Potassium Tert-butoxide

Potassium ''tert''-butoxide (or potassium ''t''-butoxide) is a chemical compound with the formula CH₃)₃COKsub>''n'' (abbr. KOtBu). This colourless solid is a strong base (pKa of conjugate acid is 17 in H₂O), which is useful in organic synthesis. The compound is often depicted as a salt, and it often behaves as such, but its ionization depends on the solvent.

Preparation

Potassium ''t''-butoxide is commercially available as a solution and as a solid, but it is often generated ''in situ'' for laboratory use because samples are so moisture- sensitive and older samples are often of low purity. It is prepared by the reaction of dry ''tert''-butyl alcohol with potassium metal. The solid is obtained by evaporating these solutions followed by heating the solid. The solid can be purified by sublimation.

Structure

It crystallizes as a tetrameric cubane-type cluster. It crystallises from tetrahydrofuran/pentane at −20 °C as BuOK·tBuOHsub>∞, which consists of straight chains linked by hydrogen bonding. Sublimation of BuOK·tBuOHsub>∞ affords the tetramer BuOKsub>4, which adopts a cubane-like structure. Mildly Lewis basic solvents such as THF and diethyl ether do not break up the tetrameric structure, which persists in the solid, in solution and even in the gas phase.

Reactions

As a base

Many modifications have been reported that influence the reactivity of this reagent. The compound adopts a complex cluster structure (the adjacent picture is a simplified cartoon), and additives that modify the cluster affect the reactivity of the reagent. For example, DMF, DMSO, hexamethylphosphoramide (HMPA), and 18-crown-6 interact with the potassium center, yielding solvent separated ion pairs such as K(DMSO)_x⁺ and ''tert''-BuO⁻. Whereas in benzene, on the other hand, the compound remains as a cluster structure, which is less basic. Even in polar solvents, it is not as strong as amide bases, e.g., lithium diisopropylamide, but stronger than potassium hydroxide. Its steric bulk inhibits the group from participating in nucleophilic addition, such as in a Williamson ether synthesis or related S_N2 reactions.

Substrates that are deprotonated by potassium ''t''-butoxide include terminal acetylenes and active methylene compounds. It is useful in dehydrohalogenation reactions. Illustrating the latter behavior, potassium ''tert''-butoxide reacts with chloroform yielding dichlorocarbene, which is useful for dichloro cyclopropanations. Potassium ''tert''-butoxide can abstract a beta-proton from alkylammonium cations, leading to the Hofmann product via an elimination reaction.

Other reactions

Potassium ''tert''-butoxide catalyzes the reaction of hydrosilanes and heterocyclic compounds to give the silyl derivatives, with release of H₂.

Safety

Potassium ''tert''-butoxide is a very strong base that rapidly attacks living tissue.

Potassium ''tert''-butoxide forms explosive mixtures when treated with dichloromethane.

Related compounds

* Sodium tert-butoxide
* Lithium tert-butoxide

References

{{Reflist

Alkoxides
Reagents for organic chemistry
Non-nucleophilic bases
Tert-butyl compounds
Potassium compounds