Superbase

For the Superbase database, see Superbase (database).

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 can exist in water. Such bases are valuable in organic synthesis and are fundamental to physical organic chemistry. Superbases have been described and used since the 1850s.[1] 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. Superbases also have a corrosive effect.

Definitions

IUPAC defines superbases simply as a "compound having a very high basicity, such as lithium diisopropylamide."[2] 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."[3]

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).[4]

Classes of superbases

There are three main classes of superbases: organic, organometallic and inorganic.

Organic

Organic superbases are almost always neutral, nitrogen-containing species. Despite enormous proton affinity, organosuperbases are prized for their heightened reactivity tempered by low nucleophilicity and relatively mild conditions of use. 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.[5]

Organometallic

Organometallic compounds of reactive metals are often superbases, including organolithium and organomagnesium (Grignard reagent) compounds. Another type of organic 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.[6]

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.

See also

References

  1. ↑ "BBC - h2g2 - History of Chemistry - Acids and Bases". Retrieved 2009-08-30.
  2. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "superacid".
  3. ↑ Caubère, P (1993). "Unimetal super bases". Chemical Reviews 93: 2317–2334. doi:10.1021/cr00022a012.
  4. ↑ Raczynska, E. D.; Decouzon, M.; Gal, J.-F. et al. (1998). "Superbases and superacids in the gas phase". Trends in Organic Chemistry 7: 95–103.
  5. ↑ Superbases for Organic Synthesis Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.
  6. ↑ Schlosser, M. (1988). "Superbases for organic synthesis". Pure Appl. Chem. 60 (11): 1627–1634. doi:10.1351/pac198860111627.