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LiHMDS
Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula . It is commonly abbreviated as LiHMDS or Li(HMDS) (lithium hexamethyldisilazide - a reference to its conjugate acid HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents, it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species. Preparation LiHMDS is commercially available, but it can also be prepared by the deprotonation of bis(trimethylsilyl)amine with ''n''-butyllithium. This reaction can be performed ''in situ''. : Once formed, the compound can be purified by sublimation or distillation. Reactions and applications As a base LiHMDS is often used in organic chemistry as a strong non-nucleophilic base. Its conjugate acid has a p''K''a of ~26, making it is less basic than other lithium bases, such as LDA (p''K''a of conjugate acid ~36). It is relatively more sterically hindered and hen ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Organolithium
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric. History and de ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bis(trimethylsilyl)amine
Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula CH3)3Sisub>2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An electron diffraction study shows that silicon-nitrogen bond length (173.5 pm) and Si-N-Si bond angle (125.5°) to be similar to disilazane (in which methyl groups are replaced by hydrogen atoms) suggesting that steric factors are not a factor in regulating angles in this case. This colorless liquid is a reagent and a precursor to bases that are popular in organic synthesis and organometallic chemistry. Additionally, HMDS is also increasingly used as molecular precursor in chemical vapor deposition techniques to deposit silicon carbonitride thin films or coatings. Synthesis and derivatives Bis(trimethylsilyl)amine is synthesized by treatment of trimethylsilyl chloride with ammonia: :2 (CH3)3SiCl + 3 NH3 → CH3)3Sisub>2NH + 2 NH4Cl Ammo ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sodium Bis(trimethylsilyl)amide
Sodium bis(trimethylsilyl)amide is the organosilicon compound with the formula . This species, usually called NaHMDS (sodium hexamethyldisilazide), is a strong base used for deprotonation reactions or base-catalyzed reactions. Its advantages are that it is commercially available as a solid and it is soluble not only in ethers, such as THF or diethyl ether, but also in aromatic solvents, like benzene and toluene by virtue of the lipophilic TMS groups. NaHMDS is quickly destroyed by water to form sodium hydroxide and bis(trimethylsilyl)amine. Structure Although the Na–N bond is polar covalent as a solid, when dissolved in nonpolar solvents this compound is trimeric, consisting of a central ring. Applications in synthesis NaHMDS is used as a strong base in organic synthesis. Typical reactions: *To deprotonate ketones and esters to generate enolate derivatives. *Generate carbenes by dehydrohalogenation of halocarbons. These carbene reagents add to alkenes to give substituted c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Acetylide
In chemistry, an acetylide is a compound that can be viewed as the result of replacing one or both hydrogen atoms of acetylene (ethyne) by metallic or other cations. Calcium carbide is an important industrial compound, which has long been used to produce acetylene for welding and illumination. It is also a major precursor to vinyl chloride. Other acetylides are reagents in organic synthesis. Nomenclature The term acetylide is used loosely. It apply to an acetylene , where R = H or a side chain that is usually organic. The nomenclature can be ambiguous with regards to the distinction between compounds of the type and . When both hydrogens of acetylene are replaced by metals, the compound can also be called carbide, e.g. calcium carbide , which is calcium acetylide. When only one hydrogen atom is replaced, the anion may be called hydrogen acetylide or the prefix ''mono''- may be attached to the metal, as in monosodium acetylide or sodium hydrogen acetylide, . An acetylide may b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Lithium Diisopropylamide
Lithium diisopropylamide (commonly abbreviated LDA) is a chemical compound with the molecular formula . It is used as a strong base and has been widely utilized due to its good solubility in non-polar organic solvents and non-nucleophilic nature. It is a colorless solid, but is usually generated and observed only in solution. It was first prepared by Hamell and Levine in 1950 along with several other hindered lithium diorganylamides to effect the deprotonation of esters at the α position without attack of the carbonyl group. Preparation and structure LDA is commonly formed by treating a cooled (0 to −78 °C) mixture of tetrahydrofuran and diisopropylamine with ''n''-butyllithium. When dissociated, the diisopropylamide anion can become protonated to form diisopropylamine. Diisopropylamine has a p''K''a value of 36. Therefore, its conjugate base is suitable for the deprotonation of compounds with greater acidity, importantly, such weakly acidic compounds (carbon acids ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hexane
Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as a cheap, relatively safe, largely unreactive, and easily evaporated non-polar solvent, and modern gasoline blends contain about 3% hexane. The term hexanes refers to a mixture, composed largely (>60%) of ''n''-hexane, with varying amounts of the isomeric compounds 2-methylpentane and 3-methylpentane, and possibly, smaller amounts of nonisomeric C5, C6, and C7 (cyclo)alkanes. These "hexanes" mixtures are cheaper than pure hexane and are often used in large-scale operations not requiring a single isomer (e.g., as cleaning solvent or for chromatography). Isomers Uses In industry, hexanes are used in the formulation of adhesive, glues for shoes, leather products, and roofing. They are also used to extract cooking oils (such as canola oil ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are Lewis bases. ''Nucleophilic'' describes the affinity of a nucleophile to bond with positively charged atomic nuclei. Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance's nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge, and nucleophilic addition. Nucleophilicity is closely related to basicity. The difference between the two is, that basicity is a thermodynamic property (i.e. relates to an equilibrium state), but nucleophilicity is a kinetic property ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Enolate
In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl () compounds. Rarely isolated, they are widely used as reagents in the Organic synthesis, synthesis of organic compounds. Bonding and structure Enolate anions are electronically related to allyl anions. The anionic charge is delocalized over the oxygen and the two carbon sites. Thus they have the character of both an alkoxide and a carbanion. Although they are often drawn as being simple salts, in fact they adopt complicated structures often featuring aggregates. Preparation Deprotonation of enolizable ketones, aromatic alcohols, aldehydes, and esters gives enolates. With strong bases, the deprotonation is quantitative. Typically enolates are generated from using lithium diisopropylamide (LDA). Often, as in conventional Claisen condensations, Mannich reactions, and aldol condensations, enolates are generated in low concentrations with alkoxide bases. Under such conditions, they ex ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Claisen Condensation
The Claisen condensation is a carbon–carbon bond forming reaction that occurs between two esters or one ester and another carbonyl compound in the presence of a strong base. The reaction produces a β-keto ester or a β- diketone. It is named after Rainer Ludwig Claisen, who first published his work on the reaction in 1887. The reaction has often been displaced by diketene-based chemistry, which affords acetoacetic esters. Requirements At least one of the reagents must be enolizable (have an α-proton and be able to undergo deprotonation to form the enolate anion). There are a number of different combinations of enolizable and nonenolizable carbonyl compounds that form a few different types of Claisen. The base used must not interfere with the reaction by undergoing nucleophilic substitution or addition with a carbonyl carbon. For this reason, the conjugate sodium alkoxide base of the alcohol formed (e.g. sodium ethoxide if ethanol is formed) is often used, since the a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fráter–Seebach Alkylation
In organic chemistry, the Fráter–Seebach alkylation (also known as Seebach–Fráter alkylation or Fráter–Seebach reaction) is a diastereoselective alkylation of chiral beta-hydroxy esters using strong base (chemistry), bases. The reaction was first published by Georg Fráter in 1979; in 1980, Dieter Seebach reported about a similar reaction with malic acid ester. Outline and mechanism Chiral beta-hydroxy esters can be treated with two equivalents of a strong base (lithium diisopropylamide, lithium diisopropylamide (LDA) or Lithium bis(trimethylsilyl)amide, lithium bis(trimethylsilyl)amide (LHMDS) are popular choices) to both remove the proton on the alcohol and Enolization, enolize the ester. An alkylating agent (methyl iodide in the case of Fráter's publication) is then added. This attacks from the opposite face of the chiral hydroxyl group to avoid steric hindrance as shown below in the 6-membered transition state with Chelation, chelating metal ions. This reaction has ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
