Acid-Catalyzed Ring Opening of an Epoxide

Acid-Catalyzed Ring Opening of an Epoxide THE OVERALL REACTION  

Step 1 The most abundant acid in the reaction mixture, the conjugate acid of the solvent methanol, transfers a proton to the oxygen of the epoxide. 

Step 2 Methanol acts as a nucleophile toward the protonated epoxide and bonds to the more highly substituted (more carbocation-hke) carbon of the ring. The carbon-oxygen bond of the ring breaks in this step, and the ring opens. 

Step 3 Proton transfer to methanol completes the reaction and regenerates the acid catalyst. 

A method for achieving net anti hydroxylation of alkenes combines two stereospecific processes epoxidation of the double bond and hydrolysis of the derived epoxide. 

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Acid-Catalyzed Ring Opening of an Epoxide 525 Base-Catalyzed Ring Opening of an Epoxide 526... 

The acid-catalyzed ring opening of an epoxide always results in a frans-1,2,-diol, with the two carbons formerly part of the epoxide bonded to each of the two hydroxyl groups. 

MECHANISM 14.6 ACID-CATALYZED RING OPENING OF AN EPOXIDE... 

A MECHANISM FOR THE REACTION ] Acid-Cataiyzed Ring Opening of an Epoxide 525 [ A MECHANISM FOR THE REACTION ] Base-Catalyzed Ring Opening of an Epoxide 526... 

Acid-catalyzed ring opening of an unsymmetrical epoxide occurs primarily by attack of the nucleophile at the more substituted carbon atom. 

Anti hydroxylation of an alkene is readily achieved with peroxycarboxylic acids. - Acid-catalyzed ring opening of the initial product, an oxirane (epoxide), forms the monoester of a 1,2-diol, hydrolysis of which affords the parent diol. Alternative reagents which are often used for anti hydroxylation of alkenes are hydrogen peroxide with oxides of tungsten - - or selenium, - and iodine-silver benzoate (Pidvost reaction). - - ... 

The acid-catalyzed ring opening of epoxides shows a stereoselectivity typical of 8 2 reactions The nucleophile attacks anti to the leaving hydroxyl group, and the —OH groups in the glycol thus formed are anti. As a result, the acid[Pg.266]

The degree of epoxidation can be controlled by the stoichiometric addition of percarboxylic acids [124-126]. When aiming for high epoxidation levels in the polymer, the quantity of peracid needs to be increased over a 1 1 molar ratio. The concomitant high concentration of carboxylic acids in the reaction media leads to side reactions [99, 118, 124] an acid-catalyzed ring opening of epoxides by water or the carboxylic acid can occur. The resulting PBD contains vicinal hydroxyl... 

In 1996, Archer et al. reported a highly enantioselective chemoenzymatic resolution of rac-l-methyl-l,2-epoxycyclohexane using whole cells of Corynebacterium C12, giving rise to the (li ,2S)-epoxide in >99% ee (30% )deld) and the (lS,2S)-diol in 92% ee (42% yield) [134]. Further, an additional efficient chemoenzymatic deracemization process was run in tandem using the EH from Corynebacterium C12 and perchloric acid for the acid-catalyzed ring opening of the residual epoxide (Figure 8.25). 

The regiochemistry of acid-catalyzed ring-opening depends on the epoxide s structure, and a mixture of products is often formed. When both epoxide carbon atoms are either primary or secondary, attack of the nucleophile occurs primarily at the less highly substituted site—an Sf,j2-like result. When one of the epoxide carbon atoms is tertiary, however, nucleophilic attack occurs primarily at the more highly substituted site—an S>jl-like result. Thus, 1,2-cpoxypropane reacts with MCI to give primarily l-chloro-2-propanol, but 2-inethyl-1,2-epoxypropane gives 2-chloro-2-methyl-l-propanol as the major product. 

For both cycles, a concerted mechanism is suggested in which the electron-rich double bond of the alkene attacks a peroxidic oxygen of 2. It has been inferred, from experimental data, that the system may involve a spiro arrangement [3, 5 a]. The selectivity toward epoxides can be enhanced by the addition of Lewis O- or Wbases such as quinuclidine, pyridine, pyrazole or 2,2 -bipyridine to the system [3, 6d, lOg-k]. Lewis acids catalyze ring-opening reactions and diol formation. These reactions are suppressed after the addition of Lewis bases. An... 

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