In chemistry, a superbase is an extremely basic compound or substance that has a high affinity for protons. The hydroxide ion is the strongest base possible in aqueous solutions, but bases exist with much greater strengths than the bases that could possibly withstand in water/aqueous solutions(without ionizing it). Such bases are valuable in organic synthesis and are fundamental to physical organic chemistry. Superbases have been described and used since the 1850s. Reactions involving superbases often require special techniques since they are destroyed by water and atmospheric carbon dioxide as well as oxygen. Inert atmosphere techniques and low temperatures minimize these side reactions.
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Definitions
Superbase is defined as an organic compound whose basicity is greater than that of proton sponge, which has a pKa. Strong superbases can be prepared from extending the hydrogen bond network of multiple amino groups substituted on an aromatic core. Superbases are of great interest to practicing organic chemists due to their reactivity as well as good solubility in organic solvents. Superbases are also important environmentally as they have recently been found to participate in CO2 fixation.
IUPAC defines superbases simply as a "compound having a very high basicity, such as lithium diisopropylamide." Caubère defines superbases qualitatively but more precisely: "The term superbases should only be applied to bases resulting from a mixing of two (or more) bases leading to new basic species possessing inherent new properties. The term superbase does not mean a base is thermodynamically and/or kinetically stronger than another, instead it means that a basic reagent is created by combining the characteristics of several different bases."
Superbases have also been defined semi-quantitatively as any species with a higher absolute proton affinity (APA = 245.3 kcal/mol) and intrinsic gas phase basicity (GB = 239 kcal/mol) than Alder's canonical proton sponge (1,8-bis-(dimethylamino)-naphthalene).
Classes of superbases
Three main classes of superbases are recognized: organic, organometallic and inorganic.
Organic
Organic superbases are almost always charge-neutral, nitrogen-containing species. Despite enormous proton affinity, organosuperbases exhibit low nucleophilicity. Increasingly important in organic synthesis, these include the phosphazenes, amidines, and guanidines. Other organic compounds also meet the physicochemical or structural definitions of 'superbase'. Proton chelators like the aromatic proton sponges and the bispidines are also superbases. Multicyclic polyamines, like DABCO might also be loosely included in this category.
Organometallic
Organometallic compounds of reactive metals can be superbases, including organolithium and organomagnesium (Grignard reagent) compounds. Another type of organometallic superbase has a reactive metal exchanged for a hydrogen on a heteroatom, such as oxygen (unstabilized alkoxides) or nitrogen (metal amides such as lithium diisopropylamide). A desirable property in many cases is low nucleophilicity, i.e. a non-nucleophilic base. Unhindered alkyllithiums, for example, cannot be used with electrophiles such as carbonyl groups, because they attack the electrophiles as nucleophiles.
The Schlosser base (or Lochmann-Schlosser base), the combination of n-butyllithium and potassium tert-butoxide, is a commonly used superbase. n-Butyllithium and potassium tert-butoxide form a mixed aggregate of greater reactivity than either reagent alone and with distinctly different properties in comparison to tert-butylpotassium.
Inorganic
Inorganic superbases are typically salt-like compounds with small, highly charged anions, e.g. lithium nitride. Alkali and earth alkali metal hydrides potassium hydride and sodium hydride are superbases. Such species are insoluble in all solvents owing to the strong cation-anion interactions, but the surfaces of these materials are highly reactive and slurries are useful in synthesis.
Structure and mechanism
It has been shown that if a compound has more than two amino groups that are close to each other, upon protonation, an extended network of intramolecular hydrogen bonding (from the NH3+) can help stabilize the conjugated acid (see Figure 1). Moreover, the higher number of amino groups present in the compound, the more degrees of intramolecular hydrogen bondings can be found within the conjugated acid (hydrogen from one NH3+ can participate in the hydrogen bond system with other NH3+ groups surrounding it) (see Figure 2). Therefore, the proton affinity of system with more amino groups is higher than those with fewer amino groups [2], thus enabling superbases.
Superbases in organic chemistry
Superbases are used in organocatalysis.
Phase-transfer catalysis is widely used in conjunction with superbases. Within the category of catalysts, particular emphasis has recently been placed on bifunctional catalysts combining both hydrogen bonding and phase-transfer motif. Figure 4 demonstrates how this phase-transfer-catalyst would work. Note that it has the backbone scaffold of a superbase. In this structure, there is a lipophilic shell and a hydrophilic binding region (see Figure 4). So the molecule can actively bring the substrates together in the interface between the aqueous face and the organic face. The molecule’s full potential for favorable intermolecular interaction is believed to benefit from (1) a π-polarizable naphthyl ring that would allow for π−π stacking interactions, (2) a pair of highly diffuse cations that would facilitate the transport of anions into a lipophilic organic phase, and (3) a H-bonding region for molecular recognition. [6]