Lithium monoxide anion: a ground-state triplet with the strongest base to date

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Strong Bases Design: Predicted Limits of Basicity

Brønsted superbases have wide applications in organic chemistry due to their ability to activate C-H bonds. The strongest neutral bases to date are substituted aminophosphazenes developed in the late 1980s by Reinhard Schwesinger. Since then, much effort has been expended to create even stronger neutral bases. In this article, the reasons for the instability of very basic compounds are investigated by means of high-level quantum-chemical calculations. Theoretical basicity limits are suggested for solutions as well as for the gas phase. A record-breaking superbase most likely to be synthesizable and stable at ambient conditions is proposed. Hexamethylphosphoramide is considered a reliable ionizing solvent for superbases.

Multi-Ion Bridged Pathway of N-Oxides to 1,3-Dipole Dilithium Oxide Complexes

Roussi's landmark work on the generation of 1,3-dipoles from tertiary amine N-oxides has not reached its full potential since its underlying mechanism is neither well explored nor understood. Two competing mechanisms were previously proposed to explain the transformation involving either an iminium ion or a diradical intermediate. Our investigation has revealed an alternative mechanistic pathway that explains experimental results and provides significant insights to guide the creation of new N-oxide reagents beyond tertiary alkylamines for direct synthetic transformations. Truhlar's M06-2x functional and Møller-Plesset second-order perturbation theory with Dunning's [jul,aug]-cc-pv[D,T]z basis sets and discrete-continuum solvation models were employed to determine activation enthalpies and structures. During these mechanistic explorations, we discovered a unique multi-ion bridged pathway resulting from the rate-determining step, which was energetically more favorable than other alternate mechanisms. This newly proposed mechanism contains no electrophilic intermediates, strengthening the reaction potential by broadening the reagent scope and limiting the possible side reactions. This thoroughly defined general mechanism supports a more direct route for improving the use of N-oxides in generating azomethine ylide-dilithium oxide complexes with expanded functional group tolerance and breadth of chemistry.

Characterization of the alkali metal oxalates (MC2O4-) and their formation by CO2 reduction via the alkali metal carbonites (MCO2-)

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations. The critical step in the reductive process is the coordination of CO2 to an alkali metal anion, forming a metal carbonite MCO2- able to subsequently receive a second CO2 molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO2 in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li-Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO2, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C2O42- moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.

Unexpected Rearrangement of Dilithiated Isoindoline-1,3-diols into 3-Aminoindan-1-ones via N-Lithioaminoarylcarbenes: A Combined Synthetic and Computational Study

The reaction of 2-aryl-3-hydroxyisoindolin-1-ones with the s-BuLi·TMEDA system in THF at -78 °C, affording a series of diastereomeric 3-aminoindan-1-ones via a novel rearrangement of the isoindolinone scaffold, is reported. It is proposed that α-elimination of LiOH from the transient N,O-dilithiated hemiaminal carbenoids leads to the formation of singlet carbenes followed by their trapping via an intramolecular C-H insertion. An alternative explanation based on an intramolecular Mannich reaction seem much less probable. A mechanistic-type study that combines spectroscopic data of the products and calculation results, with a special focus on the diverse lithiated intermediates that are most likely to engage in the title process (particularly those with internal Li bonds), is presented. The MP2 approach, including the NPA and QTAIM data, provided insight into structures and properties of all these species. Two reaction routes A and B appeared to be possible for the postulated carbene mechanism. An unusual metamorphosis of the CCN atom triad, from a near sp 1-azaallene-type in more stable noncarbene Li enolates to a roughly sp² type in their carbene keto tautomers, is recognized in one of these pathways (route B). Dominant forms of resonance structures for the aforementioned tautomeric systems that have seven-membered quasi rings stabilized by Li⁺ ions bridging the N and carbonyl O atoms are indicated. Large computational difficulties arising from a huge impact of internal Li⁺ complexation on conformational preferences and electronic properties of carbonyl group-bearing lithium derivatives are also discussed. The new γ-keto carbene species under study belong to a subclass of acyclic aminoarylcarbenes.

Ab initio analysis on potential superbases of several hyperlithiated species: Li3F2O and Li3F2OHn (n = 1, 2)

A systematic investigation on the potential basicity of the novel hyperlithiated species Li₃F₂O and Li₃F₂(OH)_n (n = 1, 2) based upon the superalkali cluster Li₃F₂ was conducted using high-level ab initio techniques.

Dissociation of Mg(ii) and Zn(ii) complexes of simple 2-oxocarboxylates – relationship to CO2fixation, and the Grignard and Barbier reactions

The glyoxylate and pyruvate carboxylates have been complexed to Mg(ii) and Zn(ii) to investigate the intrinsic interactions of these important biochemical species in the gas phase.

An unexpected acid-catalyzed decomposition reaction of cilnidipine and pranidipine to the decarboxylative bridged tricyclic products via cascade rearrangements

A gas-phase Carroll rearrangement occurring during electrospray ionization tandem mass spectrometry led to the discovery of bridged tricyclic degradation products from cilnidipine and pranidipine.

Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion

Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions are shown as a route to preparing powerful ‘superbases’.

Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions have been proposed as one route to prepare and access a range of new and powerful “superbases”. Paradoxically, while the additional electrons in polyanions increase basicity they serve to diminish the electron binding energy and thus, it had been thought, hinder experimental synthesis. We report the synthesis and isolation of the ortho-diethynylbenzene dianion (ortho-DEB^2–) and present observations of this novel species undergoing gas-phase proton-abstraction reactions. Using a theoretical model based on Marcus–Hush theory, we attribute the stability of ortho-DEB^2– to the presence of a barrier that prevents spontaneous electron detachment. The proton affinity of 1843 kJ mol^–1 calculated for this dianion superbase using high-level quantum chemistry calculations significantly exceeds that of the lithium monoxide anion, the most basic system previously prepared. The ortho-diethynylbenzene dianion is therefore the strongest base that has been experimentally observed to date.

Ab initio investigations on the gas phase basicity and nonlinear optical properties of FLinOH species (n = 2–5)

The superalkali hydroxide (FLi₅OH) possesses alkalide characteristics which is responsible for its remarkable mean hyperpolarizability i.e. NLO properties.

Gas-phase synthesis and reactivity of Cu+–benzyne complexes

Cu⁺–benzyne complexes bearing ligands (L) were synthesized and their addition reactivity was studied in the gas phase using electrospray ionization ion trap mass spectrometry.

Investigation of coordination of Mg(II) cations to 2-pyrimidinyloxy-N-arylbenzylamines by electrospray mass spectrometry: insights for Mg(II) catalyzed Smiles rearrangement reactions

The CH(3)OH solutions of pyrimidinyloxy-N-arylbenzylamines (1-5) in the presence of Mg(II)X(2) salts (X = Cl or ClO(4)) were investigated by electrospray ionization mass spectrometry and tandem mass spectrometry (MS/MS) subsequently, showing that the cationic Mg(II) complexes 1-5·MgX(+) were important active complexes or intermediates for initiating interesting Smiles rearrangement reactions in both the gas and solution phases. By using different MgX(2) salts and selecting a set of reactants with different substitutes, the role of the counter-ion (X(-)) and the structure effect of the reactants on the Mg(II) catalyzed Smiles rearrangement reactions were studied. Moreover, the solvent effect on Mg(II) catalyzed Smiles rearrangement reactions was revealed by studying the CH(3)OH adduct complexes of 1-5·MgCl(+), which showed that the coordination of CH(3)OH to the Mg(II) center in the complexes decreased the reaction tendency. The mechanisms involved in the gas-phase Mg(II) catalyzed Smiles rearrangement reactions were proposed on the basis of MS/MS experiments and theoretical computations, showing some unique chemistries initiated by introducing Mg(II) into the template molecules.

Old acid, new chemistry. Negative metal anions generated from alkali metal oxalates and others

A brief search in Sci Finder for oxalic acid and oxalates will reward the researcher with a staggering 129,280 hits. However, the generation of alkali metal and silver anions via collision-induced dissociation of the metal oxalate anion has not been previously been reported, though Tian and coworkers recently investigated the dissociation of lithium oxalate. The exothermic decomposition of alkali metal oxalate anion to carbon dioxide in the collision cell of a triple quadrupole mass spectrometer leaves no place for the electron to reside, resulting in a double electron-transfer reaction to produce an alkali metal anion. This reaction is facilitated by the negative electron affinity of carbon dioxide and, as such, the authors believe that metal oxalates are potentially unique in this respect. The observed dissociation reactions for collision with argon gas (1.7-1.8 × 10(-3) mbar) for oxalic acid and various alkali metal oxalates are discussed and summarized. Silver oxalate is also included to demonstrate the propensity of this system to generate transition-metal anions, as well.

Gas-phase synthesis of hydrodiphenylcyclopropenylium via nonclassical Favorskii rearrangement from alkali-cationized alpha,alpha'-dibromodibenzyl ketone

The gas-phase synthesis of hydrodiphenylcyclopropenylium from alkali-cationized alpha,alpha'-dibromodibenzyl ketone (1) via nonclassical Lewis-acid-induced Favorskii rearrangement has been studied by electrospray ionization/tandem mass spectrometry (ESI-MS/MS) and theoretical methods, showing that cations [1-Br](+) by debromination from 1 and 1.M(+)(M = Li or Na) by alkali-metal cationization of 1 could convert into the protonated diphenylcyclopropenone 2.H(+) by collision-induced dissociation in the gas phase. A concerted mechanism for the Lewis-acid-induced Favorskii rearrangement from alkali-metal-cationized alpha,alpha'-dibromodibenzyl ketone was proposed and studied, based on mass spectrometric results and theoretical methods.

Gas-phase synthesis and reactivity of the lithium acetate enolate anion, -CH2CO2Li

Aerial pingpong: The lithium acetate enolate anion, the prototypical lithium salt of an alpha-deprotonated carboxylate, was prepared in the gas phase by electrospray ionization (ESI) and collision-induced ionization (CID). Its structure, reactivity, and energetics are presented along with the results of high-level computations.