[7440-09-7] · K · Potassium · (MW 39.10)
(reducing agent for aromatic rings,1 carbonyl compounds,2 and various functional groups,3 including alcohols4 in the presence of crown ethers; reagent for the preparation of Rieke's metals;5 reacts with proton donors;6 promotes SRN1 arylation reactions;7 modified reactivity occurs with deposition on alumina,8 insertion in graphite,9 or dispersion by ultrasound10)
Physical Data: mp 64 °C; bp 760 °C; d 0.86 g cm-3; soft silvery (fresh cut) metal.
Solubility: sol liquid ammonia, 1,2-ethylenediamine, aniline; slightly sol ethers.
Form Supplied in: widely available as ingots, sticks, under mineral oil, or in ampules; common impurities are other alkali metals, aluminum, calcium, boron, silicon.
Handling, Storage, and Precautions: reacts violently with water, oxygen, halogens. In air, potassium forms the yellow peroxide KO2, which is dangerous in contact with organic compounds. Reacts with many solvents; inert in saturated and some aromatic hydrocarbons. For safe handling, see Fieser and Fieser.6 Potassium fires should be treated with inert powders or sand.
1 Birch- and Benkeser-type reductions of aromatic rings are achieved by potassium dissolved in liquid Ammonia or amines. However, potassium is used less often than Lithium or Sodium. The reducing ability of potassium is illustrated by the total reduction of the B-ring of equilenin to estrone, which cannot be achieved with sodium (eq 1).11
Proton donors are frequently used in potassium reductions. The accepted mechanism consists of a sequence of single-electron transfer (SET), a protonation, and a second SET. The anion thus formed can be trapped by an electrophile to give the reduction-alkylation product (eq 2).12
Alkali metal reduction of ketones to alcohols in the presence or absence of proton donors has been studied in detail.2 The use of potassium decreases the amount of pinacol coupling products,13 and increases the reaction rate and yields of desired product.14 The stereoselectivity, which depends on the structure of the intermediate ketyl radical anion, is frequently lower with K in comparison to lighter alkali metals. Recently, reductions with K have been conducted in THF under sonication with stereoselectivity similar to that of the usual method (eq 3).15
When an unsaturated functional group is closely positioned with respect to the carbonyl group, the ketyl radical anion adds to the multiple bond producing products of reductive cyclization (eq 4).16
In the case of a,b-unsaturated carbonyl groups,2a,17 the radical anion is readily transformed to the dianion, especially with K. When protonation occurs, an enolate results which can be trapped with an electrophile, giving rise to alkylation on the a-carbon. The stereoselectivity at the b-position does not seem to be strongly dependent on the nature of the metal, although exceptions are known (eq 5).18
In the absence of a proton source, the dianion can be alkylated on the b-carbon. The increased yields obtained in some cases with potassium result from the higher reactivity of the potassium dianion, as compared with reactions with lithium or sodium (eq 6).17,19
Various functional groups are reduced by potassium in liquid ammonia (eqs 7-9).3,20,21 In many cases the reaction is accomplished in the presence of crown ethers, which generate K+K- ion pairs.
Alcohol deoxygenation occurs4 via carboxylic, thiocarbamic (eq 10),22 and phosphoric esters (eq 11).23 Free benzylic alcohols can be reduced with potassium in the presence of 4,4´-di-t-butylbiphenyl in THF under sonication.24
Potassium reduces metal salts to finely dispersed metals in refluxing THF or DME (eq 12). Activated metal slurries can be prepared, which exhibit a high reactivity with organic compounds.5 The method was improved by replacing potassium with lithium in the presence of an electron transport agent.25
Potassium reacts with acidic hydrogen compounds such as alcohols. This reaction is used to prepare the useful potassium alkoxides (e.g. Potassium t-Butoxide).6 Some potassium amides, such as 3-aminopropanamide, are readily prepared in quantities up to 25 mmol from the metal and 1,3-propanediamine in the presence of a trace of iron(III) nitrate under sonication (eq 13).26
Deprotonation-alkylation of g-lactones has been reported. In the case of acylation, it occurs only on the carbon atom (eq 14).27
Substitution of aromatic halides, phosphate esters, trialkylammonium groups, and sulfides by an anionic nucleophile occurs with complete regioselectivity in the presence of potassium in liquid ammonia or DMSO (eq 15).7 These reactions are now more generally effected with a potassium alkoxide under photochemical activation.28
Potassium on Alumina is prepared by melting the metal in the presence of Al2O3. Potassium-Graphite is obtained by vaporizing the metal on graphite. Both reagents are sensitive to oxygen and moisture. Various reductive processes take place with alkenes, nitriles, or halides (eqs 16 and 17).8,29
Sonication of potassium metal in aromatic solvents (toluene, xylene) produces a silvery blue suspension. The reaction of this reagent with water is much milder than that of potassium metal. This suspension can behave as a base or as a single-electron transfer agent. The Dieckman cyclization of diethyl adipate (eq 18)10 and the dimerization of succinates (eq 19)30 occur readily at rt in excellent yield.
The carbon-sulfur bond in sulfolene is cleaved to a radical anion, which can be alkylated to provide open chain sulfones. The inconvenience of long irradiation times was solved by running the reaction in the presence of proton sources, water, or phenol (eq 20).31
Université Paul Sabatier de Toulouse, France