Oxiranes structure effects

For oxirane rings an IR absorption around 890 cm-1 is characteristic. This is also observed in the case of K-region epoxides and can be used for diagnostic purposes, but it is not sensitive enough to provide detailed structural information. The oxepins ordinarily do not show this band. Ultraviolet spectroscopy has been invaluable in studying the dynamic equilibrium between the arene oxides and oxepins. The solvent variation of UV spectra has also been exploited very effectively.8... 

The stereochemistry of the cyclopropyl as well as oxirane moiety in these P-lactams was determined based on the single crystal X-ray analysis of P-lactam 34 and the 4-nitrobenzoyl derivative (37) of 3-hydroxy-P-lactam 32.71 The extremely high diastereoselectivity observed in these cyclopropanation and epoxidation reactions can be explained by taking into account the highly organized transition state structures illustrated in Figure 1. The directing effect of the C-3... 

When styrene oxide was reacted in alcohols in the presence of an Al(OTf)3 catalyst, the nucleophile attacked at the benzyl carbon.28 The effect of catalyst loading and the effect of changing the structure of the reacting alcohols was investigated. The reactions of oxiranes with alkyl substituents were not regiospecific. More catalyst was needed when bulkier alcohols were used. 

Block copolymers characterised by different backbone structures of well-defined block lengths have been obtained from oxiranes and other heterocyclic monomers in the presence of catalysts that are effective at bringing about living polymerisations. Aida et al. [127,188,189,195,196] applied aluminium porphyrins and Teyssie et al. [125,197,198] applied bimetallic /i-oxoalkoxidcs for block copolymerisations in systems involving oxirane lactone, oxirane oxirane/cyclic acid anhydride, and oxirane/cyclic acid anhydride lactone as block forming units and obtained respective polyether polyester and polyester polyester block copolymers. Such copolymers seem to be of exceptionally wide potential utility [53]. 

Oxirane has a higher boiling point (10.5°) than cyclopropane (—32.9°), which indicates its more polar character. Its dipole moment has been determined by numerous authors via the dielectric constant (e.g.. Refs. ) and the Stark effect. The results agree well 1.9D. The structures of aryl-substituted oxiranes have also been determined by means of dipole moment measurements. ... 

The rearrangement of oxiranes with various structures in the presence of AICI3, ZnCl2, and other Lewis acids has been examined by taking into consideration the strengths of the Lewis acids, the molecular structure, and steric and electronic fac-The effects of Lewis acid and the solvent have been investigated in the... 

The mechanism and stereochemistry of the reduction with metal hydride has been dealt with in many articles. It may be stated in general that the structure of the oxirane and the nature of the reducing agent exert great effects on the rate and pathway of the reaction. 

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. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

In contrast to the dianhydrohexopyranoses, their deoxy derivatives 30, 131, 132, and 133 have an oxirane ring that is quite unsymmet-rically substituted384,742 with regard to polar effects. Consequently, in reactions with nucleophiles, polar effects may operate more effectively than steric effects, and this may result in oxirane-ring cleavage at the most electron-deficient position (as indicated by the arrows). Depending on the structure of the dianhydro derivative, the steric... 

End-labelling of the active centres with diphenyl chlorophosphate, combined with31P NMR spectroscopy, was shown to be an effective method for the determination of structure and concentration of the active species in the polymerization of several oxirane compounds.96 98 The chemical shifts of31P signals from the diphenylphosphoryl end-groups are sensitive to the structure of polymer chain end to which it is attached. Thus the 31P NMR spectrum provides information whether the propagating anion is secondary or primary. A similar... 

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