Browsing by Subject "chiral secondary alcohols"
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Item Mechanistic Studies on Covalent Assemblies of Metal-Mediated Hemi-Aminal Ethers(2015-09) Jo, Hyun Hwa; Edupuganti, Ramakrishna; You, Lei; Dalbyc, Kevin N.; Anslyn, Eric V.; Jo, Hyun Hwa; Edupuganti, Ramakrishna; Anslyn, Eric V.The use of reversible covalent bonding in a four-component assembly incorporating chiral alcohols was recently reported to give a method for determining the enantiomeric excess of the alcohols via CD spectroscopy. Experiments that probe the mechanism of this assembly, which consists of 2-formylpyridine (2-PA), dipicolylamine (DPA), Zn(II) and alcohols to yield zinc complexes of tren-like ligands, are presented. The studies focus upon the mechanism of conversion of a hemi-aminal (1) to a hemi-aminal ether (3), thereby incorporating the fourth component. It was found that molecular sieves along with 3 to 4 equivalents of alcohol are required to drive the conversion of 1 to 3. Attempts to isolate an intermediate in this reaction via addition of strong Lewis acids led to the discovery of a five-membered ring pyridinium salt (5), but upon exposure to Zn(II) and alcohols gave different products to the assembly. This was interpreted to support the intermediacy of an iminium species. Kinetic studies reveal that the conversion of 1 to 3 is zero-order in alcohol in large excesses of alcohol, supporting rate-determining formation of an intermediate prior to reaction with alcohol. Further, the magnitudes of the rate constants for interconversion of 1 and 3 are similar, supporting the notion that there are similar rate-determining steps (rds) for the forward and reverse reactions. Hammett plots show that the rds involves creation of a negative charge (interpreted as the loss of positive charge), supporting the notion that the decomplexation of Zn(II) from the assemblies to generate apo-forms of 1 and 3 is rate-determining. The individual mechanistic conclusions are combined to create a qualitative reaction coordinate diagram for the interconversion of 1 and 3.Item Synthesis and Structural Analyses of Phenylethynyl-Substituted Tris(2-Pyridylmethyl)Amines and their Copper(II) Complexes(2016-05) Lim, Jaebum; Lynch, Vincent M.; Edupuganti, Ramakrishna; Ellington, Andrew; Anslyn, Eric V.; Lim, Jaebum; Lynch, Vincent M.; Edupuganti, Ramakrishna; Ellington, Andrew; Anslyn, Eric V.Three new tris(2-pyridylmethyl) amine-based ligands possessing phenylethynyl units have been prepared using Sonogashira couplings and substitution reactions. Copper(II) complexes of those tetradentate ligands have also been synthesized. Solid-state structures of the six new compounds have been determined by single-crystal X-ray diffraction analyses. Examination of the molecular structures of the ligands revealed the expected triangular geometries with virtually undeformed carbon-carbon triple bonds. While the tertiary nitrogen of the free ligands seem to be prevented from participation in supramolecular non-covalent interactions by the pyridyl hydrogen at the 3-position, the pyridyl nitrogens play a crucial role in the packing mode of the crystal structure. The nitrogens form weak hydrogen bonds, varied in length between 2.32 and 2.66 angstrom, with the pyridyl hydrogen of its neighbouring molecule. The [N center dot center dot center dot H-C] contacts enforce one-dimensional columnar assemblies on ligands that organize into wall-like structures, which in turn assemble into three-dimensional structures through CH-pi interactions. Structural analyses of Cu(II) complexes of the ligands revealed propeller-like structures caused by steric crowding of three pyridine ligands. The copper complexes of the ligands having three phenylethynyl substituents showed a remarkably deformed carbon-carbon triple bond enforced by a steric effect of the three phenyl groups. Most significantly, a total of seventy non-covalent interactions, classified into twelve types of hydrogen-involving short contacts, were identified in this study. The phenylethynyl substituent participated in forty-two interactions as a hydrogen bond acceptor, and its role was more distinctive in the crystal structures of the Cu(II) complexes.