Pyrolysis alkenes

An important and frequently observed phenomenon in alkene pyrolysis is the ready equilibration of E and Z isomers at FVP temperatures above 500 °C. The apparently contrathermodynamic conversion of the E into the Z isomer has been quantified over the range 500-900 °C for stilbene, cinnamyl alcohol and cinnamonitrile37. In the last case, the proportion of Z isomer increases to 38% at 900 °C. In certain cases the diradical implicit in the isomerization process can be trapped by an intramolecular reaction and this is exemplified by the formation of 2-phenylindane in low yield from FVP of 56 at 700 °C37. The cis cyclobutene diester 58 is assumed to be formed as an intermediate in the FVP of the bicyclic sulphone 57 at 775 °C by loss of SO2 and ethylene. Under these conditions, however, it reacts further to give equal proportions of the E diesters 59... 

The complete set of the reference kinetic parameters for cyc/o-alkane and alkene pyrolysis is reported in Table VII. 

Important structural information was also obtained from pyrolysis of PRB A at 400 °C carried out under an helium flow so as to minimize secondary reactions (67). GC-EIMS of the pyrolysate fractions obtained by CC and AgN03-Si02 TLC showed the predominance of hydrocarbons ca 55% of the total pyrolysate). Regular series of C13 to C31 n-alkanes and n-alk-l-enes, formed by cracking of C-C bonds, are the major constituents of this fraction. They are accompanied by minor series of n-alkylbenzenes, n-alkyl- and n-alkenylcyclohexanes and n-trans-alkenes. Pyrolysis also provided a complex mixture of unidentified ketones and a series of n-fatty acids dominated by palmitic and oleic acids. The recovery of fatty acids on pyrolysis of PRB A, although isolation of this resistant material required drastic basic and acid treatments, indicates that the corresponding esters are sterically protected in the polymeric network. 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

In contrast, the ultrasonic irradiation of organic Hquids has been less studied. SusHck and co-workers estabHshed that virtually all organic Hquids wiU generate free radicals upon ultrasonic irradiation, as long as the total vapor pressure is low enough to allow effective bubble coUapse (49). The sonolysis of simple hydrocarbons (for example, alkanes) creates the same kinds of products associated with very high temperature pyrolysis (50). Most of these products (H2, CH4, and the smaller 1-alkenes) derive from a weU-understood radical chain mechanism. 

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... 

Pyrolysis. The pyrolysis of simple esters of the formula RCOOCR R CHR 2 to form the free acid and an alkene is a general reaction that is used for producing olefins ... 

Perhaps one of the most exciting developments in the chemistry of quinoxalines and phenazines in recent years originates from the American University of Beirut in Lebanon, where Haddadin and Issidorides first made the observation that benzofuroxans undergo reaction with a variety of alkenic substrates to produce quinoxaline di-AT-oxides in a one-pot reaction which has subsequently become known as the Beirut reaction . Many new reactions tend to fall by the wayside by virtue of the fact that they are experimentally complex or require starting materials which are inaccessible however, in this instance the experimental conditions are straightforward and the starting benzofuroxans are conveniently prepared by hypochlorite oxidation of the corresponding o-nitroanilines or by pyrolysis of o-nitrophenyl azides. 

Pyrolysis of alkanes is referred to as eraeking. Alkanes from the paraffins (kerosene) fraetion in the vapor state are passed through a metal ehamher heated to 400-700°C. Metallie oxides are used as a eatalyst. The starting alkanes are broken down into a mixture of smaller alkanes, alkenes, and some hydrogen. 

The pyrolysis of sodium chlorodinuoroacetate is still a widely used, classical method for generating difluorocarbene, especially with enol and allyl acetates [48, 49, 50, 51] (equation 21) A convenient alternative that avoids the hygroscopic salt uses methyl chlorodifluoroacetate with 2 equivalents of a lithium chlonde-hexa-methylphosphoric triamide complex at 75-80 °C in triglyme [52], Yields are excellent with electron-rich olefins but are less satisfactory with moderately nucleophilic alkenes (4-5% yields for 2-bulenes)... 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids299 (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism246,300 and thus facilitates the desired stereospecific synthesis301. The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene301. 

In a related reaction, pyrolysis of allylic ethers that contain at least one a hydrogen gives alkenes and aldehydes or ketones. The mechanism is also pericyclic"" ... 

In contrast to this behavior stabilized sulfonyl ylides 125 lose PhjP under flash vacuum pyrolysis conditions to give sulfonyl carbenes 127 which evolve to the formation of various products among which the alkenes 129, observed in all the cases (Scheme 35) [129]. 

More recently, a number of reports dealing with 1,3-sulfonyl shifts which proceed by other mechanisms have been published. For example, Baechler and coworkers suggested that the higher activation enthalpy observed for the isomerization of the deuterium labeled methallyl sulfone 72 in nitrobenzene at 150°C as compared to the corresponding sulfide, together with the positive entropy of activation may be taken as evidence for a homolytic dissociation mechanism (equation 44). A similar mechanism has also been suggested by Little and coworkers for the gas-phase thermal rearrangement of deuterium labelled allyl sec-butyl sulfone, which precedes its pyrolysis to alkene and sulfur dioxide. 

Scheme 6.8 gives some examples of ketene-alkene cycloadditions. In Entry 1, dimethylketene was generated by pyrolysis of the dimer, 2,2,4,4-tetramethylcyclobutane-l,3-dione and passed into a solution of the alkene maintained at 70° C. Entries 2 and 3 involve generation of chloromethylketene by dehydrohalo-genation of a-chloropropanoyl chloride. Entry 4 involves formation of dichloroketene. Entry 5 is an intramolecular addition, with the ketene being generated from a 2-pyridyl ester. Entries 6, 7, and 8 are other examples of intramolecular ketene additions. 

Amine oxide pyrolysis occurs at temperatures of 100°-150°C. The reaction can proceed at room temperature in DMSO.323 If more than one type of (3-hydrogen can attain the eclipsed conformation of the cyclic TS, a mixture of alkenes is formed. The product ratio parallels the relative stability of the competing TSs. Usually more of the /f-alkene is formed because of the larger steric interactions present in the TS leading to the Z-alkene, but the selectivity is generally not high. 

Gas phase pyrolysis of dimethylchlorophosphine leads to the formation of the reactive phospha-alkene (166), which has the... 

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