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Superbase

A superbase is an extremely strong base in chemistry, characterized by its particularly high affinity for protons.^[1] There is no universally accepted quantitative definition, but superbases typically exhibit proton affinities exceeding 1048 kJ/mol or pK_a values greater than 25 for their conjugate acids, surpassing common strong bases like sodium hydroxide in non-aqueous solvents.^[2] Suprebases are classified into organic, organometallic, and inorganic categories, with examples including phosphazenes, amidines like DBU, and metal alkyls such as n-butyllithium.^[1] They are of theoretical interest and valuable in organic synthesis for deprotonating weak acids, such as carbonyl compounds to form enolates, often under inert conditions due to their reactivity with water and air.^[2] The concept of superbases has been explored since the 1850s, with significant advancements in the 20th century leading to non-nucleophilic variants for selective reactions.^[3]

Introduction

Definition and Characteristics

Superbases are compounds exhibiting exceptionally high basicity, defined by the International Union of Pure and Applied Chemistry (IUPAC) as bases having a very high affinity for protons, exemplified by lithium diisopropylamide.^[4] This distinguishes them from conventional bases such as sodium hydroxide, whose conjugate acid (water) has a pKa of approximately 15.7 in aqueous media.^[5] Their defining trait is the ability to deprotonate extremely weak acids, including hydrocarbons with pKa values exceeding 30.^[6] The basicity of a superbase B is quantified through the reaction
\ce{B + H+ -> BH+}
where the proton affinity (PA) represents the negative of the enthalpy change, often surpassing 1000 kJ/mol in the gas phase for these compounds.^[7] A key benchmark for organic superbases is 1,8-bis(dimethylamino)naphthalene, known as the proton sponge, which possesses a pK_{BH^+} of 18.6 in acetonitrile.^[8] Organic superbases are frequently engineered to exhibit low nucleophilicity, thereby facilitating selective proton abstraction without competing nucleophilic reactions. Superbases encompass organic, organometallic, and inorganic variants, with detailed classifications addressed elsewhere.

Historical Development

The use of inorganic superbases for deprotonation reactions dates back to the mid-19th century, with compounds such as sodium amide (NaNH₂) being employed in early organic syntheses to generate carbanions and facilitate alkylation processes.^[9] These strong bases, derived from alkali metals and ammonia, marked the initial practical application of highly basic reagents in chemistry, though their reactivity often required careful handling due to moisture sensitivity.^[10] In the 1960s and 1970s, the field advanced with the development of organometallic superbases, particularly the Lochmann-Schlosser base (also known as LICKOR superbase), introduced independently by Ludvik Lochmann in 1966 and Manfred Schlosser in 1967 through the combination of alkyllithium compounds with potassium alkoxides.^[11] This reagent enhanced selectivity in deprotonations, enabling metalation of less acidic hydrocarbons and broadening synthetic utility in organometallic chemistry.^[12] The 1980s saw the emergence of neutral organic superbases, exemplified by phosphazene bases developed by Reinhard Schwesinger in 1987, which offered high basicity without the nucleophilicity of charged species.^[13] Building on this, John G. Verkade introduced proazaphosphatrane superbases in 1989, featuring caged structures that provided steric shielding and improved stability for catalytic applications.^[14] In the 2000s, computational studies expanded understanding of inorganic superbases, with quantum chemical calculations predicting cesium oxide (Cs₂O) as the strongest neutral inorganic base in the gas phase, with a proton affinity of approximately 1412 kJ/mol, based on extensions of the Hunter-Lias scale.^[15] As of 2020, bifunctional iminophosphorane superbases have emerged as key catalysts for asymmetric synthesis, enabling enantioselective conjugate additions of nitroalkanes to enones with high stereocontrol.^[16]

Classification

Organic Superbases

Organic superbases, particularly neutral variants such as amidines, guanidines, phosphazenes, and proazaphosphatranes, are typically prepared through multi-step synthetic routes starting from readily available amine or amidine precursors. For bicyclic amidines like 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), synthesis proceeds in three stages from caprolactam and acrylonitrile, involving initial addition of the lactam to the nitrile, followed by hydrogenation and cyclization under basic conditions.^[17] This method yields DBU in moderate to good efficiency, leveraging the ring strain in the bicyclic framework to enhance basicity. Guanidine-based superbases, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), are synthesized by heating cyanamide or guanidine derivatives with bis(3-aminopropyl)amine in the presence of a strong acid catalyst at 140–180 °C for 7–9 hours, affording the bicyclic product in 95–97% yield using inexpensive, non-toxic reagents.^[18] This condensation approach exploits the nucleophilicity of amines toward the electrophilic cyanamide carbon, forming the characteristic guanidine motif central to these compounds' high basicity. Phosphazene superbases are commonly prepared via the Staudinger reaction, where peralkylated triaminophosphanes react with bulky alkyl azides to form phosphazides, which undergo thermal denitrogenation—often solvent-free—to yield the P1-phosphazene bases in moderate to excellent yields (up to quantitative for phosphazide intermediates).^[19] This two-step, one-pot process from commercial phosphane precursors enables access to sterically hindered variants, with the iminophosphorane functionality providing the electron-rich nitrogen lone pair responsible for superbasicity. Proazaphosphatranes, known as Verkade superbases, are synthesized through cyclization of triamino-phosphonium precursors, followed by deprotonation with potassium tert-butoxide (KO^tBu) to liberate the free base, achieving high yields such as 93% for key derivatives while avoiding unwanted polymerization.^[20] These air-sensitive compounds require purification under inert atmospheres to prevent hydrolysis or oxidation, often involving distillation or recrystallization in glovebox conditions, which can limit scalability despite overall efficient routes.

Organometallic Superbases

Organometallic superbases consist of mixtures involving organolithium or organosodium compounds combined with alkali metal alkoxides, leveraging the synergy between these components to achieve exceptional basicity. A prototypical example is the Lochmann-Schlosser base, prepared from n-butyllithium (n-BuLi) and potassium tert-butoxide (t-BuOK), which demonstrates an effective pKa of approximately 50 and enables the deprotonation of challenging substrates like benzene (pKa = 43) at ambient temperatures.^[21]^[22] These systems are distinguished by their metal-containing nature, incorporating alkali metals to facilitate higher reactivity compared to purely organic bases. Variants of Schlosser's base include modifications with alternative alkyl groups, such as neopentyllithium paired with potassium tert-butoxide, or substitutions involving sodium or lithium counterparts, which maintain the core mixed-metal architecture while tuning solubility and stability in non-polar solvents. Another representative example is lithium hexamethyldisilazide (LiHMDS), an organolithium amide that, when combined with additives like alkoxides, forms enhanced mixed aggregates to amplify its deprotonation capabilities in selective reactions.^[23] The unique behaviors of these superbases stem from the formation of mixed aggregates, which promote synergistic effects that boost basicity and reactivity. In these complexes, the interaction between the organometallic and alkoxide components reduces nucleophilicity while preserving strong Brønsted basicity, allowing for clean, selective deprotonation of C-H or N-H bonds without competing side reactions. This enhancement is exemplified by the equilibrium:

\ce{RLi + ROM ⇌ [RLi \cdot ROM]}

where the resulting aggregate exhibits increased basicity beyond that of the individual reagents.^[24] Such structural motifs are critical for applications in directed metalation, underscoring the role of aggregate formation in controlling reactivity profiles.

Inorganic Superbases

Inorganic superbases encompass salt-like compounds, mainly alkali and alkaline earth metal hydrides and oxides, characterized by their exceptional basicity arising from highly charged, compact anions such as hydride (H⁻) and oxide (O²⁻). These materials form ionic lattices with high melting points, typically exceeding 600°C for hydrides like LiH (mp 680°C) and NaH (mp ~800°C), rendering them stable in solid form but highly reactive toward protic species. For instance, they vigorously react with water or any proton source, liberating hydrogen gas and forming the corresponding metal hydroxide or alkoxide.^[25]^[26] Among metal hydrides, alkali metal examples such as lithium hydride (LiH), sodium hydride (NaH), and potassium hydride (KH) exhibit pKa values for their conjugate acid (H₂) in the range of 35–38 in aprotic solvents like DMSO or THF, enabling deprotonation of carbon acids with pKa up to ~35. Calcium hydride (CaH₂), an alkaline earth variant, shares similar basicity (pKa ~35) and is particularly valued in solid-state applications for its ability to generate dry hydrogen or act as a desiccant without introducing metallic impurities. The deprotonation mechanism typically proceeds via MH + RH → M⁺ + R⁻ + H₂, where MH represents the metal hydride and RH is the substrate acid, highlighting their role as hydride donors in proton abstraction.^[27]^[28] Metal oxides, particularly those of heavier alkali metals, represent the pinnacle of inorganic basicity in the gas phase. Cesium oxide (Cs₂O) is predicted to be the strongest, with a proton affinity (PA) of 1443 kJ/mol, surpassing most known neutral bases and confirmed through high-level quantum chemical calculations including DFT methods from the early 2000s. This extreme basicity stems from the large, polarizable Cs⁺ cation stabilizing the O²⁻ anion, though Cs₂O remains largely theoretical for practical use due to its reactivity and handling challenges. Early preparations of such hydrides date back to the 1850s, underscoring their long-recognized potential in basic transformations.^[29]^[30]^[31]

Physicochemical Properties

Basicity and Proton Affinity

The basicity of superbases is quantified primarily through the pKa of their conjugate acids in non-aqueous solvents such as dimethyl sulfoxide (DMSO) and acetonitrile, where values exceeding 25 indicate superbasic strength. For instance, phosphazene superbases exhibit pKa values greater than 35 in these solvents, with the P4-t-Bu phosphazene reaching an extrapolated pKa of 42.1 in acetonitrile. These measurements allow differentiation from conventional bases like ammonia, whose conjugate acid has a pKa of approximately 10.5 in DMSO.^[32]^[27] Established scales provide benchmarks for superbase basicity across classes. The Bordwell pKa table, compiled from equilibrium acidity measurements in DMSO, lists organic superbases such as tetramethylguanidine with a pKa of 23.3 for its conjugate acid, while stronger examples like certain amidines approach 26. In the gas phase, proton affinity (PA) serves as a solvent-free metric, with ammonia at 853.6 kJ/mol and inorganic superbases like Cs2O reaching 1442.9 kJ/mol, highlighting the extreme proton-accepting capacity of oxide species compared to nitrogen bases.^[27]^[33]^[29] Experimental determination of these values relies on equilibrium methods, such as monitoring the proton transfer reaction B + HA ⇌ BH⁺ + A⁻ via titration or spectrophotometry in aprotic solvents to establish relative acidities. For exceptionally strong superbases where direct measurement is challenging, density functional theory (DFT) calculations, often combined with implicit solvation models like IPCM, predict pKa values in acetonitrile with errors below 2 units by computing free energy differences for protonation. An example is the proton sponge (1,8-bis(dimethylamino)naphthalene), whose gas-phase PA of 1012 kJ/mol underscores its enhanced basicity through intramolecular stabilization.^[34] Solvent effects significantly modulate observed basicity, with protic solvents like water leveling strong bases by hydrogen-bonding to the conjugate anion, whereas aprotic solvents such as acetonitrile reveal intrinsic strengths without such interference. Stability of the conjugate acid further enhances basicity through charge delocalization, as seen in phosphazenes where electron-withdrawing groups distribute the positive charge across the framework, elevating pKa beyond 40 in low-polarity media.^[35]

Nucleophilicity and Reactivity

Organic superbases, such as phosphazenes, generally exhibit low nucleophilicity due to pronounced steric hindrance around the basic nitrogen center, which shields it from electrophilic attack and minimizes unwanted side reactions.^[36] For instance, the phosphazene superbase P4-t-Bu, featuring multiple tert-butyl groups, demonstrates this property through its bulky substituents that restrict access to the reactive site, allowing it to function primarily as a proton abstractor rather than a nucleophile.^[36] Similarly, proton sponges like 1,8-bis(dimethylamino)naphthalene display low nucleophilicity despite their high proton affinity, as steric congestion between the adjacent nitrogen lone pairs hinders nucleophilic interactions.^[37] In contrast, organometallic superbases often possess high nucleophilicity, which can lead to competing side reactions such as alkylations during deprotonation attempts.^[38] This heightened nucleophilicity arises from the polar carbon-metal bonds, enabling facile nucleophilic addition to carbonyls or halides, as seen in n-butyllithium (n-BuLi).^[39] The Lochmann-Schlosser base, a mixed aggregate of n-BuLi and potassium tert-butoxide, mitigates this to some extent by reducing nucleophilicity relative to pure n-BuLi, thereby enhancing selectivity for proton abstraction over addition pathways.^[38] Reactivity profiles of superbases are markedly influenced by environmental factors, with many displaying extreme sensitivity to air and moisture. Inorganic variants like sodium hydride (NaH) are pyrophoric and can ignite spontaneously upon exposure to moist air due to rapid hydrolysis and hydrogen evolution.^[40] Organometallic superbases exacerbate this issue through thermal instability, often decomposing above 0°C via β-hydride elimination pathways that generate alkenes and metal hydrides.^[41] Steric bulk plays a pivotal role in modulating nucleophilicity across superbase classes; in P4-t-Bu, the tert-butyl moieties create a protective envelope that suppresses nucleophilic behavior while preserving basicity.^[36] For organometallics, aggregate formation further refines reactivity by promoting selective deprotonation; mixed lithium-potassium clusters in the Lochmann-Schlosser base, for example, stabilize the reagent and direct it toward kinetically favored proton transfers over indiscriminate nucleophilic attacks.^[39] These structural features underscore the balance between basic strength and controlled reactivity essential for synthetic applications.

Synthesis and Preparation

Organic Superbases

Organometallic and Inorganic Superbases

Organometallic superbases, such as the Lochmann-Schlosser base, are typically prepared in situ by mixing an alkyllithium compound with a potassium alkoxide under strictly inert conditions to prevent decomposition or side reactions. For example, the Lochmann-Schlosser superbase is generated by combining n-butyllithium (n-BuLi) with potassium tert-butoxide (t-BuOK) in hexane at -78°C, ensuring controlled reactivity during deprotonation processes.^[21] This method leverages the synergistic basicity enhancement from the mixed metal centers, often requiring cryogenic temperatures to maintain stability. Variants of organometallic superbases include lithium amides like lithium bis(trimethylsilyl)amide (LiHMDS), which is synthesized via deprotonation of hexamethyldisilazane (HMDS) with n-BuLi in an inert atmosphere, typically at room temperature or slightly elevated to facilitate the reaction.^[43] These preparations demand rigorous exclusion of air and moisture, as the organometallic species are highly air-sensitive and can ignite upon exposure. Inorganic superbases, such as alkali metal hydrides, are prepared through direct reduction of the corresponding metal with hydrogen gas at elevated temperatures. Sodium hydride (NaH), for instance, is produced industrially by reacting molten sodium with hydrogen at approximately 300–370°C under pressure, yielding a fine powder that serves as a potent base in various reactions.^[44] Lithium hydride (LiH) and NaH are commercially available as powders, often dispersed in mineral oil to mitigate reactivity with atmospheric moisture.^[45] Handling these superbases presents significant challenges due to their extreme sensitivity to air and water. Organometallic variants require Schlenk line techniques, involving vacuum/inert gas manifolds for manipulations like transfers and filtrations, to maintain an oxygen- and moisture-free environment.^[46] Inorganic hydrides are stored under mineral oil or in sealed containers to prevent hydrolysis, with careful dispensing using dry syringes or spatulas in a glovebox or fume hood to avoid ignition risks.^[45]

Applications

Superbase has been widely used for developing and deploying database applications, particularly in small businesses, industry, government, and academia. Its intuitive interface and relational capabilities make it suitable for end-user tasks as well as more complex systems.^[47] Early versions of Superbase, starting from the Commodore 64 in 1983, were applied in personal and small-scale data management, such as contact lists and basic inventory tracking. As it evolved to platforms like Amiga, Atari ST, and Windows, it supported business applications including accounting systems, enterprise resource planning (ERP) and manufacturing resource planning (MRP) packages, production control, and membership or customer relationship management.^[48] In modern iterations like Superbase NG, released as a cross-platform solution for Windows and Linux, the software facilitates rapid application development for data-intensive tasks. It is used for bespoke business workflows, remote data access in cloud-based web applications, and migrating legacy systems to support real-time multi-user environments via the PPCS engine. Examples include optimizing inventory management, customer tracking, and secure data sharing across devices. As of November 2025, Superbase Software Ltd promotes it for cost-effective, scalable solutions in sectors requiring backward compatibility with older data formats.^[49]^[50]

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[66]
Quantum chemical study on the activity of organic superbase ...
The base t-Bu-P4 is more active than traditional organolithium superbases in the alkylation reactions of arylacetic esters [22] and dioxanones [21]. In addition ...