Showing papers by "Edamana Prasad published in 2002"
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TL;DR: The results obtained indicate that coordination or chelation is possible in the transition state geometry for SmI(2)/ketone systems even in the presence of the sterically demanding ligand HMPA.
Abstract: The effect of HMPA on the electron transfer (ET) rate of samarium diiodide reduction reactions in THF was analyzed for a series of ketones (2-butanone, methyl acetoacetate, and N,N-dimethylacetoacetamide) and alkyl iodides (1-iodobutane and 2-iodobutane) with stopped flow spectrophotometric studies. Activation parameters for the ET processes were determined by temperature-dependence studies over a range of 30-50 degrees C. The ET rate constants and the activation parameters obtained for the above systems in the presence of different equivalents of HMPA were compared to understand the mechanism of action of HMPA on various substrates. The results obtained from these studies indicate that coordination or chelation is possible in the transition state geometry for SmI(2)/ketone systems even in the presence of the sterically demanding ligand HMPA. After the addition of 4 equiv of HMPA the ET rate and activation parameters for ketone reduction by Sm is unaffected by further HMPA addition while a linear dependence of ET rate on the equivalents of HMPA was found in the SmI(2)/alkyl iodide system. The results of these studies are consistent with an inner-sphere-type ET for the reduction of ketones by SmI(2) (and SmI(2)[bond]HMPA complexes) and an outer-sphere-type ET for the reduction of alkyl iodides by SmI(2) or SmI(2)[bond]HMPA complexes.
93 citations
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TL;DR: In this paper, the rate constants for the reduction of 2-butanone, methylacetoacetate, N, N-dimethylacetopoacetamide, and a series of 4-and 2-substituted acetophenone derivatives by SmI2 were determined in dry THF using stopped-flow absorption decay experiments.
Abstract: The rate constants for the reduction of 2-butanone, methylacetoacetate, N, N-dimethylacetoacetamide, and a series of 4‘- and 2‘-substituted acetophenone derivatives by SmI2 were determined in dry THF using stopped-flow absorption decay experiments. Activation parameters for the electron-transfer processes in each series of compounds were determined by a temperature-dependence study over a range of 30 to 50 °C. Two types of reaction pathways are possible for these electron-transfer processes. One proceeds through coordination (Scheme 1) while the other involves chelation (Scheme 2). The results described herein unequivocally show that both coordination and chelation provide highly ordered transition states for the electron-transfer process but the presence of a chelation pathway dramatically increases the rate of the reduction of these substrates by SmI2. The ability of various functional groups to promote a chelated reaction pathway plays a crucial role in determining the rate of the reaction. Among the 2...
49 citations
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TL;DR: The mechanism of reduction of alkyl iodides and ketones by [Sm(N(SiMe(3)) (2))(2)] has more inner-sphere character than reduction by SmI(2) or Sm-(HMPA) complexes.
Abstract: The reductant [Sm{N(SiMe3)2}2] was examined by cyclic voltammetry and UV−vis spectroscopy. Rate constants and activation parameters for the reduction of 1-iodobutane, 2-butanone, and methylacetoacetate by [Sm{N(SiMe3)2}2] were measured in THF by stopped-flow absorption decay experiments. Comparison with SmI2 and SmI2−HMPA shows that the redox potential of [Sm{N(SiMe3)2}2] is intermediate between the SmI2-based reductants, yet it reduces alkyl iodides and ketones at a faster rate than the powerful combination of SmI2 and HMPA. The activation data for reduction of alkyl iodides and ketones by [Sm{N(SiMe3)2}2] are consistent with highly ordered transition states having low activation barriers. All of these results taken together suggest that the mechanism of reduction of alkyl iodides and ketones by [Sm{N(SiMe3)2}2] has more inner-sphere character than reduction by SmI2 or Sm−(HMPA) complexes. The change in the ET mechanism is attributed to the unique structure of the [Sm{N(SiMe3)2}2] complex.
39 citations