Asuperbase is acompound that has a particularly high affinity forprotons. Superbases are of theoretical interest and potentially valuable inorganic synthesis.[1][2] Superbases have been described and used since the 1850s.[3][4]
Organic superbases are charge-neutral compounds with basicities greater than that ofproton sponge (pKBH+ = 18.6 in MeCN)."[1] In a related definition: any species with a higher absoluteproton affinity (APA = 245.3 kcal/mol) and intrinsic gas phase basicity (GB = 239 kcal/mol) than proton sponge.[6] Common superbases of this variety featureamidine,guanidine, andphosphazene functional groups. Strong superbases can be designed by utilizing various approaches[7][8][9] to stabilize the conjugate acid, up to the theoretical limits of basicity.[10]
Organometallic superbases, sometimes called Lochmann–Schlosser superbases, result from the combination ofalkali metalalkoxides andorganolithium reagents.[11] Caubère defines superbases as "bases resulting from a mixing of two (or more) bases leading to new basic species possessing inherent new properties. The termsuperbase 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."[12]
Organic superbases are mostly charge-neutral,nitrogen containing species, where nitrogen act as a proton acceptor. These include the phosphazenes,phosphanes, amidines, and guanidines. Other organic compounds that meet the physicochemical or structural definitions of 'superbase' include protonchelators like the aromatic proton sponges and thebispidines.[14][15]Multicyclicpolyamines, likeDABCO might also be loosely included in this category.[4]Phosphanes and carbodiphosphoranes are also strong organosuperbases.[16][17][18][19]
Despite enormous proton affinity, many organosuperbases can exhibit lownucleophilicity.
TheSchlosser base (or Lochmann-Schlosser base), the combination ofn-butyllithium andpotassiumtert-butoxide, is commonly cited as a superbase.n-Butyllithium and potassiumtert-butoxide form a mixed aggregate of greater reactivity than either component reagent.[24]
Inorganic superbases are typicallysalt-like compounds with small, highly charged anions, e.g.lithium hydride,potassium hydride, andsodium hydride. Such species are insoluble, but the surfaces of these materials are highly reactive andslurries are useful in synthesis.Caesium oxide is probably the strongest base according to quantum-chemical calculations.[10]
^abPuleo, Thomas R.; Sujansky, Stephen J.; Wright, Shawn E.; Bandar, Jeffrey S. (2021). "Organic Superbases in Recent Synthetic Methodology Research".Chemistry – A European Journal.27 (13):4216–4229.doi:10.1002/chem.202003580.PMID32841442.S2CID221326865.
^Pozharskii, Alexander F.; Ozeryanskii, Valery A. (2012). "Proton Sponges and Hydrogen Transfer Phenomena".Mendeleev Communications.22 (3):117–124.doi:10.1016/j.mencom.2012.05.001.
^Raczynska, Ewa D.; Decouzon, Michele; Gal, Jean-Francois; Maria, Pierre-Charles; Wozniak, Krzysztof; Kurg, Rhio; Carins, Stuart N. (3 June 2010). "ChemInform Abstract: Superbases and Superacids in the Gas Phase".ChemInform.31 (33): no.doi:10.1002/chin.200033267.
^Formica, Michele; Rozsar, Daniel; Su, Guanglong; Farley, Alistair J. M.; Dixon, Darren J. (2020). "Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis".Accounts of Chemical Research.53 (10):2235–2247.doi:10.1021/acs.accounts.0c00369.PMID32886474.S2CID221503523.
^Pozharskii, Alexander F.; Ozeryanskii, Valery A. (2012). "Proton Sponges and Hydrogen Transfer Phenomena".Mendeleev Communications.22 (3):117–124.doi:10.1016/j.mencom.2012.05.001.
^Ishikawa, Tsutomu, ed. (2009).Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts. John Wiley & Sons.doi:10.1002/9780470740859.ISBN9780470740859.
