Solvent Effects on a Diels-Alder Reaction Involving a Cationic Diene: Consequences of the Absence of Hydrogen-Bond Interactions for Accelerations in Aqueous Media

Cycloaddition reactions via "on water" protocol reactions: A density functional theory study

In this work, the reactions of quadricyclane with dimethyl azodicarboxylate (DMAD) and of quadricyclane with diethyl azodicarboxylate (DEAD) in gas phase and in water environments were studied by a first-principles investigation within the framework of auxiliary density functional theory (ADFT). For these type of organic reactions is known that water is required to accelerate them. Since the reason of why this occur is still unknown, this work aims to gain insight into this reaction mechanism. For this investigation, the generalized gradient approximation as well as a hybrid functional were employed. The obtained optimized structures for the reactants, of the products and of the transition states are reported, together with the corresponding frequency analysis results and the reaction profiles. Along the proposed concerted reaction mechanism, a critical points search of the electron density and a charge analysis were performed. The calculated potential energy barriers of these reactions in gas phase and in water environments are compared. In agreement with experiment, the obtained results indicate that both reactions occur faster in water than in gas phase. This study shows that there is a change in the polarity of the two most important carbon atoms of the formed compounds along the reactions and that the decrease of the activation energy barrier which occurs in liquid phase in these reactions is because the structures of the main transition states are stabilized by the water environment. Therefore, the here obtained results demonstrate the important role played by the water-molecule framework into the activation energy barrier and structures of the molecules that participate in the DMAD and DEAD cycloaddition reactions.

Force-mediated molecule release from double network hydrogels

The incorporation of mechanosensitive linkages into polymers has led to materials with dynamic force responsivity. Here we report oxanorbornadiene cross-linked double network hydrogels that release molecules through a force-mediated retro Diels-Alder reaction. The molecular design and tough double network of polyacrylamide and alginate promote significantly higher activation at substantially less force than pure polymer systems. Activation at physiologically relevant forces provides scope for instilling dynamic mechanochemical behavior in soft biological materials.

Analytical energy gradient for the second-order Møller-Plesset perturbation theory coupled with the reference interaction site model self-consistent field explicitly including spatial electron density distribution

Solvatochromic shifts of the activation free energies are important aspects to consider for reaction control. To predict the energies, the stationary points in a solution must be accurately determined along the reaction pathway. In this study, the second-order Møller-Plesset perturbation (MP2) theory combined with the reference interaction site model was applied using our fitting approach, and the MP2 analytical energy gradient was determined. The coupled-cluster energy and thermal correction were calculated using the MP2 optimized geometry with solvent effect, and the activation free energies of the Diels-Alder reaction between cyclopentadiene and methyl vinyl ketone are within an error of 2 kcal/mol compared with the experimental data.

Theoretical insight into the on-water catalytic effect in the biogenesis of triterpene dimers: from one-step to two-step hetero Diels–Alder reactions

An alternative pathway to the hetero Diels–Alder reaction for the biogenic origin of triterpene dimers is presented here. In this new pathway, the explicit water molecules take a fundamental role.

Coordination-Cage-Catalysed Hydrolysis of Organophosphates: Cavity- or Surface-Based?

The hydrophobic central cavity of a water-soluble M8 L12 cubic coordination cage can accommodate a range of phospho-diester and phospho-triester guests such as the insecticide "dichlorvos" (2,2-dichlorovinyl dimethyl phosphate) and the chemical warfare agent analogue di(isopropyl) chlorophosphate. The accumulation of hydroxide ions around the cationic cage surface due to ion-pairing in solution generates a high local pH around the cage, resulting in catalysed hydrolysis of the phospho-triester guests. A series of control experiments unexpectedly demonstrates that-in marked contrast to previous cases-it is not necessary for the phospho-triester substrates to be bound inside the cavity for catalysed hydrolysis to occur. This suggests that catalysis can occur on the exterior surface of the cage as well as the interior surface, with the exterior-binding catalysis pathway dominating here because of the small binding constants for these phospho-triester substrates in the cage cavity. These observations suggest that cationic but hydrophobic surfaces could act as quite general catalysts in water by bringing substrates into contact with the surface (via the hydrophobic effect) where there is also a high local concentration of anions (due to ion pairing/electrostatic effects).

Synthesis of an amphiphilic tetrazine derivative and its application as a liposomal component to accelerate release of encapsulated drugs

Tetrazine irreversibly reacts with dienophiles, and its derivatives find wide applications in the fields of biochemistry and biophysics. We have synthesized an amphiphilic tetrazine derivative (2-hexadecyl-N-(6-(6-(pyridin-2-yl)-1,2,4,5-tetrazine-3-yl)pyridin-3-yl)octadecanamide; 1), which has a hydrophilic tetrazine structure and hydrophobic alkyl chains. Liposomes composed of compound 1 and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) (PTz-liposome) were prepared. In search of a new drug delivery system (DDS), we investigated the viability of inverse electron-demand Diels-Alder, a reaction between tetrazine and 2-norbornene, on the surface of the liposomes to change membrane fluidity and promote spatial and temporal controlled release of the encapsulated drugs. Compound 1 was synthesized with a yield of 71%. MS analysis after incubation of 2-norbornene with PTz-liposome revealed the binding of 2-norbornene to tetrazine. Indium-111-labeled diethylenetriaminepentaacetic acid (¹¹¹In-DTPA) was encapsulated inside PTz-liposome to evaluate the leakage of free ¹¹¹In-DTPA from the liposomes quantitatively. After 24 h of adding 2-norbornene, the release percentage for PTz-liposome was significantly higher than that for the control liposome (without tetrazine structure). Furthermore, the membrane fluidity of the PTz-liposome was increased by adding 2-norbornene. These results suggested that the combination of dienophile and liposome containing a newly synthesized tetrazine derivative can be used as a controlled release DDS carrier.

Ab Initio Study of Solvent Effects on Rate of 1,3-Dipolar Cycloadditions of Benzonitrile Oxide and Various Dipolarophiles

Ab initio molecular orbital calculations have been used to investigate the structures and the transition states of 1,3-dipolar cycloadditions between benzonitrile oxide with ethylene, cyclopentene, acrylonitrile and tetracyanoethylene in heptane and water: calculations reveal enhanced hydrogen bonding of a water molecule to the transition states for the cycloaddition 1,3-dipolar of reaction of benzonitrile oxide with cyclopentene, the optimal interaction energies being 0.7 kcal/mol more favourable for hydrogen bonding to the oxygen atom in the transition states than for the reactants.

On the mechanism of retro-Diels–Alder reaction of partially saturated 2-pyrones to produce biorenewable chemicals

Partially saturated 2-pyrone molecules undergo ring-opening and decarboxylation via retro-Diels–Alder (rDA) reaction.

On-resin Diels–Alder reaction with inverse electron demand: an efficient ligation method for complex peptides with a varying spacer to optimize cell adhesion

The DAR_invon resin is a new orthogonal reaction in peptide synthesis and the benefits for cell adhesion are discussed.

Effects of Surfactants on the Rate of Chemical Reactions

Surfactants are self-assembled compounds that depend on their structure and electric charge can interact as monomer or micelle with other compounds (substrates). These interactions which may catalyze or inhibit the reaction rates are studied with pseudophase, cooperativity, and stoichiometric (classical) models. In this review, we discuss applying these models to study surfactant-substrate interactions and their effects on Diels-Alder, redox, photochemical, decomposition, enzymatic, isomerization, ligand exchange, radical, and nucleophilic reactions.

Acceleration of disproportionation of aromatic alcohols through self-emulsification of reactants in water

Exploration of new and effective routes to conduct organic reactions in water using the special properties of water/organics is of great importance. In this work, we performed the disproportionation of various aromatic alcohols in water and in different organic solvents. It was demonstrated that the disproportionation reactions of the alcohols were accelerated more effectively in water than organic-solvent-based or solvent-free reactions. A series of control experiments were conducted to study the mechanism of the accelerated reaction rate in water. It was shown that the reactants could emulsify the reactant/water systems at the reaction conditions owing to their amphiphilic nature. The regularly orientated reactant molecules at the water/reactant droplet interface improved the contact probability of the reactive groups and the Pd nanocatalysts, which is one of the main reasons for the enhanced reaction rate in water. Controlling the self-emulsification of amphiphilic reactant/water systems has great application potential for optimizing the rate and/or selectivity of many organic reactions.

Protruding interfacial OH groups and 'on-water' heterogeneous catalysis

The key aspect of the remarkable organic catalysis that is observed to occur at the organic/water phase boundary, the so-called 'on-water' catalysis (Narayan et al 2005 Angew. Chem. 44 3275), was recently proposed to be the protruding OH groups of water molecules at the interface that interact with the transition state (TS) via hydrogen bonding and lower activation barriers (Jung and Marcus 2007 J. Am. Chem. Soc. 129 5492). In particular, the cycloaddition reaction of quadricyclane (Q) with dimethyl azodicarboxylate (DMAD) on-water was calculated to be more than 100,000 times more efficient in terms of rate constant than the neat reaction. In this paper, we review and consider a related reaction of Q with dimethyl acetylenedicarboxylate, where nitrogen, a good H-bond acceptor, in DMAD is replaced by carbon, a poor H-bond acceptor. A very low rate acceleration of acetylenedicarboxylate on-water relative to the neat reaction is obtained theoretically, as compared to DMAD on-water, due to the relatively low H-bonding ability of acetylenedicarboxylate with water at the TS relative to the reactants. We suggest that there may also be an 'intrinsic steric effect' or orientational advantage in the on-water catalysis in general, and both electronic and steric effects may be in operation for the smaller on-water catalysis for the cycloaddition reaction of quadricyclane and acetylenedicarboxylate. A preliminary quantum mechanical/molecular mechanical (QM/MM) simulation including 1264 water molecules for the on-water reaction of DMAD + Q also suggests that there are indeed approximately two-four more H-bonds between the TS and the dangling OH groups than between the reactants and the surface.

Quantum mechanical/molecular mechanical modeling finds Diels-Alder reactions are accelerated less on the surface of water than in water

Quantum and molecular mechanics calculations for the Diels-Alder reactions of cyclopentadiene with 1,4-naphthoquinone, methyl vinyl ketone, and acrylonitrile have been carried out at the vacuum-water interface and in the gas phase. In conjunction with previous studies of these cycloadditions in dilute solution, a more complete picture of aqueous environmental effects emerges with implications for the origin of observed rate accelerations using heterogeneous aqueous suspensions, "on water" conditions. The pure TIP4P water slab maintains the bulk density and hydrogen-bonding properties in central water layers. The bulk region merges to vacuum over a ca. 5 A band with progressive diminution of the density and hydrogen bonding. The relative free energies of activation and transition structures for the reactions at the interface are found to be intermediate between those calculated in the gas phase and in bulk water; i.e., for the reaction with 1,4-naphthoquinone, the DeltaDeltaG(++) values relative to the gas phase are -3.6 and -7.3 kcal/mol at the interface and in bulk water, respectively. Thus, the results do not support the notion that a water surface is more effective than bulk water for catalysis of such pericyclic reactions. The trend is in qualitative agreement with expectations based on density considerations and estimates of experimental rate constants for the gas phase, a heterogeneous aqueous suspension, and a dilute aqueous solution for the reaction of cyclopentadiene with methyl vinyl ketone. Computed energy pair distributions reveal a uniform loss of 0.5-1.0 hydrogen bond for the reactants and transition states in progressing from bulk water to the vacuum-water interface. Orientational effects are apparent at the surface; e.g., the carbonyl group in the methyl vinyl ketone transition structure is preferentially oriented into the surface. Also, the transition structure for the 1,4-naphthoquinone case is buried more in the surface, and the free energy of activation for this reaction is most similar to the result in bulk water.

Linear-dendritic supramolecular complexes as nanoscale reaction vessels for "green" chemistry. Diels-Alder reactions between fullerene C60 and polycyclic aromatic hydrocarbons in aqueous medium

This study describes the first Diels-Alder (DA) reaction performed in aqueous medium with highly hydrophobic compounds-fullerene (C 60) as the dienophile and anthracene (An) or tetracene (Tet) as the dienes, respectively. The reactions are performed in nanocontainers, constructed by self-assembly of linear-dendritic amphiphilic copolymers with poly(ethylene glycol), PEG or poly(ethylene oxide), PEO as the hydrophilic blocks and poly(benzyl ether) monodendrons as the hydrophobic fragments: G3PEO13k, dG3 and dG2. Comparative studies under identical conditions are carried out with an amphiphilic linear-linear copolymer, poly(styrene)1800- block-PEO2100, PSt-PEO, and the nonionic surfactant Igepal CO-720, IP720. The binding affinity of supermolecules built of these amphiphiles toward the DA reagents decreases in the following order: G3PEO13k > dG3 > PSt-PEO > dG2 > IP720. The kinetic constant of binding is evaluated for tetracene and decreases in a similar fashion: 5 x 10 (-7) M/min (G3PEO13k), through 4 x 10 (-7) M/min (PSt-PEO) down to 1.5 x 10 (-7) M/min for IP720. The mobility of substrates encapsulated in the micellar core, estimated by pyrene fluorescence decay, is 95-121 ns for the micelles of the linear-dendritic copolymers and notably higher for PSt-PEO (152 ns), revealing the much denser interior of the linear analogue. The apparent kinetic constant for the DA reaction of C 60 and Tet within the G3PEO13k supermolecule in aqueous medium is markedly higher than in organic solvent (toluene), 208 vs 1.82 M /min. With G3PEO13k the conversions reach 49% for the DA reaction between C 60 and An, and 55% for C 60 and Tet. Besides the monoadduct (26.5% yield) the reaction with An produces exclusively increasing amounts of D 2 h -symmetric antipodal bis-adduct, whose yield reaches up to 22.5% after 48 h. In addition to the environmentally friendly conditions notable advantages of the synthetic strategy described are the extended stability of the linear-dendritic nanovessels, the easy collection of the products formed, and the recovery and reuse of unreacted reagents and linear-dendritic copolymers.

On the Theory of Organic Catalysis “on Water”

A molecular origin of the striking rate increase observed in a reaction on water is studied theoretically. A key aspect of the on-water rate phenomenon is the chemistry between water and reactants that occurs at an oil-water phase boundary. In particular, the structure of water at the oil-water interface of an oil emulsion, in which approximately one in every four interfacial water molecules has a free ("dangling") OH group that protrudes into the organic phase, plays a key role in catalyzing reactions via the formation of hydrogen bonds. Catalysis is expected when these OH's form stronger hydrogen bonds with the transition state than with the reactants. In experiments more than a 5 orders of magnitude enhancement in rate constant was found in a chosen reaction. The structural arrangement at the "oil-water" interface is in contrast to the structure of water molecules around a small hydrophobic solute in homogeneous solution, where the water molecules are tangentially oriented. The latter implies that a breaking of an existing hydrogen-bond network in homogeneous solution is needed in order to permit a catalytic effect of hydrogen bonds, but not for the on-water reaction. Thereby, the reaction in homogeneous aqueous solution is intrinsically slower than the surface reaction, as observed experimentally. The proposed mechanism of rate acceleration is discussed in light of other on-water reactions that showed smaller accelerations in rates. To interpret the results in different media, a method is given for comparing the rate constants of different rate processes, homogeneous, neat and on-water, all of which have different units, by introducing models that reduce them to the same units. The observed deuterium kinetic isotope effect is discussed briefly, and some experiments are suggested that can test the present interpretation and increase our understanding of the on-water catalysis.

Substrate dependence in aqueous Diels-Alder reactions of cyclohexadiene derivatives with 1,4-benzoquinone

A reactivity difference based on the position of substituents on cyclohexa-1,3-diene was observed for the title reaction. The effect of water as solvent was more distinct for 1-methyl-4-isopropylcyclohexa-1,3-diene than for 2-methyl-5-isopropylcyclohexa-1,3-diene or non-substituted cyclohexa-1,3-diene. The effect of NaCl (salting-out) and guanidium chloride (salting-in) was also large for 1-methyl-4-isopropylcyclohexa-1,3-diene.

Low-melting sugar–urea–salt mixtures as solvents for Diels–Alder reactions

Sweet solutions are obtained upon heating mixtures of simple carbohydrates, urea and inorganic salts to moderate temperatures, to give new chiral media for organic reactions.

The use of hydrophilic groups in aqueous organic reactions

The representative examples of beneficial effects of hydrophilic groups in aqueous organic reactions are described, including the Diels-Alder reactions, hetero Diels-Alder reactions, Claisen rearrangement, radical reactions, and transition metal-catalyzed reactions. Although the low solubility of organic molecules in water has been a bane in aqueous organic reactions, the incorporation of hydrophilic groups into the substrate structure can overcome the solubility problem and at the same time enhance the hydrophobic effect. In some cases, interesting micellar effects are observed because of the amphiphilic natures of such molecules. The emerging concept of "removable hydrophilic group," in which the solubility problems have been alleviated, yet the initial product can still be transformed into a variety of products with the removal of the hydrophilic groups, is also described.