Barton-McCombie deoxygenation: Facts, Discussion Forum, and Encyclopedia Article

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Barton-McCombie deoxygenation

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	Barton-McCombie deoxygenation
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	Topics Barton-McCombie deoxygenation Topic Home The Barton-McCombie deoxygenation is an organic reactionOrganic reaction Organic reactions are chemical reactions involving organic compounds.... in which an hydroxyHydroxy In chemical nomenclature, the prefix *hydroxy* indicates the presence of a hydroxyl functional group '.... functional groupFunctional group In organic chemistry functional groups are specific groups of atoms within molecules, that are responsible for the characte... in an organic compoundOrganic compound An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; theref... is replaced by a hydrideHydride A hydride is a compound of hydrogen with more electropositive elements.... to give an alkyl groupAlkane An alkane is an acyclic saturated hydrocarbon.... . It is named for the British chemists Sir Derek Harold Richard BartonDerek Harold Richard Barton Sir Derek Harold Richard Barton was a British physical chemist and Nobel Prize Laureate.... and Stuart W. McCombie. This deoxygenation reaction is a radical substitutionRadical substitution Summary In Organic chemistry, a radical substitution reaction is a substitution reaction involving free radicals as a reactive inter... . In the related Barton decarboxylation the reactant is a carboxylic acidCarboxylic acid Carboxylic acids are organic acids characterized by the presence of a carboxyl group, which has the formula --OH, usua... . Mechanism The reaction mechanismReaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occ... consists of a catalytic radical initiation step and a propagation step. The alcoholAlcohol In chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substit... (1) is first converted into a xanthateXanthate Xanthates are the salts and esters of a xanthic acid, ROCSH or O-esters of dithiocarbonic acid where R is any organic residu... (2). The other reactant tributyltin hydrideOrganotin Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents.... 3 is decomposed by AIBN 8 into a tributyltin radicalRadical (chemistry) In chemistry, radicals are atomic or molecular species with unpaired electrons on an otherwise open shell configuration.... 4. The tributyltin radical abstracts the xanthate group from 2 leaving an alkylFacts About Alkyl An alkyl is a univalent radical containing only carbon and hydrogen atoms arranged in a chain.... radical 5 and tributyltin xanthate (7). The sulfurSulfur Sulfur or sulphur is the chemical element in the periodic table that has the symbol S and atomic number 16.... tinTin Tin is a chemical element in the periodic table that has the symbol Sn and atomic number 50.... bond in this compound is very stable and provides the driving forceStandard enthalpy change of reaction The standard enthalpy change of reactionis the enthalpy change that occurs in a system when one mole of matter is transforme... for this reaction. The alkyl radical in turn abstracts a hydrogen atom from a new molecule of tributyltin hydride generating the desired deoxygenated product (6) and a new radical species ready for propagation. Variations Alternative hydride sources Main disadvantage of this reaction is the use of the tin hydride which is toxic, expensive and difficult to remove from the reaction mixture. One alternative is the use of tributyltin anhydride as the radical source and poly(methylhydridesiloxane)PMHS PMHS or poly is a polymer with the general structure --.... (PMHS) as the hydrideHydride A hydride is a compound of hydrogen with more electropositive elements.... source . Phenyl chlorothionoformate used as the starting material ultimately generates carbonyl sulfideCarbonyl sulfide Carbonyl sulfide is a colourless gas at room temperature with an unpleasant odor.... . Trialkyl boranes An even more convenient hydrogen donor is provided by trialkylboraneOrganoborane Organoborane or organoboron compounds are chemical compounds comprised of boron and carbon.... -water complexes such as trimethylborane contaminated with small amounts of water. In this catalytic cycleCatalytic cycle Summary A catalytic cycle in chemistry is a concept that appreciates the notion that in a chemical reaction a catalyst is often firs... the reaction is initiated by air oxidation of the trialkylborane 3 by air to the methyl radical 4. This radical reacts with the xanthate 2 to S-methyl-S-methyl dithiocarbonate 7 and the radical intermediate 5. The CH₃B.H₂O complex 3 provides a hydrogen for recombining with this radical to the alkane 6 leaving behind diethyl borinic acid and a new methyl radical. It is found by theoretical calculationsComputational chemistry Computational chemistry is a branch of chemistry that uses the results of theoretical chemistry incorporated into efficient ... that that a O-H homolysisHomolysis In chemistry, homolysis is chemical bond dissociation of a neutral molecule generating two free radicals.... reaction in the borane-water complex is endothermicEndothermic reaction In Chemistry an endothermic reaction is one in which the reactants have less energy than the products, and thus a net input ... with an energy similar to that of the homolysis reaction in tributyltin hydride but much lower than the homolysis reaction of pure water. Scope A variation of this reaction was used as one of the steps in the total synthesisTotal synthesis A total synthesis is the complete chemical synthesis of complex organic molecules from simple, commercially available precur... of azadirachtinAzadirachtin Azadirachtin is a chemical compound belonging to the limonoids.... : In another variation the reagent is the imidazoleImidazole Imidazole is a heterocyclic aromatic organic compound.... 1,1'-thiocarbonyldiimidazole (TCDI), for example in the total synthesis of pallescensin B.. TCDI is especially good to primary alcohols because there is no resonance stabilization of the xanthate because the nitrogen lonepair is involved in the aromatic sextet. The reaction also applies to S-alkylxanthates. With triethylboraneTriethylborane Triethylborane, also called triethylborine and triethylboron, is an organoborane, a near-colorless to yellowish ... as a novel metal-free reagent, the required hydrogen atoms are abstracted from protic solvents, the reactor wall or even (in strictly anhydrous conditions) the borane itself. External links Chemical & Engineering NewsChemical & Engineering News Overview Chemical & Engineering News is a weekly chemistry trade magazine published by the American Chemical Society....