Superbase Explained

A superbase 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]

Definitions

Generically IUPAC defines a superbase as a "compound having a very high basicity, such as lithium diisopropylamide." Superbases are often defined in two broad categories, organic and organometallic.

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.[5] Common superbases of this variety feature amidine, guanidine, and phosphazene functional groups. Strong superbases can be designed by utilizing various approaches[6] [7] [8] to stabilize the conjugate acid, up to the theoretical limits of basicity.[9]

Organometallic superbases, sometimes called Lochmann–Schlosser superbases, result from the combination of alkali metal alkoxides and organolithium reagents.[10] 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."[11]

Organic superbases

thumb|290 px|left|Protonation of Verkade base. Its conjugate acid has a pKa of 32.9 in acetonitrile.[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 proton chelators like the aromatic proton sponges and the bispidines.[13] [14] Multicyclic polyamines, like DABCO might also be loosely included in this category.[4] Phosphanes and carbodiphosphoranes are also strong organosuperbases.[15] [16] [17] [18]

Despite enormous proton affinity, many organosuperbases can exhibit low nucleophilicity.

Superbases are used in organocatalysis.[19] [20]

Organometallic

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 alkoxides) or nitrogen (metal amides such as lithium diisopropylamide).[21]

The Schlosser base (or Lochmann-Schlosser base), the combination of n-butyllithium and 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.[22]

Inorganic

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.

See also

References

  1. 10.1002/chem.202003580. Organic Superbases in Recent Synthetic Methodology Research. 2021. Puleo. Thomas R.. Sujansky. Stephen J.. Wright. Shawn E.. Bandar. Jeffrey S.. Chemistry – A European Journal. 27. 13. 4216–4229. 32841442. 221326865 .
  2. Pozharskii. Alexander F.. Ozeryanskii. Valery A.. Proton Sponges and Hydrogen Transfer Phenomena. Mendeleev Communications. en. 22. 3. 117–124. 10.1016/j.mencom.2012.05.001. 2012.
  3. Web site: BBC - h2g2 - History of Chemistry - Acids and Bases . 2009-08-30.
  4. Superbases for Organic Synthesis Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.
  5. Raczynska. Ewa D.. Decouzon. Michele. Gal. Jean-Francois. Maria. Pierre-Charles. Wozniak. Krzysztof. Kurg. Rhio. Carins. Stuart N.. ChemInform Abstract: Superbases and Superacids in the Gas Phase. ChemInform. 3 June 2010. 31. 33. no. 10.1002/chin.200033267.
  6. Maksić. Zvonimir B.. Kovačević. Borislav. Vianello. Robert. 2012-10-10. Advances in Determining the Absolute Proton Affinities of Neutral Organic Molecules in the Gas Phase and Their Interpretation: A Theoretical Account. Chemical Reviews. en. 112. 10. 5240–5270. 10.1021/cr100458v. 22857519 . 0009-2665.
  7. 10.1021/acs.accounts.0c00369. Bifunctional Iminophosphorane Superbase Catalysis: Applications in Organic Synthesis. 2020. Formica. Michele. Rozsar. Daniel. Su. Guanglong. Farley. Alistair J. M.. Dixon. Darren J.. Accounts of Chemical Research. 53. 10. 2235–2247. 32886474. 221503523 .
  8. 10.1021/acs.accounts.1c00297. Katarina. Vazdar. Davor. Margetić. Borislav. Kovačević. Jörg. Sundermeyer. Ivo. Leito. Ullrich. Jahn. Design of Novel Uncharged Organic Superbases: Merging Basicity and Functionality. Accounts of Chemical Research. 54. 15. 3108–3123. 2021. 34308625 . 236430307 .
  9. 10.1021/acs.jpca.2c00521. Strong Bases Design: Predicted Limits of Basicity. Andrey. Kulsha. Ekaterina. Ragoyja. Oleg. Ivashkevich. J. Phys. Chem. A. 2022. 126. 23. 3642–3652. 35657384 . 2022JPCA..126.3642K . 249313043 .
  10. 10.1002/chem.202002812. Structural Motifs of Alkali Metal Superbases in Non‐coordinating Solvents. 2021. Klett. Jan. Chemistry – A European Journal. 27. 3. 888–904. 33165981. 7839563.
  11. Caubère . P . 1993 . Unimetal Super Bases . Chemical Reviews . 93 . 6. 2317–2334 . 10.1021/cr00022a012 .
  12. 10.1002/047084289X.rn00702.pub2. Proazaphosphatrane. Encyclopedia of Reagents for Organic Synthesis. 2012. Verkade. John G.. Urgaonkar. Sameer. 978-0471936237 .
  13. Pozharskii. Alexander F.. Ozeryanskii. Valery A.. Proton Sponges and Hydrogen Transfer Phenomena. Mendeleev Communications. en. 22. 3. 117–124. 10.1016/j.mencom.2012.05.001. 2012.
  14. Barić. Danijela. Dragičević. Ivan. Kovačević. Borislav. 2013-04-19. Design of Superbasic Guanidines: The Role of Multiple Intramolecular Hydrogen Bonds. The Journal of Organic Chemistry. en. 78. 8. 4075–4082. 10.1021/jo400396d. 23445344 . 0022-3263.
  15. Kovačević. Borislav. Maksić. Zvonimir B.. 2006. High basicity of phosphorus–proton affinity of tris-(tetramethylguanidinyl)phosphine and tris-(hexamethyltriaminophosphazenyl)phosphine by DFT calculations. Chemical Communications. en. 14. 1524–1526. 10.1039/b517349c. 16575448 . 1359-7345.
  16. Ullrich. Sebastian. Kovačević. Borislav. Xie. Xiulan. Sundermeyer. Jörg. 2019. Phosphazenyl Phosphines: The Most Electron-Rich Uncharged Phosphorus Brønsted and Lewis Bases. Angewandte Chemie International Edition. en. 58. 30. 10335–10339. 10.1002/anie.201903342. 31037821 . 140304424 . 1521-3773.
  17. Mehlmann. Paul. Mück-Lichtenfeld. Christian. Tan. Tristan T. Y.. Dielmann. Fabian. 2017-05-02. Tris(imidazolin-2-ylidenamino)phosphine: A Crystalline Phosphorus(III) Superbase That Splits Carbon Dioxide. Chemistry - A European Journal. en. 23. 25. 5929–5933. 10.1002/chem.201604971. 27779340 .
  18. Ullrich. Sebastian. Kovačević. Borislav. Koch. Björn. Harms. Klaus. Sundermeyer. Jörg. 2019. Design of non-ionic carbon superbases: second generation carbodiphosphoranes. Chemical Science. en. 10. 41. 9483–9492. 10.1039/C9SC03565F. 2041-6520. 6993619. 32055322.
  19. MacMillan. David W. C.. The advent and development of organocatalysis. Nature. 455. 7211. 304–308. 10.1038/nature07367. 2008Natur.455..304M. 2008. 18800128 . 205215034 .
  20. Book: Ishikawa . Tsutomu . Superbases for Organic Synthesis: Guanidines, Amidines, Phosphazenes and Related Organocatalysts . 2009 . John Wiley & Sons . 9780470740859 . 10.1002/9780470740859.
  21. 10.32737/2221-8688-2022-3-325-340. Superbases in Organic Synthesis. Chemical Problems. 2022. 20. 4. 325–340. Trofimov, B.A.. Schmidt, E.Yu.. 253832800 .
  22. Schlosser, M. . 1988 . Superbases for organic synthesis . Pure Appl. Chem. . 60 . 11 . 1627–1634 . 10.1351/pac198860111627. free .

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