Experimental Basicities of Phosphazene, Guanidinophosphazene, and Proton Sponge Superbases in the Gas Phase and Solution

Strong Bases and beyond: The Prominent Contribution of Neutral Push-Pull Organic Molecules towards Superbases in the Gas Phase

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted-Lowry theory, was constructed in the gas-phase (PA-proton affinity and/or GB-gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push-pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid-base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push-pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined.

Highly Efficient Darzens Reactions Mediated by Phosphazene Bases under Mild Conditions

The highly basic and poorly nucleophilic phosphazene base P₁‐t‐Bu promotes the Darzens condensation of α‐halo esters with aromatic aldehydes affording α,β‐epoxy esters in nearly quantitative yields under mild conditions and in short reaction times. The more basic P₄‐t‐Bu phosphazene was found useful with low reactivity aldehydes. These reactions can be performed in aprotic organic solvents of low polarity, thus minimizing the hydrolysis of α,β‐epoxy esters which often accompanies the base‐promoted Darzens condensations.

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.

Acid catalysis through N-protonation in undistorted carboxamides: improvement of amide proton sponge acylating ability

Acid catalysis of weakly distorted or undistorted carboxamides in acyl-migration reactions proceeding through N-protonation is the process with low probability in contrast to O-protonation. This circumstance made the experimental study...

Quantitative electrospray ionization efficiency scale: 10 years after

RATIONALE

The first comprehensive quantitative scale of the efficiency of electrospray ionization (ESI) in the positive mode by monoprotonation, containing 62 compounds, was published in 2010. Several trends were found between the compound structure and ionization efficiency (IE) but, possibly because of the limited diversity of the compounds, some questions remained. This work undertakes to align the new data with the originally published IE scale and carry out statistical analysis of the resulting more extensive and diverse data set to derive more grounded relationships and offer a possibility of predicting logIE values.

METHODS

Recently, several new IE studies with numerous compounds have been conducted. In several of them, more detailed investigations of the influence of compound structure, solvent properties, or instrument settings have been conducted. IE data from these studies and results from this work were combined, and the multilinear regression method was applied to relate IE to various compound parameters.

RESULTS

The most comprehensive IE scale available, containing 334 compounds of highly diverse chemical nature and spanning 6 orders of magnitude of IE, has been compiled. Several useful trends were revealed.

CONCLUSIONS

The ESI ionization efficiency of a compound by protonation is mainly affected by three factors: basicity (expressed by pK_aH in water), molecular size (expressed by molar volume or surface area), and hydrophobicity of the ion (expressed by charge delocalization in the ion or its partition coefficient between a water-acetonitrile mixture and hexane). The presented models can be used for tentative prediction of logIE of new compounds (under the used conditions) from parameters that can be computed using commercially available software. The root mean square error of prediction is in the range of 0.7-0.8 log units.

Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality

ConspectusOne of the constant challenges of synthetic chemistry is the molecular design and synthesis of nonionic, metal-free superbases as chemically stable neutral organic compounds of moderate molecular weight, intrinsically high thermodynamic basicity, adaptable kinetic basicity, and weak or tunable nucleophilicity at their nitrogen, phosphorus, or carbon basicity centers. Such superbases can catalyze numerous reactions, ranging from C-C bond formation to cycloadditions and polymerization, to name just a few. Additional benefits of organic superbases, as opposed to their inorganic counterparts, are their solubility in organic reaction media, mild reaction conditions, and higher selectivity. Approaching such superbasic compounds remains a continuous challenge. However, recent advances in synthetic methodology and theoretical understanding have resulted in new design principles and synthetic strategies toward superbases. Our computational contributions have demonstrated that the gas-phase basicity region of 350 kcal mol^-1 and even beyond is easily reachable by organosuperbases. However, despite record-high basicities, the physical limitations of many of these compounds become quickly evident. The typically large molecular weight of these molecules and their sensitivity to ordinary reaction conditions prevent them from being practical, even though their preparation is often not too difficult. Thus, obviously structural limitations with respect to molecular weight and structural complexity must be imposed on the design of new synthetically useful organic superbases, but strategies for increasing their basicity remain important.The contemporary design of novel organic superbases is illustrated by phosphazenyl phosphanes displaying gas-phase basicities (GB) above 300 kcal mol^-1 but having molecular weights well below 1000 g·mol^-1. This approach is based on a reconsideration of phosphorus(III) compounds, which goes along with increasing their stability in solution. Another example is the preparation of carbodiphosphoranes incorporating pyrrolidine, tetramethylguanidine, or hexamethylphosphazene as a substituent. With gas-phase proton affinities of up to 300 kcal mol^-1, they are among the top nonionic carbon bases on the basicity scale. Remarkably, the high basicity of these compounds is achieved at molecular weights of around 600 g·mol^-1. Another approach to achieving high basicity through the cooperative effect of multiple intramolecular hydrogen bonding, which increases the stabilization of conjugate acids, has recently been confirmed.This Account focuses on our efforts to produce superbasic molecules that embody many desirable traits, but other groups' approaches will also be discussed. We reveal the crucial structural features of superbases and place them on known basicity scales. We discuss the emerging potential and current limits of their application and give a general outlook into the future.

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

Proton-coupled electron transfer reactivities of electronically divergent heme superoxide intermediates: a kinetic, thermodynamic, and theoretical study

Heme superoxides are one of the most versatile metallo-intermediates in biology, and they mediate a vast variety of oxidation and oxygenation reactions involving O2(g). Overall proton-coupled electron transfer (PCET) processes they facilitate may proceed via several different mechanistic pathways, attributes of which are not yet fully understood. Herein we present a detailed investigation into concerted PCET events of a series of geometrically similar, but electronically disparate synthetic heme superoxide mimics, where unprecedented, PCET feasibility-determining electronic effects of the heme center have been identified. These electronic factors firmly modulate both thermodynamic and kinetic parameters that are central to PCET, as supported by our experimental and theoretical observations. Consistently, the most electron-deficient superoxide adduct shows the strongest driving force for PCET, whereas the most electron-rich system remains unreactive. The pivotal role of these findings in understanding significant heme systems in biology, as well as in alternative energy applications is also discussed.

Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙-), and doubly reduced benzoquinone ([Q]2-) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol-1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid-Q/[Q]˙-/[Q]2- adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol-1, and the capacity to transfer a hydride by about 30 kcal mol-1.

Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

A series of tetradentate tris(phosphinimine) ligands (^R3P₃tren) was developed and bound to Cu^I to form the trigonal pyramidal, C_3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F₄] (1PR3) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, 1PMe3, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (−780 mV vs. Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of 1PR3via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy-plane that destabilize 2e (d_xy/d_x²−y²) orbital sets and uniquely stabilized a₁ (d_z²) orbitals. Support is provided that the a₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z-axes of 1PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1PMe3 to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (2PMe3), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo-C_s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal C_s-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' properties via a method orthogonal to classic ligand field tuning.

Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials.

Symmetry/Asymmetry of the NHN Hydrogen Bond in Protonated 1,8-Bis(dimethylamino)naphthalene

Experimental and theoretical results are presented based on vibrational spectra and motional dynamics of 1,8-bis(dimethylamino)naphthalene (DMAN) and its protonated forms (DMANH+ and the DMANH+ HSO4− complex). The studies of these compounds have been performed in the gas phase and solid-state. Spectroscopic investigations were carried out by infrared spectroscopy (IR), Raman, and incoherent inelastic neutron scattering (IINS) experimental methods. Density functional theory (DFT) and Car–Parrinello molecular dynamics (CPMD) methods were applied to support our experimental findings. The fundamental investigations of hydrogen bridge vibrations were accomplished on the basis of isotopic substitutions (NH → ND). Special attention was paid to the bridged proton dynamics in the DMANH+ complex, which was found to be affected by interactions with the HSO4− anion.

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

Synthesis and Basicity Studies of Quinolino[7,8-h]quinoline Derivatives

Quinolino[7,8-h]quinoline is a superbasic compound, with a pK_aH in acetonitrile greater than that of 1,8-bis(dimethylaminonaphthalene) (DMAN), although its synthesis and the synthesis of its derivatives can be problematic. The use of halogen derivatives 4,9-dichloroquinolino[7,8-h]quinoline (16) and 4,9-dibromoquinolino[7,8-h]quinoline (17) as precursors has granted the formation of a range of substituted quinolinoquinolines. The basicity and other properties of quinolinoquinolines can be modified by the inclusion of suitable functionalities. The experimentally obtained pK_aH values of quinolino[7,8-h]quinoline derivatives show that N⁴,N⁴,N⁹,N⁹-tetraethylquinolino[7,8-h]quinoline-4,9-diamine (26) is more superbasic than quinolino[7,8-h]quinoline. Computationally derived pK_aH values of quinolinoquinolines functionalized with dimethylamino (NMe₂), 1,1,3,3-tetramethylguanidino (N═C(NMe₂)₂) or N,N,N',N',N″,N″-hexamethylphosphorimidic triamido (N═P(NMe₂)₃) groups are significantly greater than those of quinolino[7,8-h]quinoline. Overall, electron-donating functionalities are observed to increase the basicity of the quinolinoquinoline moiety, while the substitution of electron-withdrawing groups lowers the basicity.

1,2,5,6-Tetrakis(guanidino)-Naphthalenes: Electron Donors, Fluorescent Probes and Redox-Active Ligands

New redox-active 1,2,5,6-tetrakis(guanidino)-naphthalene compounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable further in two one-electron steps to the tetracations, are reported. Protonation switches on blue fluorescence, with the fluorescence intensity (quantum yield) increasing with the degree of protonation. Reactions with N-halogenosuccinimides or N-halogenophthalimides led to a series of new redox-active halogeno- and succinimido-/phthalimido-substituted derivatives. These highly selective reactions are proposed to proceed via the tri- or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications and further to tri- and tetracations. The integration of redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values and also allows reversible oxidation in two potentially separated one-electron steps.

Engineering non-ionic carbon super- and hyperbases by a computational DFT approach: substituted allenes have unprecedented cation affinities

DFT calculations reveal that allenes substituted by a cyclopropene or a methylenecyclopropene group, offer suitable scaffolds for tailoring powerful carbon bases. The protonation at C(sp) site provide superbases with PAs = 879–1218 kJ mol⁻¹.

Validation and extension of the gas-phase superacidity scale

RATIONALE

In recent years it has become increasingly evident that the previously reported experimental gas-phase acidity (GA) values of several strong acids differ markedly from the corresponding high-level computational values. In this work, the superacidic part of the current gas-phase acidity scale was validated and extended.

METHODS

For that, the strongly acidic section of the gas-phase acidity scale was remeasured using the equilibrium Fourier transform ion cyclotron resonance (FTICR-MS) method, adding new compounds and introducing methodological changes. In particular, a novel approach for anchoring the scale was used - the results were anchored to the computational (W1BD) GA values of trifluoromethanesulfonic acid and bis(fluorosulfonyl)imide (291.3 and 286.2 kcal mol-1 , respectively).

RESULTS

The newly measured section consists of 20 gas-phase superacids and its consistency standard deviation is 0.2 kcal mol-1 , indicating good consistency. In contrast to the previously reported experimental gas-phase acidities for a number of important superacids, the current results are consistent with high-level theoretical GA values. Structure-acidity relationships based on the current results as well as available MeCN and DCE acidity data were described and explained.

CONCLUSIONS

The introduced methodological innovations were found to be adequate and strong evidence is presented in support of the current GA values of the strong acids.

Gas-Phase Deprotonation of Benzhydryl Cations: Carbene Basicity, Multiplicity, and Rearrangements

Many fundamental properties of carbenes, particularly basicity, remain poorly understood. Herein, an experimental and computational examination of the deprotonation of a series of benzhydryl cations has been undertaken. These studies represent the first attempt at providing experimental values for diarylcarbene basicities. Pathways to deprotonation, including whether the singlet or triplet carbene is formed, are probed. Because diarylcarbenes are expected to be among the strongest organic bases known, assessing the energetics of protonation of these species is of fundamental importance for a wide range of chemical processes.

Formation of Non-Natural α,α-Disubstituted Amino Esters via Catalytic Michael Addition

The enolate monoanion of amino esters is explored, and the first catalytic Michael addition of α-amino esters is demonstrated. These studies indicate that the acidity of the αC-H is the primary factor determining reactivity. Thus, polyfluorophenylglycine amino esters yield novel α-amino esters in the presence of a catalytic amount of a guanidine-derived base and Michael acceptors. Reactivity requires an acidic N-H, which is accomplished using common protecting groups such as N-Bz, N-Boc, and N-Cbz. Calculations and labeling experiments provide insight into the governing principles in which a key C-to-N proton transfer occurs, resulting in an expansion of the scope to include a number of natural amino esters. The study culminates with a late-stage functionalization of peptidic γ-secretase inhibitor, DAPT.

Electronic Interactions in Iminophosphorane Superbase Complexes with Carbon Dioxide

Iminophosphoranes or phosphazenes are an important class of compounds with increasing use in synthetic organic chemistry as neutral organic superbases exhibiting low nucleophilicity. Their electronic structure and therefore their properties strongly depend on substitution, but there have been very few theoretical studies devoted to this topic, and more specifically to the formation of electron donor-acceptor complexes of iminophosphoranes with electrophiles. In this work, we have investigated the interaction with carbon dioxide at different ab initio levels. Carbon dioxide usually behaves as a Lewis acid and the reaction with iminiphosphoranes has been described as a nonconventional aza-Wittig process leading to isocyanates. The reaction can be conducted in supercritical CO₂ conditions (carbon dioxide acts as both solvent and reactant), which is a promising strategy in the context of green chemistry. Our calculations have been carried out at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level for model systems and at the M06-2X/6-611+G(d,p) level for a larger species used in experiments. The electronic interactions and the interaction energies are analyzed and discussed in detail using the natural bond orbital method. Proton affinities and gas-phase basicities are provided as well.

Enhanced Basicity of Push-Pull Nitrogen Bases in the Gas Phase

Nitrogen bases containing one or more pushing amino-group(s) directly linked to a pulling cyano, imino, or phosphoimino group, as well as those in which the pushing and pulling moieties are separated by a conjugated spacer (C═X)_n, where X is CH or N, display an exceptionally strong basicity. The n-π conjugation between the pushing and pulling groups in such systems lowers the basicity of the pushing amino-group(s) and increases the basicity of the pulling cyano, imino, or phosphoimino group. In the gas phase, most of the so-called push-pull nitrogen bases exhibit a very high basicity. This paper presents an analysis of the exceptional gas-phase basicity, mostly in terms of experimental data, in relation with structure and conjugation of various subfamilies of push-pull nitrogen bases: nitriles, azoles, azines, amidines, guanidines, vinamidines, biguanides, and phosphazenes. The strong basicity of biomolecules containing a push-pull nitrogen substructure, such as bioamines, amino acids, and peptides containing push-pull side chains, nucleobases, and their nucleosides and nucleotides, is also analyzed. Progress and perspectives of experimental determinations of GBs and PAs of highly basic compounds, termed as "superbases", are presented and benchmarked on the basis of theoretical calculations on existing or hypothetical molecules.

Electronic nature of zwitterionic alkali metal methanides, silanides and germanides - a combined experimental and computational approach

Zwitterionic group 14 complexes of the alkali metals of formula [E(SiMe₂OCH₂CH₂OMe)₃M], where E = C, Si or Ge and M = Li, Na or K, have been prepared, structurally characterized and their electronic nature was investigated by computational methods.

Zwitterionic group 14 complexes of the alkali metals of formula [C(SiMe₂OCH₂CH₂OMe)₃M], (M-1), [Si(SiMe₂OCH₂CH₂OMe)₃M], (M-2), [Ge(SiMe₂OCH₂CH₂OMe)₃M], (M-3), where M = Li, Na or K, have been prepared, structurally characterized and their electronic nature was investigated by computational methods. Zwitterions M-2 and M-3 were synthesized via reactions of [Si(SiMe₂OCH₂CH₂OMe)₄] (2) and [Ge(SiMe₂OCH₂CH₂OMe)₄] (3) with MOBu^t (M = Li, Na or K), resp., in almost quantitative yields, while M-1 were prepared from deprotonation of [HC(SiMe₂OCH₂CH₂OMe)₃] (1) with LiBu^t, NaCH₂Ph and KCH₂Ph, resp. X-ray crystallographic studies and DFT calculations in the gas-phase, including calculations of the NPA charges confirm the zwitterionic nature of these compounds, with the alkali metal cations being rigidly locked and charge separated from the anion by the internal OCH₂CH₂OMe donor groups. Natural bond orbital (NBO) analysis and the second order perturbation theory analysis of the NBOs reveal significant hyperconjugative interactions in M-1–M-3, primarily between the lone pair and the antibonding Si–O orbitals, the extent of which decreases in the order M-1 > M-2 > M-3. The experimental basicities and the calculated gas-phase basicities of M-1–M-3 reveal the zwitterionic alkali metal methanides M-1 to be significantly stronger bases than the analogous silanides M-2 and germanium M-3.

(15)N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.