Electronic effects, epoxide ring

A study carried out with various K-region arene oxides shows that the reactivity order of epoxides with sodium cresolate in DMSO is 29 > 28 > 1, 253 > 4 > 254. The poor regioselectivity observed in the reactions of 28 and 29 with the cresolate is attributed to the insensitivity of the reaction to electronic influences as well as by the inherent absence of the electronic effects on the condensed benzo ring.24... 

A comprehensive picture at the molecular level has been developed for the titan-ocene-mediated epoxide ring opening through electron transfer. The investigations, carried out by experimental and computational techniques, have shown that the most reactive Ti(II) species is the dimer in its half-open structure and that the selectivity of the ring opening is governed by steric effects.393... 

The alkoxycarbonyl group does not have a strong directing effect on the ring opening of epoxides by amines (Scheme 4.76), and steric or electronic effects of the other substituents can be more important to the outcome of these reactions. The... 

Several catalysts have been found for the ring opening of epoxides. For instance, cyclohexene oxide gave an excellent yield of the trans-fi-amino alcohol when treated with either aromatic or aliphatic amines in the presence of a scandium triflate catalyst.21 Aromatic epoxides react in a regiospeciflc reaction at the benzylic carbon when treated with aromatic amines and scandium triflate but at the least substituted carbon of the epoxide ring when aliphatic amines react. Electronic effects are more important in the reactions of the aromatic epoxides whereas steric factors control the reaction with aliphatic epoxides. 

Alkenyloxirans 16 are reactive allylating agents used under neutral conditions. The epoxy ring is opened by Pd(0) to form 7r-allylpalladium with generation of an alkoxide anion, which abstracts a proton from a pronucleophile to produce the a-hydroxy- 7r-allylpalladium 17. Reductive elimination of 17 affords either 1,4-adducts 18 or 1,2-adducts 19 [8,9]. The 1,4-adducts are mainly obtained under usual conditions due to the electronic effect of the epoxide oxygen. The 1,4-adducts 18 are allylic alcohols, and can be used again for the allylation after esterification to yield 19a. 

There are two factors competing to control the regiochemistry electronic eflFects vs. steric effects. The former favors attack at the more substituted position, while the latter favors attack at the less substituted position. To determine which factor is dominant, we must analyze the epoxide. When the epoxide possesses a tertiary position, the electronic effect will be dominant. When the epoxide possesses only primary and secondary positions, the steric effect will be dominant. The regiochemistry of acid-catalyzed ring opening is just one example where steric effects and electronic effects compete. As we progress through the course, we will see other examples of electronic vs. steric effects. 

In this case, we deal with the electronic effect ofa neighboring sihcon substituent, but simply properly chosen reaction conditions can efficiently determine the outcome of epoxide ring openings too. 

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