Pages in category "Non-nucleophilic bases"
The following 21 pages are in this category, out of 21 total. This list may not reflect recent changes (learn more).
The following 21 pages are in this category, out of 21 total. This list may not reflect recent changes (learn more).
1. Lithium diisopropylamide – Lithium diisopropylamide is a chemical compound with the molecular formula 2NLi. It is used as a base and has been widely accepted due to its good solubility in non-polar organic solvents. It is a solid, but is usually generated and observed only in solution. LDA is commonly formed by treating a cooled solution of diisopropylamine with n-butyllithium. When dissociated, the anion can become protonated to form Diisopropylamine. Diisopropylamine has pKa value of 36, therefore, its conjugate base is suitable for the deprotonation of compounds with greater acidity. Importantly, such weakly acidic compounds carbon acids of the type R2CHZ, where Z = CR, CO2R, conventional protic functional groups such as alcohols and carboxylic acids are of course readily deprotonated. Like most organolithium reagents, LDA is not a salt, but is highly polar and it forms aggregates in solution, with the extent of aggregation depending on the nature of the solvent. In THF its structure is primarily that of a solvated dimer, in nonpolar solvents such as toluene, it forms a temperature-dependent oligomer equilibrium. At room temperature trimers and tetramers are the most likely structures, with increasing temperature the aggregation extends to pentameric and higher oligomeric structures. Solid LDA is pyrophoric, but its solutions are generally not, as such it is commercially available as a solution in polar aprotic solvents such as THF and ether, however for small scale use, it is common and more cost effective to prepare LDA in situ. The deprotonation of carbon acids can proceed with either kinetic or thermodynamic reaction control, kinetic controlled deprotonation requires a base that is sterically hindered. For example, in the case of phenylacetone, deprotonation can produce two different enolates, LDA has been shown to deprotonate the methyl group, which is the kinetic course of the deprotonation. A weaker base such as an alkoxide, which deprotonates the substrate. An alternative to the base is to use a strong base which is present at a lower concentration than the ketone. For instance, with a slurry of sodium hydride in THF or dimethylformamide, a ketone molecule might be deprotonated at the kinetic site. This enolate may then encounter other ketones and the thermodynamic enolate will form through the exchange of protons, LDA can, however, act as a nucleophile under certain conditions. Lithium amide Lithium bisamide Lithium tetramethylpiperidide
2. Lithium tetramethylpiperidide – Lithium tetramethylpiperidide is a chemical compound with the molecular formula C9H18LiN. It is used as a base, being comparable to LiHMDS in terms of pKa. It is synthesised by the deprotonation of 2,2,6, recent reports show that this reaction can also be performed 0 °C. The compound is stable in a THF/ethylbenzene solvent mixture and commercially available as such, like many lithium reagents it has a tendency to aggregate, forming a tetramer in the solid state
3. Lithium bis(trimethylsilyl)amide – Lithium bisamide is a lithiated organosilicon compound with the formula LiN2. It is commonly abbreviated as LiHMDS and is used as a strong non-nucleophilic base. Like many lithium reagents it has a tendency to aggregate and will form a trimer in the absence of coordinating species. LiHMDS is commercially available, but it can also be prepared by the deprotonation of bisamine with n-butyllithium and this reaction can be performed in situ. HN2 + C4H9Li → LiN2 + C4H10 Once formed, the compound can be purified by sublimation or distillation, LiHMDS is often used in organic chemistry as a strong non-nucleophilic base. It has a pKa of ~26 making it is less basic that other lithium bases, such as LDA and it can be used to form various organolithium compounds including acetylides, or lithium enolates. As such it finds use in a range of coupling reactions, particularly carbon-carbon bond forming reactions such as the Fráter–Seebach alkylation, LiHMDS can react with a wide range of metal halides, via a salt metathesis reaction, to give metal bisamides. The steric bulk of the ligands causes their complexes to be discrete and monomeric, having a built-in base, these compounds conveniently react with protic ligand precursors to give other metal complexes and hence are important precursors to more complex coordination compounds. LiHMDS is volatile and has been discussed for use for atomic layer deposition of lithium compounds, like many organolithium reagents, lithium bisamide can form aggregates in solution. The extent of aggregation depends on the solvent, in coordinating solvents such as ethers and amines the so-called monomer and dimer are prevalent. In the monomeric and dimeric state, one or two solvent molecules bind to lithium centers, with ammonia as donor base lithium bisamide forms a trisolvated monomer that is stabilized by intermolecular hydrogen bonds. In noncoordinating solvents, such as aromatics or pentane, the complex oligomers predominate, in the solid state structure is trimeric. Lithium amide Lithium diisopropylamide Lithium tetramethylpiperidide
4. 2,2,6,6-Tetramethylpiperidine – 2,2,6, 6-Tetramethylpiperidine or TMP or HTMP is an organic compound of the amine class. In appearance, it is a liquid and has a fishy. This amine is used in chemistry as a base because it can dissolve in organic solvents unlike inorganic bases such as potassium hydroxide. TMP is the material for an even stronger base lithium tetramethylpiperidide. There are many ways to synthesise TMP, one method starts with a conjugate addition reaction of ammonia to phorone. The intermediate triacetone amine is then reduced in a Wolff-Kishner reaction,2, 6-Dimethylpiperidine Pempidine DataSheet I DataSheet II TMP applied in synthesis in Organic Syntheses
5. Potassium tert-butoxide – Potassium tert-butoxide is the chemical compound with the formula K+3CO−. This colourless solid is a base which is useful in organic synthesis. It exists as a tetrameric cubane-type cluster and it is often seen written in chemical literature as potassium t-butoxide. It is prepared by the reaction of dry tert-butyl alcohol with potassium metal, the solid is obtained by evaporating these solutions followed by heating the solid. The solid can be purified by sublimation at 220 °C and 1 mmHg, potassium tert-butoxide crystallises from THF/pentane at −20 °C as ∞, which consists of infinite one-dimensional chains linked by hydrogen bonding. Sublimation of ∞ affords the tetramer 4, which adopts a cubane-like structure, mild Lewis basic solvents such as THF and diethyl ether do not break up the tetrameric structure, which persists in the solid, in solution and even in the gas phase. The tert-butoxide species is useful as a strong, non-nucleophilic base in organic chemistry. It is not as strong as amide bases, e. g. lithium diisopropylamide and its steric bulk inhibits the group from participating in nucleophilic addition, such as in a Williamson ether synthesis or an SN2 reaction. Substrates that are deprotonated by potassium t-butoxide include terminal acetylenes and active methylene compounds and it is useful in dehydrohalogenation reactions. Potassium tert-butoxide catalyzes the reaction of hydrosilanes and heterocyclic compounds to give the silyl derivatives, many modifications have been reported that influence the reactivity of this reagent. The compound adopts a complex structure, and additives that modify the cluster affect the reactivity of the reagent. For example, DMF, DMSO, hexamethylphosphoramide, and 18-crown-6 interact with the potassium center, schlossers base, a mixture of the alkoxide and an alkyl lithium compound, is a related but stronger base
6. Sodium bis(trimethylsilyl)amide – Sodium bisamide is the organosilicon compound with the formula 2NNa. This species, usually called NaHMDS, is a base used for deprotonation reactions or base-catalyzed reactions. NaHMDS is quickly destroyed by water to form sodium hydroxide and bisamine, although the N-Na bond is polar covalent as a solid, when dissolved in nonpolar solvents this compound is trimeric, consisting of a central Na3N3 ring. NaHMDS is used as a base in organic synthesis, typical reactions, To deprotonate ketones and esters to generate enolate derivatives. Generate carbenes by dehydrohalogenation of halocarbons and these carbene reagents add to alkenes to give substituted cyclopropanes and cyclopropenes. To deprotonation of phosphonium salts, generating Wittig reagents, NaHMDS is also used as a base to deprotonate other compounds containing weakly acidic O-H, S-H, and N-H bonds. NaHMDS is reagent to convert alkyl halides to amines in a two step process that begins with N-alkylation followed by hydrolysis of the N-Si bonds. 2NNa + RBr → 2NR + NaBr 2NR + H2O → 2O + RNH2 This method has been extended to aminomethylation via the reagent 3Si)2NCH2OMe, which contains a displaceable methoxy group
7. 1,8-Diazabicyclo(5.4.0)undec-7-ene – 1, 8-Diazabicycloundec-7-ene, or more commonly DBU, is a chemical compound and belongs to the class of amidine compounds. It is used in organic synthesis as a catalyst, a ligand. Although DBU is typically produced synthetically, it is also an alkaloid isolated from the sponge Niphates digitalis, the biosynthesis of DBU has been proposed to begin with suberic aldehyde and diaminopropane. As a reagent in chemistry, DBU is used as a catalyst, a complexing ligand. It is also used as a agent for epoxy. It is used in fullerene purification with trimethylbenzene, and it is used as a catalyst in polyurethane production. It has a strong catalyst effect for the reactions of alicyclic and aliphatic isocyanates and it also exhibited its dual character in the synthesis of aryl- & styryl- terminal acetylenes
8. 1,5-Diazabicyclo(4.3.0)non-5-ene – 1, 5-Diazabicyclonon-5-ene is a chemical compound with the formula C7H12N2. It is a base used in organic synthesis. A related compound with related functions is 1, 8-diazabicycloundec-7-ene, the relatively complex nature of the formal names for DBU and DBN reflects the fact that these compounds are bicyclic and contain several functional groups. The compounds are employed for dehydrohalogenation reactions as well as base-catalyzed rearrangements
9. Phosphazene – Phosphazenes are a class of chemical compounds in which a phosphorus atom is covalently linked to a nitrogen atom by a double bond and to three other atoms or radicals by single bonds. While other substitutions produce relatively persistent compounds, in organic synthesis the term refers to species with three amino substituents bound to phosphorus. The compounds are unusually stable examples of the class of molecules and have a remarkable proton affinity. As such, they are one of the eminent examples of neutral, two examples are hexachlorocyclotriphosphazene and bisiminium chloride. Phosphazenes are also known as iminophosphoranes and phosphine imides, phosphinimide ligands can be used in catalysis. Phosphazene bases are strong non-metallic non-ionic and low-nucleophilic bases and they are stronger bases than regular amine or amidine bases such as Hünigs base or DBU. Protonation takes place at a doubly bonded nitrogen atom, related to phosphazene bases are the proazaphosphatrane bases, which have a saturated P3 structure and protonate at phosphorus. Though the simplest phosphazene superbase, P1-Me, was first synthesized in 1975, chemists assumed that the compounds were highly unstable, the species was regarded at that time as little more than an academic curiosity. By now phosphazene bases are established reagents in organic synthesis, besides organic synthesis, phosphazene bases are used as basic titrants in non-aqueous acid-base titration. Their advantages for this are, they are strong bases in many solvents and their conjugate acids are inert
10. Potassium bis(trimethylsilyl)amide – Potassium bisamide is the chemical compound with the formula 2NK. It is a strong, non-nucleophilic base with an approximate pKa of 26, in the solid state, the unsolvated compound is dimeric, with two potassium and two nitrogen atoms forming a square. This compound is soluble in solvents and conducts electricity poorly in solution. This is attributed to strong ion pairing
11. Triisopropylamine – Triisopropylamine is an organic chemical compound consisting of three isopropyl groups bound to a central nitrogen atom. As a hindered tertiary amine, it can be used as a base and as a stabilizer for polymers, however, its applications are limited by its relatively high cost. Triisopropylamine is notable as being the most sterically crowded amine currently known, the even more crowded tri-tert-butyl-amine has never been successfully synthesized, although the existence of 2,2,4, 4-Tetramethyl-3-t-butyl-pentane-3-ol implies that it may be possible. The average C-N-C angle was claimed to be 119. 2° and this peculiarity was attributed to steric hindrance by the bulky isopropyl radicals. However, in 1998 X-ray diffraction analysis of the crystallized solid showed that the C3N core is actually pyramidal, however the researchers could not rule out the crystal field effect as the cause of the asymmetry. The C-C-C planes of the groups are slightly tilted relative to the threefold symmetry axis of the C3N core. It can be prepared from diisopropylamine by a developed by Bock and others