Ring-opening reactions: radicals

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Another approach for the ring expansion of epoxides uses low-valent iron complexes which open epoxides under reductive conditions, as reported by Hilt et al. [106]. The iron complexes are reduced and after coordination of the epoxide to the iron center an electron transfer initiates the radical-type ring opening of the epoxide. Under formal insertion of an alkene, regioselective formation of tetrahy-drofurans was observed (Scheme 9.46). The reaction is applicable to a broad range of acceptor-substituted alkenes bearing another double or triple bond system in conjugation with the inserted carbon-carbon double bond. 

The discovery of muonic radicals in 1978 by Roduner opened up a broad field of interesting experiments. Aromatic substitution is normally studied by the measurement of the yields of stable products. This includes not only the site of addition of the reactant, but also the splitting off of the atom that is substituted. With the MuSR technique only the first step is studied exclusively. Another development concerns the measurements of the reactions of muonic radicals dimerization, ring opening, and ring closure. The observations of different reaction rates of the three isomeric muonic cyclohexadienyl radicals from anisole with quinones also opens a new field of investigation. 

This barrier does not exist for the C(8)-OH adduct [c/. reaction (74)]. This radical can moreover undergo the 1,2-H-shift reaction (75), leading to a species whose reduction [reaction (80)] gives rise [reaction (79)] to a product that can hydrolyze to an amino-formamidopyrimidine, FAPY. These are well-documented purine radiolysis products. In the presence of an oxidant, e.g. Fe(CN)6, these transformation reactions (ring-opening and/or 1,2-H-shift) can... 

It was found when this monomer was treated with dl-tert-butyl peroxide at 130" and the reaction was stopped below 30% conversion, a soluble polymer was obtained having a structure of a polycarbonate with pendant methylene groups. The structure of the polymer was established by elemental analysis as well as Infrared and NMR spectroscopy. A very similar polymer could be attained by treatment of the monomer with boron trlfluorlde etherate at low conversions. The mechanism of the polymerization appeared to Involve a radical double ring-opening according to the following mechanslm (12) ... 

Apparently the intermediate radical undergoes ring opening to give the carbonyl radical. In the sealed tube only a little decarbonylation takes place and the copol3mier contains mostly the g-diketone structure. When the reaction is carried out at atmospheric pressure, much more decarbonylation takes place and the copolymer consists of mostly the simple ketone units. Since ketene dimer is an article of commerce, this monomer appears to be an excellent start-x ing material for the introduction of keto groups into an addition polymer. 

Cossy, J., Aclinou, R, Bellosta, V., Furet, N., Baranne-Lafont, J., Sparfel, D., and Souchaud, C., Radical anion ring opening reactions via photochemically induced electron transfer. Tetrahedron... 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

In spirooxaziridines like (114), /3-scission proceeds with ring opening. Stoichiometric amounts of iron(II) salt in acidic solution lead to the dicarboxylic acid derivative (115). The radical undergoes some interesting reactions with added unsaturated compounds. For example, pyridine yields a mixture of 2- and 4-alkylation products in 80% yield. Catalytic amounts of iron(II) ion are sufficient here since the adduct of the radical with pyridine is oxidized by iron(III) ion to the final product (116), thus regenerating iron(II) ion (68TL5609). 

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