Transition-State Charge Stabilization through Multiple Non-covalent Interactions in the Guanidinium-Catalyzed Enantioselective Claisen Rearrangement

The mechanism by which chiral arylpyrrole-substituted guanidinium ions promote the Claisen rearrangement of O-allyl α-ketoesters and induce enantioselectivity was investigated by experimental and computational methods. In addition to stabilization of the developing negative charge on the oxallyl fragment of the rearrangement transition state by hydrogen-bond donation, evidence was obtained for a secondary attractive interaction between the π-system of a catalyst aromatic substituent and the cationic allyl fragment. Across a series of substituted arylpyrrole derivatives, enantioselectivity was observed to vary predictably according to this proposal. This mechanistic analysis led to the development of a new p-dimethylaminophenyl-substituted catalyst, which afforded improvements in enantioselectivity relative to the parent phenyl catalyst for a representative set of substrates.Chemistry and Chemical Biolog

A New Family of Nucleophiles for Photoinduced, Copper-Catalyzed Cross-Couplings via Single-Electron Transfer: Reactions of Thiols with Aryl Halides Under Mild Conditions (0 °C)

Building on the known photophysical properties of well-defined copper–carbazolide complexes, we have recently described photoinduced, copper-catalyzed N-arylations and N-alkylations of carbazoles. Until now, there have been no examples of the use of other families of heteroatom nucleophiles in such photoinduced processes. Herein, we report a versatile photoinduced, copper-catalyzed method for coupling aryl thiols with aryl halides, wherein a single set of reaction conditions, using inexpensive CuI as a precatalyst without the need for an added ligand, is effective for a wide range of coupling partners. As far as we are aware, copper-catalyzed C–S cross-couplings at 0 °C have not previously been achieved, which renders our observation of efficient reaction of an unactivated aryl iodide at −40 °C especially striking. Mechanistic investigations are consistent with these photoinduced C–S cross-couplings following a SET/radical pathway for C–X bond cleavage (via a Cu(I)–thiolate), which contrasts with nonphotoinduced, copper-catalyzed processes wherein a concerted mechanism is believed to occur

Selective Nitrite Reduction at Heterobimetallic CoMg Complexes

Heme-containing nitrite reductases bind and activate nitrite by a mechanism that is proposed to involve interactions with Brønsted acidic residues in the secondary coordination sphere. To model this functionality using synthetic platforms that incorporate a Lewis acidic site, heterobimetallic CoMg complexes supported by diimine–dioxime ligands are described. The neutral (μ-NO_2)CoMg species 3 is synthesized from the [(μ-OAc)(Br)CoMg]+ complex 1 by a sequence of one-electron reduction and ligand substitution reactions. Data are presented for a redox series of nitrite adducts, featuring a conserved μ-(η^1-N:η^1-O)-NO_2 motif, derived from this synthon. Conditions are identified for the proton-induced N–O bond heterolysis of bound NO_2– in the most reduced member of this series, affording the [(NO)(Cl)CoMg(H_2O)]+ complex 6. Reduction of this complex followed by protonation leads to the evolution of free N_2O. On the basis of these stoichiometric reactivity studies, the competence of complex 1 as a NO_2– reduction catalyst is evaluated using electrochemical methods. In bulk electrolysis experiments, conducted at −1.2 V vs SCE using Et_3NHCl as a proton source, N_2O is produced selectively without the competing formation of NH_3, NH_2OH, or H_2

Breaking the Correlation between Energy Costs and Kinetic Barriers in Hydrogen Evolution via a Cobalt Pyridine-Diimine-Dioxime Catalyst

A central challenge in the development of inorganic hydrogen evolution catalysts is to avoid deleterious coupling between the energetics of metal site reduction and the kinetics of metal hydride formation. In this work, we combine theoretical and experimental methods to investigate cobalt diimine-dioxime catalysts that show promise for achieving this aim by introducing an intramolecular proton shuttle via a pyridyl pendant group. Using over 200 coupled-cluster-level electronic structure calculations of the Co-based catalyst with a variety of pyridyl substituents, the energetic and kinetic barriers to hydrogen formation are investigated, revealing nearly complete decoupling of the energetics of Co reduction and the kinetics of intramolecular Co hydride formation. These calculations employ recently developed quantum embedding methods that allow for local regions of a molecule to be described using high-accuracy wavefunction methods (such as CCSD(T)), thus overcoming significant errors in the DFT-level description of transition-metal complexes. Experimental synthesis and cyclic voltammetry of the methyl-substituted form of the catalyst indicate that protonation of the pendant group leaves the Co reduction potential unchanged, which is consistent with the theoretical prediction that these catalysts can successfully decouple the electronic structures of the transition-metal and ligand-protonation sites. Additional computational analysis indicates that introduction of the pyridyl pendant group enhances the favorability of intramolecular proton shuttling in these catalysts by significantly reducing the energetic barrier for metal hydride formation relative to previously studied cobalt diimine-dioxime catalysts. These results demonstrate a promising proof of principle for achieving uncoupled and locally tunable intramolecular charge-transfer events in the context of homogeneous transition-metal catalysts

Mega-evolutionary dynamics of the adaptive radiation of birds

The origin and expansion of biological diversity is regulated by both developmental trajectories and limits on available ecological niches. As lineages diversify, an early and often rapid phase of species and trait proliferation gives way to evolutionary slow- downs as new species pack into ever more densely occupied regions of ecological niche space. Small clades such as Darwin’s finches demonstrate that natural selection is the driving force of adaptive radiations, but how microevolutionary processes scale up to shape the expansion of phenotypic diversity over much longer evolutionary timescales is unclear. Here we address this problem on a global scale by analysing a crowd-sourced dataset of three-dimensional scanned bill morphology from more than 2,000 species. We find that bill diversity expanded early in extant avian evolutionary history, before transitioning to a phase dominated by packing of morphological space. However, this early phenotypic diversification is decoupled from temporal variation in evolutionary rate: rates of bill evolution vary among lineages but are comparatively stable through time. We find that rare, but major, discontinuities in phenotype emerge from rapid increases in rate along single branches, sometimes leading to depauperate clades with unusual bill morphologies. Despite these jumps between groups, the major axes of within-group bill-shape evolution are remarkably consistent across birds. We reveal that macroevolutionary processes underlying global-scale adaptive radiations support Darwinian and Simpsonian ideas of microevolution within adaptive zones and accelerated evolution between distinct adaptive peaks

On the Quasi One-Dimensional Structure of Two-Dimensional Cellular Detonations in a Duct

Thesis (Master's)--University of Washington, 2015Adaptive mesh refinement combined with a WENO-TCD hybrid numerical method are used to simulate cellular detonations in ducts using a detailed chemical mechanism linked to the inviscid Euler equations. Using this computational setup, we were able to reproduce the detonation cells and structure that other researchers have produced in their work and then used the results to validate what is called the cross-current dynamics theory, developed by Kurosaka & Tsuboi (2014). This theory uses the conservation laws, Rankine-Hugoniot jump condition, and detonation front curvature to determine the velocity directly behind the detonation front. Comparisons of the velocity ratio calculated from the cross-current dynamics theory and the data from the simulation verify the accuracy of the theory. Next we simulate a detonation propagating from the closed end of a duct and compare the one-dimensional ZND solution, which we used to initiate the two-dimensional detonations, to the area-averaged properties and the properties of particles tracked along their pathlines from the detonation front to their sonic points. Despite the complex structures that appear within the detonation, the one-dimensional solution proves to also model the structure of the area-averaged and particle properties. Disagreements between the particle properties and the one-dimensional solution are concentrated near the detonation front where the transverse wave and Mach stem introduce larger jumps in the flow properties than in the one-dimensional case. We also show the particle pathlines are dominated by a one-dimensional motion with slight drifts in the vertical direction downstream from the detonation front. By reviewing the particles’ v-velocity to u-velocity ratio in the reference frame attached to the detonation front, we observe the quick transition the particles experience from a two-dimensional to a quasi one-dimensional motion. These findings give us new found appreciation of the quasi one-dimensional nature of two-dimensional detonations