A Phosphorus Bisylide: Exploring a New Class of Superbases with Two Interacting Carbon Atoms as Basicity Centers

Exploiting the (-C-H···C-) Interaction to Design Cage-Functionalized Organic Superbases and Hyperbases: A Computational Study

A set of carbon center-based P-ylidesubstituting bases have been exploited computationally with pentacyclo[5.4.0.02,6.03,10.05.9]undecane (PCU) and pentacyclo [6.4.0.02,7.03,11.06,10] dodecane (PCD) scaffolds using the B3LYP-D3/6-311+G(d,p) level of theory. The proton affinities calculated in the gas phase are in the range of superbases and hyperbases. The Atomsin-Molecules and Natural Bond Orbital calculations reveal that the -C-H···C- interaction plays a substantial role in improving the basicity, and tuning the -C-H···C- interaction can enhance the basicity of such systems. The free activation energy for proton exchange for PCD and PCU scaffolds substituted with P-ylide is substantially low. The computed results reveal the strength and nature of such - C-H···C- interactions compared to the -N-H···N- hydrogen bonds. The isodesmic reactions suggest that the superbasicity achieved using these frameworks arises from a combination of several factors, such as the ring strain of the bases in their unprotonated form, steric repulsion, and the intramolecular -C-H···C- interaction.

Modeling pKa of the Brønsted Bases as an Approach to the Gibbs Energy of the Proton in Acetonitrile

A simple but efficient computational approach to calculate pK_a in acetonitrile for a set of phosphorus, nitrogen, and carbon bases was established. A linear function that describes relations between the calculated ΔG’_a.sol(BH⁺) and pK_a values was determined for each group of bases. The best model was obtained through the variations in the basis set, in the level of theory (density functionals or MP2), and in the continuum solvation model (IPCM, CPCM, or SMD). The combination of the IPCM/B3LYP/6-311+G(d,p) solvation approach with MP2/6-311+G(2df,p)//B3LYP/6-31G(d) gas-phase energies provided very good results for all three groups of bases with R² values close to or above 0.99. Interestingly, the slopes and the intercepts of the obtained linear functions showed significant deviations from the theoretical values. We made a linear plot utilizing all the conducted calculations and all the structural variations and employed methods to prove the systematic nature of the intercept/slope dependence. The interpolation of the intercept to the ideal slope value enabled us to determine the Gibbs energy of the proton in acetonitrile, which amounted to −258.8 kcal mol⁻¹. The obtained value was in excellent agreement with previously published results.

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.

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.

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.

Utilizing the Azaazulene Scaffolds in the Design of New Organic Superbases

New neutral organic superbases with 1-azaazulene(s) as a molecular backbone are computationally designed, employing two basic substituents: dimethylaminocyclopropen-imines (CPI) and dimethylaminocyclopropeniminophosphazenes (CPI-P). Their proton affinities, gas basicities, and pK _a values in acetonitrile are obtained using density functional theory. Azaazulenes substituted with CPI have a computed PA in the gas phase ranging between 272.9 and 306.8 kcal mol^-1, with pK _a values in acetonitrile between 28.8 and 36 units. The substitution with the CPI-P group resulted in even stronger superbases, with a PA from 296.5 to 335.2 kcal mol^-1 and corresponding pK _a values from 33.9 to 50 units. This exceptionally strong thermodynamic basicity is accompanied by very high kinetic basicity as well; contrary to typical proton sponges, the release of a proton from the conjugate superbase does not demand high activation energy. Because synthetic routes for both substituents and azaazulenes are already known, newly designed superbases represent suitable targets for synthesis and application.

Design of non-ionic carbon superbases: second generation carbodiphosphoranes

The edge-cutting design, synthesis and characterization of the so far strongest non-ionic carbon superbases is presented.

A new generation of carbodiphosphoranes (CDPs), incorporating pyrrolidine, tetramethylguanidine, or tris(dimethylamino)phosphazene as substituents is introduced as the most powerful class of non-ionic carbon superbases on the basicity scale to date. The synthetic approach as well as NMR spectroscopic and structural characteristics in the free and protonated form are described. Investigation of basicity in solution and in the gas phase by experimental and theoretical means provides the to our knowledge first reported pK_BH⁺ values for CDPs in the literature and suggest them as upper tier superbases.

Carbon Atom as an Extremely Strong Nucleophilic and Electrophilic Center: Dendritic Allenes Are Powerful Organic Proton and Hydride Sponges

Gas-phase proton affinities (PAs) and hydride affinities (HAs) of organic bases possessing an allene moiety and substituted with methyl, dimethylamino, cyano, and vinyl substituents were examined with the B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) model. It was shown that a number of superbases and hyperbases can be obtained, as well as the potent hydride sponges. Methyl or dimethylamino substituents increased the proton affinity of the parent molecule, and the cyano substituents increased its hydride affinity. When the vinyl substituents are placed on allene, both the hydride and the proton affinities increased. A disubstituted allene with two dimethylamino groups is the smallest studied superbase, whereas the allene tetrasubstituted with four vinyl groups gives the smallest superbase possessing only alkene substituents. By introducing the vinyl group as a repeating subunit, one can obtain dendritic structures with the investigated substituents determining its properties. By changing the dimethylamino with the cyano group, a dendrimeric molecule can change from a hyperbase with a proton affinity of 324.6 kcal mol^-1 to a very strong hydride ion acceptor with a hydride affinity of 205.4 kcal mol^-1, while possessing the same proton or hydride ion attachment site.