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{{short description|Extremely strong base}}
{{for|similarly named articles|Superbase (disambiguation)}}
{{Acids and bases}}
A '''superbase''' is a [[Chemical compound|compound]] that has a particularly high affinity for [[Hydron (chemistry)|protons]]. Superbases are of theoretical interest and potentially valuable in [[organic synthesis]].<ref name=Puleo>{{cite journal |doi=10.1002/chem.202003580|title=Organic Superbases in Recent Synthetic Methodology Research|year=2021|last1=Puleo|first1=Thomas R.|last2=Sujansky|first2=Stephen J.|last3=Wright|first3=Shawn E.|last4=Bandar|first4=Jeffrey S.|journal=Chemistry – A European Journal|volume=27|issue=13|pages=4216–4229|pmid=32841442|s2cid=221326865 }}</ref><ref>{{Cite journal|last1=Pozharskii|first1=Alexander F.|last2=Ozeryanskii|first2=Valery A.|title=Proton Sponges and Hydrogen Transfer Phenomena|journal=Mendeleev Communications|language=en|volume=22|issue=3|pages=117–124|doi=10.1016/j.mencom.2012.05.001|year=2012}}</ref> Superbases have been described and used since the 1850s.<ref>{{cite web |title= BBC - h2g2 - History of Chemistry - Acids and Bases |url=https://www.bbc.co.uk/dna/h2g2/alabaster/A708257 |access-date=2009-08-30}}</ref><ref name=Ishi>''Superbases for Organic Synthesis'' Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.</ref>

==Definitions==
Generically [[International Union of Pure and Applied Chemistry|IUPAC]] defines a superbase as a "compound having a very high [[Base (chemistry)|basicity]], such as [[lithium diisopropylamide]]."<ref>{{GoldBookRef | title = superacid | file = S06135}}</ref> Superbases are often defined in two broad categories, [[Organic chemistry|organic]] and [[Organometallic chemistry|organometallic]].

Organic superbases are charge-neutral compounds with basicities greater than that of [[proton sponge]] (1,8-bis(dimethylamino)naphthalene, pK<sub>BH<sup>+</sup></sub> = 18.6 in acetonitrile).<ref name=Puleo/> 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.<ref>{{cite journal|last1=Raczynska|first1=Ewa D.|last2=Decouzon|first2=Michele|last3=Gal|first3=Jean-Francois|last4=Maria|first4=Pierre-Charles|last5=Wozniak|first5=Krzysztof|last6=Kurg|first6=Rhio|last7=Carins|first7=Stuart N.|title=ChemInform Abstract: Superbases and Superacids in the Gas Phase|journal=ChemInform|date=3 June 2010|volume=31|issue=33|pages=no|doi=10.1002/chin.200033267}}</ref> Common superbases of this variety feature [[amidine]], [[guanidine]], and [[phosphazene]] functional groups. Strong superbases can be designed by utilizing various approaches<ref>{{Cite journal|last1=Maksić|first1=Zvonimir B.|last2=Kovačević|first2=Borislav|last3=Vianello|first3=Robert|date=2012-10-10|title=Advances in Determining the Absolute Proton Affinities of Neutral Organic Molecules in the Gas Phase and Their Interpretation: A Theoretical Account|url=https://pubs.acs.org/doi/10.1021/cr100458v|journal=Chemical Reviews|language=en|volume=112|issue=10|pages=5240–5270|doi=10.1021/cr100458v|pmid=22857519 |issn=0009-2665|url-access=subscription}}</ref><ref>{{cite journal |doi=10.1021/acs.accounts.0c00369|title=Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis|year=2020|last1=Formica|first1=Michele|last2=Rozsar|first2=Daniel|last3=Su|first3=Guanglong|last4=Farley|first4=Alistair J. M.|last5=Dixon|first5=Darren J.|journal=Accounts of Chemical Research|volume=53|issue=10|pages=2235–2247|pmid=32886474|s2cid=221503523 }}</ref><ref>{{cite journal|doi=10.1021/acs.accounts.1c00297|first1=Katarina|last1=Vazdar|first2=Davor|last2=Margetić|first3=Borislav|last3=Kovačević|first4=Jörg|last4=Sundermeyer|first5=Ivo|last5=Leito|first6=Ullrich|last6=Jahn|title=Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality|journal=Accounts of Chemical Research|volume=54|issue=15|pages=3108–3123|year=2021|pmid=34308625 |s2cid=236430307 |url=http://fulir.irb.hr/7728/ }}</ref> to stabilize the conjugate acid, up to the theoretical limits of basicity.<ref name="limits">{{cite journal|doi=10.1021/acs.jpca.2c00521|title=Strong Bases Design: Predicted Limits of Basicity|first1=Andrey|last1=Kulsha|first2=Ekaterina|last2=Ragoyja|first3=Oleg|last3=Ivashkevich|journal=J. Phys. Chem. A|year=2022|volume=126|issue=23|pages=3642–3652|pmid=35657384 |bibcode=2022JPCA..126.3642K |s2cid=249313043 }}</ref>

Organometallic superbases, sometimes called Lochmann–Schlosser superbases, result from the combination of [[alkali metal]] [[Alkoxide|alkoxides]] and [[Organolithium reagent|organolithium]] reagents.<ref>{{cite journal |doi=10.1002/chem.202002812|title=Structural Motifs of Alkali Metal Superbases in Non‐coordinating Solvents|year=2021|last1=Klett|first1=Jan|journal=Chemistry – A European Journal|volume=27|issue=3|pages=888–904|pmid=33165981|pmc=7839563}}</ref> 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."<ref>{{cite journal | last1 = Caubère | first1 = P | year = 1993 | title = Unimetal Super Bases | journal = Chemical Reviews | volume = 93 | issue = 6| pages = 2317–2334 | doi = 10.1021/cr00022a012 }}</ref>

===Organic superbases===
[[image:VerkadeProtn.svg|thumb|290 px|left|Protonation of [[Verkade base]]. Its conjugate acid has a pK<sub>a</sub> of 32.9 in [[acetonitrile]].<ref>{{cite journal|doi=10.1002/047084289X.rn00702.pub2|title=Proazaphosphatrane|journal=Encyclopedia of Reagents for Organic Synthesis|year=2012|last1=Verkade|first1=John G.|last2=Urgaonkar|first2=Sameer|isbn=978-0471936237 }}</ref>]]
Organic superbases are mostly charge-neutral, [[nitrogen]] containing species, where nitrogen act as a proton acceptor. These include the phosphazenes, [[organophosphine|phosphanes]], amidines, and guanidines. Other organic compounds that meet the physicochemical or structural definitions of 'superbase' include proton [[Chelation|chelators]] like the aromatic proton sponges and the [[bispidine]]s.<ref>{{Cite journal|last1=Pozharskii|first1=Alexander F.|last2=Ozeryanskii|first2=Valery A.|title=Proton Sponges and Hydrogen Transfer Phenomena|journal=Mendeleev Communications|language=en|volume=22|issue=3|pages=117–124|doi=10.1016/j.mencom.2012.05.001|year=2012}}</ref><ref>{{Cite journal|last1=Barić|first1=Danijela|last2=Dragičević|first2=Ivan|last3=Kovačević|first3=Borislav|date=2013-04-19|title=Design of Superbasic Guanidines: The Role of Multiple Intramolecular Hydrogen Bonds|url=https://pubs.acs.org/doi/10.1021/jo400396d|journal=The Journal of Organic Chemistry|language=en|volume=78|issue=8|pages=4075–4082|doi=10.1021/jo400396d|pmid=23445344 |issn=0022-3263|url-access=subscription}}</ref> [[Polycyclic compound|Multicyclic]] [[polyamine]]s, like [[DABCO]] might also be loosely included in this category.<ref name=Ishi/> [[Phosphine|Phosphanes]] and carbodiphosphoranes are also strong organosuperbases''.<ref>{{Cite journal|last1=Kovačević|first1=Borislav|last2=Maksić|first2=Zvonimir B.|date=2006|title=High basicity of phosphorus–proton affinity of tris-(tetramethylguanidinyl)phosphine and tris-(hexamethyltriaminophosphazenyl)phosphine by DFT calculations|url=http://xlink.rsc.org/?DOI=b517349c|journal=Chemical Communications|language=en|issue=14|pages=1524–1526|doi=10.1039/b517349c|pmid=16575448 |issn=1359-7345|url-access=subscription}}</ref><ref>{{Cite journal|last1=Ullrich|first1=Sebastian|last2=Kovačević|first2=Borislav|last3=Xie|first3=Xiulan|last4=Sundermeyer|first4=Jörg|date=2019|title=Phosphazenyl Phosphines: The Most Electron-Rich Uncharged Phosphorus Brønsted and Lewis Bases|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201903342|journal=Angewandte Chemie International Edition|language=en|volume=58|issue=30|pages=10335–10339|doi=10.1002/anie.201903342|pmid=31037821 |s2cid=140304424 |issn=1521-3773|url-access=subscription}}</ref><ref>{{Cite journal|last1=Mehlmann|first1=Paul|last2=Mück-Lichtenfeld|first2=Christian|last3=Tan|first3=Tristan T. Y.|last4=Dielmann|first4=Fabian|date=2017-05-02|title=Tris(imidazolin-2-ylidenamino)phosphine: A Crystalline Phosphorus(III) Superbase That Splits Carbon Dioxide|url=http://doi.wiley.com/10.1002/chem.201604971|journal=Chemistry - A European Journal|language=en|volume=23|issue=25|pages=5929–5933|doi=10.1002/chem.201604971|pmid=27779340}}</ref><ref>{{Cite journal|last1=Ullrich|first1=Sebastian|last2=Kovačević|first2=Borislav|last3=Koch|first3=Björn|last4=Harms|first4=Klaus|last5=Sundermeyer|first5=Jörg|date=2019|title=Design of non-ionic carbon superbases: second generation carbodiphosphoranes|url=http://xlink.rsc.org/?DOI=C9SC03565F|journal=Chemical Science|language=en|volume=10|issue=41|pages=9483–9492|doi=10.1039/C9SC03565F|issn=2041-6520|pmc=6993619|pmid=32055322}}</ref>''

Despite enormous proton affinity, many organosuperbases can exhibit low [[Nucleophile|nucleophilicity]].

Superbases are used in [[organocatalysis]].<ref>{{Cite journal|last=MacMillan|first=David W. C.|title=The advent and development of organocatalysis|journal=Nature|volume=455|issue=7211|pages=304–308|doi=10.1038/nature07367|bibcode=2008Natur.455..304M|year=2008|pmid=18800128 |s2cid=205215034 }}</ref><ref>{{cite book |editor1-last=Ishikawa |editor1-first=Tsutomu |title=Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts |date=2009 |publisher=John Wiley & Sons |isbn=9780470740859 |doi=10.1002/9780470740859}}</ref>

===Organometallic===
[[File:LDArxn.png|thumb|Deprotonation using LDA <ref>{{cite journal|doi=10.15227/orgsyn.087.0137|title=Preparation of Ethyl 1-Benzyl-4-Fluoropiperidine-4-Carboxylate|journal=Organic Syntheses|year=2010|volume=87|pages=137|author=Jianshe Kong, Tao Meng, Pauline Ting, and Jesse Wong|doi-access=free}}</ref>.]]
Organometallic compounds of [[electropositive]] metals are superbases, but they are generally strong nucleophiles. Examples include [[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 [[alkoxide]]s) or nitrogen (metal [[Azanide|amides]] such as [[lithium diisopropylamide]]).<ref>{{cite journal|doi=10.32737/2221-8688-2022-3-325-340|title=Superbases in Organic Synthesis|journal=Chemical Problems|year=2022|volume=20|issue=4|pages=325–340|author1=Trofimov, B.A.|author2=Schmidt, E.Yu.|s2cid=253832800 }}</ref>

The [[Schlosser base]] (or Lochmann-Schlosser base), the combination of [[N-butyllithium|''n''-butyllithium]] and [[potassium tert-butoxide|potassium ''tert''-butoxide]], is commonly cited as a superbase. ''n''-Butyllithium and potassium ''tert''-butoxide form a mixed aggregate of greater reactivity than either component reagent.<ref>{{cite journal | author1 = Schlosser, M. | year = 1988 | title = Superbases for organic synthesis | journal = Pure Appl. Chem. | volume = 60 | issue = 11 | pages = 1627–1634 | doi = 10.1351/pac198860111627| doi-access = free}}</ref>

===Inorganic===
Inorganic superbases are typically [[Salt (chemistry)|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 monoxide|Caesium oxide]] is probably the strongest base according to quantum-chemical calculations.<ref name="limits"></ref>

==See also==
*[[Superacid]]
*[[Phosphazene]]

==References==
<references />

[[Category:Bases (chemistry)]]
[[Category:Superbases|*]]