Oxirane reduction mechanism

Two parallel routes for the elimination of glycol formate are suggested, involving either reaction with H2 or with cocatalyst water. The detection of formic acid in the reaction products suggests another mechanism, with initial production of formic acid from H2 and C02, followed by reaction with the oxirane. This mechanism is not favored however since the yields of glycol formates varied substantially when various substituted oxiranes were reacted. This would not have been expected in a mechanism with formic acid as an intermediate. A third mechanism, not considered by the authors, could proceed through initial production of propylene carbonate, followed by reduction to the mono- or di-formate. 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The past decade has also seen the publication of many articles on the catalytic reduction of oxiranes. An account of the mechanism and stereochemistry of the hydrogenolysis is presented in various reviews and papers discussed in the next section. A review that appeared recently on the reduction of oxiranes to alcohols covers all of these methods. 

The mechanism and stereochemistry of the reduction with metal hydride has been dealt with in many articles. It may be stated in general that the structure of the oxirane and the nature of the reducing agent exert great effects on the rate and pathway of the reaction. 

In other solvents (ether, cyclohexane, acetone), the cycloaddition does not take place, only reduction of the double bond is observed. As regards the two monooxiranes of ewdo-bicyclopentadiene, in the course of acetone-sensitized photolysis only enrio-4-oxatetracycloundec-9-ene 136 is changed. Photoreduction of the double bond proceeds by a free-radical mechanism there is no interaction between the triplet-state excited olefin and the oxirane part. ... 

The release of ring strain in epoxides is probably responsible for the high reactivity of these special ethers. HI opens epoxides under mild conditions stereospecifically to iodohydrins (Scheme 26). The mechanism is similar to the reaction of bromide with epoxides (see Section 1.7.3.3). It should be noted, however, that reduction of epoxides to alkenes may occur if vicinal diiodides are intermediately formed, which can lose I2 under the reaction conditions. With the combination of acyl chloride and Nal unstable diiodides are avoided and 2-iodoethyl esters are formed from oxiranes (Scheme 27). ° ... 

Opposite regioselectivity to that of regular metal hydride reductions has been observed in the NaBH4 reductions of oxiranes in the presence of triethylamine under photochemical conditions (hydride attacks the more highly substituted carbon) <92CC1133>. Diborane (B2H6) likewise tends to reduce oxiranes at the sterically more hindered oxirane carbon the mechanism and stereochemistry of the diborane reduction in connection with aliphatic oxiranes has been studied <82H(l8)28l>. 

Typical mechanisms for pinacol couplings are shown in Scheme 2. In the first reduction step a ketyl radical is formed which can dimerize (path A) or add to a second carbonyl group, forming a C-C bond. In path B a second one-electron reduction must then follow. For pinacol coupling reactions mediated by transition metals the insertion of a carbonyl group into the metal-carbon bond of initially formed metal oxiranes has been proposed (path C). 

Reduction of propargyl oxiranes with copper hydride results in a diastereoselective synthesis of a-hydroxyallenes. DFT studies, supported by kinetic investigations, partially validate the previously proposed reaction mechanism and attribute the high... 

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