Potassium t-Butoxide

t-BuOK

[865-47-4]  · C4H9KO  · Potassium t-Butoxide  · (MW 112.23)

(strong alkoxide base capable of deprotonating many carbon and other Brjnsted acids; relatively poor nucleophile1)

Physical Data: mp 256-258 °C (dec).

Solubility: sol/100g solvent at 25-26 °C: hexane 0.27 g, toluene 2.27 g, ether 4.34 g, t-BuOH 17.80 g, THF 25.00 g.

Form Supplied in: white, hygroscopic powder; widely available commercially; also available as a 1.0 M solution in THF.

Preparative Methods: sublimation (220 °C/1 mmHg; 180 °C/0.05 mmHg) of the commercial material prior to use is recommended. For critical experiments, the reagent should be freshly prepared prior to use. Solutions of the reagent in t-BuOH may be prepared by reaction of the anhydrous alcohol with Potassium under nitrogen,2 or the solvent may be removed (finally at 150 °C/0.1-1.0 mmHg for 2 h) and solutions prepared using other solvents.3

Handling, Storage, and Precautions: do not breathe dust; avoid contact with eyes, skin, and clothing. Reacts with water, oxygen, and carbon dioxide, and may ignite on exposure to air at elevated temperatures. Handle in an inert atmosphere box or bag and conduct reactions under an inert atmosphere in a fume hood. Store in small lots in sealed containers under nitrogen.

Introduction.

Potassium t-butoxide is intermediate in power among the bases which are commonly employed in modern organic synthesis. It is a stronger base than the alkali metal hydroxides and primary and secondary alkali metal alkoxides,1 but it is a weaker base than the alkali metal amides and their alkyl derivatives, e.g. the versatile strong base Lithium Diisopropylamide.4 The continued popularity of t-BuOK results from its commercial availability and the fact that its base strength is highly dependent on the choice of reaction solvent. It is strongly basic in DMSO, where it exists primarily as ligand-separated ion pairs and dissociated ions, but its strength is significantly decreased in solvents such as benzene, THF, and DME, where its state of aggregation is largely tetrameric. DMSO is able to enhance the basicity of the t-butoxide anion by selectively complexing with the potassium cation. Other additives, such as the dipolar aprotic solvent Hexamethylphosphoric Triamide (HMPA) and 18-Crown-6, have a similar effect. This section will provide cursory coverage of the reactions of t-BuOK in t-BuOH and in relatively nonpolar aprotic solvents. The unique features of Potassium t-Butoxide-Dimethyl Sulfoxide, Potassium t-Butoxide-Hexamethylphosphoric Triamide, and Potassium t-Butoxide-18-Crown-6, as well as those of the Potassium t-Butoxide-t-Butyl Alcohol Complex (1:1), are described elsewhere in this encyclopedia.

Alkylations.

Many bases which are weaker than t-BuOK are capable of essentially quantitative conversion of active methylene compounds into the corresponding enolates or other anions.5 However, the alkylation of diethyl malonate with a bicyclic secondary tosylate (eq 1)6 and the alkylation of ethyl n-butylacetoacetate with n-BuI (eq 2)7 provide examples of cases where the use of t-BuOK in t-BuOH is very effective. In the latter reaction, cleavage of the product via a retro-Claisen reaction is minimized with the sterically hindered base and yields obtained are higher than when Sodium Ethoxide or EtOK in EtOH, Sodium in dioxane or toluene, or Sodium Hydride in toluene are used for the enolate formation.

Potassium t-butoxide in t-BuOH or ethereal solvents is not capable of effecting quantitative formation of enolates of unactivated saturated ketones;8 also, because potassium enolates are subject to rapid proton transfer reactions, their intermolecular alkylations are complicated by equilibration of structurally isomeric enolates, polyalkylation, and aldol condensation reactions.9 Thus reactions of preformed lithium enolates (generated by deprotonation of ketones with LDA or by indirect procedures) with alkylating agents provide the method of choice for regioselective alkylation of unsymmetrical ketones.9,10 However, as illustrated in eqs 3 and 4,11,12 ketones capable of forming only a single enolate, such as symmetrical cyclic ketones and those containing a-methylene blocking groups, are readily alkylated via t-BuOK-promoted reactions. Also, t-BuOK in t-BuOH or benzene frequently has been employed for the alkylation of a,a-disubstituted aldehydes.9

As illustrated in eq 5, a,b-unsaturated ketones undergo a,a-dialkylation when treated with excess t-BuOK/t-BuOH and an alkylating agent.13 The a,a-dimethylated b,g-unsaturated ketone is formed by conversion of the initially produced a-methylated b,g-unsaturated ketone to its dienolate, which undergoes a second methylation faster than the b,g-double bond is isomerized to the a,b-position.14

In contrast to intermolecular processes, intramolecular alkylations are frequently performed with t-BuOK in various solvents.19 In eqs 6 and 7 are shown examples of endo- and exo-cycloalkylations of cyclic saturated ketones which lead to new five-membered rings.15,16

An interesting example of how a change in the base can influence the course of a cycloalkylation reaction is shown in eq 8.17 Since the reaction with t-BuOK involves equilibrating conditions, exo-cycloalkylation occurs via the more-substituted enolate, which is more thermodynamically stable. On the other hand, when LDA is used as the base, endo-cycloalkylation occurs via the kinetically formed terminal enolate.

Intramolecular alkylations of a,b-unsaturated ketones may occur at the a-, a-, or g-positions depending upon the nature of the base, the leaving group, and other structural features. A recent example involving a-cycloalkylation using t-BuOK is shown in eq 9.18

The reaction of a-halo esters, ketones, nitriles, and related compounds with appropriate organoboranes in the presence of t-BuOK can lead to replacement of the halogen with an alkyl or an aryl group.19 An example of this reaction using an a-bromo ester is shown in eq 10.20 THF is a more effective solvent than t-BuOH for alkylations of a-bromo ketones using this methodology.21 Potassium 2,6-di-t-butylphenoxide, a mild, sterically hindered base, is much more effective than t-BuOK for the alkylation of highly reactive a-halo ketones, e.g. bromoacetone, and a-halo nitriles.22

Condensation Reactions.

Traditionally, intermolecular aldol condensation reactions have been performed under equilibrating conditions using weaker bases than t-BuOK in protic solvents.23 Since the mid-1970s, new methodology has focused on directed aldol condensations which involve the use of preformed Lithium and Group 2 enolates,24 Group 13 enolates,25 and transition metal enolates.26 Although examples of the use of t-BuOK in intramolecular aldol condensations are limited, complex diketones (eq 11)27 and keto aldehydes (eq 12)28 have been cyclized with this base.

t-BuOK is the most commonly used base for the Darzens condensation of an a-halo ester with a ketone or an aromatic aldehyde to yield an a,b-epoxy or glycidic ester.29 In the example shown in eq 13,30 the aldol step is reversible and the epoxide ring closure step is rate limiting. This leads to the product with the ester group and the bulky b-substituent trans. However, in other systems the opposite stereochemical result often occurs because the aldol condensation step is rate limiting. In addition to esters, a wide variety of a-halo compounds that contain electron-withdrawing groups may participate in these types of reaction.29c

The Dieckmann cyclization of diesters and related reactions has found an enormous amount of use in the synthesis of five-, six-, seven-, and even larger-membered rings.31 In unsymmetrical systems, steric effects and the stability of the product enolates determine the regiochemistry of the reaction. As shown in eqs 14 and 15,32,33 t-BuOK is an effective base for these reactions when used in t-BuOH or other solvents such as benzene. In the former example, 40% of unchanged starting material is recovered when MeONa/PhH is used to effect the cyclization.

The nature of the base can profoundly influence the regiochemistry of the reaction. t-BuOK favors kinetic control in the reaction shown in eq 16 and the product derived from cyclization of the enolate having a b-amino group is obtained. However, when EtONa/EtOH is employed, the more stable b-keto ester enolate resulting from thermodynamic control is obtained.34 In addition to diesters, dinitriles, ε-keto esters, ε-cyano esters, ε-sulfinyl esters, and ε-phosphonium esters may participate in these reactions.31

t-BuOK is a better base than EtONa for the Stobbe condensation (eq 17) because yields are higher, reaction times are shorter, and frequent side reactions of ketone or aldehyde reduction are avoided.35

Although the use of t-BuOK for the generation of unstabilized ylides from phosphonium salts has been rare, it is the base of choice for the generation of Methylenetriphenylphosphorane for the Wittig reaction of hindered ketones (eq 18).36 No 1,1-di-t-butylethylene is obtained from di-t-butyl ketone when NaH/DMSO is used to generate the ylide.37

Elimination Reactions.

t-BuOK is a widely used base for both a- and b-elimination reactions. It is the most effective base in the conventional alkoxide-haloform reaction for the generation of dihalocarbenes.38 This procedure still finds general use (eq 19),39 but since it requires anhydrous conditions, it has been replaced to a degree by use of phase-transfer catalysts.40 Vinylidene carbenes have also been produced from the reaction of a-halo allenes with t-BuOK.41

Substrates containing a host of leaving groups, such as alkyl chlorides (eqs 20 and 21),42,43 bromides (eqs 22-24),44-46 tosylates, mesylates (eq 25),47 and even sulfenates and sulfinates (eq 26),48 undergo b-eliminations with t-BuOK in the solid phase or in t-BuOH or various nonpolar solvents.

The regiochemical and stereochemical results of b-eliminations of 2-substituted acyclic alkanes with t-BuOK in various solvents have been extensively investigated.49 Space does not permit the details of these investigations to be presented, but a few brief generalizations may be noted. (1) t-BuOK normally gives more of the terminal alkene than primary alkoxide bases but less than extremely bulky bases such as potassium tricyclohexylmethoxide. (2) A greater proportion of the terminal alkene is produced in solvents such as t-BuOH, where the base is highly aggregated, than in DMSO where it is substantially monomeric and partially dissociated. (3) The lower the state of aggregation of the base, the higher the trans:cis ratio of the disubstituted 2-alkene. (4) As base aggregation increases, the syn:anti elimination ratio increases. As far as the trans:cis ratio is concerned, cyclic halides give the opposite results from open-chain systems, i.e. this ratio is higher when base aggregation is greater (eq 27).50

Fragmentation Reactions.

Because it is a relatively strong base and a relatively weak nucleophile, t-BuOK has been the most popular base for effecting Grob-type fragmentations of cyclic 1,3-diol derivatives.51 In t-BuOH or nonpolar solvents, sufficiently high concentrations of alkoxide ions of 1,3-diol derivatives are produced for fragmentations to proceed smoothly if relatively good leaving groups are present (eq 28).52 When relatively poor leaving groups, e.g. sulfinates, are involved, it is necessary to increase the strength of t-BuOK by the addition of a dipolar aprotic solvent (eq 29).53 Other base-solvent combinations which may offer advantages over t-BuOK/t-BuOH for certain fragmentation substrates include NaH/THF, DMSO- Na+/DMSO, and LAH/ether.51

Isomerizations of Unsaturated Compounds.

t-BuOK is an effective base for bringing about migrations of double bonds in alkenes and alkynes via carbanion intermediates,1 but since the base promotes these reactions most effectively in DMSO, they will be described in more detail under Potassium t-Butoxide-Dimethyl Sulfoxide. Important examples of enone deconjugations with t-BuOK/t-BuOH which proceed via di- and trienolate intermediates are shown in eqs 30 and 31.54,55 Potassium t-pentoxide is effective in promoting the latter reaction, but various lithium amide bases are not, apparently because they deprotonate the enone at the a-position regioselectively. The isomerization of a,b-unsaturated imines to alkenyl imines (eq 32) is an important step in an alternative method for reduction-alkylation of a,b-unsaturated ketones.56

Ketone Cleavage Reactions.

t-BuOK in ether containing water is the medium of choice for the cleavage of nonenolizable ketones such as nortricyclanone (eq 33).57 Optically active tertiary a-phenyl phenyl ketones are cleaved in fair yields with high retention of configuration with the base in t-BuOH or PhH (eq 34).58 t-BuOK is more effective than t-BuONa or Lithium t-Butoxide in these reactions.

Michael Additions.

t-BuOK is one of an arsenal of bases that can be used in Michael addition reactions.59 A catalyst prepared by impregnation of xonotlite with the base promotes the Michael addition of the b-diketone dimedone to 2 mol of MVK (eq 35).60 Unsymmetrical ketones like 2-methylcyclohexanone yield Michael adducts primarily at the more substituted a-carbon atom (eq 36).8a Active methylene compounds undergo double Michael additions to enynes in the presence of t-BuOK (eq 37).61 This type of reaction has been used in the total synthesis of griseofulvin. A tricyclo[5.3.1.01,5]undecanedione, a precursor of (±)-cedrene, is available by a highly regioselective intramolecular Michael addition using t-BuOK (eq 38).62

Oxidation Reactions.

The use of t-BuOK to convert organic substrates to carbanionic species which react with molecular oxygen via a radical process has been reviewed.1 Ketones and esters are the most common substrates for these reactions. Oxidations of unsymmetrical ketones occur via the more thermodynamically stable potassium enolates; tetrasubstituted enolates yield stable hydroperoxides, while hydroperoxides derived from trisubstituted enolates are further oxidized to a-diketones or their enol forms.9a

Rearrangement Reactions.

Benzil is converted into the ester of benzylic acid in high yield upon treatment with t-BuOK in t-BuOH/PhH (eq 39).63 Lower yields are obtained if the individual solvents alone are used or if the base is replaced by MeONa or EtONa.

Monotosylates of cis-1,2-diols, such as that derived from a-pinene, undergo pinacol-type rearrangements upon treatment with t-BuOK (eq 40).64 The trans isomer is converted to the epoxide under the same conditions (eq 41).64 4-Benzoyloxycyclohexanone undergoes a mechanistically interesting rearrangement to benzoylcyclopropanepropionic acid when treated with t-BuOK/t-BuOH (eq 42).65

The Ramberg-Bäcklund rearrangement of a-halo sulfones is frequently carried out with t-BuOK.66 This reaction provides a useful route to deuterium-labeled alkenes (eq 43).67 t-BuOK is presumably the active base in a modification which involves the direct conversion of a sulfone into an alkene with KOH and a mixture of t-BuOH/CCl4.68 The base converts the sulfone to the a-sulfonyl carbanion, which undergoes chlorination with CCl4 by a single-electron transfer process. Proton abstraction from the a-position of the a-chloro ketone by the base yields a thiirane 1,1-dioxide which loses SO2 to yield the alkene. t-BuOK is frequently used directly in this modified procedure (eq 44).69 Cyclic enediynes are available from the corresponding a-chloro sulfones using t-BuOK to effect the Ramberg-Bäcklund rearrangement.70

Bromomethylenecyclobutanes undergo ring enlargement reactions to 1-bromocyclopentenes when heated in the presence of solid t-BuOK (eq 45).71

Reduction Reactions.

A modification of the Wolff-Kishner reduction, which is particularly useful for the reduction of a,b-unsaturated ketones, involves the reaction of carbonyl hydrazones and semicarbazones with t-BuOK in PhMe at reflux.72 The reduction of (±)-3-oxo-a-cadinol to (±)-a-cadinol and its isomer (eq 46) provides an example of this method.73

Related Reagents.

n-Butyllithium-Potassium t-Butoxide; Potassium Amide; Potassium Hexamethyldisilazide; Potassium t-Butoxide-Benzophenone; Potassium t-Butoxide-t-Butyl Alcohol Complex; n-Butyllithium-Potassium t-Butoxide; Potassium t-Butoxide-18-Crown-6; Potassium t-Butoxide-Dimethyl Sulfoxide; Potassium t-Butoxide-Hexamethylphosphoric Triamide; Potassium Diisopropylamide; Potassium t-Heptoxide; Potassium Hydroxide; Potassium 2-Methyl-2-butoxide.


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Drury Caine

The University of Alabama, Tuscaloosa, AL, USA



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