- Concerted and stepwise dissociative electron transfers. Oxidability of the leaving group and strength of the breaking bond as mechanism and reactivity governing factors illustrated by the electrochemical reduction of α-substituted acetophenones
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The cyclic voltammetric investigation of a series of α-substituted acetophenones allowed the identification of the concerted and stepwise character of the dissociative electron transfer reaction, and, in the stepwise cases, the determination of the cleava
- Andrieux, Claude P.,Savéant, Jean-Michel,Tallec, André,Tardivel, Robert,Tardy, Caroline
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- Photoreleasable protecting groups on alcohols, phosphates and diacids and the use thereof
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Alcohol, phosphate, and diacid derivatives that may be deprotected by irradiation are disclosed. The protecting group is an arylacyl or heteroarylacyl group.
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(2010/11/30)
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- Application of a new kinetic method in the investigation of cleavage reactions of haloaromatic radical anions
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A simple kinetic method based on competition kinetics is presented for the measurement of cleavage rate constants of radical anions over the range of 107 - 5 × 109 s-1 in aprotic solvents. The standard potential for the formation of the radical anions may be extracted from the kinetic analysis as well. The method employs electrochemical steady-state or optical detection techniques and is an extension of the redox catalysis approach described previously in the literature. The applicability of the method is illustrated through a systematic study of the cleavage reactions for a number of short-lived haloaromatic radical anions. Interestingly, the radical anion of iodobenzene is found to be an intermediate in the homogeneous reduction of iodobenzene, even though recent investigations have shown that the corresponding heterogeneous reduction at an electrode surface proceeds by a concerted electron transfer-bond cleavage process. The nature of the cleavage reactions is discussed in terms of the activation driving force plot of the cleavage rate constants versus the relevant Gibbs energies. While the exergonic cleavage reactions follow a simple decay mechanism taking place at the halogen site, the endergonic processes are best described as intra-molecular electron transfers from the substituent to the carbon-halogen bond. Nevertheless, the overall intrinsic barrier is found to be relatively small (27-39 kJ mol-1) and it is suggested that the endergonic reactions may proceed by a stepwise mechanism, in which a σ* radical anion is formed as an intermediate prior to the formation of the dissociated products, the aryl radical and the halide. The above conclusions were supported by semi-empirical PM3 calculations of structures and charge distributions in the radical anions.
- Enemaerke, Rasmus J.,Christensen, Torben B.,Jensen, Henrik,Daasbjerg, Kim
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p. 1620 - 1630
(2007/10/03)
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- Solvent effects on the reduction mechanism of 9-chloroanthracene, 3-nitrobenzyl chloride and 3-chloroacetophenone
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The reduction mechanism of 9-chloroanthracene, 3-nitrobenzyl chloride and 3-chloroacetophenone has been investigated in detail by means of cyclic voltammetry and controlled potential bulk electrolysis. The investigation was carried out in a number of aprotic solvents in order to elucidate the importance of solvation phenomena on the various parameters associated with the reaction mechanism, i.e., clevage, hydrogen abstraction, protonation and dimerization rate constants as well as standard potentials. In this connection, the influence of the supporting electrolyte in terms of ion pairing and double layer effects has been considered. For the radical anions of 9-chloroanthracene and 3-nitrobenzyl chloride the logarithm of the cleavage rate constant shows a linear dependence on the Gutmann acceptor number and donor number, respectively, whereas no clear-cut correlation can be observed in the case of the radical anion of 3-chloroacetophenone. The rate constant obtained for the protonation reaction between 3-nitrobenzyl anion and 3-nitrobenzyl chloride is to an even higher extent influenced by the nature of the solvent. The trends in the solvent effects are discussed. Acta Chemica Scandinavica 1998.
- Jensen, Henrik,Daasbjerg, Kim
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p. 1151 - 1164
(2007/10/03)
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- Effect of Ring Substitution on the Photochemistry of α-(Aryloxy)acetophenones
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The photochemistry of a series of 10 α-(aryloxy)acetophenones has been examined by quantum yield, product studies, and laser flash photolysis techniques.Triplet decay involves a competition of β-cleavage with intramolecular quenching by the β-aryl group, the latter being favored in all systems.Typical triplet lifetimes at room temperature range from 60 to 260 ps and are rather insensitive to the substitution pattern. β-Cleavage rate constants range between 1*107 and 3*109 s-1 and are strongly favored by electron releasing substituents, such as 4-methoxy.The determination of true quantum yields of β-cleavage requires the addition of thiophenol as a radical trap to prevent the efficient back reaction that takes place otherwise; under these conditions quantum yields range from 10-3 to 0.14, the highest value being obtained for the 4-methoxy and 4-hydroxy derivatives.
- Netto-Ferreira, J.C.,Avellar, I.G.J.,Scaiano, J.C.
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- Rapid Cleavage Reactions of Haloaromatic Radical Anions Measured with Fast-Scan Cyclic Voltammetry
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Cyclic voltammetry at scan rates from 10 mV/s to 1E6 V/s has been used to characterize the reduction of aryl and benzyl halides in acetonitrile solutions containing 0.6 M tetraethylammonium perchlorate.The use of very rapid scan rates is made possible with electrodes of micrometer dimensions.The kinetics and mechanism of the reduction process have been evaluated by digital simulation of the recorded voltammograms.The radical anion generated at the electrode surface is found to have a half-life ranging from less than 100 ns in the case of 4'-bromoacetophenone to 70 ms for the case of m-nitrobenzyl chloride.The reduction mechanism for the aryl halides is consistent with the ECE-DISP1 mechanism.Thus following initial one-electron reduction, halogen bond cleavage occurs resulting in a radical that is subsequently reduced.For the compounds with relatively long half-lives (>1ms) the results in thus work are in good agreement with prior investigations with cyclic voltammetry.Good agreement is also obtained with compounds that have a short half-life ( ca. 1 μs) which have been characterized by the technique of redox catalysis.However, the measured rate constants differ with previously reported values which were estimated or measured at the extreme time limits of classical electrochemical techniques.
- Wipf, David O.,Wightman, R. Mark
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p. 4286 - 4291
(2007/10/02)
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