Quasi‐trapped ion and electron populations at Mercury

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94682/1/grl28663.pd

Rotaxanes and Biofunctionalized Pseudorotaxanes via Thiol-Maleimide Click Chemistry

Base-catalyzed thiol-maleimide click chemistry has been applied to the synthesis of neutral donor–acceptor [2]rotaxanes in good yield. This method is extended further to the synthesis of a glutathione-functionalized [2]pseudorotaxane, a precursor to integrated conjugates of interlocked molecules with proteins and enzymes

Structural control at the organic–solid interface

The structure–function relationships of a series of bistable [2]rotaxane and [2]pseudorotaxane-based devices have been evaluated across different length scales. The switching characteristics of bistable [2]rotaxanes and self-assembled [2]pseudorotaxanes, which can be controlled chemically, electrochemically, or photochemically, enable them to function as prototypes of molecular machines. The switching processes are operative, not only in solution, but also in a wide variety of condensed phases. The universality of the switching mechanism demonstrates that these functional organic materials can be incorporated onto solid metallic and inorganic supports for device applications, despite the fact that interactions at the organic substrate interface can influence molecular structure and function. Through iterative design–analysis feedback loops that focus upon fine-tuning device performance, based on molecular structures and molecule-substrate interactions, the fabrication of functioning micro-actuators, nanovalves and light-harvesting devices has been achieved

Experimental and Theoretical Studies of Selective Thiol–Ene and Thiol–Yne Click Reactions Involving <i>N</i>‑Substituted Maleimides

A combination of experimental and computational methods has been used to understand the reactivity and selectivity of orthogonal thiol–ene and thiol–yne ″click″ reactions involving <i>N</i>-allyl maleimide (<b>1</b>) and <i>N</i>-propargyl maleimide (<b>2</b>). Representative thiols methyl-3-mercaptopropionate and β-mercaptoethanol are shown to add exclusively and quantitatively to the electron poor maleimide alkene of <b>1</b> and <b>2</b> under base (Et₃N) initiated thiol-Michael conditions. Subsequent radical-mediated thiol–ene or thiol–yne reactions can be carried out to further functionalize the remaining allyl or propargyl moieties in near quantitative yields (>95%). Selectivity, however, can only be achieved when base-initiated thiol-Michael reactions are carried out first, as radical-mediated reactions between equimolar amounts of thiol and <i>N</i>-substituted maleimides give complex mixtures of products. CBS-QB3 calculations have been used to investigate the energetics and kinetics of reactions between a representative thiol (methyl mercaptan) with <i>N</i>-allyl and <i>N</i>-propargyl maleimide under both base-initiated and radical-mediated conditions. Calculations help elucidate the factors that underlie the selective base-initiated and nonselective radical-mediated thiol–ene/yne reactions. The results provide additional insights into how to design selective radical-mediated thiol–ene/yne reactions

Spectroscopic and Computational Investigations of The Thermodynamics of Boronate Ester and Diazaborole Self-Assembly

The solution phase self-assembly of boronate esters, diazaboroles, oxathiaboroles, and dithiaboroles from the condensation of arylboronic acids with aromatic diol, diamine, hydroxythiol, and dithiol compounds in chloroform has been investigated by ¹H NMR spectroscopy and computational methods. Six arylboronic acids were included in the investigations with each boronic acid varying in the substituent at its 4-position. Both computational and experimental results show that the para-substituent of the arylboronic acid does not significantly influence the favorability of forming a condensation product with a given organic donor. The type of donor, however, greatly influences the favorability of self-assembly. ¹H NMR spectroscopy indicates that condensation reactions between arylboronic acids and catechol to give boronate esters are the most favored thermodynamically, followed by diazaborole formation. Computational investigations support this conclusion. Neither oxathiaboroles nor dithiaboroles form spontaneously at equilibrium in chloroform at room temperature. Computational results suggest that the effect of borylation on the frontier orbitals of each donor helps to explain differences in the favorability of their condensation reactions with arylboronic acids. The results can inform the use of boronic acids as they are increasingly utilized in the dynamic self-assembly of organic materials and as components in dynamic combinatorial libraries

Thiol–Ene Click Chemistry: Computational and Kinetic Analysis of the Influence of Alkene Functionality

The influence of alkene functionality on the energetics and kinetics of radical initiated thiol–ene click chemistry has been studied computationally at the CBS-QB3 level. Relative energetics (Δ<i>H</i>°, Δ<i>H</i>^⧧, Δ<i>G</i>°, Δ<i>G</i>^⧧) have been determined for all stationary points along the step-growth mechanism of thiol–ene reactions between methyl mercaptan and a series of 12 alkenes: propene, methyl vinyl ether, methyl allyl ether, norbornene, acrylonitrile, methyl acrylate, butadiene, methyl­(vinyl)­silanediamine, methyl crotonate, dimethyl fumarate, styrene, and maleimide. Electronic structure calculations reveal the underlying factors that control activation barriers for propagation and chain-transfer processes of the step-growth mechanism. Results are further extended to predict rate constants for forward and reverse propagation and chain-transfer steps (<i>k</i>_P, <i>k</i>_–P, <i>k</i>_CT, <i>k</i>_–CT) and used to model overall reaction kinetics. A relationship between alkene structure and reactivity in thiol–ene reactions is derived from the results of kinetic modeling and can be directly related to the relative energetics of stationary points obtained from electronic structure calculations. The results predict the order of reactivity of alkenes and have broad implications for the use and applications of thiol–ene click chemistry

Vibrational Properties of Boroxine Anhydride and Boronate Ester Materials: Model Systems for the Diagnostic Characterization of Covalent Organic Frameworks

The vibrational characteristics of 28 different boronic acid, boroxine anhydride, and boronate ester species have been systematically investigated using a combination of experimental infrared (IR) spectroscopy and computational modeling. IR bands characteristic to each boron-containing functionality have been categorized and assigned in conjunction with density functional theory (B3LYP/6-31G­(d)), with the aim of better understanding and distinguishing the vibrational characteristics of covalent organic frameworks (COFs) built from boronic acids. In several cases, vibrational assignments differ from those previously reported in the literature on boronic acid-based COFs. Vibrations commonly regarded as diagnostic for one functionality are found in regions of the IR spectrum where other functionalities also show characteristic peaks. The collective experimental and computational results reveal that several alternative bands in the IR region can be used to more diagnostically distinguish between boronic acid, boroxine anhydride, and boronate ester species. The results presented herein provide the tools for straightforward characterization of boroxine anhydride and boronate ester species using IR spectroscopy. The results can be applied to additional theoretical studies of larger COF-like assemblies as well as the analysis of other boronic-acid-based materials

Investigation and Demonstration of Catalyst/Initiator-Driven Selectivity in Thiol-Michael Reactions

Thiol-Michael “click” reactions are essential synthetic tools in the preparation of various materials including polymers, dendrimers, and other macromolecules. Despite increasing efforts to apply thiol-Michael chemistry in a controlled fashion, the selectivity of base- or nucleophile-promoted thiol-Michael reactions in complex mixtures of multiple thiols and/or acceptors remains largely unknown. Herein, we report a thorough fundamental study of the selectivity of thiol-Michael reactions through a series of 270 ternary reactions using ¹H NMR spectroscopy to quantify product selectivity. The varying influences of different catalysts/initiators are explored using ternary reactions between two Michael acceptors and a single thiol or between a single Michael acceptor and two thiols using three different catalysts/initiators (triethylamine, DBU, and dimethylphenylphosphine) in chloroform. The results from the ternary reactions provide a platform from which sequential quaternary, one-pot quaternary, and sequential senary thiol-Michael reactions were designed and their selectivities quantified. These results provide insights into the design of selective thiol-Michael reactions that can be used for the synthesis and functionalization of multicomponent polymers and further informs how catalyst/initiator choice influences the reactivity between a given thiol and Michael acceptor