A superbase is a compound that has a particularly high affinity for

protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron m ...

. Superbases are of theoretical interest and potentially valuable in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

. Superbases have been described and used since the 1850s.''Superbases for Organic Synthesis'' Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.

Definitions

Generically

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

defines a superbase as a "compound having a very high

basicity In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. ...

, such as lithium diisopropylamide." Superbases are often defined in two broad categories, organic and

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and ...

. Organic superbases are charge-neutral compounds with basicities greater than that of

proton sponge 1,8-Bis(dimethylamino)naphthalene is an organic compound with the formula CH(NMe) (Me = methyl). It is classified as a peri-naphthalene, i.e. a 1,8-disubstituted derivative of naphthalene. Owing to its unusual structure, it exhibits exceptional b ...

(pK_BH⁺ = 18.6 in MeCN)." 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. Common superbases of this variety feature

amidine Amidines are organic compounds with the functional group RC(NR)NR2, where the R groups can be the same or different. They are the imine derivatives of amides (RC(O)NR2). The simplest amidine is formamidine, HC(=NH)NH2. Examples of amidines inclu ...

, guanidine, and phosphazene functional groups. Strong superbases can be designed by utilizing multiple intramolecular hydrogen bonds that stabilize the conjugate acid. Organometallic superbases, sometimes called Lochmann–Schlosser superbases, result from the combination of alkali metal

alkoxides In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, wh ...

and organolithium reagents. 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."

Organic superbases

290 px, left, Protonation of Verkade base. Its conjugate acid has a pK_a of 32.9 in acetonitrile. 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

s and the bispidines. Multicyclic polyamines, like

DABCO DABCO (1,4-diazabicyclo .2.2ctane), also known as triethylenediamine or TEDA, is a bicyclic organic compound with the formula N2(C2H4)3. This colorless solid is a highly nucleophilic tertiary amine base, which is used as a catalyst and reage ...

might also be loosely included in this category.'' Phosphanes and carbodiphosphoranes are also strong organosuperbases. '' Despite enormous proton affinity, the organosuperbases can exhibit low nucleophilicity.

Organometallic

Organometallic compounds of

electropositive Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

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 In chemistry, a heteroatom () is, strictly, any atom that is not carbon or hydrogen. Organic chemistry In practice, the term is usually used more specifically to indicate that non-carbon atoms have replaced carbon in the backbone of the molecul ...

, such as

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

(unstabilized alkoxides) or nitrogen (metal amides such as lithium diisopropylamide). The

Schlosser base Schlosser's base (or Lochmann-Schlosser base) describes various superbase, superbasic mixtures of an alkyllithium compound and a potassium alkoxide. The reagent is named after Manfred Schlosser, although he uses the term ''LICKOR superbase'' (LIC de ...

(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.

Inorganic

Inorganic superbases are typically

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

-like compounds with small, highly charged anions, e.g. lithium hydride, potassium hydride, and

sodium hydride Sodium hydride is the chemical compound with the empirical formula Na H. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H− ions, in ...

. Such species are insoluble, but the surfaces of these materials are highly reactive and slurries are useful in synthesis.

Applications

Superbases are used in

organocatalysis In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic com ...

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References