Pentadienate radical

Fig. 6-5 Weighting coefficients of the atomic orbitals centred on carbon atoms [-5 in the non-bonding MO of the odd. alternant pentadiene-radical.

Lipoxygenases (Lox) are selective towards polyunsaturated fatty acids containing the c/, c/ -l,4-pentadiene moiety to produce either the 9(5)-hydroperoxide, 13(i )-hydroperoxide, or a mixture of both from linoleic and linolenic acids. These enzymes contain an iron atom in their active center. They are activated by hydroperoxides, and the Fe + is oxidized to Fe +, according to a scheme in which the pentadiene radical of linoleic acid becomes bound to the enzyme, reacts with oxygen, and the peroxyl radical formed is reduced by the enzyme to produce a hydroperoxide after reaction with a proton (1). 

FIGURE 3.19. (a) The AOs in a typical n system can be chosen to overlap in phase (b) inverting the phase of one AO in the pentadienate radical changes resonance integrals for adjacent n interactions from jS to P, (c) calculation of NBMO coefficients with the AOs of (b) (d) calculation of the n energy of union of (b) with methyl to form benzene. The result is the same as before [equation (3.29)]. 

Autoxidation of linolenic acid yields four monohydroperoxides (Table 3.28). Formation of the monohydroperoxides is easily achieved by H-abstraction from the bis-allylic groups in positions 11 and 14. The resultant two pentadiene radicals then stabilize analogously to linoleic... 

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

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. 

Zimmerman and Hoffacker also observed a regioselective reaction subjecting various aryl-substituted 1,4-pentadienes to photoinduced electron transfer using DCN and DCA. The radical cations produced underwent a regioselective cyclization wherein one electron-deficient aryl group of one diarylvinyl moiety bonds to the (3-carbon of the second diarylvinyl group (Scheme 30) [41]. 

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