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Dihydroxylation
Dihydroxylation is the process by which an alkene is converted into a vicinal diol. Although there are many routes to accomplish this oxidation, the most common and direct processes use a high-oxidation-state transition metal (typically osmium or manganese). The metal is often used as a catalyst, with some other stoichiometric oxidant present. In addition, other transition metals and non-transition metal methods have been developed and used to catalyze the reaction. Mechanism In the dihydroxylation mechanism, a ligand first coordinates to the metal catalyst (depicted as osmium), which dictates the chiral selectivity of the olefin. The alkene then coordinates to the metal through a +2cycloaddition, and the ligand dissociates from the metal catalyst. Hydrolysis of the olefin then yields the vicinal diol, and oxidation of the catalyst by a stoichiometric oxidant regenerates the metal catalyst to repeat the cycle. The concentration of the olefin is crucial to the enantiomer ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dihydroxylation
Dihydroxylation is the process by which an alkene is converted into a vicinal diol. Although there are many routes to accomplish this oxidation, the most common and direct processes use a high-oxidation-state transition metal (typically osmium or manganese). The metal is often used as a catalyst, with some other stoichiometric oxidant present. In addition, other transition metals and non-transition metal methods have been developed and used to catalyze the reaction. Mechanism In the dihydroxylation mechanism, a ligand first coordinates to the metal catalyst (depicted as osmium), which dictates the chiral selectivity of the olefin. The alkene then coordinates to the metal through a +2cycloaddition, and the ligand dissociates from the metal catalyst. Hydrolysis of the olefin then yields the vicinal diol, and oxidation of the catalyst by a stoichiometric oxidant regenerates the metal catalyst to repeat the cycle. The concentration of the olefin is crucial to the enantiomer ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sharpless Asymmetric Dihydroxylation
Sharpless asymmetric dihydroxylation (also called the Sharpless bishydroxylation) is the chemical reaction of an alkene with osmium tetroxide in the presence of a chiral quinine ligand to form a vicinal diol. The reaction has been applied to alkenes of virtually every substitution, often high enantioselectivities are realized, with the chiral outcome controlled by the choice of dihydroquinidine (DHQD) vs dihydroquinine (DHQ) as the ligand. Asymmetric dihydroxylation reactions are also highly site selective, providing products derived from reaction of the most electron-rich double bond in the substrate. It is common practice to perform this reaction using a catalytic amount of osmium tetroxide, which after reaction is regenerated with reoxidants such as potassium ferricyanide or ''N''-methylmorpholine ''N''-oxide. This dramatically reduces the amount of the highly toxic and very expensive osmium tetroxide needed. These four reagents are commercially available premixed (" AD- ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Upjohn Dihydroxylation
The Upjohn dihydroxylation is an organic reaction which converts an alkene to a ''cis'' vicinal diol. It was developed by V. VanRheenen, R. C. Kelly and D. Y. Cha of the Upjohn Company in 1976. It is a catalytic system using ''N''-methylmorpholine ''N''-oxide (NMO) as stoichiometric re-oxidant for the osmium tetroxide. It is superior to previous catalytic methods. Prior to this method, use of stoichiometric amounts of the toxic and expensive reagent osmium tetroxide was often necessary. The Upjohn dihydroxylation is still often used for the formation of ''cis''-vicinal diols; however, it can be slow and is prone to ketone byproduct formation. One of the peculiarities of the dihydroxylation of olefins is that the standard "racemic" method (the Upjohn dihydroxylation) is slower and often lower yielding than the asymmetric method (the Sharpless asymmetric dihydroxylation). Improvements to Upjohn dihydroxylation In response to these problems, Stuart Warren and co-worke ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Milas Hydroxylation
The Milas hydroxylation is an organic reaction converting an alkene to a vicinal diol, and was developed by Nicholas A. Milas in the 1930s. The cis-diol is formed by reaction of alkenes with hydrogen peroxide and either ultraviolet light or a catalytic osmium tetroxide, vanadium pentoxide, or chromium trioxide. The reaction has been superseded in synthetic chemistry by the Upjohn dihydroxylation and later by the Sharpless asymmetric dihydroxylation Sharpless asymmetric dihydroxylation (also called the Sharpless bishydroxylation) is the chemical reaction of an alkene with osmium tetroxide in the presence of a chiral quinine ligand to form a vicinal diol. The reaction has been applied to alk .... Mechanism The proposed mechanism for the Milas hydroxylation involves the initial combination of hydrogen peroxide and the osmium tetroxide catalyst to form an intermediate, which then adds to the alkene, followed by a cleavage that forms the product and regenerates the OsO4. Li ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Osmium Dihydroxylation Mechanism
Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element. When experimentally measured using X-ray crystallography, it has a density of . Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness. Osmium is among the rarest elements in the Earth's crust, making up only 50 parts per trillion ( ppt). It is estimated to be about 0.6 parts per billion in the universe and is therefore the rarest precious metal. Characteristics Physical properties Osmium has a blue-gray tint and is the densest stable element; it is approximately twice as dense as lead and narrowly denser than ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
N-Methylmorpholine N-oxide
''N''-Methylmorpholine ''N''-oxide (more correctly 4-methylmorpholine 4-oxide), NMO or NMMO is an organic compound. This heterocyclic amine oxide and morpholine derivative is used in organic chemistry as a co-oxidant and sacrificial catalyst in oxidation reactions for instance in osmium tetroxide oxidations and the Sharpless asymmetric dihydroxylation or oxidations with TPAP. NMO is commercially supplied both as a monohydrate C5H11NO2·H2O and as the anhydrous compound. The monohydrate is used as a solvent for cellulose in the lyocell process to produce cellulose fibers. Uses Solvent of cellulose NMMO monohydrate is used as a solvent in the lyocell process to produce lyocell fiber. It dissolves cellulose to form a solution called dope, and the cellulose is reprecipitated in a water bath to produce a fiber. The process is similar but not analogous to the viscose process. In the viscose process, cellulose is made soluble by conversion to its xanthate derivatives. With NMMO, cellul ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst. Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category. Catalysis is ubiquitous in chemical industry of all kinds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, and Biological Chemistry'. 1232 pages. Two general types of monoalkenes are distinguished: terminal and internal. Also called α-olefins, terminal alkenes are more useful. However, the International Union of Pure and Applied Chemistry (IUPAC) recommends using the name "alkene" only for acyclic hydrocarbons with just one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cycloalkadiene, etc. for cyclic ones; and "olefin" for the general class – cyclic or acyclic, with one or more double bonds. Acyclic alkenes, with only one double bond and no other functional groups (also known as mono-enes) form a homologous series of hydrocarbons with the general formula with '' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Glycol Cleavage
Glycol cleavage is a specific type of organic chemistry oxidation. The carbon–carbon bond in a vicinal diol (glycol) is cleaved and instead the two oxygen atoms become double-bonded to their respective carbon atoms. Depending on the substitution pattern in the diol, these carbonyls can be either ketones or aldehydes. Glycol cleavage is an important reaction in the laboratory because it is useful for determining the structures of sugars. After cleavage takes place the ketone and aldehyde fragments can be inspected and the location of the former hydroxyl groups ascertained. Reagents Periodic acid (HIO4), (diacetoxyiodo)benzene (PhI(OAc)2) and lead tetraacetate (Pb(OAc)4) are the most common reagents used for glycol cleavage, processes called the Malaprade reaction and Criegee oxidation, respectively. These reactions are most efficient when a cyclic intermediate can form, with the iodine or lead atom linking both oxygen atoms. The ring then fragments, with breakage of the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sharpless Asymmetric Dihydroxylation Mnemonic
Sharpless is a surname. Notable people with the surname include: *Bevan Sharpless, solar system astronomer *Karl Barry Sharpless, American chemist and Nobel prize winner *Josh Sharpless, baseball player *Christopher Sharpless, 1988 Winter Olympics bobsledder * Mattie R. Sharpless (born 1942), American diplomat *Stewart Sharpless, galactic astronomer **Sharpless catalog, a 20th-century astronomical catalog with 313 items *Isaac Sharpless, educator *Norman Sharpless, American oncologist and director of the National Cancer Institute *Disappearance of Toni Sharpless, an American nurse who disappeared in 2009 Fictional characters *A character in Madama Butterfly See also *Sharpless asymmetric dihydroxylation, a chemical reaction *Sharpless epoxidation, a chemical reaction *Sharpless oxyamination The Sharpless oxyamination (often known as Sharpless aminohydroxylation) is the chemical reaction that converts an alkene to a vicinal amino alcohol. The reaction is related to the Sharpless d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hydroxylation In The Synthesis Of Brassinosteroid
In chemistry, hydroxylation can refer to: *(i) most commonly, hydroxylation describes a chemical process that introduces a hydroxyl group () into an organic compound. *(ii) the ''degree of hydroxylation'' refers to the number of OH groups in a molecule. The ''pattern of hydroxylation'' refers to the location of hydroxy groups on a molecule or material. Hydroxylation reactions Synthetic hydroxylations Installing hydroxyl groups into organic compounds can be effected by various metal catalysts. Many such catalysts are biomimetic, i.e. they are inspired by or intended to mimic enzymes such as cytochrome P450. Whereas many hydroxylations insert O atoms into bonds, some reactions ''add'' OH groups to unsaturated substrates. The Sharpless dihydroxylation is such a reaction: it converts alkenes into diols. The hydroxy groups are provided by hydrogen peroxide, which adds across the double bond of alkenes. Biological hydroxylation In biochemistry, hydroxylation reactions are often fa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diol
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is ethylene glycol. Examples of diols in which the hydroxyl functional groups are more widely separated include 1,4-butanediol and propylene-1,3-diol, or beta propylene glycol, . Synthesis of classes of diols Geminal diols A geminal diol has two hydroxyl groups bonded to the same atom. These species arise by hydration of the carbonyl compounds. The hydration is usually unfavorable, but a notable exception is formaldehyde which, in water, exists in equilibrium with methanediol H2C(OH)2. Another example is (F3C)2C(OH)2, the hydrated form of hexafluoroacetone. Many gem-diols undergo further condensation to give dimeric and oligomeric derivatives. This reaction applies to glyoxal and related aldehydes. Vicinal diols In a vicinal diol, t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
