Radical induced isomerization

In a photosensitized reaction, radical induced isomerization can occur if the sensitizer undergoes either homolytic decomposition or hydrogen abstraction, or if the system contains impurities which give radicals on irradiation. The result may be to shift the measured photostationary state in the direction of thermodynamic equilibrium and to give anomalously high values of the quantum yields for cis-trans isomerization. 

Adhikari S, Sprinz H, Brede 0 (2001) Thiyl radical induced isomerization of unsaturated fatty acids determination of equilibrium constants. Res Chem Intermed 27 549-559 Adhikary A, Bothe E, Jain V, von Sonntag C (2000) Pulse radiolysis of the DNA-binding bisbenzimid-azole derivatives Hoechst 33258 and 33342 in aqueous solution. Int J Radiat Biol 76 1157-1166 Akhlaq MS, von Sonntag C (1986) Free-radical-induced elimination of H2S from dithiothreitol. A chain reaction. J Am Chem Soc 108 3542-3544... 

The lability of allylic iodides and their sensitivity to free radical-induced isomerization, polymerization, and decomposition reactions has discouraged kinetic studies of their substitution and isomerization reactions. 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

Reaction (26) is, in principle, reversible, and if the C-H BDE of the product RH is close to that of the S-H BDE of the thiol the reverse reaction becomes of importance. This has first been shown for 2,5-dimethyltetrahydrofurane, where thiyl radicals induce a cis-trans isomerization via a chain reaction [reactions... 

Vieira AJSC, Steenken S (1990) Pattern of OH radical reaction with adenine and its nucleosides and nucleotides. Characterization of two types of isomeric OH adduct and their unimolecular transformation reactions. J Am Chem Soc 112 6986-6994 Vieira AJSC, Steenken S (1991) Pattern of OH radical reaction with N6,N6,9-trimethyladenine. Dehy-droxylation and ring-opening of isomeric OH-adducts. J PhysChem 95 9340-9346 Vieira AJSC, Candeias LP, Steenken S (1993) Hydroxyl radical induced damage to the purine bases of DNA in vitro studies. J Chim Phys 90 881-897... 

Hankiewicz E (1995) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis II. Reactions of primary radicals with oxidants. Bull Pol Acad Sci Chem 43 41-49 Hankiewicz E (1996) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis - Part III. Consecutive reactions. Radiat Phys Chem 47 61-65 Hankiewicz E (1998) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis. IV. Reactions of primary radicals with reductants. Bull Pol Acad Sci Chem 46 455-464 Hankiewicz E, Bothe E, Schulte-Frohlinde D (1992) Hydroxyl radical-induced reactions in polyadenylic acid as studied by pulse radiolysis, part. I. Transformation reactions of two isomeric OH-adducts. Free Rad Res Commun 16 391-400... 

Interestingly, the radical cation 138 can be generated also by light-induced isomerization of cyclooctatetraene radical cation (140). The conversion of the red non-planar ion 140 (4n - 1 n electrons) upon irradiation with visible light had been observed previously [395], but the blue photo-product had not been recognized as the cyclic conjugated species 138 with 4n + 1 n electrons. This interconversion is one of only a few orbital symmetry allowed processes documented in radical cation chemistry [393]. 

Schwinn J, Sprinz H, Drossier K, Leistner S, Brede O. (1998) Thiyl radical-induced cis/tr r-isomerization of methyl linoleate in methanol and of linoleic acid residues in liposomes. Int JRadiat Biol 74 359-365. 

G-values are reported for the electron induced isomerization which is interpreted in terms of a radical-chain mechanism which involves an addition elimination sequence in which both the added and eliminated moieties are RSO - radicals. 

Itransfer cyclization Free-radical chain reactions are usually terminated by abstraction of a hydrogen atom. In contrast, standard radical cyclization of the 6-iodohexyne (1) induces isomerization to the (iodomethylene)cyclopentane (2). The formation of minor amounts of the reduced product 3 can be suppressed by use of Bu3Sn—SnBu3 and irradiation (95% yield of 2). 

Depending on the mechanism and reaction conditions, this type of transformation can be subdivided into thermal interconversions via 1,3-diradical intermediates, isomerizations via radical anions, as well as base- or acid-induced isomerizations. 

Radiolysis at 77 K of /-butyl nitrile in trichlorofiuoromethane has shown that the radical cation is produced. y-Radiolysis of 1-cyanoadamantane affords the corresponding 1-cyanoadamantyl radical which isomerizes to the 1-adamantaneiminyl radi-cal. It was also reported that a-radiolysis of nitriles 739 affords radicals and electron-impact induced formation of cyano radicals from acetonitrile, fluoroacetonitrile and chloroacetonitrile has been carried out. Pulsed radiolysis studies have also been reported and interest shown in the study of change in absorption bands brought about by phase effects on the pulse radiolysis of the liquid crystalline material 740. Several... 

Zard also used a xanthate-mediated radical cascade sequence to synthesize indolines from vinyl sulfanilides in three steps [54]. The initial peroxide-initiated intermolecular addition of the xanthate to the vinyl sulfanilide was followed by an intramolecular cyclization onto the ortho position of the aromatic ring. Further heating with DBU generated the dihydroindole through a base-induced isomerization of the olefinic bond followed by conjugate addition. 

Acetylene is one of the simplest hydrocarbons and a fundamentally important chemical in organic chemistry. The electronic structure of acetylene and its related compounds have been extensively studied both experimentally and theoretically, and a number of interesting observations have been reported. For example, a trans-bent structure has been reported for acetylene in an excited state [42, 43] and also in a radical anion form [34, 44]. The interaction between acetylene and metal atoms such as A1 and Li has been studied by inert gas matrix isolation (MI) ESR and IR methods [45-48]. Here we present our ESR study on the structural distortion in acetylene radical anion in the glassy 2-MTHE matrix, which is another example showing a mixing of the k and higher-lying a orbitals at the C=C carbons similar to that at the C=C carbons for the perfluoroalkene radical anions. In addition photo-induced isomerization reaction of the acetylene radical anion in the matrix is briefly presented. 

A third example may demonstrate the isomerization tendency of a-functionalized alkylpolysilanes. When the hydridosilane 16 was subjected to an intended radical-induced Si-Si coupling again a product of a l,2-Si,C substituent exchange was observed. The hydridosilane 16 was heated with dibenzoyl peroxide. But instead of the expected Si,Si-coupling product we obtained the benzoyloxysilane 18 (Eq. 4). The structure of 18 was elucidated by a full spectral analysis. The results of the X-ray structural analysis are shown in Figure 2. 

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