Radicals ring adduct

In the phosphoryl fullerenyl radical the unpaired electron is - similar to the alkylfullerenyl radical - delocalized over two six-membered rings adjacent to the C-P bond [10]. The rotation barrier for the radical C5oP(0)(0 Pr)2 was determined to be 4.8 kcal mol Another phosphorus-containing radical fullerenyl adduct, determined via ESR spectroscopy, is PFjjO BujCgQ. It can be obtained by photolytic reaction of Cgo with HPF4 and tert-butylperoxide in tert-butylbenzene [43]. 

A ring carbon can also be involved, however, as in the reaction of the thianthrene and phenothiazine radical cations in neat pyridine or with pyridine in an anhydrous solvent. In this reaction the 1-pyridinium group is inserted on to the benzo ring (43), apparently via nucleophilic attack on di-cations 42, in turn resulting from oxidation of the initially formed radical cation adducts (Scheme 27). In the presence of moisture the sulfoxides are again formed [84]. 

The opened ring adduct (ODC6 DO) is shown in Figure 6.7 to have two low energy channels formation of a five member ring, (formyl cyclopentenone-yl radical) and a beta scission to CO and a vinylic 0=C—C=C—C=C radical. This vinylic radical will further beta scission (unzip) to two acetylenes, two CO and one H radical or rapidly react with molecular oxygen. 

Radical ring expansion of fused cyclobutanones This reaction involves reaction of an ru-bromoalkylchloroketene with an alkene to form a cyclobutanone with an exo side chain. Treatment of this adduct with BuaSnH (AIBN) generates a radical that cyclizes to a ring annelated product because of relief of strain in the four-membered ring. This reaction can be used to append 7-, 8-, or higher-membered rings to appropriate alkenes. 

All of the examples using thio-Claisen transposition for the construction of building blocks consisted of the cyclization of the rearranged adduct. Thus, octa-hydro[2]pyridinones [160] have been prepared by radical ring closure differently substituted y-lactams [161, 162] and y-lactones (Scheme9.37) have been synthesized by electrophilic olefin heterocyclization. The influence of a sulfur substituent on the selectivity of the iodolactonization has been demonstrated by our group [115]. Indeed, whereas the sulfmyl group was not efficient (yield=60%, dr=72/ 28), an excellent 1,3-induction (dr=96/4) and yield (98%) has been obtained with an a-sulfanylamide function. 

Figure 7.12 Approximate molecular orbital energy level diagram for ring adduct radicals.

Nitrones can accept various atoms and radicals. The convincing evidence of hydrogen-atom addition to nitrones has been obtained by the example of PEN [19]. The attachment of H atoms to aromatic rings gives cyclohexadienyl radicals [81]. Adducts of PEN with H atoms are unstable and are not observed in non-polar solvents. The spin trapping of fluorinated radicals and Cl atoms by PEN takes place [82, 83]. 

A radically different course is followed when the reaction of 2-alkyl-substituted thiazoles is periormed in methanol or acetonitrile (335), 2 1 adducts containing seven-membered azepine rings (91) are being formed in which two of the original activated hydrogen atoms have altered positions (Scheme 55). A similar azepine adduct (92) was obtained by... 

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). 

The results are consistent with the rate-determining step being addition of the aryl radical to the aromatic ring, Eq. (9). Support for this mechanism is derived from the results of three other studies (a) When A -nitrosoacetanilide is decomposed in pyridine, the benzene formed by abstraction of hydrogen from pyridine by phenyl radical accounts for only 1 part in 120 of the reaction leading to phenyl-pyridines. (b) 9,9, 10,lCK-Tetrahydro-10,10 -diphenyl-9,9 -bianthryl is formed in the reaction between phenyl radicals and anthracene, probably by the addition mechanism in Eq. (11). Adducts are also formed in the reactions of benzyl radicals with anthracene- and acridine. ... 

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization."110... 

Time-resolved spectroscopy establishes the formation of an ion-radical pair as the critical reactive intermediate (both from direct excitation of the CT absorption band at 532 nm and from specific excitation of chloranil at 355 nm, see Fig. 3) which undergoes ion-pair collapse to the biradical adduct followed by the ring closure to oxetane, as summarized in Scheme 11. 

Treatment of anions derived from 2-(diphenylmethyl)benzotriazole 447 with iodine generates relatively stable radicals 448 which undergo spontaneous dimerization to adducts 449 (Scheme 73) <1998JOC9992>. When one of the phenyl rings in starting material 447 is substituted in the para position, similar dimerization occurs readily, but it... 

Shevlin and coworkers30 studied the radiolysis-induced addition of the a-hydroxy isopropyl radical to substituted 1,6-heptadienes and analogs containing a heteroatom. The radical was generated by /-irradiation of propanol solutions of various 1,6-heptadienes. It was found that the adduct to the double bond decomposed to give a compound containing a five-membered ring (equation 26). 

Further variations of the general scenario described in Scheme 4 consist in trapping adduct radical 48 before oxidation occurs7. This can be achieved if intramolecular radical additions are possible, as is the case in radical 62. Oxidation of 62 to the corresponding allyl cation is slower than 6-ew-cyclization to the chlorobenzene ring to form radical 63, which subsequently is oxidized to yield tetrahydronaphthalene 64 as the main product (equation 27). This sequence does not work well for other dienes such as 2,3-dimethyl-1,3-butadiene, for which oxidation of the intermediate allyl radical is too rapid to allow radical cyclization onto the aromatic TT-system. 

The generation of HO by pulse radiolysis provides a way for investigating the kinetics of hydroxyl spin adduct formation. For PBN and some of its 4-substituted derivatives (ranging from 4-MeO to 4-N02), rate constants in the range of (5-9) X 109 dm3 mol 1 s 1 were determined (Greenstock and Wiebe, 1982). A study of the reaction of the water-soluble 2-, 3- and 4-PyBN[23] and HO showed that most of the hydroxyl radicals became attached to the heteroaromatic ring (Neta et al., 1980 Sridhar et al., 1986). Similar findings... 

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