Potassium Methoxide-Dimethyl Sulfoxide


[865-33-8]  · CH3KO  · Potassium Methoxide-Dimethyl Sulfoxide  · (MW 70.14) (DMSO)

[67-68-5]  · C2H6OS  · Potassium Methoxide-Dimethyl Sulfoxide  · (MW 78.15)

(highly basic reagent; used occasionally for deprotonation of weakly acidic carbon acids, b-elimination, and alkene isomerization reactions2)

Physical Data: prepared in situ.

Form Supplied in: MeOK: commercially available in 95% purity; white powder. DMSO: commercially available (see Dimethyl Sulfoxide).

Preparative Methods: prepared from anhydrous MeOK and anhydrous DMSO,2b in much the same manner as Potassium t-Butoxide-Dimethyl Sulfoxide.5

Handling, Storage, and Precautions: MeOK: solid is flammable, corrosive, and hygroscopic.1 Use the same precautions as for t-BuOK/DMSO. Use in a fume hood.

Deprotonation Reactions.

MeOK in DMSO solution forms a highly basic and nucleophilic medium, since the dipolar aprotic solvent strongly coordinates with potassium cations producing activated, solvent-separated, and dissociated methoxide anions in a medium of high dielectric constant.2 Rate constants of base-catalyzed racemization and hydrogen isotope exchange reactions, as well as dehydrohalogenation and alkene isomerization reactions, indicate that MeOK/DMSO is a less basic medium than t-BuOK/DMSO. Apparently, the t-butoxide anion is not solvated as well as the methoxide ion in DMSO.2b In this dipolar, aprotic solvent it might have been expected that the order of basicity of the anions would be the same as in the gas phase, i.e. MeO- > t-BuO-.3 Solutions of t-BuOK in DMSO are known to contain dimsyl anions;4 this may have an influence on the basicity of the medium.2b

The rate of the MeOK-catalyzed racemization of (+)-2-methyl-3-phenylpropionitrile increases by 109 when the solvent is changed from MeOH to DMSO.5 In MeOH the methoxide ion has relatively lower activity because it is strongly hydrogen bonded by the solvent molecules. MeOK catalyzes the hydrogen-tritium exchange reaction between tritiated toluene and DMSO, but the reaction rate is ca. 250 times faster when t-BuOK is employed as the base.2b

Substitution and Elimination Reactions.

Upon treatment with alkoxide bases in 83.7 mol % DMSO/alcohol mixtures, 1-bromodecane gives mixtures of alkyl decyl ethers via substitution (SN2) reactions and 1-decene via an elimination (E2) reaction (eq 1).6 The E2/SN2 ratio is 0.14 for MeOK, but it is ca. 1.8 for t-BuOK; thus under these conditions the methoxide ion is a much better nucleophile than the t-butoxide ion. The addition of 18-Crown-6 to the above solution has very little effect on the overall reaction rate or the E2/SN2 ratio when MeOK is employed as the base. The same is true for KOH and EtOK. However, in the presence of the crown ether the reaction is ca. 3.6 times faster, and the E2/SN2 ratio increases slightly when t-BuOK is used as the base. From linear extrapolation of the Brønsted relation (ln k vs. H-), it has been suggested that at very high values of the acidity function (H-), where the anions would be largely desolvated, the order of basicity of alkoxide ions should be MeO- > EtO- > t-BuO-, i.e. the same as in the gas phase.6

Alkene Isomerization Reactions.

Both MeOK and t-BuOK effect the isomerization of 1-butene to cis-2-butene primarily in DMSO (eq 2).7,2b However, the rates are markedly dependent upon the anion; the reaction is ca. 125 times faster with t-BuOK than with MeOK. The stereoselectivity of the reaction is presumably controlled by the fact that the cis-allylic anion produced upon deprotonation of the alkene is more thermodynamically stable than its trans isomer.7

Related Reagents.

Potassium t-Butoxide-Dimethyl Sulfoxide; Potassium Methoxide-Dicyclohexano-18-crown-6; Sodium Ethoxide; Sodium Methoxide.

1. The Sigma-Aldrich Library of Chemical Safety Data, 2nd ed.; Lenga, R. E., Ed.; Sigma-Aldrich: Milwaukee, 1988; Vol. 2, p 2906.
2. (a) Cram, D. J. Fundamentals of Carbanion Chemistry; Academic: New York, 1965; Chapter 1 and pp 196-204. (b) Bank, S. JOC 1972, 37, 114.
3. Brauman, J. I.; Blair, L. K. JACS 1970, 92, 5986.
4. Brauman, J. I.; Bryson, J. A.; Kahl, D. C.; Nelson, N. J. JACS 1970, 92, 6679.
5. Cram, D. J.; Rickborn, B.; Kingsbury, C. A.; Haberfield, P. JACS 1961, 83, 3678.
6. Aksnes, G.; Stensland, P. ACS 1989, 43, 893.
7. Bank, S.; Schriesheim, A.; Rowe, C. A., Jr. JACS 1965, 87, 3244.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

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