Alkenes thermal

Because of the different chemistry of cracking processes their products have different compositions. The major product of thermal cracking is ethylene, with large amounts of Ci and C3 hydrocarbons, and C4— C15 terminal alkenes. Thermal cracking, consequently, is used mainly in olefin manufacture. 

Thermolabile 2,2,5,5-tetrasubstituted A3-l,3,4-selenadiazolines are useful in the preparation of sterically very crowded alkenes. Thermal decomposition of (172) generates episelenide (173) and provides access to the alkene by ready loss of selenium (Scheme 70)... 

Radical chloroaminations are known, using radical, transition metal ion or photochemical initiation. They also occur without overt initiation, thus anti-Markovnikov additions to terminal alkenes occur with N,N-dichlorourethane in benzene at 5-40 C (yields <= 60%)P Similar reactions occur with N,N-dichlo-roarenesulfonamides in CH2CI2 at or below room temperature (yields mostly 53-91% 10% with isobu-tylene). ° The remaining N—Cl bond is reaiUly reduced if desired with sodium sulflte. N-Halosulfoximines also add to alkenes thermally or photolytically. ... 

Diethyl N,A -dibromophosphoramidate undergoes ready Markovnikov addition to styrene or cyclohexene (92%) in ecu at -20 to -t-20 C with BF3-Et20 catalysis.Alternatively, such reactions can be carri out in refluxing CH2CI2 in the absence of catalyst, when anti-Markovnikov products (47-97%) are formed. All these compounds are versatile intermediates (Scheme 83). A few N-bromoperfluo-roamines have been added to alkenes, thermally or photolytically. ... 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Tetrasubstituted and some hindered trisubstituted alkenes react rapidly only to the monoalkylborane stage. Rarely, when the tetrasubstituted double bond is incorporated in a cycHc stmcture, does hydroboration under normal conditions fail (25—27). However, such double bonds may react under conditions of greater force (25,28—31). Generally, trialkylboranes are stable at normal temperatures, undergoing thermal dissociation at temperatures above 100°C (32—34). In the presence of B—H bonds, trialkylboranes undergo a redistribution reaction (35—38). 

Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

Ethyleneamines are used in certain petroleum refining operations as well. Eor example, an EDA solution of sodium 2-aminoethoxide is used to extract thiols from straight-mn petroleum distillates (314) a combination of substituted phenol and AEP are used as an antioxidant to control fouling during processing of a hydrocarbon (315) AEP is used to separate alkenes from thermally cracked petroleum products (316) and TEPA is used to separate carbon disulfide from a pyrolysis fraction from ethylene production (317). EDA and DETA are used in the preparation and reprocessing of certain... 

Thermal decomposition of unsubstituted 3,4,5,6-tetrahydropyridazine at 439 °C in the gas phase proceeds 55% via tetramethylene and 45% via a stereospecific alkene forming pathway. The thermal decomposition of labelled c/s-3,4,5,6-tetrahydropyridazine-3,4- f2 affords cfs-ethylene-l,2- f2, trans-ethylene-l,2-if2, c/s-cyclobutane-l,2- f2 and trans-cyclo-butane-1,2- /2 (Scheme 57) (79JA3663, 80JA3863). 

Anhydro-3-hydroxy-2-phenylthiazolo[2,3-6]thiazolylium hydroxide (407) underwent ready thermal reaction with alkynic and alkenic dipolarophiles in refluxing toluene. With the former dipolarophile sulfur was lost from the intermediate 1 1 cycloadduct (408) to give the substituted 5H-thiazolo[3,2- i]pyridin-5-ones (409). With the latter, the intermediate (410) lost H2S, also forming (409). 

Heterocycles which provide the NOC or CNO component synthon Isoxazoles can be prepared by the thermal or photolytic cleavage of a number of heterocycles, such as 1,3,5-dioxazolidone, furazans, furoxans and 1,3,2,4-dioxathiazole 2-oxides, in the presence of a reactive alkene or alkyne. 

The thermal decomposition of the silver salt of dinitrophenylmethane in the presence of an alkene produced an isoxazoline A-oxide via a proposed arylnitrocarbene (80JOC4158),... 

Azetidine N-oxides produce isoxazolidines by a thermal ring expansion (77AHC(21)207, 75GEP2365391), and nitrosobenzenes react with alkenes to provide isoxazolidines (77AHC(21)207, 79IZV1059). 

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

The most important oxirane syntheses are by addition of an oxygen atom to a carbon-carbon double bond, i.e. by the epoxidation of alkenes, and these are considered in Section 5.05.4.2.2. The closing, by nucleophilic attack of oxygen on carbon, of an OCCX moiety is dealt with in Section 5.05.4.2.1 (this approach often uses alkenes as starting materials). Finally, oxirane synthesis from heterocycles is considered in Section 5.05.4.3 one of these methods, thermal rearrangement of 1,4-peroxides (Section 5.05.4.3.2), has assumed some importance in recent years. The synthesis of oxiranes is reviewed in (B-73MI50500) and (64HC(19-1U). 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

The stabilization reactions of alkylcarbenes were used preparatively in some cases. The diazirine derived from adamantanone gave the dehydroadamantane (2l7) thermally in 96% yield 73ZOR430). Alkene formation was reported for a steroid with its C-3 atom part of a diazirine ring. At 140 °C a A-2-unsaturated steroid was formed 65JA2665). 

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