Achiral Templates in Asymmetric Catalysis: Applications in Construction of All Carbon Quaternary Centers

Conjugated olefins are readily available and inexpensive starting materials and their functionalization offers a rapid access to many important building blocks for organic synthesis. The functionalization of these olefins by asymmetric catalytic methods for the formation of C-C and C-X bonds is an active area of research. Major advances in this field are not only triggered by the development of new catalysts but also by engineering of new acceptor olefins. In our lab we have successfully developed acceptor olefins appended to alkoxyimidazoles as a novel template. Using these templated acceptors, we demonstrated methodologies to construct all carbon-quaternary centers, one of the demanding tasks in synthetic methodology development. We have also made significant efforts towards understanding the solution structures of the intermediates involved in the catalytic asymmetric reactions developed in our lab. In chapter 1, the importance of templated acceptors in the field of chemical synthesis with special emphasis on acylimidazoles is reviewed. The versatility of the N-alkylimidazole templates is showcased by their utility in several organic transformations. In chapter 2, modes of activation of acceptor olefins by catalysts, need for templated acceptors and challenges associated with designing an asymmetric catalytic process is described. Our approach in designing novel acceptors based on imidazoles is also described. In chapter 3 the synthetic utility of these novel N-alkoxyimidazole based acceptors is shown by enantioselective construction all carbon quaternary centers by Lewis-acid catalysis. We also compare the effectiveness of other templates such as oxazolidinone and N-methylimidazoles in the Lewis-acidic catalyzed Friedel-Crafts alkylation reaction. The effect of various parameters such as Lewis acid, chiral ligands, temperature, additives and solvents on the conjugate addition of Friedel-Crafts nucleophiles are presented in this chapter. In chapter 4, our research efforts toward understanding solution structures of intermediates involved in catalytic asymmetric reactions are presented. A combination of Diffusion Ordered Spectroscopy (DOSY), heteronuclear NMR spectroscopy, mass spectrometry and X-ray crystallography has been used to study the solution structure of intermediates involved in asymmetric catalytic reactions. In chapter 5, a future outlook on templated chemistry developed in our laboratory is presented. Some preliminary results pertinent to future projects are presented.ND-EPSCoR: DDA (Grant FAR0021979

Propargyl Radicals in Organic Synthesis

The appearance of propargyl radicals as synthetic intermediates has increased in recent years from a structural curiosity to a frequent occurrence. This minireview covers the synthetically relevant organic chemistry involving the intermediacy of propargyl radicals. The review is organized by the process employed in radical generation, including H-atom abstraction, propargyl-X homolyses, radical addition to enynes, reduction of polar C=X bonds, reductions of stable carbocations, and metal-mediated coupling reactions. The relevant mechanisms of generation and reaction are discussed, as are applications in syntheses of target molecules of interest

The settling of spheres in a viscoplastic fluid

Results on the variation in the steady settling velocity of spheres with a time interval between experiments in a 0.3 wt% aqueous Carbopol-941 dispersion are reported. Spheres made of stainless steel, brass, ceramics and soda lime glass ranging in nominal diameter from ¼ to ⅜ in. were used. In each experiment, four identical spheres were permitted to settle in quick succession and the time interval between such sets was varied systematically. It was found that four spheres were sufficient to reach an asymptotic settling velocity which did not change with the addition of more spheres to the set. The dependence of the drag coefficient on a modified Reynolds number based on this asymptotic settling velocity was found to be consistent with that from prior work. The steady terminal settling velocity of the first sphere was found to decrease with increasing time intervals between experiments. The results are consistent with the hypothesis of network damage caused by shear with subsequent healing

Azaaldol Condensation of a Lithium Enolate Solvated by <i>N,N,N′,N′</i>-Tetramethylethylenediamine: Dimer-Based 1,2-Addition to Imines

The lithium enolate of <i>tert</i>-amylacetate solvated by <i>N,N,N′,N′</i>-tetramethylethylenediamine (TMEDA) is shown to be a doubly chelated dimer. Adding the dimeric enolate to 4-fluorobenzaldehyde-<i>N</i>-phenylimine affords an N-lithiated β-amino ester shown to be monomeric using ⁶Li and ¹⁵N NMR spectroscopies. Rate studies using ¹⁹F NMR spectroscopy reveal reaction orders consistent with a transition structure of stoichiometry [(ROLi)₂(TMEDA)₂(imine)]^⧧. Density functional theory computations explore several possible dimer-based transition structures with monodentate and bidentate coordination of TMEDA. Supporting rate studies using <i>trans-N,N,N′,N′</i>-1,2-tetramethylcyclohexanediamine showing analogous rates and rate law suggest that TMEDA is fully chelated