Showing papers by "Edamana Prasad published in 2005"
TL;DR: Rate studies on the reduction of benzyl bromide by SmI2-water are consistent with reaction occurring through a dimeric transition state with the assembly of the activated complex requiring an equivalent of water at low concentrations but not at higher concentrations.
Abstract: The mechanistic impact of water addition to SmI2 on the ground state and rate-limiting transition state structures in the reduction of benzyl bromide was determined using UV−vis spectroscopy, cyclic voltammetry, vapor pressure osmommetry, and stopped-flow spectrophotometric studies. The results obtained from these studies show that, upon addition of water, SmI2 in THF (or DME) becomes partially water-solvated by displacing metal-coordinated solvent. Further addition of water displaces remaining bound solvent and induces a monomer−dimer equilibrium of the SmI2−water complex. Concomitant with this process, a thermodynamically more powerful reductant is created. Rate studies on the reduction of benzyl bromide by SmI2−water are consistent with reaction occurring through a dimeric transition state with the assembly of the activated complex requiring an equivalent of water at low concentrations but not at higher concentrations. The mechanistic complexity of the SmI2−water system shows that simple empirical mode...
83 citations
TL;DR: Iradiation studies clearly show that SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.
Abstract: Photoluminescence quenching studies of SmI 2 in dry THF were carried out in the presence of five different classes of compounds: ketone, alkyl chloride, nitrile, alkene and imine. The free energy change (AG°) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI 2 ) and acceptors. The bimolecular quenching constants (k q ) derived from the Stern-Volmer experiments parallel the free energy changes of the PET processes. The observed quenching constants were compared with the theoretically derived electron transfer rate constants (k e t ) from Marcus theory and found to be in good agreement when a value of λ=167 kJ mol - 1 (40 kcal mol - 1 ) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of Sm I I in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of Sm I I in THF. The activation parameters determined for the PET reactions in SmI 2 /1-chlorobutane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions. Irradiation studies clearly show that SmI 2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.
27 citations
TL;DR: The results indicate that the reducing power of LnX2 has a large impact on not only the pinacol coupling/reduction product ratio of ketones but also on other substrates in which there are competing coupling and reduction reactions.
Abstract: The use of microwave heating in lanthanide(II) halide (LnX2=SmBr2, SmI2, and YbI2) mediated reduction and coupling reactions has been investigated for a variety of functional groups including α,β-unsaturated esters, aldehydes, ketones, imines, and alkyl halides. Good to quantitative transformations were obtained within a few minutes without the addition of any co-solvents, such as hexamethyl phosphoramide (HMPA). The redox potential of YbI2 in tetrahydrofuran (THF) has been determined as −1.02±0.05 V (versus Ag/AgNO3) by cyclic voltammetry. A large selectivity difference in various reactions was observed depending on the redox potential of the LnX2 reagent. The more powerful reductant, SmBr2, afforded mainly pinacol-coupling products of ketones whereas the weaker reductant YbI2 afforded mainly reduction products. The results indicate that the reducing power of LnX2 has a large impact on not only the pinacol coupling/reduction product ratio of ketones but also on other substrates in which there are competing coupling and reduction reactions. The use of in situ generated LnX2 has also been explored and proven useful in many of these reactions.
22 citations