Asuperbase is a compound that has a particularly high affinity for protons. Superbases are of theoretical interest and potentially valuable in organic 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 of proton sponge (pKBH+ = 18.6 in MeCN)."[1] In a related definition: any species with a higher absolute proton affinity (APA = 245.3 kcal/mol) and intrinsic gas phase basicity (GB = 239 kcal/mol) than proton sponge.[6] Common superbases of this variety feature amidine, guanidine, and phosphazene 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 of alkali metalalkoxides and organolithium 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 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."[12]
Inorganic superbases are typically salt-like compounds with small, highly charged anions, e.g. lithium hydride, potassium hydride, and sodium hydride. Such species are insoluble, but the surfaces of these materials are highly reactive and slurries 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.