Showing papers by "Edamana Prasad published in 2006"

Stopped-flow kinetics of tetrazine cycloadditions; experimental and computational studies toward sequential transition states.

Abstract: The Diels-Alder cycloadditions of tetrazines (1) with alkynes (2) are expected to give bicyclic adducts (3). Kinetic measurements of the cycloadditions of 1a and 1b with 2a give DeltaG(++) = 19.2 +/- 1.0 and 11.5 +/- 1.2 kcal/mol, respectively. Stopped-flow UV studies on the reaction of 1b with 2a show an isosbestic point at 428 nm; this places an upper limit of 11.6 +/- 2.6 kcal/mol on DeltaG(++) for loss of N(2) from the putative bicyclic intermediate 3b. Calculations (B3LYP/6-31G(d,p) + ZPVE) of transition structures for the reaction of tetrazinediacid 1d with propynylamine 2c are consistent with the experimental results for the reaction of 1b with 2a. This and several related model systems reveal two interesting features of the calculated energy surfaces. First, there may be no barrier for the loss of nitrogen from structures 3 and thus there may be two sequential transition states. This also extends Berson's correlation of activation energy with reaction energy in pericyclic reactions to significantly lower barriers. Second, for the cycloaddition of 4e and 2c, there is neither an intermediate nor a transition state between TS3e and the final product 6e. It appears that the energy surface "turns a corner" in the vicinity of a structure resembling 5e. This is not a mathematically well-defined point but has chemical consequences in that the overall exothermicity of the reaction from 4e to 6e is not felt in TS3e.

Chapter 230 Samarium (II) based reductants

Abstract: 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.