Potassium Naphthalenide

[4216-48-2]  · C10H8K  · Potassium Naphthalenide  · (MW 167.28)

(functions as a base in the a-alkylation of g- and d-lactones;1 deprotonates b-lactones to provide a,b-unsaturated carboxylic acids and esters;1 reductively cleaves O,S-acetals2 and alkyl halides3,4)

Solubility: sol THF, DME.

Analysis of Reagent Purity: 39K NMR (0.2 M THF, 20 °C) 20 ppm.

Preparative Methods: a 1.0 M solution of the reagent is prepared by adding Potassium metal (1.0 equiv) to a solution of naphthalene (1.0 equiv) in THF (or DME) under an argon atmosphere. The mixture is stirred until all of the potassium metal has dissolved (ca. 2 h). The resulting solution exhibits the characteristic dark green color of the radical anion.5 Potassium is an extremely flammable solid and should always be handled with care under an argon atmosphere.

Handling, Storage, and Precautions: both air and moisture sensitive. Stock solutions of the reagent are flammable and should be handled under an inert atmosphere.

Use as a Base.

Potassium naphthalenide acts as an efficient base in the a-alkylation of g- and d-lactones (eqs 1 and 2).1 Lactone enolates prepared from alternate bases (e.g. Sodium Methoxide, Lithium Diisopropylamide) undergo competitive self-condensation and dialkylation reactions.6

b-Lactones fragment to a,b-unsaturated carboxylate anions when reacted with potassium naphthalenide/18-Crown-6. a,b-Unsaturated carboxylic acids or esters are readily prepared by subsequent protonation or alkylation of the carboxylate anions (eq 3). Products of (E)-alkene configuration are formed exclusively.

Use as a Reducing Agent.

a-Alkoxy potassium compounds are generated by the reductive cleavage of O,S-acetals with potassium naphthalenide. The O,S-acetals derived from allylic alcohols undergo reduction followed by [2,3]-Wittig rearrangement.2 Homoallylic alcohols of preferential (Z)-alkene configuration are prepared in this manner (eq 4). K-, Li-, Cs-, and Rb-naphthalenides are equally effective at initiating the [2,3]-Wittig rearrangement. Reduced yields occur with Sodium Naphthalenide.

Cyclopropyl halides react with potassium naphthalenide to form dehalogenated products (eq 5).3 The gem-dihalocyclopropane in eq 6 undergoes skeletal rearrangement with potassium naphthalenide, while the parent hydrocarbon is formed upon treatment with Sodium-Ammonia.4

Related Reagents.

Copper(I) Iodide-Potassium Naphthalenide.

1. Kowalczuk, M.; Kurcok, P.; Glówkowski, W.; Jedlinski, Z. JOC 1992, 57, 389.
2. Kruse, B.; Brückner, R. TL 1990, 31, 4425.
3. Boche, G.; Schneider, D. R.; Wintermayr, H. JACS 1980, 102, 5697.
4. Oku, A.; Tsuji, H.; Yoshida, M.; Yoshiura, N. JACS 1981, 103, 1244.
5. Keys, B. A.; Elliel, E. L.; Juaristi, E. Isr. J. Chem. 1989, 29, 171.
6. (a) Posner, G. H.; Loomis, G. L. CC 1972, 892. (b) Curtis, O. E., Jr.; Sandri, J. M.; Crocker, R. E.; Hart, H. OSC 1963, 4, 278.

Bryon A. Merrill,

3M Pharmaceuticals, St. Paul, MN, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.