Potassium t-Heptoxide


[20484-37-1]  · C7H15KO  · Potassium t-Heptoxide  · (MW 154.32)

(strong, sterically hindered alkoxide base; particularly capable of effecting Hofmann elimination of alkyl halides1)

Alternate Name: potassium triethylmethoxide.

Solubility: sol THF, DME, aromatic hydrocarbons.

Preparative Methods: 3.0 M solution of the red reagent in Et3COH is prepared by reacting potassium metal with an excess of the anhydrous alcohol at 140 °C for 2.5 h.1 Solutions of the base of approximately 2.0 M concentration in xylene or undecane are obtained by heating potassium metal with 1.2 equiv of the alcohol at 160 °C for 6.0 h.1 The reaction of the anhydrous alcohol with a slight excess of oil-free Potassium Hydride in THF or other solvents produces the reagent in 3-15 min.2 After the excess insoluble KH settles, the solution of the base is removed via a cannula or a syringe.

Handling, Storage, and Precautions: Et3COK solutions should be handled with the same precautions that are employed for Potassium t-Butoxide solutions, i.e. avoid contact with the eyes, skin, and clothing. Conduct reactions under a dry nitrogen or argon atmosphere. For critical experiments, it is recommended that the reagent be freshly prepared. Use in a fume hood.

Formation of Potassium Enolates.

The basicity of tertiary potassium alkoxide bases increases as the bulk of the alkyl substituents increases. Therefore Et3COK is a stronger base than t-BuOK or Potassium 2-Methyl-2-butoxide.3 For example, the value of the equilibrium constant for the reaction of pinacolone with Et3COK in THF to form its potassium enolate and the alcohol is 57 (eq 1), while it is only 6.7 and 10.0 for t-BuOK and EtCMe2OK, respectively.3

b-Elimination Reactions.

Et3COK is a poor nucleophile, and its steric bulk and high basicity make it an excellent reagent for effecting Hofmann-type eliminations of alkyl halides.1 As indicated in eq 2, the reaction of 2-bromo-2-methylbutane with Et3COK gives a higher 1-alkene/2-alkene ratio than when either t-BuOK or EtCMe2OK are employed under the same conditions.1a Also, other tertiary halides such as 1-chloro-1-methylcyclohexane (eq 3), the hydrochloride of a-cedrene, and 2-chloro-2,3-dimethylbutane give the less-substituted alkenes in good yields when treated with an excess of the base in xylene solution at 60 °C.1b Because the three fluorine atoms enhance the acidity of the protons at C-3, dehydrohalogenation of 2-chloro-4,4,4-trifluoro-2-methylbutane with Et3COK in Et3COH at 40 °C gives largely 4,4,4-trifluoro-2-methyl-2-butene.4 However, the corresponding 1-butene derivative is obtained if a highly hindered pyridine derivative or a metal oxide is employed as the base.4 The reaction of 2-bromobutane with Et3COK/Et3COH also gives a higher 1-butene/2-butene ratio than does t-BuOK/t-BuOH (eq 4).5

Ramberg-Bäcklund Rearrangement.

Et3COK in DME/HMPA at 70 °C is effective for promoting the Ramberg-Bäcklund reaction of the complex a-chloro sulfone shown in eq 5.6 The use of less-hindered bases causes dechlorination of the reactant or partial decomposition of the product. This reaction is a key step in the recent total synthesis of the natural product (+)-eremantholide.6

Metalation Reactions.

Mixtures of alkyllithium reagents and potassium tertiary alkoxides are powerful metallating agents for weakly acidic compounds.7 Because of higher solubilities of Et3COK and EtCMe2OK in hydrocarbons, mixtures of these bases and n-Butyllithium are more effective than n-BuLi/t-BuOK in promoting dimetalation of benzene.8

1. (a) Brown, H. C.; Moritani, I.; Okamoto, Y. JACS 1956, 78, 2193. (b) Acharya, S. P.; Brown, H. C. CC 1968, 305.
2. Brown, C. A. S 1974, 427.
3. Brown, C. A. CC 1974, 680.
4. Nicholas, P. P. JOC 1992, 57, 2741.
5. Leone, S. A.; Davis, J. D. J. Chem. Educ. 1992, 69, A175.
6. Boeckman, R. K., Jr.; Yoon, S. K.; Heckendorn, D. K. JACS 1991, 113, 9682.
7. Schlosser, M. PAC 1988, 60, 1627.
8. Lochmann, L.; Fossatelli, M.; Brandsma, L. RTC 1990, 109, 529.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

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