Oxirane opening influence

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

The mechanism of the opening in acidic medium is not so simple. Although a general reaction scheme can be written, the pathway is influenced to a great extent by the structure of the oxirane, the steric and electronic effects of the substituents, and the solvent also plays an important role. Ever-increasing attention is being paid to the concept of the microscopic structure of solvation in binary solvents. In binary solvent mixtures, the structure of the solvent around the molecule in the solvent shell differs from that of bulk solvent. 

The role of steric factors has been examined in the opening of polyfluorinated oxiranes. Accounts have been given of the alcoholysis of steroid oxiranes and their transformation with phenol.The kinetics of the reactions of oxiranes with alcohols and phenols have been reported in a number of publications. The catalytic influence of transition metals has been examined. A secondary deuterium isotope effect has been studied in the course of methanolysis, and the solvent effect has been considered in reactions with phenols. ... 

The formation of alcohols by the hydrogenolytic opening of oxiranes (epoxides) takes place over palladium at room temperature and atmospheric pressure. The addition of some perchloric or hydrochloric acid not only increases the rate of the reaction, but can also influence the direction of ring opening of unsymmetric epoxides. 

In recent studies of styrene oxide polymerization reactions we found the phenyl substituent to have a significant Influence on the course of the polymerization process, too. In our particular case, however, the influence is due not only to steric factors, but also to the inductive effects of the phenyl ring, which Influences directly the course of the oxirane ring-opening reaction. 

The butadienyl Grignard reagent (141) reacts with oxirans to yield normal ring-opened products (142) and the rearranged allenic products (143) (Scheme 7). Steric factors greatly influence the product distribution, from 100% of (142 R = H, R = Me)toproductratiosfor (142)to(143)of 91 9[R R = (CH2)4] and 60 40 (R = R = Me). The presence of AICI3 also caused a remarkable... 

One of the important secondary reactions is the opening of oxiranic cycles under the influences of carboxylic acid [213] ... 

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