Epoxides oxygen-based nucleophiles

In base, there can be no protonation of the epoxide, and no build-up of positive charge. Without protonation, the epoxide oxygen is a poor leaving group, and leaves only if pushed by a strong nucleophile the reaction becomes pure S>j2. Steric hindrance becomes the controlling factor, and methoxide attacks only the primary end of the epoxide. 

Reaction of these epoxides with Pd(0) follows the pattern of the allylic acetate reactions. The epoxide oxygen atom is the leaving group but the alkoxide intermediate is basic enough to deprotonate nucleophiles such as malonates without added base. With cyclopentadiene monoxide 265, one regio- and stereoisomer 275 is formed.45 The palladium adds to the opposite side of the... 

Asymmetric phase-transfer catalyzed cascades initiated by the conjugate addition of heteronucleophiles has been employed for the enantioselective preparation of epoxides and aziridines using a suitable oxygen- or nitrogen-based nucleophile incorporating an appropriate leaving group ready to... 

Besides addition of bases, hydrolysis of hydrated metal cations may be initiated by addition of another proton scavenger, like propylene oxide [18, 19]. Protonation of the epoxide oxygen by an acidic proton of a water ligand is followed by the epoxide ring opening through nucleophilic attack of the conjugated base. 

The oxygen atom from epoxides can also act as a nucleophile in the cycloisomerization of epoxyalkynes to furans catalyzed by TpRuCl (PPh3)(CH3CN)2 in the presence of Et3N [109]. The initial ruthenium vinyhdene species A would evolve to the mthenium furylidene B by nucleophihc attack of the epoxide oxygen, which in the presence of base would afford ruthenium furyl anion C. Protonolysis would give rise to the final product and recovery of the catalytic ruthenium species (Scheme 17). 

Epoxides, though uncharged, have a formal resemblance to cyclic bromonium ion intermediates (cf. p. 180), but unlike them are stable and may readily be isolated. They do, however, undergo nucleophilic attack under either acid- or base-catalysed conditions to yield the 1,2-diol. In either case attack by the nucleophile on a carbon atom will be on the side opposite to the oxygen bridge in (49) such attack on the epoxide will involve inversion of configuration (cf. p. 94) ... 

Simple examples shown above are the base-catalysed formation of oxygen- and nitrogen-containing ring systems. We have shown base-initiated ionization of the alcohol to an alkoxide anion in epoxide formation the anion is a better nucleophile than the alcohol. For pyrrolidine synthesis, the amino group is sufficiently nucleophilic for reaction to occur, but base is required to remove a proton from the first-formed intermediate. 

Base-catalyzed reactions, in which the nucleophile provides the driving force for ring opening, usually involve breaking the epoxide bond at the less substituted carbon, since this is the position most accessible to nucleophilic attack.86 87 The situation in acid-catalyzed reactions is more complex. The bonding of a proton to the oxygen weakens the C—O... 

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