Atom transfer radical solvent effects

The synthesis of mixed peroxides formed from /-butyl hydroperoxide and carbon-centred radicals has been studied. The reactions were strongly effected by solvents as well as catalytic amounts of Cun/Fem. The kinetic data suggest that the conditions for the Ingold-Fischer persistent radical effect are fulfilled in these cases.191 The use of Cu /Cu" redox couples in mediating living radical polymerization continues to be of interest. The kinetics of atom-transfer radical polymerization (ATRP) of styrene with CuBr and bipyridine have been investigated. The polymer reactions were found to be first order with respect to monomer, initiator and CuBr concentration, with the optimum CuBr Bipy ratio found to be 2 1.192 In related work using CuBr-A-pentyl-2-... 

Matyjaszewski, K., Nakagawa, Y., and Jasieczek, C. B. (1998). Polymerization of n-butyl acrylate by atom transfer radical polymerization. Remarkable effect of ethylene carbonate and other solvents. Macromolecules, 31(5) 1535-1541. 

Braunecker WA, Tsarevsky NV, Gennaro A, Matyjaszewski K. Thermodynamic components of the atom transfer radical polymerization equilibrium quantifying solvent effects. Macromolecules 2009 42 6348-6360. 

Feng W, Chen R, Brash JL, Zhu S (2005) Surface-initiated atom transfer radical polymerization of oligo(ethylene glycol) methacrylate effect of solvent on graft density. Macromol Rapid Commun 26(17) 1383-1388... 

Yorimitsu H, Nakamura T, Shinokuho H, Oshima K, Omoto K, Fujimoto H. Powerful solvent effect of water in radical reaction triethylborane-induced atom-transfer radical cyclization in water. J. Am. Chem. Soc. 2000 122(45) 11041-11047. 

AA sec acrylic acid abstraction sec hydrogen atom transfer abstraction v,v addition and micleophilicity 35 by aikoxy radicals 34-5, 124-5, 392 by alkoxycarbonyloxy radicals 103,127-8 by alkyl radicals 34 5, 113, 116 by f-amyloxy radicals 124 by arenethiyl radicals 132 by aryl radicals 35, 118 by benzovloxy radicals 35, 53, 120, 126 wilh MM a" 53, 120 by /-butovy radicals 35, 53, 55, 124 solvent effects 54, 55. 123 with alkenes 122 3 with ally I acrylates 122 wilh AMS 120, 123 wilh BMA 53, 123 with isopropenvl acetate 121 with MA 120 with MAN 121 with MMA 53, 55, 120.419 with VAc 121 with vinyl ethers 123... 

As to the next step, namely, the reaction of aryl radicals with nucleophiles, we should take into account the fact that air molecular orbital, which initially accommodates the incoming electron, is available in the aryl halide. The electron is subsequently transferred in-tramolecularly from the it to the o molecular orbital of the carbon-halogen bond. Aryl radicals effectively scavenge H atoms. Therefore, an abstraction of a hydrogen atom from the solvent may occur. However, in the case of nucleophiles that can act as effective traps of aryl radicals, the addition of a nucleophile to the phenyl radical takes place. At this point, let us focus on the step of addition of the nucleophile (Y ) to the intermediate radical (Ph). When a new a bond begins to form between the sp3 carbon-centered radical (H5C6) and... 

The mechanism shown in Scheme 4.9 has been proposed for the hydrogen atom transfer from phenols (ArOH) to radicals (Y ) in non-aqueous solvents, a kinetic effect ofthe solvent (S) being expected when ArOH is a hydrogen bond donor and the solvent a hydrogen bond acceptor. Steps with mechanistic rate constants k, k-1 and k>, involve proton transfer (the latter two near to the diffusion-controlled limit), and kj involves electron transfer. The step with rate constant fco involves a direct hydrogen atom transfer, and the other path around the cycle involves a stepwise alternative. 

If a hydrogen atom is abstracted from an alkane by an alkyl radical, both the initial and final state of the reaction involve neutral species and it is only the transition state where some limited charge separation can be assumed. In the case of a homolytic O—H bond fission, however, the initial state possesses a certain polarity and possible changes in polarity during the reaction depend on both the lifetime of the transition state and the nature of the attacking radical. If the unpaired electron is localized mainly on oxygen in the reactant radical, the polarity of the final state will be close to that of the initial state and any solvent effect will primarily depend on the solvation of the transition state. Solvent effects can then be expected since the electron and proton transfers are not synchronous. 

The initially formed chlorides PyX ionize to a pyridinium halide while PyR represents mixed 1,4- and 1,2-dihydropyridines.233,234 4-Nitrobenzyl halides react by a different mechanism the initial step here is electron transfer from 66 to the halide to give a nitro anion-radical which subsequently loses halide. Since this reaction involves ionic intermediates it is much more susceptible to solvent effects than the atom transfer reaction.235,236... 

Free-radical activated complexes are formed by the creation of unpaired electrons during homolytic bond cleavage. Free-radical transition-state reactions with small or negligible solvent effects are found among radical-pair formation and atom-transfer reactions such as ... 

The negligible solvent effect of this radical reaction with dibromomethane demonstrates that the activated complex for bromine atom-transfer has the same charge separation as the initial reactants. The dipole moment expected for a molecule like the pyridinyl radical is probably (0... 10) 10 ° Cm (0...3 D). Dibromomethane has a modest dipole moment of 5 10 Cm (1.5 D). Consequently, in view of the negligible solvent effect upon rate, the activated complex must also have a dipole moment between (0... 10) 10-3 Cm [214, 570]. 

It has been established that for solvents in which specific solvation is not dominant, a small solvent effect implies an atom-transfer reaction and a large solvent effect suggests an electron-transfer reaction between neutral species. The high solvent sensitivity of electron-transfer reactions between neutral molecules should provide a useful test of their occurrence [215, 570]. From Table 5-11, it can be concluded that atom-transfer, according to Eq. (5-66), is the rate-limiting step in the reaction of pyridinyl radical with... 

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