48 research outputs found
Quasiâtrapped 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 donorâacceptor [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 organicâsolid interface
The structureâfunction 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 designâanalysis 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 ThiolâEne and ThiolâYne Click Reactions Involving <i>N</i>âSubstituted Maleimides
A combination of experimental and
computational methods has been
used to understand the reactivity and selectivity of orthogonal thiolâene
and thiolâyne âłclickâł 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 ÎČ-mercaptoethanol are shown to add exclusively and quantitatively
to the electron poor maleimide alkene of <b>1</b> and <b>2</b> under base (Et<sub>3</sub>N) initiated thiol-Michael conditions.
Subsequent radical-mediated thiolâene or thiolâyne 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
thiolâene/yne reactions. The results provide additional insights
into how to design selective radical-mediated thiolâene/yne
reactions
ThiolâEne 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 thiolâene click chemistry has been studied
computationally at the CBS-QB3 level. Relative energetics (Î<i>H</i>°, Î<i>H</i><sup>⧧</sup>, Î<i>G</i>°, Î<i>G</i><sup>⧧</sup>) have
been determined for all stationary points along the step-growth mechanism
of thiolâene reactions between methyl mercaptan and a series
of 12 alkenes: propene, methyl vinyl ether, methyl allyl ether, norbornene,
acrylonitrile, methyl acrylate, butadiene, methylÂ(vinyl)Âsilanediamine,
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><sub>P</sub>, <i>k</i><sub>âP</sub>, <i>k</i><sub>CT</sub>, <i>k</i><sub>âCT</sub>) and used to model overall reaction kinetics. A relationship between
alkene structure and reactivity in thiolâene 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 thiolâene click chemistry
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 <sup>1</sup>H 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. <sup>1</sup>H 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
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/6-31GÂ(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
âclickâ 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 <sup>1</sup>H 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