2- Butenyl radical, decomposition

A complete analysis of the products reported in Fig. 1 requires some more comments on cyclopentadiene and benzene. Both are typical secondary products, and are mainly the result of successive addition and condensation reactions of alkenes and unsaturated radicals. Once a significant amount of ethylene and propylene is formed, vinyl and allyl radicals are present in the reacting system and form butadiene, via butenyl radicals. Successive addition reactions of vinyl and allyl-like radicals on alkenes and dialkenes sequentially explain the formation of cyclopentadiene and benzene. These reactions are discussed in-depth in the literature and will be also analysed in the coming paragraphs (Dente et al., 1979). It seems worthwhile mentioning that these successive reactions and interactions of small unsaturated radicals and species constitute the critical sub-mechanism for the correct evaluation of ethylene selectivity. In fact, once the primary decomposition of the hydrocarbon feed has largely completed, the primary products and mainly small alkenes can be... 

Similarly, the kinetic parameters of the decomposition reaction of the 5-hexen-l-yl radical to form C2H4 and the 3-butenyl radical... 

The first of these four phases Is a hydrogen-abstraction reaction. Using the pyrolysis of cls-2-butene as an Illustration, step 9 makes up the first phase. In this step, a hydrogen atom is abstracted from a cis-2-butene molecule by a methyl radical to form a 2-butenyl radical. The decomposition of the 2-butenyl radical In step 36 makes up the second phase. Decomposition of the 2-butenyl radical produces a butadiene molecule and a hydrogen atom. 

Cu(II) EPR signal in nitriles as solvent as well as by polarographic measurements 144>. Similarly, the EPR signal disappeared when Cu(OTf)2 was used for catalytic cyclo-propanation of olefins with diazoesters 64). In these cases, no evidence for radical-chain reactions has been reported, however. The Cu(acac)2- or Cu(hfacac)2-eatalyzed decomposition of N2CHCOOEt, N2C(COOEt)2, MeCOC(N2)COOEt and N2CHCOCOOEt in the presence of cyclopropyl-substituted ethylenes did not furnish any products derived from a cyclopropylcarbinyl - butenyl rearrangement128. These results rule out the possible participation of electron-transfer processes and radical intermediates which would arise from interaction between the olefin and a radical species derived from the diazocarbonyl compound. 

This access to a , y-bis(tricyclohexyltin)alkanes proves to be very convenient when the corresponding organic dihalides are available. When this is not the case, and when a , y-dienes are more easily accessible, a double hydrostannation of these unsaturated compounds can be used. As indicated above, this radical addition has to be conducted in less mild conditions than for the addition of tri-n-butyltin hydride, for instance, because of the bulkiness of the organotin center and also of the more nucleophilic character of the tricyclohexyltin radical. Nevertheless, the treatment of 4,4 -di(butenyl)biphenyl or 4,4 -bis(but-3-enyloxymethyl)biphenyl with tricyclohexyltin hydride at 130 °C over seven days affords the corresponding adducts 72 and 75 in good yields (Scheme 3.7.11). Due to its rapid decomposition at this temperature, the AIBN initiator has to be added in small portions for the duration of the reaction. 

Decomposition of cis- and /ra/tj-l-propenylcoppers and the phosphine complexes gives hexadienes with retention of stereochemistry about the double bond (Whitesides and Casey, 1966 Whitesides etal., 1971). 2-Butenyl-copper behaves similarly. A free-radical mechanism is unlikely to be operative... 

Copper and Silver.—Decomposition of 1-propenyl- and 2-butenyl-copper(i) and -silver(i) compounds to hexa-2,4-dienes occurs with retention of configuration. This indicates a non-radical mechanism, for intermediate vinylic radicals would be of very low configurational stability and would thus lead to extensive isomerization." The decomposition of n-propyl-copper(i) gives propene and propane, but no six-carbon molecules. It therefore seems highly unlikely that propyl radicals are intermediates. Perhaps, as in the earlier but very similar report on the absence of eight-carbon molecules from the decomposition of triphenylphosphine-n-butylcopper, the mechanism involves initially the separation of alkene,... 

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