Thermolysis. Photolysis. Reactions with Electrons

1 Thermolysis. Photolysis. Reactions with Electrons Thermolysis 

O2F2 is sufficiently stable at temperatures below its melting point of about 110 K (see above) but decomposes at temperatures below its boiling point (216 K) to give O2 and F2 [1,2]. A decomposition temperature of 195 K was cited [3, p. 152]. O2F2 is considerably less stable than previously reported [4,5] a 4% per day rate of decomposition at 113 K was found [6] in contrast to a 4.3% per day rate at 195 K [4, 5]. 

The formation of OF radicals in the first step of the reaction is unlikely, since it requires a considerably higher activation energy [8]. 

No explosions or inflammation of O2F2 occur on subjecting samples of liquid O2F2 at dry ice temperatures to the flame of a safety fuse, a 1.25-Joule electric spark, or the impact of a 22-caliber bullet [2]. 

For a mass spectrometric investigation of O2F2-O3F2 mixtures, see [15]. 

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The second type is the construction of a heterocyclic system from units containing perfluoroalkyl groups or their fragments. Each type has its own advantages and weaknesses. Thus, reactive species used in processes of the first type are perfluoroalkyl radicals and carbocations, and the processes are conducted by elaborate methods (thermolysis, photolysis, electrolysis, one-electron oxidation, etc.), whereas reactions of the second type use condensation of molecules with suitable groups and nucleophilic reactions of perfluoroolefins. 

Silyl Radicals (R3Si). Much is known about the chemistry of silyl radicals. They can be produced from thermolysis, photolysis and electron transfer reactions. With the exception of disproportionation and degradation to silenes, most of the reactions known for the carbon radicals are also known for silyl radicals. 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

The thermolysis (- 10 to -t- 25°C) or photolysis (2 > 300 nm, - 10°C) of 3//-diazirine-3-car-bonitriles generates cyanocarbenes under very mild reaction conditions (see Houben-Weyl, Vol. E19b, pl206). Their addition to electron-rich alkenes yielded cyclopropane carbonitriles in moderate yields (Table 3). The 3//-diazirines-3-carbonitriles were prepared from 3-halodiazirines, mostly in situ, by an exchange reaction with tetrabutylammonium cyanide. The cyclopropanation of (Z)- or ( )-but-2-ene proceeded nonstereospecifically, which is explained by the participation of a triplet carbene. 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

With respect to the electronic structure, nitrenes are analogous (isoelectronic) to carbenes. As in the case of diazocompounds, photolysis as well as thermolysis of azides leads to the corresponding nitrenes. Some reactions are collected in the following scheme ... 

In the thermolysis of cyclohexanone virtually all of the products arise from the (8-cleaved structure 5.23) Jwo effects are important here. In the ion the a-cleavage structure is specifically stabilized (see below) and the jff-cleavage structure 9, would be specifically destabilized by the heteroatom which is at an active site in the odd alternant n system associated with the carbonyl. The primary photolysis products of cyclohexanone 24) are much more closely correlated to its mass spectrum than the thermolysis products are. This is because n- n excitation can be relaxed by an a-cleavage (Norrish type I) analogous to 7. The analogy for the photochemical reactions is, however, far from perfect because of the stabilizing effect of the oxygen atom on the even alternant even electron ions, e.g. 10. The photolysis of... 

Reaction of carbonylcarbenes with a triple bond results in either 1,1-cycloaddition product 326 or 1,3-cycloaddition product 327. Substituted furans 327 are also accessible through photolysis, thermolysis, or catalytic rearrangement of carbonylcyclopropenes 326. However, furan formation can be imagined as a cyclization of 6ir-electron system 328, incorporating a singlet carbene or carbenoid center and a conjugated heteroatomic diene... 

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