From Carbodiimides to Carbon Dioxide: Quantification of the Electrophilic Reactivities of Heteroallenes

Efficient one-step amide formation using amino porphyrins

Amide bonds are one of the most prevalent phenomena in nature and are utilized frequently in drug and material design. However, forming amide bonds is not always efficient or high yielding, particularly when the amine used to conjugate to a carboxylic acid is a weak nucleophile. This limitation precludes many useful amino compounds from participating in conjugation reactions to form amides. A particularly valuable amino compound, which is also a very weak nucleophile, is the amino porphyrin, valued for its role as a photosensitizer, fluorescent agent, catalyst, or, upon metalation, even a very efficient contrast agent for magnetic resonance imaging (MRI). In this work, we propose fast and high-yield coupling of an unreactive amine - the amino porphyrin - to carboxylic acid via isothiocyanate conjugation. Reactions can be achieved in one step at room temperature in one hour, achieving quantitative conversion and near perfect selectivity. Both metalated and unmetalated porphyrin, as well as fluorescein isothiocyanate (FITC), demonstrated efficient conjugation. To illustrate the value of the proposed method, we created a new blood-pool MRI contrast agent that reversibly binds to serum albumin. This new blood-pool agent, known as MITC-Deox (MRI isothiocyanate that links with deoxycholic acid), substantially reduced T1 relaxation times in blood vessels in mice, remained stable for 1 hour, cleared from blood by 24 hours, and was eliminated from the body after 4 days. The proposed method for efficient amide formation is a superior alternative to existing coupling methods, opening a door to novel synthesis of MRI contrast agents and beyond.

Electrophile-Arene Affinity: An Energy Scale for Evaluating the Thermodynamics of Electrophilic Dearomatization Reactions

Rational design and development of organic reactions are lofty goals in synthetic chemistry. Quantitative description of the properties of molecules and reactions by physical organic parameters plays an important role in this regard. In this Article, we report an energy scale, namely, electrophile-arene affinity (EAA), for evaluating the thermodynamics of electrophilic dearomatization reactions, a class of important transformations that can rapidly build up molecular complexity and structural diversity by converting planar aromatic compounds into three-dimensional cyclic molecules. The acquisition of EAA data can be readily achieved by theoretically calculating the enthalpy changes (ΔH) of the hypothetical reactions of various (cationic) electrophiles with aromatic systems (taking the 1-methylnaphthalen-2-olate ion as an example in this study). Linear correlations are found between the calculated ΔH values and established physical organic parameters such as the percentage of buried volume %VBur (steric effect), Hammett's σ or Brown's σ+ (electronic effect), and Mayr's E (reaction kinetics). Careful analysis of the ΔH values leads to the rational design of a dearomative alkynylation reaction using alkynyl hypervalent iodonium reagents as the electrophiles.

Heteroallene Insertions into Tin(II) Alkoxide Bonds

A series of iso-carbamate complexes have been synthesized by the reaction of [SnII(OiPr)2] or [SnII(OtBu)2] with either aryl or alkyl isocyanates, ONC-R (R = 2,4,6-trimethylphenyl (Mes), 2,6-diisopropylphenyl (Dipp), isopropyl (iPr), cyclohexyl (Cy) and tert-butyl (tBu)). In the case of aryl isocyanates, mono-insertion occurs to form structurally characterized complexes [Sn{κ2-N,O-R-NC(OiPr)O}(μ-OiPr)]2 (1: R = Mes, 2: R = Dipp) and [Sn{κ2-N,O-R-NC(OtBu)O}(μ-OtBu)]2 (3: R = Mes, 4: R = Dipp). The complicated solution-state chemistry of these species has been explored using 1H DOSY experiments. In contrast, reactions of tin(II) alkoxides with alkyl isocyanates result in the formation of bis-insertion products [Sn{κ2-N,O-R-NC(OiPr)O}2] (5: R = iPr, and 6: R = Cy) and [Sn{κ2-N,O-R-NC(OtBu)O}2] (7: R = iPr, 8: R = Cy), of which complexes 6-8 have also been structurally characterized. 1H NMR studies show that the reaction of tBu-NCO with either [Sn(OiPr)2] or [Sn(OtBu)2] results in a reversible mono-insertion. Variable-temperature 2D 1H-1H exchange spectroscopy (VT-2D-EXSY) was used to determine the rate of exchange between free tBu-NCO and the coordinated tBu-iso-carbamate ligand for the {OiPr} alkoxide complex, as well as the activation energy (Ea = 92.2 ± 0.8 kJ mol-1), enthalpy (ΔH‡ = 89.4 ± 0.8 kJ mol-1), and entropy (ΔS‡ = 12.6 ± 2.9 J mol-1 K-1) for the process [Sn(OiPr)2] + tBu-NCO ↔ [Sn{κ2-N,O-tBu-NC(OiPr)O}(OiPr)]. Attempts to form Sn(II) alkyl carbonates by the insertion of CO2 into either [Sn(OiPr)2] or [Sn(OtBu)2] proved unsuccessful. However, 119Sn{1H} NMR spectroscopy of the reaction of excess CO2 with [Sn(OiPr)2] reveals the presence of a new Sn(II) species, i.e., [(iPrO)Sn(O2COiPr)], VT-2D-EXSY (1H) of which confirms the reversible alkyl carbonate formation (Ea = 70.3 ± 13.0 kJ mol-1; ΔH‡ = 68.0 ± 1.3 kJ mol-1 and ΔS‡ = -8.07 ± 2.8 J mol-1 K-1).

Diborane Reductions of CO2 and CS2 Mediated by Dicopper μ-Boryl Complexes of a Robust Bis(phosphino)-1,8-naphthyridine Ligand

A dinucleating 1,8-naphthyridine ligand featuring fluorene-9,9-diyl-linked phosphino side arms (PNNPFlu) was synthesized and used to obtain the cationic dicopper complexes 2, [(PNNPFlu)Cu2(μ-Ph)][NTf2]; [NTf2] = bis(trifluoromethane)sulfonimide, 6, [(PNNPFlu)Cu2(μ-CCPh)][NTf2], and 3, [(PNNPFlu)Cu2(μ-OtBu)][NTf2]. Complex 3 reacted with diboranes to afford dicopper μ-boryl species (4, with μ-Bcat; cat = catecholate and 5, with μ-Bpin; pin = pinacolate) that are more reactive in C(sp)-H bond activations and toward activations of CO2 and CS2, compared to dicopper μ-boryl complexes supported by a 1,8-naphthyridine-based ligand with di(pyridyl) side arms. The solid-state structures and DFT analysis indicate that the higher reactivities of 4 and 5 relate to changes in the coordination sphere of copper, rather than to perturbations on the Cu-B bonding interactions. Addition of xylyl isocyanide (CNXyl) to 4 gave 7, [(PNNPFlu)Cu2(μ-Bcat)(CNXyl)][NTf2], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 4 and 5 with CO2 yielded the corresponding dicopper borate complexes (8, [(PNNPFlu)Cu2(μ-OBcat)][NTf2]; 9, [(PNNPFlu)Cu2(μ-OBpin)][NTf2]), with 4 demonstrating catalytic reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS2 provided insertion products 10, {[(PNNPFlu)Cu2]2[μ-S2C(Bcat)2]}[NTf2]2, and 11, [(PNNPFlu)Cu2(μ,κ2-S2CBpin)][NTf2], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO2 insertion intermediate.

Lactone Enolates of Isochroman-3-ones and 2-Coumaranones: Quantification of Their Nucleophilicity in DMSO and Conjugate Additions to Chalcones

Owing to stereoelectronic effects, lactones often deviate in reactivity from their open-chain ester analogues as demonstrated by the CH acidity (in DMSO) of 3-isochromanone (pKa = 18.8) and 2-coumaranone (pKa = 13.5), which is higher than that of ethyl phenylacetate (pKa = 22.6). We have now characterized the reactivity of the lactone enolates derived from 3-isochromanone and 2-coumaranone by following the kinetics of their Michael reactions with p-quinone methides and arylidenemalonates (reference electrophiles) in DMSO at 20 °C. Evaluation of the experimentally determined second-order rate constants k2 by the Mayr-Patz equation, lg k2 = sN(N + E), furnished the nucleophilicity parameters N (and sN) of the lactone enolates. By localizing their position on the Mayr nucleophilicity scale, the scope of their electrophilic reaction partners becomes predictable, and we demonstrate a novel catalytic methodology for a series of carbon-carbon bond-forming reactions of lactone enolates with chalcones under phase transfer conditions in toluene.

Diversity of transformation of heteroallenes on acenaphthene-1,2-diimine aluminum oxide

The reactions of oxide [(dpp-bian)Al(μ2-O)2Al(dpp-bian)] (1) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with phenyl- or cyclohexylisocyanates result in the formation of carbonimidate derivatives [(dpp-bian)Al(μ-O)(μ-RNCO2)Al(dpp-bian)] (R = Ph, 2; Cy, 3). Addition of N,N'-dicyclohexylcarbodiimide to compound 1 leads to the formation of ureate complex [(dpp-bian)Al(μ-O)(μ-(CyN)2CO)Al(dpp-bian)] (4). The reactions of the oxide 1 with pinacolborane and catecholborane afford oxo-bridged hydride [{(dpp-bian)Al(H)}(μ-O){Al(OBpin)(dpp-bian)}] (5) and compound [{(dpp-bian)Al(OBCat)}2(μ-O)] (7), respectively. Insertion of cyclohexylisocyanate into the Al-H bond of compound 5 gives CO insertion product [{(dpp-bian)Al(OC(H)NCy)}(μ-O){Al(OBpin)(dpp-bian)}] (6). New compounds have been characterized by ESR and IR spectroscopy; their molecular structures have been established by single-crystal X-ray analysis. The oxide 1 serves as a catalyst for the hydroboration of heteroallenes (isocyanates, carbodiimides) with pinacolborane.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs)

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Switchable Organocatalysis from N-heterocyclic Carbene-Carbodiimide Adducts with Tunable Release Temperature

N-Heterocyclic carbenes (NHCs) are powerful organocatalysts, but practical applications often require in situ generation from stable precursors that "mask" the NHC reactivity via reversible binding. Previously established "masks" are often simple small molecules, such that the NHC structure is used to control both catalytic activity and activation temperature, leading to undesirable tradeoffs. Herein, we show that NHC-carbodiimide (CDI) adducts can be masked precursors for switchable organocatalysis and that the CDI substituents can control the reaction profile without changing the NHC structure. Large electronic variations on the CDI (e.g., alkyl versus aryl) drastically change the catalytically active temperature, whereas smaller perturbations (e.g., different para-substituted phenyls) tune the catalyst release within a narrower window. This control was demonstrated for three classic NHC-catalyzed reactions, each influencing the NHC-CDI equilibrium in different ways. Our results introduce a new paradigm for controlling NHC organocatalysis as well as present practical considerations for designing appropriate masks for various reactions.

Redox-Mediated pH Swing Systems for Electrochemical Carbon Capture

ConspectusThe rising levels of atmospheric CO2 and their resulting impacts on the climate have necessitated the urgent development of effective carbon capture technologies. Electrochemical carbon capture systems have emerged as a potential alternative to conventional thermal systems based on amine solutions due to their modularity, energy efficiency, and lower environmental impact. Among these, aqueous electrochemical pH swing systems that capitalize on the pH dependence of dissolved inorganic carbon (CO2/HCO3-/CO32-) speciation to capture and release CO2 are of particular interest as they can be flexible in system design and in the range of electrochemical potentials used as well as being environmentally benign. In this Account, we present our recent findings in pH swing-based electrochemical carbon capture using redox-active materials, paving the way toward a sustainable solution for mitigating CO2 emissions.In the first section, we discuss the utilization of molecular redox-active organic materials in electrochemical carbon capture by the pH swing method. This electrochemical system configuration involves homogeneous aqueous electrolytes containing molecular redox-active compounds combined with inert carbon-based electrodes. We first present the development of redox-active amine and oxygen-insensitive neutral red (NR)-based systems. Notably, the discovery of 1-aminopyridinium (1-AP) as an electrochemically reversible compound enables efficient pH swing, leading to an impressive electron utilization of 1.25 mol of CO2 per mole of electrons. Additionally, we explore an oxygen-insensitive neutral red/leuconeutral red (NR/NRH2) redox system, which demonstrates potential applicability to direct air capture (DAC) systems with ambient air as a feed gas.The second section focuses on the utilization of inorganic nanomaterials for redox-active electrodes for pH swing-based electrochemical carbon capture. In this system configuration, we employ redox-active electrodes for inducing reversible pH swings in aqueous electrolytes without interrupting other ionic species, except protons. Specifically, we explore the effectiveness of manganese oxide (MnO2) electrodes for achieving selective CO2 removal from simulated flue gas. We then demonstrate a bismuth/silver (Bi/BiOCl, Ag/AgCl) nanoparticle electrode system as a sodium-insensitive pH swing system for extracting dissolved inorganic carbon (DIC) from simulated seawater with high electrochemical energy efficiency.Overall, these advances in pH swing-based electrochemical carbon capture offer promising preliminary solutions for combating climate change by capturing CO2 from dilute sources such as flue gas and ambient air as well as enabling direct carbon removal from ocean water. While these systems have demonstrated impressive energy efficiency and environmental benefits using redox-active materials, they represent only the beginning of our research journey. Further development and optimization are currently underway as we strive to unlock their full potential for large-scale implementation, paving the way toward a sustainable and carbon-neutral future.

Electrocatalytic Desulfurizative Amination of Thioureas to Guanidines

Guanidine has been known as an important class of N-containing molecules with a wide range of applications. Described here is a selective and efficient electrochemical approach to the synthesis of guanidines from easily accessible thioureas and amines. The key to success for this reaction is the in situ generation of a hypervalent iodine reagent as a catalyst from iodoarene by anodic oxidation. This mild desulfurizative amination presents ample substrate scope and good functional group tolerance without the use of extra stoichiometric chemical oxidants. As only electrons serve as the oxidation reagents, this method offers a more straightforward and sustainable manner toward versatile guanidines, including late-stage functionalization of pharmaceutically relevant molecules.

Pushing the Upper Limit of Nucleophilicity Scales by Mesoionic N-Heterocyclic Olefins

A series of mesoionic, 1,2,3-triazole-derived N-heterocyclic olefins (mNHOs), which have an extraordinarily electron-rich exocyclic CC-double bond, was synthesized and spectroscopically characterized, in selected cases by X-ray crystallography. The kinetics of their reactions with arylidene malonates, ArCH=C(CO2 Et)2 , which gave zwitterionic adducts, were investigated photometrically in THF at 20 °C. The resulting second-order rate constants k2 (20 °C) correlate linearly with the reported electrophilicity parameters E of the arylidene malonates (reference electrophiles), thus providing the nucleophile-specific N and sN parameters of the mNHOs according to the correlation lg k2 (20 °C)=sN (N+E). With 21 Collapse

Key Words

• DFT Calculations
• Kinetics
• Mesoionic Compounds
• Methyl Cation Affinities

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MESH Headings Collapse

Grants

• 390677874-RESOLV Deutsche Forschungsgemeinschaft
• HA 8832/1-1 Deutsche Forschungsgemeinschaft
• 101077332 HORIZON EUROPE European Research Council
• J-4592 Österreichische Forschungsförderungsgesellschaft

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Affiliation(s)

Andreas Eitzinger Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Justus Reitz Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany
Patrick W Antoni Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany
Herbert Mayr Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Armin R Ofial Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377, München, Germany
Max M Hansmann Fakultät für Chemie und Chemische Biologie, Technische Universität Dortmund, Otto-Hahn-Str. 6, 44227, Dortmund, Germany

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Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Postpolymerization Modification by Nucleophilic Addition to Styrenic Carbodiimides

Carbodiimides are electrophilic functional groups that react with select nucleophiles under mild conditions. However, their potential as platforms for postpolymerization modification has been relatively underexplored. We describe the synthesis and radical polymerization of a styrenic carbodiimide which undergoes rapid nucleophilic addition with primary and secondary alkyl amines under ambient conditions, even in the presence of other protic nucleophiles. The monomer is amenable to both free and controlled radical (co)polymerization, and we further demonstrate the utility of this approach by preparing covalent adaptable networks through guanylation of the styrenic carbodiimide with difunctional amines. These materials exhibit a variation in relaxation times according to both the guanidine structure and concentration, providing a facile means for tuning dynamic behavior.

Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework - a computational study

In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(III)-H intermediates are critical hydride-delivery agents. For bipyridine-supported complexes, a unique transformation named 'Cp* non-innocence' leads to the appearance of (Cp*H)Rh(I) intermediates, which are purported to exhibit enhanced hydride-delivery capabilities. In this work, DFT calculations performed to compare the role of these complexes in hydrogenation reveal that (Cp*H)Rh(I) intermediates do not compete with the conventional pathway (involving Cp*Rh(III)-H); instead they can lead to sequential hydrogenation of the Cp* framework, and potentially, catalyst degradation. Thus, caution is warranted when invoking the truly homogeneous nature of hydrogenation catalysis mediated by this popular class of complexes.

The computational road to reactivity scales

Reactivity scales are useful research tools for chemists, both experimental and computational. However, to determine the reactivity of a single molecule, multiple measurements need to be carried out, which is a time-consuming and resource-intensive task. In this Tutorial Review, we present alternative approaches for the efficient generation of quantitative structure-reactivity relationships that are based on quantum chemistry, supervised learning, and uncertainty quantification. First published in 2002, we observe a tendency for these relationships to become not only more predictive but also more interpretable over time.

Nucleophilic Addition/Electrocyclization Strategy toward Polyheterocyclic-Fused Quinoline-2-thiones in Green Solvent

A metal-free and base-free cyclization reaction of ortho-heteroaryl anilines with isothiocyanatobenzene for the synthesis of diverse polyheterocyclic-fused quinoline-2-thiones was developed in PEG-200. This protocol features green solvent, lack of requirement for a metal and base, short reaction time consumption, and facile isolation via simple filtration. Furthermore, this protocol is easy to scale up which demonstrates outstanding synthetic scalability.

Computational Insights into Malononitrile-Based Carbanions for CO2 Capture

Although anionic N and O sites have been widely used in chemisorption of CO₂, carbanions are much less explored for CO₂ capture. Here we employ ab initio calculations and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations to examine the interaction between CO₂ and the malononitrile carbanion, [CH(CN)₂]^-. We have explored the potential energy surface of CO₂ binding by scanning the C-C distance between CO₂ and the central C site of the carbanion. We find that CO₂ prefers to bind to the nitrile group physically rather than to form a C-C bond via the carboxylation reaction at the sp² C site. Moreover, the two -CN groups can attract two CO₂ molecules at equal strength. The presence of an alkali metal ion enhances both physical and chemical interactions of CO₂ with the malononitrile carbanion. QM/MM MD simulations further confirm the preference of physical interaction in the condensed ionic liquid phase with a phosphonium cation. Our findings suggest that ionic liquids based on the malononitrile carbanion may have a high CO₂ solubility for carbon capture.

Thermodynamic, kinetic, and mechanistic studies of the thermal guanidine metathesis reaction

We describe studies of the thermal guanidine metathesis (TGM) reaction, a reversible transformation that results in exchange of N-substituents of the guanidine functional group. By comparing the effects of discrete structural variations, we find that steric congestion is an important factor in determining both the equilibrium guanidine composition and the reaction kinetics. The alkyl versus aryl nature of N-substitution also plays an essential role in the reaction rate, up to the point that minimal TGM reactivity is observed when the guanidine contains wholly alkyl substituents. Furthermore, we demonstrate that TGM occurs under thermodynamic control and present evidence that it proceeds by a dissociative mechanism, supported by direct observation of a carbodiimide intermediate.

Late-Stage Carbon-14 Labeling and Isotope Exchange: Emerging Opportunities and Future Challenges

Carbon-14 (¹⁴C) is a gold standard technology routinely utilized in pharmaceutical and agrochemical industries for tracking synthetic organic molecules and providing their metabolic and safety profiles. While the state of the art has been dominated for decades by traditional multistep synthetic approaches, the recent emergence of late-stage carbon isotope labeling has provided new avenues to rapidly access carbon-14-labeled biologically relevant compounds. In particular, the development of carbon isotope exchange has represented a fundamental paradigm change, opening the way to unexplored synthetic transformations. In this Perspective, we discuss the recent developments in the field with a critical assessment of the literature. We subsequently discuss research directions and future challenges within this rapidly evolving field.

Aromaticity-promoted CS2 activation by heterocycle-bridged P/N-FLPs: a comparative DFT study with CO2 capture

Carbon dioxide (CO₂) capture has attracted considerable attention from both experimental and theoretical chemists. In comparison, carbon disulfide (CS₂) activation is less developed. Here, we carry out a thorough comparative density functional theory study to examine the reaction mechanisms of CS₂ activation by five-membered heterocycle-bridged P/N frustrated Lewis pairs (FLPs). Calculations suggest that despite a weaker carbon-sulfur bond, all the CS₂ activations have higher reaction barriers than the CO₂ capture, which could be attributed to electrostatic repulsion between FLPs and CS₂ caused by the reversed polarity of CS in CS₂ rather than the electrostatic attraction in CO₂ capture. In addition, aromaticity is found to play an important role in CS₂ capture as it stabilizes both the transition states and products in heterocycle-bridged FLPs. All these findings could be useful for experimentalists to realize small molecule activations by FLPs.

Basicity as a Thermodynamic Descriptor of Carbanions Reactivity with Carbon Dioxide: Application to the Carboxylation of α,β-Unsaturated Ketones

The utilization of carbon dioxide as a raw material represents nowadays an appealing strategy in the renewable energy, organic synthesis, and green chemistry fields. Besides reduction strategies, carbon dioxide can be exploited as a single-carbon-atom building block through its fixation into organic scaffolds with the formation of new C-C bonds (carboxylation processes). In this case, activation of the organic substrate is commonly required, upon formation of a carbanion C^-, being sufficiently reactive toward the addition of CO₂. However, the prediction of the reactivity of C^- with CO₂ is often problematic with the process being possibly associated with unfavorable thermodynamics. In this contribution, we present a thermodynamic analysis combined with density functional theory calculations on 50 organic molecules enabling the achievement of a linear correlation of the standard free energy (ΔG⁰) of the carboxylation reaction with the basicity of the carbanion C^-, expressed as the pK_a of the CH/C^- couple. The analysis identifies a threshold pK_a of ca 36 (in CH₃CN) for the CH/C^- couple, above which the ΔG⁰ of the carboxylation reaction is negative and indicative of a favorable process. We then apply the model to a real case involving electrochemical carboxylation of flavone and chalcone as model compounds of α,β-unsaturated ketones. Carboxylation occurs in the β-position from the doubly reduced dianion intermediates of flavone and chalcone (calculated ΔG⁰ of carboxylation in β = -12.8 and -20.0 Kcalmol^-1 for flavone and chalcone, respectively, associated with pK_a values for the conjugate acids of 50.6 and 51.8, respectively). Conversely, the one-electron reduced radical anions are not reactive toward carboxylation (ΔG⁰ > +20 Kcalmol^-1 for both substrates, in either α or β position, consistent with pK_a of the conjugate acids < 18.5). For all the possible intermediates, the plot of calculated ΔG⁰ of carboxylation vs. pK_a is consistent with the linear correlation model developed. The application of the ΔG⁰ vs. pK_a correlation is finally discussed for alternative reaction mechanisms and for carboxylation of other C=C and C=O double bonds. These results offer a new mechanistic tool for the interpretation of the reactivity of CO₂ with organic intermediates.

Reaction of potassium phosphide KP(iPr)Ter with chalcogens, heteroallenes and an acyl chloride

The reactivity of the secondary phosphide KP(iPr)Ter (1) (Ter = 2,6-bis-(2,4,6-trimethylphenyl)phenyl) toward small molecules is reported. Phosphide 1 displays distinct nucleophilic character and reacts selectively with chalcogens (S₈, Se_x), heteroallenes (CO₂, nPrNCS), and an acyl chloride (AdCOCl) to give the corresponding dichalcogenophosphinates (2a, 3), phosphanyl formate (5), thiocarbamoylphosphane (6a), or acylphosphane (7a), respectively. Furthermore the follow-up chemistry of these products was investigated. 2a was converted to a PSPS ligand (2b) which forms a Au(I) complex (2c) with (Me₂S)AuCl. Likewise, a gold complex of 7a was prepared. All species were isolated and fully characterized.

Investigation on the Stoichiometry of Carbon Dioxide in Isotope-Exchange Reactions with Phenylacetic Acids

The functionalization of carbon dioxide (CO2) as a C1 building block has attracted enormous attention. Carboxylation reactions, in particular, are of major interest for applications in isotope labeling. Due to the inexpensive nature of CO2, information about its stoichiometric use is generally unavailable in the literature. Because of the rarity and limited availability of CO2 isotopomers, this parameter is of concern for applications in carbon-isotope labeling. We investigated the effects of the stoichiometry of labeled CO2 on carbon isotope exchange of phenyl­acetic acids. Both thermal and photocatalytic procedures were studied, providing insight into product outcome and isotope incorporation. Preliminary results on isotope-dilution effects of carbonate bases in photocatalytic carboxylation reactions have also been obtained.

Mechanochemical Release of N-Heterocyclic Carbenes from Flex-Activated Mechanophores

We have discovered a new flex-activated mechanophore that releases an N-heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC-carbodiimide (NHC-CDI) adducts and demonstrates an important first step toward flex-activated designs capable of further downstream reactivities. Since the flex-activation is non-destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.

Nucleophilic Reactivities of Thiophenolates

The nucleophilic reactivities of substituted thiophenolates were determined by following the kinetics of their reactions with a series of quinone methides (reference electrophiles) in DMSO at 20 °C. The experimentally determined second-order rate constants were analyzed according to the Mayr-Patz equation log k = s_N(N + E) to derive the nucleophile-specific reactivity parameters N and s_N for ten thiophenolate ions.

Fast Carbon Isotope Exchange of Carboxylic Acids Enabled by Organic Photoredox Catalysis

Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp³) acids with labeled CO₂. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [¹¹C]CO₂ is demonstrated.

Soluble, crystalline, and thermally stable alkali CO2 - and carbonite (CO2 2-) clusters supported by cyclic(alkyl)(amino) carbenes

The mono- and dianions of CO2 (i.e., CO2 - and CO2 2-) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO2 reduction and fixation chemistry. However, CO2 anions are highly unstable and difficult to study. As such, examples of stable compounds containing these ions are extremely limited; the unadulterated alkali salts of CO2 (i.e., MCO2, M2CO2, M = alkali metal) decompose rapidly above 15 K, for example. Herein we report the chemical reduction of a cyclic (alkyl)(amino) carbene (CAAC) adduct of CO2 at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO2 - and CO2 2- clusters. One-electron reduction of CAAC-CO2 adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC-CO2)] n (M = Li, Na, K, 2-4) analogous to the alkali CO2 - radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M2(CAAC-CO2)] n (5-8) with reduced CO2 units which are structurally analogous to the carbonite anion CO2 2-. It is notable that crystalline clusters of these alkali-CO2 salts may also be isolated via the "one-pot" reaction of free CO2 with free CAAC followed by the addition of alkali metals - a process which does not occur in the absence of carbene. Each of the products 2-8 was investigated using a combination of experimental and theoretical methods.

Pharmacological characterization of a new series of carbamoylguanidines reveals potent agonism at the H2R and D3R

Even today, the role of the histamine H₂ receptor (H₂R) in the central nervous system (CNS) is widely unknown. In previous research, many dimeric, high-affinity and subtype-selective carbamoylguanidine-type ligands such as UR-NK22 (5, pK_i = 8.07) were reported as H₂R agonists. However, their applicability to the study of the H₂R in the CNS is compromised by their molecular and pharmacokinetic properties, such as high molecular weight and, consequently, a limited bioavailability. To address the need for more drug-like H₂R agonists with high affinity, we synthesized a series of monomeric (thio)carbamoylguanidine-type ligands containing various spacers and side-chain moieties. This structural simplification resulted in potent (partial) agonists (guinea pig right atrium, [³⁵S]GTPγS and β-arrestin2 recruitment assays) with human (h) H₂R affinities in the one-digit nanomolar range (pK_i (139, UR-KAT523): 8.35; pK_i (157, UR-MB-69): 8.69). Most of the compounds presented here exhibited an excellent selectivity profile towards the hH₂R, e.g. 157 being at least 3800-fold selective within the histamine receptor family. The structural similarities of our monomeric ligands to pramipexole (6), a dopamine receptor agonist, suggested an investigation of the binding behavior at those receptors. The target compounds were (partial) agonists with moderate affinity at the hD_2longR and agonists with high affinity at the hD₃R (e.g. pK_i (139, UR-KAT523): 7.80; pK_i (157, UR-MB-69): 8.06). In summary, we developed a series of novel, more drug-like H₂R and D₃R agonists for the application in recombinant systems in which either the H₂R or the D₃R is solely expressed. Furthermore, our ligands are promising lead compounds in the development of selective H₂R agonists for future in vivo studies or experiments utilizing primary tissue to unravel the role and function of the H₂R in the CNS.

Origin of Free Energy Barriers of Decarboxylation and the Reverse Process of CO2 Capture in Dimethylformamide and in Water

In aqueous solution, biological decarboxylation reactions proceed irreversibly to completion, whereas the reverse carboxylation processes are typically powered by the hydrolysis of ATP. The exchange of the carboxylate of ring-substituted arylacetates with isotope-labeled CO₂ in polar aprotic solvents reported recently suggests a dramatic change in the partition of reaction pathways. Yet, there is little experimental data pertinent to the kinetic barriers for protonation and thermodynamic data on CO₂ capture by the carbanions of decarboxylation reactions. Employing a combined quantum mechanical and molecular mechanical simulation approach, we investigated the decarboxylation reactions of a series of organic carboxylate compounds in aqueous and in dimethylformamide solutions, revealing that the reverse carboxylation barriers in solution are fully induced by solvent effects. A linear Bell-Evans-Polanyi relationship was found between the rates of decarboxylation and the Gibbs energies of reaction, indicating diminishing recombination barriers in DMF. In contrast, protonation of the carbanions by the DMF solvent has large free energy barriers, rendering the competing exchange of isotope-labeled CO₂ reversible in DMF. The finding of an intricate interplay of carbanion stability and solute-solvent interaction in decarboxylation and carboxylation could be useful to designing novel materials for CO₂ capture.

Transition-metal-free synthesis of 5-amino-1,2,3-triazoles via nucleophilic addition/cyclization of carbodiimides with diazo compounds

A facile and transition-metal-free synthesis of functionalized 5-amino-1,2,3-triazoles with good functional-group tolerance and regioselectivity via nucleophilic addition/cyclization has been described.

Polyurethanes from Direct Organocatalytic Copolymerization of p-Tosyl Isocyanate with Epoxides

The direct copolymerization of p-tosyl isocyanate (TSI) with epoxides, initiated by onium salts in the presence of trialkylborane, to produce polyurethanes is reported. The rate of copolymerization and the (regio)selectivity were investigated in relation to the trialkylborane and the initiator used. Under optimized conditions such copolymerizations have been successfully performed for a wide range of epoxides, including ethylene oxide, propylene oxide, 1-octene oxide, cyclohexene oxide, and allyl glycidyl ether. These copolymerizations afford a new category of polyurethanes, clear of side products such as cyclic oxazolidinone, isocyanurate, and poly(isocyanate) linkages. The experimental conditions used in this work are compatible with those for the organocatalytic (co)polymerization of other oxygenated monomers and CO₂ , holding the potential for their terpolymerization with p-tosyl isocyanate and the development of new materials with unprecedented properties.

Computational Study on the Fate of Oxidative Directing Groups in Ru(II), Rh(III), and Pd(II) Catalyzed C-H Functionalization

Activation of C-H bonds assisted by a directing group is indispensable in organic synthesis. Among them, utilizing oxidative directing groups that can serve as an internal oxidant to drive the Mⁿ/Mⁿ⁺² catalytic cycle has recently become a promising strategy. A survey of published reactions involving N-alkoxyamides or N-acyloxyamides reveals that not all N-O bonds act as an internal oxidant. We have therefore systematically investigated the effect of the oxidative groups on a model reaction catalyzed by Ru(II), Rh(III), and Pd(II) complexes. DFT calculations show that N-methoxy and N-acyloxy groups oxidize Ru(II) to Ru(IV) and Rh(III) to Rh(V), but cannot oxidize a cyclo-Pd(II) intermediate to Pd(IV). The stability of the metal imido intermediate 7-M (M = Ru, Rh, and Pd) controls whether the oxidation occurs or not. N-Acyloxy groups show a more pronounced selectivity than N-methoxy to oxidize Ru(II) and Rh(III) species, while no distinctive effect is observed for Pd(II).

Thermal Guanidine Metathesis for Covalent Adaptable Networks

We demonstrate that a dynamic chemical reaction that we term thermal guanidine metathesis (TGM) can serve as the basis for covalent adaptable network (CAN) materials. CANs are a class of cross-linked polymers that transition from thermoset to thermoplastic-like rheological behavior upon significant activation of reversible exchange reactions within the network and thus can be reprocessed. Small molecule studies indicate the TGM reaction proceeds by a dissociative mechanism, and guanidine-cross-linked network polymers can be reprocessed at elevated temperature. These TGM-based CANs exhibit dynamic behavior, such as dissolution in the presence of monofunctional exchange partners and stress relaxation above T_g. Additionally, differences in the activation energies obtained by small molecule kinetic studies and stress relaxation analysis are consistent with key predictions of the Semenov-Rubinstein model of thermoreversible gelation of highly cross-linked networks.