Bases. potassium tert-butoxide

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

Yet another base, potassium tert-butoxide, induces oc-elimination of dichlorofluoromethanc to form 1-chioro-l-fluorocyclopropanes with alkenes.5... 

A mild base such as diisopropylethylamine or DBU can be used for the Homer-Wads worth-Emmons reaction in the presence of a complexing agent such as lithium chloride. These conditions are useful for sensitive substrates, such as chiral compounds with an enolizable stereocentre. For example, the enone 66 is formed in high optical purity under these conditions (2.81), whereas use of the base potassium tert-butoxide gives the product as a racemic mixture. 

Sterically hindered bases that are poor nucleophiles are called nonnucleophilic bases. Potassium tert-butoxide [K "OC(CH3)3] is a strong, nonnucleophiUc base. 

In situ generated NHC-Cu-TEMPO complex 34 [eqn (12.3)] was explored in the aerobic oxidation of primary alcohols to the corresponding aldehydes. " Surprisingly, the addition of a base (potassium tert-butoxide, triethylamine) had a detrimental effect on the catalytic reaction using 34 as catalyst. This fact contrasted with other reported Cu-N-based ligands/TEMPO catalytic systems, in which the addition of base favored the reaction. The reaction proceeded more efficiently in chlorobenzene than in other solvents. Unexpectedly, [(NHC)CuX] complexes, prepared in situ from Cu powder and the corresponding imidazolium salt, or their simple combination with TEMPO, were completely inactive in the oxidation of alcohols. It was proposed that TEMPO anchored to Cu-NHC complexes facilitated the intramolecular proton abstraction, promoting the oxidation of alcohols. However, the mechanism was not further explored, and the relation between structure and activity of the catalyst remained unclear. 

Similarly sodium methoxide (NaOCHj) is a suitable base and is used m methyl alco hoi Potassium hydroxide m ethyl alcohol is another base-solvent combination often employed m the dehydrohalogenation of alkyl halides Potassium tert butoxide [K0C(CH3)3] is the preferred base when the alkyl halide is primary it is used m either tert butyl alcohol or dimethyl sulfoxide as solvent... 

If however the base itself is a crowded one such as potassium tert butoxide even pn mary alkyl halides undergo elimination rather than substitution... 

Dihalocarbenes are formed when trihalomethanes are treated with a strong base such as potassium tert butoxide The trihalomethyl anion produced on proton abstraction dissociates to a dihalocarbene and a halide anion... 

The first synthesis of a 3//-3-benzazepine, e.g. 65 (R1 = R2 = Me), was achieved by the condensation of phthalaldehyde with a bis[(alkoxycarbonyl)methyl]methylamine.24"25 With sodium methoxide as the base, A%V-bis[(methoxycarbonyl)methyl]pheiiylaniine condenses with the dialdehyde in a similar manner to give dimethyl 3-phenyl-3//-3-benzazepine-2,4-dicar-boxy late (65, Rl — Ph R2 — Me).99 However, replacement of methoxide by potassium tert-butoxide results in formation of 3-phenyl-3//-3-benzazepine-2,4-dicarboxylic acid (65, R1 = Ph R2 = H).25... 

Based on the use of 2 (R1 = H), metalated with two equivalents of potassium tert-butoxide in tetrahydrofuran, routes to (S)-phosphinotricin (79% ee) and glutamic acid (69% ee) were developed211. 

Various chiral crown ethers based on [1,1 -binaphthalene]-2.2 -diol, lactose or other chiral 1,2-di-ols were tested as catalyst in the addition of methyl benzcneacetate to methyl 2-propenoate using sodium amide or potassium tert-butoxide as the base. Some pertinent examples are given261 262 396. 

Trialkylboranes react rapidly and in high yields with a-halo ketones,a-halo esters, a-halo nitriles, and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides) in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives. Potassium tert-butoxide is often a suitable base, but potassium 2,6-di-tert-butylphenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two tert-buXy groups prevents the base from coordinating with the R3B. The trialkylboranes are prepared by treatment of 3 mol of an alkene with 1 mol of BH3 (15-16). With appropriate boranes, the R group transferred to a-halo ketones, nitriles, and esters can be vinylic, or (for a-halo ketones and esters) aryl. " °... 

However, thermolysis of the phosphonium salts (X=+PPh3) leads directly to the indolic products without need of acid catalyst or PPh3, and thus may not proceed via a normal Wittig pathway. Alternatively, Hughes has effected a solid-phase version of this reaction employing a polymer-hound phosphonium salt and potassium tert-butoxide as base <96TL7595>. In this case, the phosphine oxide by-product remains bound to the polymer resin. 

A bulky base such as potassium tert-butoxide in tert-butyl alcohol favors the formation of the less substituted alkene in dehydrohalgenation reactions. 

In 2002, Leadbeater and Torenius reported the base-catalyzed Michael addition of methyl acrylate to imidazole using ionic liquid-doped toluene as a reaction medium (Scheme 6.133 a) [190], A 75% product yield was obtained after 5 min of microwave irradiation at 200 °C employing equimolar amounts of Michael acceptor/donor and triethylamine base. As for the Diels-Alder reaction studied by the same group (see Scheme 6.91), l-(2-propyl)-3-methylimidazolium hexafluorophosphate (pmimPF6) was the ionic liquid utilized (see Table 4.3). Related microwave-promoted Michael additions studied by Jennings and coworkers involving indoles as heterocyclic amines are shown in Schemes 6.133 b [230] and 6.133 c [268], Here, either lithium bis(trimethylsilyl)amide (LiHMDS) or potassium tert-butoxide (KOtBu) was em-... 

The same group recently disclosed a related free radical process, namely an efficient one-pot sequence comprising a homolytic aromatic substitution followed by an ionic Homer-Wadsworth-Emmons olefination, for the production of a small library of a,/3-unsaturated oxindoles (Scheme 6.164) [311]. Suitable TEMPO-derived alkoxy-amine precursors were exposed to microwave irradiation in N,N-dimethylformam-ide for 2 min to generate an oxindole intermediate via a radical reaction pathway (intramolecular homolytic aromatic substitution). After the addition of potassium tert-butoxide base (1.2 equivalents) and a suitable aromatic aldehyde (10-20 equivalents), the mixture was further exposed to microwave irradiation at 180 °C for 6 min to provide the a,jS-unsaturated oxindoles in moderate to high overall yields. A number of related oxindoles were also prepared via the same one-pot radical/ionic pathway (Scheme 6.164). 

Feuer and co-workers ° conducted extensive studies into alkaline nitration with nitrate esters, exploring the effect of base, time, stoichiometry, concentration, solvent, and temperature on yields and purity. Reactions are generally successful when the substrate a-proton acidity is in the 18-25 p A a range. Alkoxide bases derived from simple primary and secondary aliphatic alcohols are generally not considered compatible in reactions using alkyl nitrates. Optimum conditions for many of these reactions use potassium tert-butoxide and amyl nitrate in THF at —30 °C, although in many cases potassium amide in liquid ammonia at —33 °C works equally well. 

The classical procedure for the Wolff-Kishner reduction—i.e. the decomposition of the hydrazone in an autoclave at 200 °C— has been replaced almost completely by the modified procedure after Huang-Minion The isolation of the intermediate is not necessary with this variant instead the aldehyde or ketone is heated with excess hydrazine hydrate in diethyleneglycol as solvent and in the presence of alkali hydroxide for several hours under reflux. A further improvement of the reaction conditions is the use of potassium tert-butoxide as base and dimethyl sulfoxide (DMSO) as solvent the reaction can then proceed already at room temperature. ... 

The possibility of acetylating the pyrrole nitrogen atom of 9a was investigated. The reaction was performed in the presence of different bases, and the highest yield of the corresponding compound 9b was achieved for potassium tert-butoxide. As should be expected, even trace amounts of this compound are absent from the acylation products of thienopyrrole 9a under Friedel-Crafts conditions <20020L387>. 

When treated with a strong base such as butyllithium or potassium tert-butoxide, 2-isocyano-tV[(S)-l-phenylethyl]propanamide (1) forms an enolate 2 which is not alkylated at low temperatures. Instead it rearranges on warming and cyclizes to give the enolate of 3,5-dihydro-5-methyl-3-[(,3 )-1-phenylctbylJ-4//-imidazol-4-onc (3) which can be alkylated with benzylic halides with excellent diastereoselectivities4,13. 3-Halopropenes or haloalkanes give much lower diastereoselectivities. 

Similarly, the intramolecular alkylation of lithium 3-(4-camphorsuIfonyloxybutyl)-2,4,6-trimeth-ylphenolate using potassium tert-butoxide as base, leads under reflux to bicyclo[4.4.0]decadiene derivatives5,6. 

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