Diazo Esters as Dienophiles in Intramolecular (4 + 2) Cycloadditions: Computational Explorations of Mechanism

Reactivity, Pathways, and Iodinated Disinfection Byproduct Formation during Chlorination of Iodotyrosines Derived from Edible Seaweed

Iodine derived from edible seaweed significantly enhances the formation of iodinated disinfection byproducts (I-DBPs) during household cooking. Reactions of chlorine with monoiodotyrosine (MIT) and diiodotyrosine (DIT) derived from seaweed were investigated. Species-specific second-order rate constants (25 °C) for the reaction of hypochlorous acid with neutral and anionic MIT were calculated to be 23.87 ± 5.01 and 634.65 ± 75.70 M-1 s-1, respectively, while the corresponding rate constants for that with neutral and anionic DIT were determined to be 12.51 ± 19.67 and 199.12 ± 8.64 M-1 s-1, respectively. Increasing temperature facilitated the reaction of chlorine with MIT and DIT. Based on the identification of 59 transformation products/DBPs from iodotyrosines by HPLC/Q-Orbitrap HRMS, three dominant reaction pathways were proposed. Thermodynamic results of computational modeling using density functional theory revealed that halogen exchange reaction follows a stepwise addition-elimination pathway. Among these DBPs, 3,5-diiodo-4-hydroxy-benzaldehyde and 3,5-diiodo-4-hydroxy-benzacetonitrle exhibited high toxic risk. During chlorination of MIT and DIT, iodinated trihalomethanes and haloacetic acids became dominant species at common cooking temperature (80 °C). These results provide insight into the mechanisms of halogen exchange reaction and imply important implications for the toxic risk associated with the exposure of I-DBPs from household cooking with iodine-containing food.

The Role of Aromatic Alcohol Additives on Asymmetric Organocatalysis Reactions: Insights from Theory

The presence of an aromatic additive has been seen to enhance, often significantly, the enantioselectivity and yield in asymmetric organocatalysis. Considering their success across a dizzying range of organocatalysts and organic transformations, it would seem unlikely that a common principle exists for their functioning. However, the current investigations with DFT suggest a general principle: the phenolic additive sandwiches itself, through hydrogen bonding and π⋅⋅⋅π stacking, between the organocatalyst coordinated electrophile and nucleophile. This is seen for a wide range of experimentally reported systems. That such complex formation leads to enhanced stereoselectivity is then demonstrated for two cases: the cinchona alkaloid complex (BzCPD), catalysing thiocyanation (2-naphthol additive employed), as well as for L-pipecolicacid catalysing the asymmetric nitroaldol reaction with a range of nitro-substituted phenol additives. These findings, indicating that dual catalysis takes place when phenolic additives are employed, are likely to have a significant impact on the field of asymmetric organocatalysis.

Comprehensive Theoretical Study of Cp*IrIII-Catalyzed Intermolecular Enantioselective Allylic C-H Amidation: Reaction Mechanism, Electronic Processes, and Regioselectivity

Density functional theory was used to elucidate the reaction mechanism of Cp*Ir^III-catalyzed intermolecular regioselective C(sp³)-H amidation of alkenes with methyl dioxazolones. All substrates, intermediates, and transition states were fully optimized at the ωB97XD/6-31G(d,p) level (LANL2DZ(f) for Ir). The computational results revealed that this amidation occurred through the Ir^III/Ir^V catalytic cycle, involving four important elementary steps: C-H bond activation, oxidative addition of methyl dioxazolone, reductive elimination, and proto-demetalation, and the first was the rate-determining step. The C-H bond activation showed good α- and branch-regioselectivity, decided by the distortion energy of 2-pentene and the interaction energy of the transition state, respectively. The oxidative addition of dioxazolone occurred in one elementary step with CO₂ disassociation. The reductive elimination showed good branch-regioselectivity determined by the distorted energy of the allyl group. In the proto-demetalation, hydrogen directly transferred from the oxygen atom to the nitrogen atom. Moreover, to clarify the effect of the substituted groups, selected 12 substrates were also discussed in this text.

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Origins of Stereospecificity and Divergent Reactivity of Pd-Catalyzed Cross Coupling with α,α-Disubstituted Alkenyl Hydrazones

This article presents an exploration of stereospecificity and divergent reactivity of Pd-catalyzed α,α-disubstituted alkenyl hydrazones to synthesize 1,4-dienes in the Z configuration and vinylcyclopropane. We calculated the energy profiles of four α,α-disubstituted alkenyl hydrazones. The results show that the energy profiles of the whole catalytic cycle are basically the same before the syn-carbopalladation step. Subsequent syn-β-C elimination yields skipping dienes, or direct β-H elimination yields vinylcyclopropane. Current theoretical calculations reveal that the stereospecificity and the divergent reactivity of reactions result from the competition between syn-β-C elimination and β-H elimination. The C-C bond rotation and subsequent syn-β-C elimination step control the stereospecificity of the reaction by changing the olefin stereostructure from E to Z configuration. The steric factor of α-substituted groups mediates the transformation between syn-β-C elimination and β-H elimination. The results are of great significance for the scientific design of substrates to achieve accurate synthesis of target products.

Total Synthesis and Prediction of Ulodione Natural Products Guided by DFT Calculations

A biomimetic synthetic strategy has resulted in a two-step total synthesis of (±)-ulodione A and the prediction of two potential natural products, (±)-ulodiones C and D. This work was guided by computational investigations into the selectivity of a proposed biosynthetic Diels-Alder dimerization, which was then utilized in the chemical synthesis. This work highlights how biosynthetic considerations can both guide the design of efficient synthetic strategies and lead to the anticipation of new natural products.

Computational insight into the mechanism and stereoselectivity of cycloaddition between donor-acceptor spirocyclopropane and aldehyde catalyzed by Brønsted acid TsOH

The mechanism and diastereoselectivity of the cycloaddition reaction between D-A spirocyclopropane and aldehydes, catalyzed by para-toluenesulfonic acid (TsOH) in dichloromethane to produce 2,5-disubstituted tetrahydrofuran-type lignans, have been investigated by density functional theory (DFT) at the M06-2X/6-311+G(d,p)//B3LYP-D3/6-31G(d,p) level combined with the solvation SMD model. Our calculations show that the entire reaction process includes three stages: the activation of the D-A cyclopropane by Brønsted acid, TsOH, the nucleophilic attack of the aldehyde on the spirocyclopropane, and the formation of the final product, 2,5-disubstituted tetrahydrofuran. It was concluded from the conceptual density functional theory (CDFT) reactivity index analysis that aldehydes with electron-rich substituents are more nucleophilic and more favorable for the reaction to proceed. Furthermore, based on the analyses of energetics as well as the noncovalent interaction (NCI) and reduced density gradient (RDG) in the key transition states, the origin of stereoselectivity was revealed to be determined thermodynamically rather than kinetically. The present work explains the experimental phenomenon well, and provides useful theoretical information for the future design of similar reactions.

BH9, a New Comprehensive Benchmark Data Set for Barrier Heights and Reaction Energies: Assessment of Density Functional Approximations and Basis Set Incompleteness Potentials

The calculation of accurate reaction energies and barrier heights is essential in computational studies of reaction mechanisms and thermochemistry. To assess methods regarding their ability to predict these two properties, high-quality benchmark sets are required that comprise a reasonably large and diverse set of organic reactions. Due to the time-consuming nature of both locating transition states and computing accurate reference energies for reactions involving large molecules, previous benchmark sets have been limited in scope, the number of reactions considered, and the size of the reactant and product molecules. Recent advances in coupled-cluster theory, in particular local correlation methods like DLPNO-CCSD(T), now allow the calculation of reaction energies and barrier heights for relatively large systems. In this work, we present a comprehensive and diverse benchmark set of barrier heights and reaction energies based on DLPNO-CCSD(T)/CBS called BH9. BH9 comprises 449 chemical reactions belonging to nine types common in organic chemistry and biochemistry. We examine the accuracy of DLPNO-CCSD(T) vis-a-vis canonical CCSD(T) for a subset of BH9 and conclude that, although there is a penalty in using the DLPNO approximation, the reference data are accurate enough to serve as a benchmark for density functional theory (DFT) methods. We then present two applications of the BH9 set. First, we examine the performance of several density functional approximations commonly used in thermochemical and mechanistic studies. Second, we assess our basis set incompleteness potentials regarding their ability to mitigate basis set incompleteness errors. The number of data points, the diversity of the reactions considered, and the relatively large size of the reactant molecules make BH9 the most comprehensive thermochemical benchmark set to date and a useful tool for the development and assessment of computational methods.

Click-functionalized hydrogel design for mechanobiology investigations

The advancement of click-functionalized hydrogels in recent years has coincided with rapid growth in the fields of mechanobiology, tissue engineering, and regenerative medicine. Click chemistries represent a group of reactions that possess high reactivity and specificity, are cytocompatible, and generally proceed under physiologic conditions. Most notably, the high level of tunability afforded by these reactions enables the design of user-controlled and tissue-mimicking hydrogels in which the influence of important physical and biochemical cues on normal and aberrant cellular behaviors can be independently assessed. Several critical tissue properties, including stiffness, viscoelasticity, and biomolecule presentation, are known to regulate cell mechanobiology in the context of development, wound repair, and disease. However, many questions still remain about how the individual and combined effects of these instructive properties regulate the cellular and molecular mechanisms governing physiologic and pathologic processes. In this review, we discuss several click chemistries that have been adopted to design dynamic and instructive hydrogels for mechanobiology investigations. We also chart a path forward for how click hydrogels can help reveal important insights about complex tissue microenvironments.

Enantioselective synthesis of chiral tetrasubstituted allenes: harnessing electrostatic and noncovalent interactions in a bifunctional activation model for N-triflylphosphoramide catalysis

DFT calculation reveals that the oxygen activation model is preferred than the nitrogen activation model due to the preferred chiral electrostatic environment.

The mechanism of conversion of substituted glycals to chiral acenes via Diels-Alder reaction: a computational study

Synthesis of linearly fused aromatic systems using a glycal-based diene with an aryne is a long-standing topic of interest in glycal chemistry. We have examined the mechanistic pathways for the transformation of substituted glycals to chiral fused aromatic cores via Diels-Alder (DA) reaction using the SMDACN-M06-2X/6-31G(d) level of theory. The DA reactions of E (1a) and Z (1a') forms of C-2 alkenyl glycal and an aryl glycal (1b) as a diene were examined with a benzyne intermediate generated as a dienophile. The computational results reveal that 1a and 1b can only be transformed into the fused aromatic cores by the base-catalyzed reaction because a [1,5] sigmatropic hydrogen shift is not feasible. The activation free energy barrier for the base-catalyzed proton abstraction process is 4.2 kcal mol-1 and there is almost no barrier for stereoisomeric 1a DA-complexes. The activation free energy barrier values for stereoisomeric 1b DA-complexes for the base-catalyzed proton abstraction process are 10.8 and 12.4 kcal mol-1. The appropriate orientation of glycal-ring-oxygen and hydrogen at the 5th position of Z (1a') forms of C-2 alkenyl glycal facilitates the [1,5] sigmatropic hydrogen shift; however, the base-catalyzed reaction is energetically more favored than the former case. The rate-determining step for 1a and 1a' is the ring-opening step (18.2 and 19.5 kcal mol-1 for the S-stereoisomer), whereas the DA adduct formation step is the rate-determining step for 1b (16.1 kcal mol-1 for the S-stereoisomer). The structural analysis reveals the formation of the preferred S-stereoisomer over the R-stereoisomer with the respective dienes.

Mechanistic details of metal-free cyclization reaction of organophosphorus oxide with alkynes mediated by 2,6-lutidine and Tf2 O

Theoretical investigations have elucidated the mechanism of metal-free electrophilic phosphinative cyclization of alkynes reaction reported by Miura and coworkers. Two competitive mechanisms I and II were explored without or with 2,6-lutidine. Both of I and II involve transformation of P(V) to P(III), electrophilic addition, ring opening and cyclization/cyclization, hydrogen-transfer, and oxidation. The rate-determining step of mechanism I and competitive less-step II is electrophilic [2 + 1] cycloaddition and electrophilic addition via single CP bond formation with activation barrier of 13.5 and 10.6 kcal/mol, respectively. Our calculation results suggested that the cumulative effect of the isomer of 2,6-lutidine and Tf₂ O as well as TfO^- affects the title reaction to some extent, and simultaneously activates key reaction sites and reverses the polarities of them via the formation of abundant noncovalent interactions to decrease activation barriers of TSs. In addition, the effects of two series substituents on reactivity of phosphine oxide were investigated. Therefore, our study will serve as useful guidance for more efficient metal-free synthesis of organophosphorus compounds mediated by pyridine reagents.

Chiral Phosphoric Acid-Catalyzed Enantioselective Direct Arylation of Iminoquinones: A Case Study of the Model Selectivity

Chiral phosphoric acid (CPA)-catalyzed enantioselective arylation reactions have attracted immense attention recently. However, the preferential activation model in the stereodetermining step is controversial, and hence, the origin of enantioselectivity is still far from being understood. Two stereochemical models are provided on the basis of the asymmetric arylations of iminoquinones with naphthylamines (reaction 1) or naphthols (reaction 2) catalyzed by (R/S)-TRIP to explain the high enantioselectivity and the effect of CPAs scaffolds. Unexpectedly, our calculations reveal that substrate naphthylamines or naphthols prefer enantioselective aminal formation model II or 1,4-addition model I, respectively, which is the reverse of Tan's and Xu's model. The different noncovalent and steric interactions between catalysts and substrates are responsible for the observed model preference. Moreover, the enantioselectivity arises from distortion (reaction 1) and noncovalent interactions (reaction 2) that discriminate between the diastereomeric transition states. We further investigated the effect of SPINOL-based CPAs on the enantioselectivity and found that the more rigid skeleton and a smaller binding pocket lead to lower enantioselectivity as compared with that of BINOL-based CPA. The new insights into the reaction activation model rationalize the stereoselectivity outcome of direct asymmetric arylation reactions, and our general model can be extended to related transformations.

A Computational Mechanistic Study of Pd(II)-Catalyzed Enantioselective C(sp3)-H Borylation: Roles of APAO Ligands

A computational mechanistic study has been performed on Pd(II)-catalyzed enantioselective reactions involving acetyl-protected aminomethyl oxazolines (APAO) ligands that significantly improved reactivity and selectivity in C(sp³)-H borylation. The results support a mechanism including initiation of C(sp³)-H bond activation generating a five-membered palladacycle and ligand exchange, followed by HPO₄^2--promoted transmetalation. These resulting Pd(II) complexes further undergo sequential reductive elimination by coordination of APAO ligands and protonation to afford the enantiomeric products and deliver Pd(0) complexes, which will then proceed by oxidation and deprotonation to regenerate the catalyst. The C(sp³)-H activation is found to be the rate- and enantioselectivity-determining step, in which the APAO ligand acts as the proton acceptor to form the two enantioselectivity models. The results demonstrate that the diverse APAO ligands control the enantioselectivity by differentiating the distortion and interaction between the major and minor pathways.

A mechanism exploration of stereodivergent coupling of aldehydes and alkynes catalyzed synergistically by rhodium and amine

The stereodivergent coupling of alkynes and aldehydes with a synergistic catalyst approach using rhodium and amine.

Mechanistic insight into the silver-catalyzed cycloaddition synthesis of 1,4-disubstituted-1,2,3-triazoles: the key role of silver

By using DFT calculations, we disclose the reason why silver(i) catalytically promotes the activity and regioselectivity of the cycloaddition of 1,4-disubstituted-1,2,3-triazoles.

The control effects of different scaffolds in chiral phosphoric acids: a case study of enantioselective asymmetric arylation

DFT calculations disclosed that the sign of enantioselectivity in chiral-phosphoric-acid catalyzed reactions can be tuned by BINOL- or SPINOL-derived backbones.

Mechanistic investigation-inspired activation mode of DBU and the function of the α-diazo group in the reaction of the α-amino ketone compound and EDA: [DBU-H]+-DMF-H2O and α-diazo as strong N-terminal nucleophiles

DFT calculations disclosed a dramatic electronic turnover of the α-diazo group based on an unexpected DBU activation mode.

Mechanistic insights into N-Bromosuccinimide-promoted synthesis of imidazo[1,2-a]pyridine in water: Reactivity mediated by substrates and solvent

The mechanism of N-Bromosuccinimide (NBS) promoted synthesis of imidazo[1,2-a]pyridine in water as well as the effective activation modes of NBS was investigated by Density Functional Theory (DFT) calculations. Two main mechanisms that differ in the reaction sequence of substrate were explored: styrene with NBS then followed by 2-aminopyridine (M1) or simultaneously with NBS and 2-aminopyridine (M2), and water-assisted M2 is the more favored one. We found that the adding sequence of 2-aminopyridine affects profoundly on the title reaction. Moreover, upon the assistance of water and NBS, the preferential mechanistic scenario involves three major processes: nucleophilic addition, stepwise H-shift and intramolecular cyclization, three-step deprotonation, rather than a classical bromonium ion species. Specifically, the cooperative interaction of NBS and water plays a critical role in the title reaction. Water acts as solvent, reactant, anchoring, stabilizer, and catalyst. NBS promotes the above three processes by the effective forms of Br⁺ /Br^- , succinimide, and its ethanol isomer. Furthermore, noncovalent interactions between catalysts and substrates are responsible for the different reactive activities of M1 and M2. Our results indicate that simultaneous adding of all reactants is recommended toward economical synthesis. © 2018 Wiley Periodicals, Inc.

Mechanistic insight on water and substrate catalyzed the synthesis of 3-(1H-indol-3-yl)-2-(4-methoxybenzyl)isoindolin-1-one: Driving by noncovalent interactions

The mechanisms of the synthesis of 2-substituted-3-(1H-indol-3-yl)-isoindolin-1-one derivatives have been investigated theoretically under unassisted, self-assisted, and water-assisted conditions. Being different from previously proposed catalyst-free by Hu et al., our results show that the title mechanism can be altered and accelerated by solvent and substrate 2. Two types of mechanisms have been developed by DFT calculations differ in the reaction sequence of substrates 1 with 3 (M1) or 2 (M2) followed by 2 (M1) or 3 (M2), and water-assisted M1 is the most favored one. It was found that the nucleophilicity of substrate 3 is stronger than that of 2. Our calculations suggest that the water-assisted pathway in M1 is the most favorable case, which undergoes nucleophilic addition and H-shift, C-N bond formation and water elimination, and intramolecular cyclization and water elimination. The rate-determining step is the nucleophilic attack process. Moreover, we also explored the effect of nucleophilic attack of the nitrogen of (4-methoxyphenyl)methanamine on hydroxyl or carbonyl group carbon of phthalaldehydic acid on the activation energy. More importantly, we found that water molecules play a critical role in the whole reaction, not only act as solvent but also as an efficient catalyst, proton shuttle, and stabilizer to stabilize the structures of transition states and intermediates via π···H-O, O···H-N, O···H-C, and O···H-O interactions. The origin of the different reactivity of M1 and M2 is ascribed to the pivotal noncovalent interactions exist between catalyst (water and substrate 2) and reactants. © 2018 Wiley Periodicals, Inc.

Probing the Structural and Electronic Effects on the Origin of π-Facial Stereoselectivity in 1-Methylphosphole 1-Oxide Cycloadditions and Cyclodimerization

We have examined the π-facial stereoselectivity in the Diels-Alder reactions of phosphole oxides computationally. The experimentally observed syn-cycloadditions have been rationalized with the Cieplak model and distortion-interaction model. The natural bond orbital analysis suggests that the hyperconjugative interactions are energetically preferred between the antiperiplanar methyl group present in the -P=O unit and the developing incipient (-C-C-) bond in syn-adducts in accordance with the Cieplak model. The distortion-interaction analysis carried out for syn and anti transition states of Diels-Alder reactions of 1-substituted phosphole 1-oxide with different dienophiles reveals that the syn selectivity is favored by distortions and interaction energies compared with the anti selectivity. The formation of a syn adduct is also stabilized by the π_CC-σ*_PO orbital interaction, and the repulsive n-π interaction destabilizes the anti adduct that leads to the 7.0 kcal/mol thermodynamic preference for the former adduct. Furthermore, the distortion-interaction model rationalizes the formation of stereospecific products in these Diels-Alder reactions, which however is not explicable with the much-debated Cieplak model.

Study on the mechanism of platinum(ii)-catalyzed asymmetric ring-opening addition of oxabicyclic alkenes with arylboronic acids

The mechanism of an asymmetric ring-opening (ARO) addition of oxabicyclic alkenes catalyzed by a platinum(ii) catalyst was investigated by M06-2X/6-311G(d,p) using density functional theory (DFT). All the structures were optimized in the solvent model density (SMD) solvation model (solvation = the mixture of H2O/CH2Cl2 1 : 10, v/v) for consistence with experimental conditions. The overall mechanism is considered as a four-step reaction including transmetalation, carboplatinum, β-oxygen elimination, and hydrolysis. The transmetalation and carboplatinum steps are multi-step processes, and both the regioselectivity and the enantioselectivity lie in the carboplatinum process. Based on the natural population analysis (NPA) and the orbital composition analysis of oxabicyclic alkenes, the preferable coordination site with a platinum(ii) center is considered as the bridging oxygen atom by exo-coordination because of the less steric hindrance and the stronger electronic effect. This coordination is thought of as origin of the regioselectivity and the enantioselectivity, which is different from that proposed previously. The Gibbs free energy profiles show that the rate-determining step involves the migration of an aryl group from the platinum(ii) center to one of the closer enantiotopic carbon atoms in an alkene of the oxabicyclic alkenes. The theoretically predicted enantiomeric excess (ee) value of 82% for this reaction is very close to the experimental ee value of 80%. It was found that the hydrogen bonds between the oxabicyclic alkenes and water molecules promotes the platinum(ii) catalyst leaving the reaction system effortlessly and entering the next catalysis recycle. In the overall catalytic cycle, the highest free energy barrier is 30.1 kcal mol-1 and the process releases an energy of 26.3 kcal mol-1. The results confirm that the Pt(ii)-catalyzed ARO reactions take place at mild experimental conditions, which is consistent with the experiment observations. Thus, this study is important for understanding the catalytic behavior of the transition metal platinum(ii) in an asymmetric ring-opening reaction.

Preparation and Characterization of a Small Library of Thermally-Labile End-Caps for Variable-Temperature Triggering of Self-Immolative Polymers

The reaction between furans and maleimides has increasingly become a method of interest as its reversibility makes it a useful tool for applications ranging from self-healing materials, to self-immolative polymers, to hydrogels for cell culture and for the preparation of bone repair. However, most of these applications have relied on simple monosubstituted furans and simple maleimides and have not extensively evaluated the potential thermal variability inherent in the process that is achievable through simple substrate modification. A small library of cycloadducts suitable for the above applications was prepared, and the temperature dependence of the retro-Diels-Alder processes was determined through in situ ¹H NMR analyses complemented by computational calculations. The practical range of the reported systems ranges from 40 to >110 °C. The cycloreversion reactions are more complex than would be expected based on simple trends expected based on frontier molecular orbital analyses of the materials.

Diels-Alder Click-Cross-Linked Hydrogels with Increased Reactivity Enable 3D Cell Encapsulation

Engineered hydrogels have been extensively used to direct cell function in 3D cell culture models, which are more representative of the native cellular microenvironment than conventional 2D cell culture. Previously, hyaluronan-furan and bis-maleimide polyethylene glycol hydrogels were synthesized via Diels-Alder chemistry at acidic pH, which did not allow encapsulation of viable cells. In order to enable gelation at physiological pH, the reaction kinetics were accelerated by replacing the hyaluronan-furan with the more electron-rich hyaluronan-methylfuran. These new click-cross-linked hydrogels gel faster and at physiological pH, enabling encapsulation of viable cells, as demonstrated with 3D culture of 5 different cancer cell lines. The methylfuran accelerates Diels-Alder cycloaddition yet also increases the retro Diels-Alder reaction. Using computational analysis, we gain insight into the mechanism of the increased Diels-Alder reactivity and uncover that transition state geometry and an unexpected hydrogen-bonding interaction are important contributors to the observed rate enhancement. This cross-linking strategy serves as a platform for bioconjugation and hydrogel synthesis for use in 3D cell culture and tissue engineering.

Ruthenium(II)-Catalyzed C-H Difluoromethylation of Ketoximes: Tuning the Regioselectivity from the meta to the para Position

A highly para-selective C_Ar -H difluoromethylation of ketoxime ethers under ruthenium catalysis has been developed. A wide variety of ketoxime ethers are compatible with the reaction, which leads to the corresponding para-difluoromethylated products in moderate to good yield. A mechanistic study clearly showed that chelation-assisted cycloruthenation is the key factor in the para selectivity of the difluoromethylation of ketoxime ethers. Density functional theory was used to gain a theoretical understanding of the para selectivity.

A computational mechanistic study of substrate-controlled competitive O–H and C–H insertion reactions catalyzed by dirhodium(ii) carbenoids: insight into the origin of chemoselectivity

DFT calculations disclosed the chemoselectivity of rhodium carbenoid and water co-catalyzed O–H and C–H insertion reactions with three 1,3-diketone substrates.

Using a traceless directing group for the silver-mediated synthesis of 3-trifluoromethylpyrazoles: a computational study on the mechanism and origins of regioselectivity

The silver-mediated one-pot synthesis of 3-trifluoromethylpyrazoles using a traceless directing group was investigated by density functional theory (DFT) calculations.

Cycloaddition reactions of enoldiazo compounds

Enoldiazo esters and amides have proven to be versatile reagents for cycloaddition reactions that allow highly efficient construction of various carbocycles and heterocycles. Their versatility is exemplified by (1) [2+n]-cycloadditions (n = 3, 4) by the enol silyl ether units of enoldiazo compounds with retention of the diazo functionality to furnish α-cyclic-α-diazo compounds that are themselves subject to further transformations of the diazo functional group; (2) [3+n]-cycloadditions (n = 1-5) by metallo-enolcarbenes formed by catalytic dinitrogen extrusion from enoldiazo compounds; (3) [2+n]-cycloadditions (n = 3, 4) by donor-acceptor cyclopropenes generated in situ from enoldiazo compounds that produce cyclopropane-fused ring systems. The role of dirhodium(ii) and the emergence of copper(i) catalysts are described, as are the different outcomes of reactions initiated with these catalysts. This comprehensive review on cycloaddition reactions of enoldiazo compounds, with emphasis on methodology development, mechanistic insight, and catalyst-controlled chemodivergence, aims to provide inspiration for future discoveries in the field and to catalyze the application of enoldiazo reagents by the wider synthetic community.