Edamana Prasad

Mechanistic study of samarium diiodide-HMPA initiated 5-exo-trig Ketyl-Olefin coupling: the role of HMPA in post-electron transfer steps.

The mechanistic importance of HMPA and proton donors (methanol, 2-methyl-2-propanol, and 2,2,2-trifluoroethanol) on SmI2-initiated 5-exo-trig ketyl-olefin cyclizations has been examined using stopped-flow spectrophotometric studies. In the presence of HMPA, the rate order of proton donors was zero and product studies showed that they had no impact on the diastereoselectivity of the reaction. Conversely, reactions were first-order in HMPA, and the additive displayed saturation kinetics at high concentrations. These results were consistent with HMPA being involved in a rate-limiting step before cyclization, where coordination of the intermediate ketyl to the sterically congested Sm(III)HMPA both stabilizes the intermediate and inhibits cyclization. Liberation of the contact ion pair through displacement by an equivalent of HMPA provides a solvent-separated ion pair releasing the steric constraint to ketyl-olefin cyclization. The mechanism derived from rate studies shows that HMPA is important not only in increasing the reduction potential of Sm(II) but also in enhancing the inherent reactivity of the radical anion intermediate formed after electron transfer through conversion of a sterically congested contact ion pair to a solvent-separated ion pair. The mechanistic complexity of the SmI2-HMPA-initiated ketyl-olefin cyclization is driven by the high affinity of HMPA for Sm(III), and these results suggest that simple empirical models describing the role of HMPA in more complex systems are likely to be fraught with a high degree of uncertainty.

Effect of crown ethers on the ground and excited state reactivity of samarium diiodide in acetonitrile.

Electron transfer from the ground and excited states of Sm[15-crown-5](2)I(2) complex to a series of electron acceptors (benzaldehyde, acetophenone, benzophenone, nitrobenzene, benzyl bromide, benzyl chloride, 1-iodohexane, and 1,4-dinitrobenzene) was investigated in acetonitrile. Electron transfer from the ground state of the Sm(II)-crown system to aldehydes and ketones has a significant inner sphere component indicating that the oxophilic nature of Sm(II) prevails in the system even in the presence of bulky ligands such as 15-crown-5 ether. Activation parameters for the ground state electron transfer were determined, and the values were consistent with the proposed mechanistic models. Since crown ethers stabilize the photoexcited states of Sm(II), the photochemistry of Sm[15-crown-5](2)I(2) system in solution state has been investigated in detail. The results suggest that photoinduced electron transfer from Sm(II)-crown systems to a wide variety of substrates is feasible with rate constant values as high as 10(7) M(-1) s(-1). The results described herein imply that the present difficulty of manipulating the extremely reactive excited state of Sm(II) in solution phase can be overcome through stabilizing the excited state of the divalent metal ion by careful design of the ligand systems.

Rate and Mechanistic Investigation of Eu(OTf)2-Mediated Reduction of Graphene Oxide at Room Temperature

We describe a fast, efficient, and mild approach to prepare chemically reduced graphene oxide (rGO) at room temperature using divalent europium triflate {Eu(OTf)2}. The characterization of solution-processable reduced graphene oxide has been carried out by various spectroscopic (FT-IR, UV-visible absorption, and Raman), microscopic (TEM and AFM), and powder X-ray diffraction (XRD) techniques. Kinetic study indicates that the bimolecular rate constants for the reduction of graphene oxide are 13.7 ± 0.7 and 5.3 ± 0.1 M(-1) s(-1) in tetrahydrofuran (THF)-water and acetonitrile (ACN)-water mixtures, respectively. The reduction rate constants are two orders of magnitude higher compared to the values obtained in the case of commonly used reducing agents such as the hydrazine derivative, sodium borohydride, and a glucose-ammonia mixture. The present work introduces a feasible reduction process for preparing reduced graphene oxide at ambient conditions, which is important for bulk production of GO. More importantly, the study explores the possibilities of utilizing the unique chemistry of divalent lanthanide complexes for chemical modifications of graphene oxide.

Self-Assembly-Directed Aerogel and Membrane Formation from a Magnetic Composite: An Approach to Developing Multifunctional Materials.

Herein, we report the preparation of an aerogel and a membrane from a magnetic composite material by tuning the self-assembly at the molecular level. The gel exhibits an excellent oil absorption property, and the membrane shows a remarkable autonomous self-healing property. The composite is formed from an organosilicon-modified poly(amidoamine) (PAMAM) dendrimer, which is linked with iron oxide nanoparticles and poly(vinyl alcohol). Upon the addition of a cross-linker (formaldehyde), the system undergoes a fast self-assembly and gelation process. The aerogel, obtained after drying of the hydrogel, was modified with 1- bromohexadecane at room temperature and utilized for the removal of oil from water with 22.9 g/g absorption capacity. Intriguingly, the same system forms a membrane with 97% autonomous self-healing ability, in the absence of the cross-linker. The membrane was used to remove the salt content from water with an efficiency of 85%. The control experiments suggest that the presence of the magnetic material (iron oxide) plays a key role in the formation of both the aerogel and membrane.

Chapter 230 Samarium (II) based reductants

Publisher Summary This chapter discusses the synthesis of Sm(II)-based reductants and their use in organic and inorganic synthesis. It focuses on the synthesis and utility of homoleptic Sm(II) reductants containing halide (I, Br, Cl) and cyclopentadienyl (Cp and Cp*) ligands and provides a brief description of amide (–N(SiMe 3 ) 2 ), alkoxide ligands, and pyrazolylborate ligands. The chapter also discusses the reactivity of Sm(II). A great deal of scientific effort has been directed towards understanding the reactivity and behavior of Sm(II) reductants containing iodide and pentamethylcyclopentadienyl ligands. Therefore, the chapter focuses on these two classes of Sm(II)-based reductants. Novel approaches to enhancing the reactivity Sm(II)-based reductants are likely to lead to protocols that mimic the beneficial behavior of additives such as HMPA, but provide safer alternatives. As these advances are exploited and others are developed, the use of Sm(II) reagents are likely to continue and grow for the foreseeable future.

Augmenting Photoinduced Charge Transport in a Single-Component Gel System: Controlled In Situ Gel-Crystal Transformation at Room Temperature

Smart single-component materials with versatile functions require pre-programming of a higher order molecular assembly. An electroactive supergelator (c=0.07 wt %) triphenylamine core-appended poly(aryl ether) dendron (TPAPAE) is described, where substantial dendritic effects improve the order and crystallinity by switching the local minima from self-assembled molecular wires to thermodynamically favorable global minima of ordered crystals, ripened within the fibers. Controlled in situ phase change at room temperature ultimately stabilized the mixed valence states in the single-component supramolecular assembly with photoluminescence and photoinduced charge transport amplified by two orders of magnitude.