Bases Potassium hydride

Reactions were performed at — 20 to 0 °C in a mixture of tetrahydrofuran and dichloromethane in the presence of 1.1 equivalents of the nucleophile with a catalytic amount of base (potassium hydride) and dibenzo-18-crown-6. High diastereoselectivities were achieved, as only the two rronj-diastereomers 15 and 16 were found after chromatographic workup. In subsequent reactions, these Michael adducts can readily be dehalogenated and converted to amino acids. 

Cycloalkanones Five- and six-membered cyclic ketones can also be prepared by reaction of methyl methylthiomethyl sulfoxide (1) and a 1, co-dihalo-( or ditosyloxy)alkane (2) in the presence of 2 eq. of base (potassium hydride or w-butyllithium) in THF at -70 to 20°. The product (3) is hydrolyzed to the ketone (4) by acid. Ogura et al. suggest that 2 eq. of base are required because... 

In 2006, Matsumoto and Tomioka developed a chiral symmetric N-heterocyclic earbene generated from a dihydroimidazolium salt and strong base (potassium hydride or /r-butyllithium) and applied these catalysts to asymmetric intramolecular Stetter reactions with up 80% ee. It is remarkable to note that high temperature (reflux in toluene) is crucial for the formation of cyclic products with relatively high enantioseleetivity. Racemization of the product was almost sufficiently suppressed by adjusting the ratio of base to dihydroimidazolium salt to 1 2. 

Potassium, Pubidium, and Cesium idjdrides. Although all the other alkah metal hydrides have been synthesized and some of the properties measured, only potassium hydride [7693-26-7] is commercially available. KH is manufactured in small amounts and sold as a mineral oil dispersion. It is a stronger base than NaH and is used to make the strong reducing agent KBH(C2H )2 and the super bases RNHK and ROK (6). 

Potassium Hydride. Potassium hydride [7693-26-7] KH, made from reaction of molten potassium metal with hydrogen at ca 200°C, is suppHed in an oil dispersion. Pressure Chemical Company (U.S.) is a principal suppHer. KH is much more effective than NaH or LiH for enolization reactions (63,64). Use of KH as a base and nucleophile has been reviewed (65). 

The next step of the Peterson olefination allows for the control of the E Z-ratio of the alkene to be formed by proper choice of the reaction conditions. Treatment of /3-hydroxysilanes 5 with a base such as sodium hydride or potassium hydride leads to preferential -elimination to give alkene 3a as major... 

Among the compounds capable of forming enolates, the alkylation of ketones has been most widely studied and applied synthetically. Similar reactions of esters, amides, and nitriles have also been developed. Alkylation of aldehyde enolates is not very common. One reason is that aldehydes are rapidly converted to aldol addition products by base. (See Chapter 2 for a discussion of this reaction.) Only when the enolate can be rapidly and quantitatively formed is aldol formation avoided. Success has been reported using potassium amide in liquid ammonia67 and potassium hydride in tetrahydrofuran.68 Alkylation via enamines or enamine anions provides a more general method for alkylation of aldehydes. These reactions are discussed in Section 1.3. 

In the presence of a very strong base, such as an alkyllithium, sodium or potassium hydride, sodium or potassium amide, or LDA, 1,3-dicarbonyl compounds can be converted to their dianions by two sequential deprotonations.79 For example, reaction of benzoylacetone with sodium amide leads first to the enolate generated by deprotonation at the more acidic methylene group between the two carbonyl groups. A second equivalent of base deprotonates the benzyl methylene group to give a dienediolate. 

The alkylation of benzene by alkylpotassium compounds has been reported by Bryce-Smith (S9) and is probably due to the increased base strength of organopotassium compounds over organosodium compounds. The potassium hydride eliminated in the cyclization reaction may add to ethylene to form ethylpotassium, which then may react with the aromatic to yield ethane and a benzylic carbanion [Reactions (16) and (17)]. 

The base employed by Brown and Yamashita was the potassium salt of 1,3-diaminopropane, prepared by reaction of potassium hydride with the solvent of the reaction, 1,3-diaminopropane. The reagent is very effective, and yields of isomerically pure products are high, but potassium hydride is hazardous, expensive and difficult to handle. 

M-H (metal hydride) Sodium hydride Potassium hydride Strong, not nucleophilic base Deprotonation of weak organic acids with acidities as high as pKa 25... 

Potassium hydride (KH) is a reactive base possessing a potassium cation and a hydrogen anion (hydride ion). The hydride ion reacts as any other base mentioned thus far and extracts acidic protons generating hydrogen gas and leaving behind anions with associated potassium cations. In this case, the dimethyl cyanomethyl-phosphonate anion, F, is formed. 

This principle can be extended to ketones whose enolates have less dramatic differences in stability. We said in Chapter 21 that, since enols and enolates are alkenes, the more substituents they carry the more stable they are. So, in principle, even additional alkyl groups can control enolate formation under thermodynamic control. Formation of the more stable enolate requires a mechanism for equilibration between the two enolates, and this must be proton transfer. If a proton source is available— and this can even be just excess ketone—an equilibrium mixture of the two enolates will form. The composition of this equilibium mixture depends very much on the ketone but, with 2-phenylcyclo-hexanone, conjugation ensures that only one enolate forms. The base is potassium hydride it s strong, but small, and can be used under conditions that permit enolate equilibration. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplanar transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward. 

Potassium enolates of aldehydes, Enolates of aldehydes are somewhat difficult to generate because of competing polymerization by base. They have been obtained recently in high yield by use of potassium hydride in THF at 0° and successfully alkylated, sulfenylated with diphenyl disulfide, and converted into o-iodo aldehydes by iodine. The last two reactions have not been observed previously. Sulfenylation of aldehydes has previously used indirectly generated lithium enolates and a reactive sulfenyl chloride. All three reactions are useful, however, for aldehydes with only one a-proton. Otherwise yields of monosubstituted aldehydes are low and largely by-products are obtained. 

Triphenylmethylpotassinm (Tritylpotassinm), (CeHslsCK. Mol. wt. 282.43. The base is prepared by reaction of triphenylmethane with potassium hydride. 

Alkylation of N-substituted trifluoroocetamides. N-alkyl or N-aryl trifluoro-acetamides can be alkylated by primary bromides or iodides by use of potassium hydride in combination with 18-crown-6 as base. This reaction is a useful route to secondary amines (equation I). ... 

Hydroxymethylaziridine 67 undergoes ring opening in the presence of either carbon- or heteroatom-based nucleophiles upon treatment with 2 equiv of potassium hydride to provide the t)7aminoalcohol derivative 69. The key step of the reaction is considered to be an aza-Payne rearrangement of the deprotonated aziridine methanol to the... 

---