Potassium tert-butoxide

Potassium ''tert''-butoxide is the chemical compound with the formula K⁺(CH₃)₃CO⁻. This colourless solid is a strong base (pKa of conjugate acid around 17), which is useful in organic synthesis. It exists as a tetrameric cubane-type cluster. It is often seen written in chemical literature as potassium ''t''-butoxide. The compound is often depicted as a salt, and it often behaves as such, but it is not ionized in solution.

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 sensitive and older samples are often of poor quality. 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 at 220 °C and 1 mmHg. Sublimation can also take place at 140 °C and 0.01 hPa. It is advisable to cover the raw material with glass wool, as potassium tert-butanolate tends to "bounce", so parts can be thrown up during the sublimation. The anhydrous removal using an inert sublimation apparatus is particularly advantageous.

Structure

Potassium ''tert''-butoxide 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. Mild 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.

Applications

The ''tert''-butoxide species is itself useful as a strong, non-nucleophilic base in organic chemistry.Drury Caine "Potassium t-Butoxide" in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons, New York, 2006. . Article Online Posting Date: September 15, 2006 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 an S_N2 reaction. Substrates that are deprotonated by potassium ''t''-butoxide include terminal acetylenes and active methylene compounds. It is useful in dehydrohalogenation reactions.

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

Modifications

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, enhancing the basicity of the butoxide. Schlosser's base, a mixture of the alkoxide and an alkyl lithium compound, is a related but stronger base.

Reactions

Potassium ''tert''-butoxide reacts with chloroform yielding dichlorocarbene, the reaction can result in ignition. Potassium ''tert''-butoxide should never be added to dichloromethane, as the reaction of 1,5g of potassium ''tert''-butoxide with drops of dichloromethane can result in ignition over 2min.

As a base, potassium tert-butoxide can extract a beta-proton and form the Hofmann product via an elimination reaction. This reaction has a high synthetic value as it can set up further reactions of the resultant alkene, especially regiochemical reactions.

Related compounds

* Sodium tert-butoxide
* Lithium tert-butoxide

References

{{Reflist

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