Oxirane ring stability

Organotin epoxide monomers 30,100 104) containing fairly reactive oxirane rings and both C=C and Sn—C bonds can be used as starting components for the synthesis of new polymers, chemically active stabilizers and biocides for polymeric materials, e.g. for PVC. 

In the case of Pt and Pd, the surface oxidation can also take place, but the resulting metal ions can be easily reduced in the presence of hydrogen. According to this fact, the observed difference in the regioselectivity of the rupture of the oxirane ring on Ni and Cu or Pt and Pd can be explained on the basis of the different stability of the metal ions in a hydrogen atmosphere.289... 

The relative stability of the oxiranes to bases at room temperature allows the selective ammonolysis of carboxylic ester groups with ammonia in methanol,49 and, conversely, free hydroxyl groups may be esterified with acid chlorides or anhydrides in pyridine without affecting the oxirane ring. Hydroxyl groups may also be converted into alkyl ethers by using either the Haworth50 or the Purdie-Irvine procedure.5152... 

In the oxirane ring opening reaction (Eq. (2)), it is possible to prepare the ketone 5, the acetonide 10, and the allylic alcohol 11 selectively by controlling the reactivity of the intermediate carbenium ion 12 under some specified reaction conditions. In order to prepare 5 preferentially, a high concentration of perchlorate ion must be employed in CH2CI2, ClCHjCH Cl, or THF. In this system, an incipient carbenium ion 12 would be associated with perchlorate ion and stabilized so that the deprotona-... 

Fusion of an oxirane ring to a pyranose ring also deforms it, and thereby lowers its stability. The composition of 2,3-anhydro-D-mannose in aqueous solution,165 as determined by g.l.c. of the trimethylsilyl derivatives, is 23 7 65 5. This is remarkably similar to the composition of a solution of 2,3-O-isopropylidene-L-rhamnose. For 2,3-anhydro-D-allose, the ratios are166 41 12 5 42 (or 41 5 12 42). In this case, although the proportion of furanose forms is substantial, there is no clear preponderance of the //-furanose form, presumably because OH-1 and OH-2 are trans but OH-1 is quasi-equatorial by contrast, in the (preponderant)... 

Substitution of methyl groups on the oxirane ring tilts the stability of the tautomers in favor of oxepin. Thus 1-methylbenzene oxide (154) exists as 2-methyloxepin (155), in rapid equilibrium with the benzene oxide tautomer 154.74 The AH has been calculated as 0.4 0.02 kcal/mol, i.e., 1.3 kcal/mol... 

In the cases of 170, 171,75-79 and 45, the resonance energy of the benzene moiety ( 39 kcal) trips the stability order irretrievably on the side of oxepin in the first two cases and in favor of arene oxide in the case of 45. Tautomerism, which ordinarily is manifested with systems having small energy differences, is therefore ruled out. However, in the case of the 9,10-oxide this limitation does not exist, but even so, the compound exists only in the oxepin form (70),8 possibly because of a very high strain on the oxirane ring in the epoxy form. 

The reaction of an oxirane with a nucleophilic carbanion or an ylide is a versatile method for the synthesis of cyclopropanes. Oxiranes and stabilized carbanions or ylides bearing different substituents are readily prepared from a variety of starting materials. - The inherent polarity and strain of the three-membered ring makes oxiranes particularly susceptible to transformation to cyclopropanes by intermolecular ring opening. 

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