Pentadiene radical, hydrogen

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

We have demonstrated that intermolecularly, amidyl radicals preferentially abstract an allylic hydrogen rather than add to a TT bond of olefins such as cyclohexene and 1,3-pentadiene (33). This reactivity pattern is completely reversed in intramolecular reactions as shown in the following examples of alkenyl mitro-samide photolysis. In every case, the amidyl radicals generated from photolysis preferentially attack the ir bonds intramolecu-... 

Exercise 13-1 1,4-Pentadiene is different from propene in some of its chemical properties for example, removal of the hydrogens at the 3-position by attack of radicals is much easier than the removal of those on the methyl group of propene. Explain why this should be so. (The rules of Section 6-5B will be helpful in this connection.)... 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

In one of the earliest investigations of regioselectivity in radical addition reactions to polyenes, the addition of hydrogen bromide to 1,3-butadiene was observed to yield mainly the 1,4-addition product in the presence of peroxides. The preference for attack at the Cl position of 1,3-butadiene has subsequently been observed for a large number of radicalsOnly for the addition of the methyl radical has the ratio of addition to the Cl vs C2 actually been measured. A value of Cl C2 = 1.0 0.01 has been found . For all other cases, products arising from attack at C2 have not been reported. This is also true for radical addition to 2,3-dimethyl-l,3-butadiene . Additions to 1,3-pentadiene occur predominantly at the Cl position due to the steric effect exerted by the terminal methyl This is a reflection of the reduced... 

FIGURE 10.19 Structure of the radical complexes 3, 4, and 5 formally derived from abstraction of a hydrogen atom from the neutral pentadiene-Fe(CO)3 complex and the mesomeric forms of 3, 4, and 5. The charges of the hydrogen atoms are summed into the carbon atoms. Spin densities are given in parentheses.160 Adapted with permission from Wiley-InterScience. 

Matthews and Strange [42] reported a similar reaction of isoprene with sulfur dioxide in the presence of hydrogen chloride. Seyer and King [Id] reported that 1,3-cyclohexadiene reacted with sulfur dioxide to give a white amorphous compound, as earlier reported by Hofmann and Damm [43]. 2-Methyl- and 4-methyl-1,3-pentadiene were reported by Morris and Van Winkle [44] to react with sulfur dioxide to yield a cyclic sulfone and some hydrocarbon polymer. Starkweather [45] in 1945 reported that chloroprene (2-chloro-l,3-butadiene) reacted with sulfur dioxide in an emulsion system to give a copolymer. Poly-sulfone is the major product when radical initiators are used. Cyclic products predominate when radical inhibitors (hydroquinone) or temperatures in excess of the ceiling temperature are used. For example ... 

Mixture with trans isomer. - Radical also generated from 1,4-pentadiene and t-butoxyl and ethoxyl radicals. ) INDO calculations of spin densities. ) da/dr = 2.7 10 TK from 103 to 182 K. ) Assignment to particular hydrogens uncertain. ") du/dr=1.5-10- TK froml03tol82K. ... 

X-ray studies have revealed a long iron-iron bond (3.14 A) in [ Fe(CO)a-(i -allyl) 2], in agreement with the ability of the dimer to dissociate into Fe(CX))3-(allyl) radicals. The complex slowly decomposes in solution to give ferracyclo-pentadiene complexes such as (25) via hydrogen abstraction and carbon-carbon coupling reactions. "... 

The LOXs abstract a hydrogen radical from the C-3 of a cfr,ci5 -l,4-pentadiene system present in PUFAs. So far, six distinct LOX enzymes have been reported in mammals. The overall shape of the LOXs is cylindrical and ca lOOA in length and ca 45 A in width. In 15-lipoxygenase (15-LOX), two Ca -binding sites are present in its membrane-binding domain. The 15-LOX catalytic domain consists of approximately 20 helices. A high-resolution structure of human 15-LOX was reported very recently by Newcomer and coworkers... 

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