Base-Catalyzed Ring Opening of an Epoxide

In a base-catalyzed ring opening of an epoxide, the nucleophilic attack is on the less substituted (less hindered) carbon atom in the epoxide ring. The mechanism is shown below. 

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

Reymond, S. Brunei, J. M. Buono, G. (2000) New development in the enantioselective ring opening of meso-epoxides by various ion silicon sources catalyzed by an o-methoxyaryldiazaphosphonamide Lewis base., Tetrahedron Asymmetry, 11 4441-4445. 

Scheme 5. An alternative mechanism. Radical addition (instead of base-catalyzed abstraction) leading to species X is followed by formation of a ferryl heme species (Compound II) and a proposed epoxide species (55). Formation of an epoxide has also been suggested from computational work (51) (it was also considered in an earlier study, but initially considered energetically unfavorable in the gas phase (50)). Electrophilic addition (Scheme 4) could also involve epoxide formation through a similar (two-electron) mechanism. Possible ring opening of the epoxide is also indicated (third step).

Epoxides can also undergo base-catalyzed ring opening. Such reactions do not occur with other ethers, but they are possible with epoxides (because of ring strain), provided that the attacking nucleophile is also a strong base such as an alkoxide ion or hydroxide ion. 

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... 

There are three principal ionic ring-opening polymerization reactions of epoxides acid-initiated, base-initiated, and coordinate-initiated polymerizations. The acid-initiated reaction involves addition of an active hydrogen compound, HY, such as ethanol, to an epoxide ring and is catalyzed by an acid, HX, such as perchloric acid. The reaction sequence involves formation of an oxonium complex, followed by ring opening by an 8 2 cleavage of an oxonium carbon bond. 

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