Solvent effects, atom transfer

Another example of a solvent-dependent atom-transfer reaction is hydrogen abstraction by chlorine atoms during the photochemical chlorination of hydrocarbons with molecular chlorine for an excellent review, see reference [571]. Russel reported that in the photochlorination of 2,3-dimethylbutane, according to reaction scheme (5-68), certain solvents do not have any effect on the selectivity of the reaction as measured by the rate ratio whereas other solvents increase this ratio significantly (c/. 

Ladanyi B M and Hynes J T 1986 Transition state solvent effects on atom transfer rates in solution J. Am. Ohem. Soc. 108 585-93... 

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... 

Taskinen and Nummelin (op. cit.) reported many other isomer equilibria in their paper. Most of these used cyclohexane as the solvent and I2 as the catalyst and so are not confounded by solvent effects. However, these authors noted that hydrogen atom transfer induced disproportionation (to form the aromatic benzene) dominates this reaction for the case of 49/50 isomerization and so they needed alternative reaction conditions. 

The rationalization of the conformational anomeric effect solely based on electrostatic interactions fails to account for these solvent effects. Another interpretation based on bond polarizability in 1,1-dialkoxyalkyl systems calls electronic transfer from a non bonding electron pair of one oxygen atom to the empty cr c 0 orbital from the other alkoxy substituent (Fig. 10).16... 

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-... 

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. 

However, even for the simple methyl transfer reactions, there is considerable confusion and some disagreement about the details of the mechanism. Some authors (Sneen, 1973) have suggested that ionization of RX always precedes attack by the nucleophile, while others have maintained that the nucleophile attacks the covalent substrate. Extensive references to both points of view are given by McLennan (1976). In the present review the application of the Marcus theory of atom transfer (Marcus, 1968a) allows us to deduce values of the parameter a which describes the symmetry of the transition state. We shall compare this information about the transition state with that from changing the solvent, from isotope effects, and from Hammett relations. We shall then attempt to deduce a model for the transition state which is consistent for all the different types of data. 

Near the TS things change. The rapid evolution of the light components of the system (electrons and H atoms involved in a transfer process) makes the adiabatic approximation questionable. Also the sudden time dependent perturbation we introduced in Section 1.1.3 to describe solvent effects on electronic transitions is not suitable. We are considering here an intermediate case for which the time dependent perturbation theory does not provide simple formulae to support our intuitive considerations. Other descriptions have to be defined. 

Substituent or solvent effects may be similar for concerted and stepwise processes. It has been shown that provided the rates of reverse reactions are almost independent of changes in oxidation potential, plots of E°, the standard reduction potential for the half cell (8) against log kf for a series of acceptors, Ox +, reacting with a hydride donor must have a slope of 30 mV/ log unit whether the rate-limiting step is hydride transfer, or hydrogen-atom transfer, or electron transfer (Kurz and Kurz, 1978). 

As we have discussed in depth elsewhere, despite the similarities in the structures of hypericin and hypocrellin, which are centered about the perylene quinone nucleus, their excited-state photophysics exhibit rich and varied behavior. The H-atom transfer is characterized by a wide range of time constants, which in certain cases exhibit deuterium isotope effects and solvent dependence. Of particular interest is that the shortest time constant we have observed for the H-atom transfer is 10 ps. This is exceptionally long for such a process, 100 fs being expected when the solute H atom does not hydrogen bond to the solvent [62]. That the transfer time is so long in the perylene quinones has been attributed to the identification of the reaction coordinate with skeletal motions of the molecule [48, 50]. 

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