Potassium tert-butoxide


Potassium tert-butoxide is the chemical compound with the formula K+3CO−. This colourless solid is a strong base, 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.

Structure

Potassium tert-butoxide crystallises from tetrahydrofuran/pentane at −20 °C as ∞, which consists of infinite one-dimensional chains linked by hydrogen bonding. Sublimation of ∞ affords the tetramer 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. 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 SN2 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, and additives that modify the cluster affect the reactivity of the reagent. For example, DMF, DMSO, hexamethylphosphoramide, 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 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.