Radical couphng

This observation opens up a new possibility in the formation of C - O bonds for the already comphcated oxygenation reactions of organic substrates, i.e., the non-innocent behavior of the olefin in open-shell transition metal olefin complexes can allow a direct radical couphng of dioxygen with the coordinated olefin. 

Radical chain reactions are comprised of three distinct parts initiation, propagation steps, and termination. The initiation portion involves one or more elementary reactions that produce a radical that can participate in one of the propagation steps. The propagation sequence is where the desired products are formed it consists of two or more reactions in which one product of each elementary reaction is a radical that serves as a reactant in another step of the sequence. Radicals are destroyed in termination steps that give nonradical products by radical-radical couphng and disproportionation reactions. 

The -catalyzed hydride transfer from BNAH to Q is known to proceed via a -promoted ET from BNAH to Q, followed by a proton transfer from the resulting BNAH" to the Q" /(Mg ) complex and the subsequent fast ET from BNA to QH /Mg (117, 143). The change in the type of reaction depending on the Lewis acidity of the metal ion can be explained well by the ET mechanism in Scheme 23. The initial rate-determining ET from BNAH to Q results in the formation of radical ion pair (BNAH + and Q ), where Q forms 1 1 and 1 2 complexes with Sc. This result is followed by fast radical couphng between Q and BNAH to give the zwitterionic intermediate that is eventually... 

Solid supported catalysts have been utihzed in ATRA and ATRC reactions in order to facilitate reaction work-up and enable catalyst recycling. In one example, V-alkyl-2-pyridylmethanimines were immobihzed on sihca and in conjunction with Cu Br or Cutl utilized in ATRC reactions of trichloro-, dichloro- and monobromo-substrates. Efficient 5-exo and 5-endo cyclizations could be mediated using this immobilized system, however, as coimnonly encountered, the solid supported catalyst was much less active than its homogeneous counterpart. The decrease in the catalytic activity was not induced by catalyst leaching, but rather accumulation of copper(II) deactivator. The accumulation of deactivator during ATRA or ATRC process was result of irreversible and often diffusion controlled radical-radical couphng reactions (A 1.0xl0 M- s- ). 

A recently discovered quantitative radical chain-couphng reaction of polymer precursors preformed by CMRP, which is referred to as cobalt-mediated radical couphng (CMRC), was first observed when PAN-Co(acac)2 chains were treated with a large excess of isoprene at room temperature [58]. Instead of the formation of an expected PAN-h-polyisoprene (PI) diblock copolymer, a weU-defined homo PAN sample, with an exactly twofold higher molar mass compared to the precursor, was recovered. Interestingly, this coupling reaction proved to be quantitative. 

It is clear that a-oxoalkyl radicals have the potential to add to unsaturated chemical bonds. Thus, metal-free, peroxide-mediated radical couphng reactions of... 

Li P, Zhao J, Xia C, Li F (2015) The development of carbene-stabilized N-O radical couphng strategy in metal-free regioselective C-H azidation of quinohne N-oxides. Org Chem Front 2 1313-1317... 

Although it is now established that CIDNP has a quite different origin from DNP, the two effects were initially thought to be related. Thus the Overhauser effect, in which saturation of unpaired electron spins leads to polarization of nuclei coupled to the electrons through the hyperfine couphng constant (ajj), can be observed in organic radicals, and CIDNP 3... 

CIDNP has also been reported in reactions of organomercurials. Emission is observed from the couphng product of p-methylbenzyl-mercuric bromide and triphenylmethyl bromide (Beletskaya et al., 1971), while thermolysis of organomercury derivatives of tin such as t-C4H9HgSn(CH3)3 gave mixtures of isobutene and isobutane (by disproportionation of uncorrelated pairs of t-butyl radicals) showing A/E polarization (Mitchell, 1972). 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

Seventeen-electron species have also been found to form complexes with noble gases. For example, the two paramagnetic radicals RrMn(00)5 and [KrFe(00)5]+ have been detected by EPR spectroscopy by Morton, Perutz, and co-workers following the y-radiolysis of HMn(00)5 and Fe(00)5 in laypton matrices at 77 and 20 K, respectively (37). Evidence for the interaction of Kr with the unpaired electron on the metal center came from the observation of hyperfine couphng with a single Kr nucleus in the EPR spectra of these species. As an example, the EPR spectrum obtained from y-radiolysis of HMn(CO)5 in a matrix of krypton enriched to 42% in the isotope Kr (I = ) is shown in Fig. 5. The spectrum shows the resonances of the Mn(CO)5 radical with characteristic decets of satellites due to hyperfine interaction between the unpaired spin on Mn and a Kr nucleus. 

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly... 

From CIDNP patterns it is possible to derive magnetic properties of free radicals as magnitudes and signs of hyperfine couphng constants, g factors and nuclear relaxation times. 

Benelli, C., Caneschi, A., Fabretti, A.C., Gatteschi, D., and Pardi, L. (1990) Ferromagnetic couphng of gadolinium(lll) ions and nitronyl nitroxide radicals in an essentially isotropic way. Inorganic Chemistry, 29, 4153-4155. 

Enholm [12] has also prepared an enantiomerically pure soluble polymer support 82 by couphng xylose-derived chiral auxiliary 81 with 77 (Scheme 18). The chiral support was then treated with bromopropionic acid 83 to give substrate 84. Eree radical allyl transfer from allyltributyltin imder thermal conditions provided 85 in 93% yield, and basic cleavage from the resin gave (R)-(-)-2-methylpent-4-enoic acid 86 in 80% yield and 97% ee, with a 92% yield of recovered 82. Previous studies of the same process in solution had found the addition of Lewis acids to be crucial for high selectivities to be obtained. Interestingly, the addition of Lewis acids to the reaction on polymer support led to cleavage of the carbohydrate from the polymer backbone. En-... 

Aliphatic iodides, and especially secondary and tertiary representatives, are subject to hydride ehmination and are not generally useful substrates in transition metal catalysed coupling reactions. The last reaction in Scheme 14 [31], for instance, cannot be executed using current transition metal-based technology. In contrast, vinyl and aryl iodides, which are superb partners in many classical metal-induced couphng reactions, are very poor substrates in the present radical process because of the high energy of vinyl and aryl radicals. The two methods thus nicely complement each other. 

Further to its ability to perform ally lie and benzyhc oxidations, " t-butylperoxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields. o,o-Couphng dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

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