A Simple and Facile Approach to Aliphatic N-Substituted Functional Eight-Membered Cyclic Carbonates and Their Organocatalytic Polymerization.

Aliphatic N-substituted functional eight-membered cyclic carbonates were synthesized from N-substituted diethanolamines by intramolecular cyclization. On the basis of the N-substituent, three major subclasses of carbonate monomers were synthesized (N-aryl, N-alkyl and N-carbamate). Organocatalytic ring opening polymerization (ROP) of eight-membered cyclic carbonates was explored as a route to access narrowly dispersed polymers of predictable molecular weights. Polymerization kinetics was highly dependent on the substituent on the nitrogen atom and the catalyst used for the reaction. The use of triazabicyclodecene (TBD), instead of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), as the catalyst for the N-alkyl substituted monomers significantly enhanced the rate of polymerizations. Computational studies were performed to rationalize the observed trends for TBD catalyzed polymerizations. With the optimal organocatalyst all monomers could be polymerized generating well-defined polymers within a timespan of ≤2 h with relatively high monomer conversion (≥80%) and low molar-mass dispersity (D(M) ≤ 1.3). Both the glass transition temperatures (T(g)) and onset of degradation temperatures (T(onset)) of these polymers were found to be N-substituent dependent and were in the range of about -45 to 35 °C and 230 to 333 °C, respectively. The copolymerization of the eight membered monomers with 6-membered cyclic comonomers including commercially available l-lactide and trimethylene carbonate produced novel copolymers. The combination of inexpensive starting materials, ease of ring-closure and subsequent polymerization makes this an attractive route to functional polycarbontes.

Development of a method for detecting trace metals in aqueous solutions based on the coordination chemistry of hexahydrotriazines.

The detection of trace amounts (<10 ppb) of heavy metals in aqueous solutions is described using 1,3,5-hexahydro-1,3,5-triazines (HTs) as chemical indicators and a low cost fluorimeter-based detection system. This method takes advantage of the inherent properties of HTs to coordinate strongly with metal ions in solution, a fundamental property that was studied using a combination of analytical tools (UV-Vis titrations, (1)H-NMR titrations and computational modeling). Based on these fundamental studies that show significant changes in the HT UV signature when a metal ion is present, HT compounds were used to prepare indicator strips that resulted in significant fluorescence changes when a metal was present. A portable and economical approach was adopted to test the concept of utilizing HTs to detect heavy metals using a fluorimeter system that consisted of a low-pressure mercury lamp, a photo-detector, a monolithic photodiode and an amplifier, which produces a voltage proportional to the magnitude of the visible fluorescence emission. Readings of the prepared HT test strips were evaluated by exposure to two different heavy metals at the safe threshold concentration described by the U.S. Environmental Protection Agency (EPA) for Cr(3+) and Ag(2+) (100 µg L(-1) and 6.25, respectively). This method of detection could be used to the presence of either metal at these threshold concentrations.

Organic acid-catalyzed polyurethane formation via a dual-activated mechanism: unexpected preference of N-activation over O-activation of isocyanates.

A systematic study of acid organocatalysts for the polyaddition of poly(ethylene glycol) to hexamethylene diisocyanate in solution has been performed. Among organic acids evaluated, sulfonic acids were found the most effective for urethane formations even when compared with conventional tin-based catalysts (dibutyltin dilaurate) or 1,8-diazabicyclo[5.4.0]undec-7-ene. In comparison, phosphonic and carboxylic acids showed considerably lower catalytic activities. Furthermore, sulfonic acids gave polyurethanes with higher molecular weights than was observed using traditional catalyst systems. Molecular modeling was conducted to provide mechanistic insight and supported a dual activation mechanism, whereby ternary adducts form in the presence of acid and engender both electrophilic isocyanate activation and nucleophilic alcohol activation through hydrogen bonding. Such a mechanism suggests catalytic activity is a function of not only acid strength but also inherent conjugate base electron density.

Computational investigations on base-catalyzed diaryl ether formation.

We report investigations with the dispersion-corrected B3LYP density functional method on mechanisms and energetics for reactions of group I metal phenoxides with halobenzenes as models for polyether formation. Calculated barriers for ether formation from para-substituted fluorobenzenes are well correlated with the electron-donating or -withdrawing properties of the substituent at the para position. These trends have also been explained with the distortion/interaction energy theory model which show that the major component of the activation energy is the energy required to distort the arylfluoride reactant into the geometry that it adopts at the transition state. Resonance-stabilized aryl anion intermediates (Meisenheimer complexes) are predicted to be energetically disfavored in reactions involving fluorobenzenes with a single electron-withdrawing group at the para position of the arene, but are formed when the fluorobenzenes are very electron-deficient, or when chelating substituents at the ortho position of the aryl ring are capable of binding with the metal cation, or both. Our results suggest that the presence of the metal cation does not increase the rate of reaction, but plays an important role in these reactions by binding the fluoride or nitrite leaving group and facilitating displacement. We have found that the barrier to reaction decreases as the size of the metal cation increases among a series of group I metal phenoxides due to the fact that the phenoxide becomes less distorted in the transition state as the size of the metal increases.

Mechanisms of organocatalytic amidation and trans-esterification of aromatic esters as a model for the depolymerization of poly(ethylene) terephthalate.

We describe investigations with B3LYP density functional theory to probe mechanisms for the organocatalyzed depolymerization of poly(ethylene) terephthalate (PET) into ester and amide products. These investigations utilize model systems involving the trans-esterification and amidation of methylbenzoate (MB) with ethylene glycol (EG), ethylenediamine (EDA), and ethanolamine (EA) organocatalyzed by 1,5,7-triazabicyclododecene (TBD) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Mechanisms for reactions in which TBD acts as the lone catalyst have been compared with pathways in which TBD and DBU catalyze these processes with an additional molecule of the amine or alcohol acting as a cocatalyst. Calculations suggest that the combination of an organocatalyst with a molecule of an alcohol or amine cocatalyst is slightly more activating than a lone catalyst. Our results predict that nucleophilic attack is the rate-determining step in reactions involving EDA and EG and that TBD is a better catalyst than DBU in the amidation of MB with EDA; in addition, both organocatalysts activate alcohols more than amines during nucleophilic attack. Amidation and trans-esterification possess similar barriers for reactions involving EA; but the amide, which is the thermodynamic product, is preferentially formed instead of the ester.

Hydrogen-bonding catalysts based on fluorinated alcohol derivatives for living polymerization.

Recognize this! A hydrogen-bonding motif based on hexafluorinated alcohol derivatives (see picture; O red, F yellow) activates electrophilic substrates. The catalytic activity of the hydrogen-bonded systems was demonstrated for the ring-opening polymerization of a variety of strained heterocycles. Narrowly dispersed polymers with predictable molecular weights were obtained with end-group fidelity.

The reaction mechanism for the organocatalytic ring-opening polymerization of l-lactide using a guanidine-based catalyst: hydrogen-bonded or covalently bound?

We have investigated two alternative mechanisms for the ring-opening polymerization of l-lactide using a guanidine-based catalyst, the first involving acetyl transfer to the catalyst, and the second involving only hydrogen bonding to the catalyst. Using computational chemistry methods, we show that the hydrogen bonding pathway is considerably preferred over the acetyl transfer pathway and that this is consistent with experimental information.

Dominant Decomposition Pathways for Ethereal Solvents in Li-O2 Batteries.

The promise of high specific energies for Li-O2 batteries has driven research toward the development of new compatible materials for this emerging technology. Obtained energies, however, fall short of the theoretical values partly due to parasitic chemistries arising from organic solvent decomposition during battery cycling. Electrolyte solvent and salt decomposition have also been identified as limiting factors for rechargeability of the battery. Although lithium trifluorosulfonamide (LiTFSI) dissolved in 1,2-dimethoxyethane (DME) has been shown to be a promising solvent/electrolyte candidate for Li-O2 batteries, significant challenges remain, namely minimizing decomposition of both the solvent and electrolyte salt during battery cycling. Herein, we provide spectroscopic labeling studies to identify the source of H2 at high potentials during charge and propose a decomposition pathway for DME to lithium formate and acetate products at low potentials. NMR studies were preformed to show that DME decomposes to lithium formate and acetate in aqueous Li2O2, products which are also observed after D2O workups on cathodes after discharge. Finally, we use density functional theory (DFT) to elucidate a mechanistic pathway for DME decomposition that is based on known organic oxidation processes.

N-Heterocyclic Carbene-Catalyzed Ring Opening Polymerization of ε-Caprolactone with and without Alcohol Initiators: Insights from Theory and Experiment.

Computational investigations with density functional theory (DFT) have been performed on the N-heterocyclic carbene (NHC) catalyzed ring-opening polymerization of ε-caprolactone in the presence and in the absence of a methanol initiator. Much like the zwitterionic ring opening (ZROP) of δ-valerolactone which was previously reported, calculations predict that the mechanism of the ZROP of caprolactone that occurs without an alcohol present involves a high-barrier step involving ring opening of the zwitterionic tetrahedral intermediate formed after the initial nucleophilic attack of NHC on caprolactone. However, the operative mechanism by which caprolactone is polymerized in the presence of an alcohol initiator does not involve the analogous mechanism involving initial nucleophilic attack by the organocatalytic NHC. Instead, the NHC activates the alcohol through hydrogen bonding and promotes nucleophilic attack and the subsequent ring-opening steps that occur during polymerization. The largest free energy barrier for the hydrogen-bonding mechanism in alcohol involves nucleophilic attack, while that for both ZROP processes involves ring opening of the initially formed zwitterionic tetrahedral intermediate. The DFT calculations predict that the rate of polymerization in the presence of alcohol is faster than the reaction performed without an alcohol initiator; this prediction has been validated by experimental kinetic studies.

Supramolecular motifs in dynamic covalent PEG-hemiaminal organogels.

Dynamic covalent materials are stable materials that possess reversible behaviour triggered by stimuli such as light, redox conditions or temperature; whereas supramolecular crosslinks depend on the equilibrium constant and relative concentrations of crosslinks as a function of temperature. The combination of these two reversible chemistries can allow access to materials with unique properties. Here, we show that this combination of dynamic covalent and supramolecular chemistry can be used to prepare organogels comprising distinct networks. Two materials containing hemiaminal crosslink junctions were synthesized; one material is comprised of dynamic covalent junctions and the other contains hydrogen-bonding bis-hemiaminal moieties. Under specific network synthesis conditions, these materials exhibited self-healing behaviour. This work reports on both the molecular-level detail of hemiaminal crosslink junction formation as well as the macroscopic behaviour of hemiaminal dynamic covalent network (HDCN) elastomeric organogels. These materials have potential applications as elastomeric components in printable materials, cargo carriers and adhesives.

Computational and experimental studies on the mechanism of formation of poly(hexahydrotriazine)s and poly(hemiaminal)s from the reactions of amines with formaldehyde.

Combined experimental and computational studies have been performed on the mechanism of formation of poly(hexahydrotriazine) and hemiaminal dynamic covalent network (PHT and HDCN) thermosetting polymers from the reactions of diamines with formaldehyde (Science 2014, 344, 732-735). Results suggest that these polymers are formed by a mechanism involving the water promoted stepwise addition of amines with formaldehyde in preference to dimerization or cyclotrimerization of imine intermediates or self-catalysis by the amine reagents. The predicted mechanism also explains experimentally observed electronic effects for hexahydrotriazine formation.

Experimental and computational studies on the mechanism of zwitterionic ring-opening polymerization of δ-valerolactone with N-heterocyclic carbenes.

Experimental and computational investigations of the zwitterionic ring-opening polymerization (ZROP) of δ-valerolactone (VL) catalyzed by the N-heterocyclic carbenes (NHC) 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene (1) and 1,3,4,5-tetramethyl-imidazol-2-ylidene (2) were carried out. The ZROP of δ-valerolactone generates cyclic poly(valerolactone)s whose molecular weights are higher than predicted from [VL]0/[NHC]0. Kinetic studies reveal the rate of polymerization is first order in [VL] and first order in [NHC]. Density functional theory (DFT) calculations were carried out to elucidate the key steps involved in the ring-opening of δ-valerolactone and its subsequent oligomerization. These studies have established that the initial steps of the mechanism involve nucleophilic attack of the NHC on δ-valerolactone to form a zwitterionic tetrahedral intermediate. DFT calculations indicate that the highest activation barrier of the entire mechanism is associated with the ring-opening of the tetrahedral intermediate formed from the NHC and δ-valerolactone, a result consistent with inefficient initiation to generate reactive zwitterions. The large barrier in this step is due to the fact that ring-opening requires a partial positive charge to develop next to the directly attached NHC moiety which already bears a delocalized positive charge.

Recyclable, strong thermosets and organogels via paraformaldehyde condensation with diamines.

Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4'-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young's moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.

Polymerizing Base Sensitive Cyclic Carbonates Using Acid Catalysis.

Organic acids were explored as a means to expand the library of cyclic carbonate monomers capable of undergoing controlled ring-opening polymerization. Various nitrogenous bases have proven incredibly adept at polymerizing cyclic carbonates; however, their use has largely precluded monomers with an acidic proton. Molecular modeling of acid catalysis provided new mechanistic insight, wherein a bifunctional activation pathway was calculated. Depending on acid structure, modeling experiments showed both monomer carbonyls and propagating hydroxyl groups undergo hydrogen bonding activation. The dual activation mechanism suggests acid strength, as well as conjugate base effects, play vital roles in catalyzing cyclic carbonate polymerizations. Moreover, the use of acid catalysis was shown to be compatible with amide-containing monomers while promoting controlled polymerizations.

QM/MM-Based Fitting of Atomic Polarizabilities for Use in Condensed-Phase Biomolecular Simulation.

Accounting for electronic polarization effects in biomolecular simulation (by using a polarizable force field) can increase the accuracy of simulation results. However, the use of gas-phase estimates of atomic polarizabilities αi usually leads to overpolarization in condensed-phase systems. In the current work, a combined QM/MM approach has been employed to obtain condensed-phase estimates of atomic polarizabilities for water and methanol (QM) solutes in the presence of (MM) solvents of different polarity. In a next step, the validity of the linear response and isotropy assumptions were evaluated based on the observed condensed-phase distributions of αi values. The observed anisotropy and low average values for the polarizability of methanol's carbon atom in polar solvents was explained in terms of strong solute-solvent interactions involving its adjacent hydroxyl group. Our QM/MM estimates for atomic polarizabilities were found to be close to values used in previously reported polarizable water and methanol models. Using our estimate for αO of methanol, a single set of polarizable force field parameters was obtained that is directly transferable between environments of different polarity.

Catalyst Chelation Effects in Organocatalyzed Ring-Opening Polymerization of Lactide.

(-)-Sparteine is a proven organocatalyst for the ring-opening polymerization (ROP) of l-lactide, which affords polymers of controlled molecular weight and narrow polydispersity. The recent worldwide shortage of (-)-sparteine has necessitated the identification of simple and cost-effective replacement ROP catalysts. A series of commercially available molecules was first identified through molecular modeling and then subsequently investigated for polymerizing l-lactide. The modeling proved very useful at predicting spatial relationships and nitrogen geometries that greatly aided in the rapid identification of various alkyl amines as alternative organocatalysts.

Consensus Computational Ligand-Based Design for the Identification of Novel Modulators of Human Estrogen Receptor Alpha.

We describe the first targeted validation of fFLASH, a molecular similarity program from IBM that has been previously proposed as suitable for the virtual screening (VS) of compound libraries based on explicit 3D flexible superimpositions, as part of its deployment within a novel consensus ligand-based virtual screening cascade. A virtual screening protocol using fFLASH for the human estrogen receptor alpha (ERα) was advanced and benchmarked against screens completed using established commercial screening softwares - Catalyst and ROCS. The optimised protocol was applied to a ∼6000 member physical screening collection and virtual 'hits' sourced and biologically assayed. The approach identified a novel, potent and highly selective partial antagonist of the ERα. This study firstly validates the clique detection algorithm utilised by fFLASH and secondly, emphasises the benefits of the consensus approach of employing more than one program in a VS protocol.

Catalytic insights into acid/base conjugates: highly selective bifunctional catalysts for the ring-opening polymerization of lactide.

Herein, we report the use of acid/base complexes for the ring opening polymerization of (L)-lactide. The salt formed from an equimolar reaction of benzoic acid and DBU produced a catalyst capable of controlled polymerizations allowing for targeted molecular weights and narrow polydispersities. The use of computational analysis suggests a bifunctional catalytic mechanism wherein both the monomer and propagating hydroxyl group are activated toward polymerization.

Accounting for polarization cost when using fixed charge force fields. II. Method and application for computing effect of polarization cost on free energy of hydration.

Polarization cost is the energy needed to distort the wave function of a molecule from one appropriate to the gas phase to one appropriate for some condensed phase. Although it is not currently standard practice, polarization cost should be considered when deriving improved fixed charge force fields based on fits to certain types of experimental data and when using such force fields to compute observables that involve changes in molecular polarization. Building on earlier work, we present mathematical expressions and a method to estimate the effect of polarization cost on free energy and enthalpy implied by a charge model meant to represent a solvated state. The charge model can be any combination of point charges, higher-order multipoles, or even distributed charge densities, as long as they do not change in response to environment. The method is illustrated by computing the effect of polarization cost on free energies of hydration for the neutral amino acid side chain analogues as predicted using two popular fixed charge force fields and one based on electron densities computed using quantum chemistry techniques that employ an implicit model to represent aqueous solvent. From comparison of the computed and experimental hydration free energies, we find that two commonly used force fields are too underpolarized in their description of the solute-water interaction. On the other hand, a charge model based on the charge density from a hybrid density functional calculation that used an implicit model for aqueous solvent performs well for hydration free energies of these molecules after the correction for dipole polarization is applied. As such, an improved description of the density (e.g., B3LYP, MP2) in conjunction with an implicit solvent (e.g., PCM) or explicit solvent (e.g., QM/MM) approach may offer promise as a starting point for the development of improved fixed charge models for force fields.

Accounting for polarization cost when using fixed charge force fields. I. Method for computing energy.

Although it is not currently standard practice, the cost to change the electronic polarization from one appropriate for the gas phase to that implied by the charge model should be considered when deriving force fields based on fits to certain types of experimental data and for using force fields to compute observables that involve changes in molecular polarization. We present mathematical expressions and a method to estimate this polarization cost implied by a fixed charge model force field, where the fixed charge model can be any combination of point charges, higher-order multipoles, or even distributed charge densities, as long as they do not change in response to environment. These expressions illuminate the relationship between polarization costs associated with fixed charge models, self-polarization energies of polarizable models, and quantum chemical based approaches that use continuum representations of the solvent, such as the self-consistent reaction field and polarizable continuum models. The technique takes account of the tensorial nature of the polarizabilities and includes quadrupole as well as dipole polarization. The consistency of this approach to one that estimates polarization cost using an implicit solvent quantum chemistry method (PCM) is demonstrated.