[11135-81-2] · KNa · Sodium-Potassium Alloy · (MW 62.09)
(preparation of organopotassium compounds;1 in combination with TMSCl, greatly enhances acyloin condensations;2 dehalogenation of dihalides;3 preparation of g, d-unsaturated alcohols via fragmentation of g-halo ketones;4 in combination with crown ethers is a useful reducing agent for alkynes5,6 and alkyl fluorides7,8)
Physical Data: mp -11 °C.
Form Supplied in: available in two alloy forms: 56% K, 44% Na; 78% K, 22% Na.
Preparative Method: can be prepared by mixing 1 part Sodium and 5 parts Potassium (by weight) in refluxing xylene until melted. The melt is then carefully mixed with a glass stirring rod, keeping the alloy in one large globule. Upon cooling to room temperature, the alloy will remain liquid and can easily be transferred via a glass pipet.1
Handling, Storage, and Precautions: flammable solid; handle under inert atmosphere.
Potassium derivatives can be generated from a number of organic substrates via the action of Na-K alloy (eqs 1-3).1 The generated organopotassium adducts are soluble and stable in anhydrous diethyl ether. Similarly, the treatment of either organosodium or organolithium compounds with Na-K alloy results in the more reactive organopotassium species.
The combination of Chlorotrimethylsilane (TMSCl) with Na-K alloy greatly facilitates the acyloin condensation.2 This action results in smooth conversion of diesters to disiloxenes (eqs 4 and 5). Sodium metal can also be used, with similar results, to facilitate the acyloin condensation.
Sodium-potassium alloy is also useful in the dehalogenation of dihalides. Sodium-potassium alloy in ethanol reacts with 1,4-dibromobicyclo[2.2.2]octane to give the Grob fragmentation product, 1,4-dimethylenecyclohexane (eq 6). However, the use of an inert solvent such as cyclohexane affords bicyclo[2.2.2]octane in modest yield (eq 7).3
The reaction of g-halo ketones with Na-K alloy in anhydrous ether gives rise to products where 1,4-reductive elimination of halide proceeds with cleavage of the 2,3 carbon-carbon bond.4 Under these conditions, 1-chloroadamantan-4-one is converted to 7-methylbicyclo[3.3.1]nonan-2-ol (eq 8).
The dissolving metal reduction of alkynes typically involves an alkali metal in liquid ammonia.5 However, it has been demonstrated that dissolving metal reductions utilizing t-butyl alcohol and Na-K alloy in THF with catalytic amounts of 18-Crown-6 proceed smoothly.6 The solubility of alkali metals in THF can be greatly increased with the aid of an appropriate crown ether or cryptate. Thus 4-octyne can be reduced to the corresponding octene (3:1 mixture of trans:cis) (eq 9).
The reductive cleavage of unactivated C-F bonds is a difficult transformation in organic synthesis. The carbon-fluorine bond is known to strongly resist the usual reductive conditions.7 However, the use of sodium-potassium alloy and Dicyclohexano-18-crown-6 in diglyme is effective in the reductive cleavage of unactivated carbon-fluorine bonds.8 Thus 3b-fluorocholest-5-ene is successfully reduced to cholest-5-ene in high yield (eq 10).
3M Pharmaceuticals, St. Paul, MN, USA