Hexadienes reactions with hydrogen atoms

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

This type of coiled conformation, in which a certain number of monomer units are present in a certain number of turns, would cause the terminal carbon atom of a free radical, formed by the scission of a carbon-carbon bond in the polymer chain backbone, to be in close proximity to, and to interact with, a specific carbon atom, or a hydrogen atom linked to a specific carbon atom, in the turn. Thus, in an isotactic polyethylene molecule, which contains three monomer units per turn and is represented by the structure in Fig. 2, the scission of the Qg - Cm bond would bring C 6 in close proximity to or C 2 or the hydrogen atoms linked to any one of them. If Qe containing the unpaired electron attacks the C i, a six-membered ring may be formed feq. (15)]. It has been observed that in an intramolecular cyclization, the fastest reactions are those which proceed via six-membered rings However, cyclohexane (I) may lose one or more hydrogen atoms to a free radical and form hexene, hexadiene, hexane, and other compounds as shown below. 

Before examining the reaction of deactivated alkenes, the phosphonium salt synthesis was applied to 1,3-dienes.21 When ( )-6-phenyl-l,3-hexadiene was treated with equimolar amounts of PPh3 and CF3S03H in the presence of RhH(PPh3)4 (2.5 mol%) in THF at 0 °C for 3 h, ( )-(6-phenyl-3-hexenyl)triphenylphosphonium salt was obtained in 89% yield after anion exchange with LiPF6 and recrystallization (Scheme 16). The addition of phosphine and hydrogen occurred at the 1- and 2-carbon atoms of the 1,3-diene, respectively. The reaction of (7)-1,3-dienes was then performed for comparison. 

Yang and Burton studied reductive radical additions of iododifluoroacetate 37 to olefins 38 and dienes catalyzed by 6-17 mol% of a catalyst generated from NiCl2 and stoichiometric amounts of zinc in the presence of water (Fig. 8) [90, 91]. Olefins gave the reductive addition products 40a in 60-83% yield, while 1,5-hexadiene or 1,8-nonadiene provided double addition products exclusively in 55% and 73% yield. 1,7-Hexadiene gave an inseparable mixture of the expected acyclic double addition product and a tandem addition/cyclization product, in which the former dominated. The radical nature of the addition is supported by inhibition of the reaction by para-dinitrobenzene. The reaction proceeds probably via initially formed atom transfer product 39, which is subsequently reduced by nickel(0) and zinc. This is supported by deuterium incorporation, when D20 was used instead of water. No deuterium incorporation was observed with THF-dg, thus ruling out hydrogen transfer from the solvent. 

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