Potassium 2-Methyl-2-butoxide

[41233-93-6]  · C5H11KO  · Potassium 2-Methyl-2-butoxide  · (MW 126.26)

(strong, sterically hindered alkoxide base)

Alternate Name: potassium t-pentoxide.

Form Supplied in: 1.5-2.0 M solution in benzene.

Solubility: sol THF, DME, aromatic hydrocarbons.

Preparative Methods: amber-colored 0.15-0.20 M EtCMe2OH solutions of the reagent are prepared in situ by refluxing oxide-free potassium metal in the anhydrous alcohol for 3-4 h.1 Solutions of the base which are 1.0-2.0 M in THF, benzene, or toluene are prepared by treating oil-free Potassium Hydride with an equivalent amount of anhydrous t-pentyl alcohol in the anhydrous solvent under an inert atmosphere.2

Handling, Storage, and Precautions: solutions should be handled with the same precautions that are used for solutions of t-BuOK (see Potassium t-Butoxide). Avoid contact with the eyes, skin, and clothing. Conduct all reactions under a dry nitrogen or argon atmosphere. In the absence of air and moisture, solutions of the base in benzene or toluene may be stored in a refrigerator for up to 1 year without significant deterioration.2b However, for critical experiments it is recommended that the reagent be freshly prepared. Use in a fume hood.

Enolate Formation and Alkylation.

The basicity of tertiary potassium alkoxides increases as the bulk of the alkyl substituents increases.2a For example, the equilibrium constant for the conversion of pinacolone into its enolate anion by EtCMe2OK in THF (eq 1) is 10, while it is 6.7 for the less bulky base t-BuOK, and 57 and 710, respectively, for the more bulky bases Et3COK and Cy3COK.2a Increased steric hindrance, which interferes with aggregation and solvation of the ion pair, presumably accounts for this.

Although potassium alkoxides can produce potassium enolates of most ketones in high concentrations, alkylation of these species is frequently a problem because of their tendency to undergo rapid proton transfer reactions which lead to polyalkylation, aldol condensation, and equilibration of structural isomers in unsymmetrical systems.3 However, monoalkylation products are obtained in satisfactory yields even with cyclopentanone, which is highly prone to side reactions, using EtCMe2OK in DME as the base and a large excess of a reactive alkylating agent such as Iodomethane or Benzyl Bromide with a very short reaction time (eq 2).4 The a-alkylation of a,b-unsaturated ketones such as 1-carvone is also accomplished in good yield using this base/solvent system and a reactive alkylating agent (eq 3).4 The weakly acidic enone 1,10-dimethyl-D1,9-2-octalone is converted into its linearly conjugated dienolate with EtCMe2OK in benzene. This species can be a-methylated with MeI in 93% yield;5 the yield is only 24% if t-BuOK is employed as the base.6

Wittig Reactions.

Potassium alkoxides are frequently used as bases in Wittig reactions because the potassium cation does not strongly associate with the oxyanion of the betaine intermediate, and thus decomposition of this species to the alkene is relatively rapid.7 The Wittig alkenation of sterically hindered 20-keto steroids such as pregnenolone with various triphenylalkylphosphoranes occurs in good yields using EtCMe2OK in benzene as the base/solvent system (eq 4).2b (E)-20(22)-Dehydro steroid derivatives are the exclusive products. These products are isolated in poor yields when EtCMe2ONa in benzene is used to generate the phosphorane from the phosphonium salt.8

Dehydrohalogenation Reactions.

As illustrated for the dehydrobromination of 2-bromo-2-methylbutane in eq (5), an increase in the steric hindrance of the potassium alkoxide base increases the 1-alkene/2-alkene ratio of the b-elimination products.9 EtCMe2OK gives a 1-alkene/2-alkene ratio which is intermediate between that of the less bulky t-BuOK and the more bulky Et3COK.9

Lethargic Reactions.

Highly hindered acetophenone derivatives are converted to their oximes by treatment with Hydroxylamine hydrochloride in the presence of 2.0 equiv of EtCMe2OK in EtCMe2OH at rt for 1 month or longer (eq 6).1b Efforts to force the reaction by using higher temperatures result in lower yields. The NHOH- anion is presumed to be the reactive species.1b

Rearrangement Reactions.

The reagent is used to extrude selenium from Se-acylmethyl selenocarboxylates to produce b-diketones (eq 7),10 and for rearranging nitronates into hydroxamates (eq 8).11

Metalation Reactions.

Mixtures of alkyllithium reagents and tertiary potassium alkoxides are powerful metalating agents for weakly acidic compounds.12 For example, phenylpotassium is produced from the combination of n-Butyllithium and EtCMe2OK in benzene.13

Related Reagents.

Potassium t-Butoxide; Potassium t-Heptoxide.

1. (a) FF 1967, 1, 905. (b) Pearson, D. E.; Keaton, O. D. JOC 1963, 28, 1557.
2. (a) Brown, C. A. CC 1974, 680. (b) Schow, S. R.; McMorris, T. C. JOC 1979, 44, 3760.
3. (a) Caine, D. In Carbon-Carbon Bond Formation; Augustine, R. L., Ed.; Dekker: New York, 1979; Vol. 1, Chapter 2. (b) Caine, D. COS 1991, 3, 1.
4. Edwards, H. N.; Wycpalek, A. F.; Corbin, N. C.; McChesney, J. D. SC 1978, 8, 563.
5. Mukherjee, S. L.; Dutta, P. C. JCS 1960, 67.
6. Yanagita, M.; Hirakura, M.; Seki, F. JOC 1958, 23, 841.
7. Fitjer, L.; Quabeck, U. SC 1985, 15, 855.
8. Schmit, J. P.; Piraux, M.; Pilette, J. F. JOC 1975, 40, 1586.
9. Brown, H. C.; Moritani, I.; Okamoto, Y. JACS 1956, 78, 2193.
10. Ishihara, H.; Hirabayashi, Y. CL 1978, 1007.
11. Denmark, S. E.; Dappen, M. S.; Cramer, C. J. JACS 1986, 108, 1306.
12. Schlosser, M. PAC 1988, 60, 1627.
13. Lochmann, L. CCC 1987, 52, 2710.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

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