditions was converted into diene 96 and not to the rearrangement product 94 (equation 33). This result is explained by the larger size of the ring, which is far less strained than that in diene 9046. The gas-phase thermolysis of diene 91 at 126-186 °C afforded an almost equimolar mixture of bicyclic diene 92 and triene 93 which are formed, presumably, via the TMM-diradicals... 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

Flash vacuum thermolysis of 1,2,3-oxadithiolane 2-oxide has been reported to proceed via the intermediacy of 1,2-oxathietan <82JCS(P2)279>. The gas-phase thermolysis of 1,2,3-benzoxadithiole 2-oxide (37) proceeds in an analogous manner to the thermolysis of 1,3,2-benzodioxathiolane 2-oxide (49), affording cyclopentadienethione <81AG(E)570>. 

The highly reactive 2-azadiene with an allene and heterocumulene structure 126, prepared by gas-phase thermolysis of propargyl thiocyanate, has been found by the Banert group to be an efficient precursor for thiazole derivatives by nucleophilic-induced heteroannulation through carbon and... 

Table 8. Gas-Phase Thermolysis Rearrangement of Alkylidenecyclobu-tanes to Cyclohex-2-enones...

Pure crystalline 2,3-dicyanobutadiene has been prepared in high yield by gas-phase thermolysis of cyclobutene (2).2,8 Analogous thermolysis of derivatives of cyclobutene-1,2-dicarboxylic acid appears to represent general procedures for the synthesis of derivatives of butadiene-2,3-dicarboxylic acid of high purity.2,12 These... 

In the gas-phase thermolysis of 44, only the products of cycloreversion are observed (97JHC113).The unsymmetrically substituted compounds 44c and 44d seem to decompose by the two alternative ways, because both iso-cyanic and hydrazoic acid are detected simultaneously. However, from the complementary products only ethyl and phenyl isocyanate, but not the corresponding azides, are found. Compounds 44b and 44f decompose only in one way, affording methyl isocyanate and hydrazoic acid, and phenyl isocyanate and methyl azide, respectively, as the primary reaction products (Scheme 3). 

Gas-phase thermolysis of 5-methyl- and 5-phenyl-2-(trimethylsilyl)-tetrazole (47) has been studied by real-time PE spectroscopy by Bock et al. (87TL617). N2 is split off quantitatively at the spectroscopically optimized temperatures of 750 K (R = CH3) and 770 K (R = C6H5), and the substituted nitrile imines are generated exclusively. 

In the PES-studied gas-phase thermolysis of the azines 72 and 73 (96T1965) methacrylonitrile and propene were found in addition to molecular nitrogen, which is in accord with a [5 - 2 + 2+ 1] fragmentation of the five-membered rings. The primary process is the cleavage of the N-N single bond, which is followed by the decay of the iminyl radicals thus formed. 

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

The sole heterocycle derived cycloproparenone to be reported143 is the thiophene derivative 90 from gas phase thermolysis of anhydride 89 the product, like 8 and other oxocycloproparenes, was intercepted by hexa- and pentafluoroacetone (equation 19). More recent studies144 using fvp and photolysis at 12 K have concluded that 90 is probably not involved in the fvp, and is even less likely in the photolysis, as it has comparable energy to its ring-opened isomeric precursor which is expected to interact with the trapping agent in exactly the same way. 

Gas-phase thermolysis of substituted tetrazoles (46a,b and 48), the 1,4-disubstituted 1,4-dihydro-5//-tetrazol-5-oncs (49a-f) and the 1,4-disubstituted 1,4-dihydro-5/7-tetrazol-5-thiones (50b,c,e-h) have been monitored by photoelectron spectroscopy.27... 

As mentioned earlier, at 500° C and 34.5 MPa supercritical water has a small dielectric constant, a very low ion product, and behaves as a high temperature gas. These properties would be expected to minimize the role of heterolysis in the dehydration chemistry. As shown in Table 1, the conversion of ethanol to ethylene at 500° C is small, even in the presence of 0.01M sulfuric acid catalyst. The appearance of the byproducts CO, C02) CH i+ and C2H6 points to the onset of nonselective, free radical reactions in the decomposition chemistry, as would be expected in the high temperature gas phase thermolysis of ethanol. 

An equilibrium of prop-2-yl selenocyanates (84) and allenes (85) was obtained on gas-phase thermolysis (0.1-0.01 Torr 350-400°C) of 84. This isomerization by [3,3]sigmatropic rearrangements provided access to the highly reactive allenyl isoselenocyanates (85). The isoselenocyanates (85) can only be handled in solution due to their pronounced tendency to polymerize. Although these are less reactive with nucleophiles than allenyl isothiocyanates, treatment with nucleophiles afforded selenazoles in moderate yield. [95AG(E)1627]... 

The thionation of 240 with Lawesson reagent (LR) in boiling xylene led to 1,2-dithietane 241 as a single isomer in 45% yield <2006HCA991>. The dithietane 241 then underwent a gas-phase thermolysis at 850°C under a pressure of 1.5 X 10 3 Torr and 3-benzylidene-4-methyl-377-dithiolane 242 was obtained as a 1 3-E/Z mixture in 54% yield (Scheme 29). 

The gas-phase thermolysis at 850°C under 1.5 x 10-3 Torr of 243 led to a mixture of 1,2-dithiete 244 in 70% yield accompanied by two heterocyclic products <2006HCA991> (Scheme 30). The authors suggested a possible mechanism of the reaction. 

Dihydroindene 12 has been produced together with minor amounts of isomers by gas-phase thermolysis of 6-cyclopropyl-6-methylpenLafulvene. From the mixture of isomers, upon treatment with 4-methyl-3//-l,2,4-triazole-3,5(4//)-dione only the 2 1 adduct 13 is isolated12. The crystal structure analyis confirms the anti arrangement of the triazolidine rings, consistent with the exo addition of the second dienophile to the intermediate derivative. 

Rademacher and co-workers studied the gas-phase thermolysis of 4-substituted 3,3,5,5-tetramethyl-3,5-dihydo-477 pyrazoles by photoelectron spectroscopy <1996T1695>. Among the systems investigated was 76, which afforded 77 at 470 °C. The alkylidene thiirane rearranged at 640 °C to thiol 78. Thermolysis of 79 at 425 °C gave alkylidenethi-irane 3 -oxide 80, which lost SO at 700 °C to yield allene 81 (Scheme 13). 

Rademacher and co-workers also studied the gas-phase thermolysis of tetrazoles by photoelectron spectroscopy and PM3 as well as ab initio theory <1997JHC113>. The tetrazole 85 began to decompose at 350°C, forming thiirane among other products. Although the thiirane obviously comes from the 2-hydroxyethylthio group, 2-mercaptoethanol did not afford thiirane at temperatures up to 950 °C. 

Low-temperature photolysis or gas-phase thermolysis followed by low-temperature quenching of 50 provided 51, as shown by low-temperature IR spectroscopy <1996T1965>. 

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