Oxirene

The molecules of Chapters 1 and 2 were species that defy one of the basic principles of the structural theoiy of organic chemistiy (or at least the theory in its simplest form, based on experience, without invoking electronic theory) the valences of tetracoordinate carbon are directed toward the comers of a tetrahedron. They are molecules that, within the confines of the theory, should not exist. The subject of this chapter, in contrast, appears to defy no rules of the stmctural theory oxirene (this is the usual name it is based on the clumsy Hantzsch-Widman system, in which heterocyclic rings of 3, 4, etc. atoms are given special names like oxirane, oxirene, oxetane, etc. An alternative system utilizes the currently neglected logical and convenient method of replacement nomenclature oxacyclopropane, oxacyclo-propene, oxacyclobutane, etc. ), 1, looks at first glance like a normal molecule. No unusual stereochemical constraints are imposed on the molecule, all atoms have 

On reflection two moderately disturbing features emerge having a three-membered ring oxirene is strained [1, 2, 3,4, 5], and (Fig. 3.1) one electron pair on the oxygen is at least potentially able to enhst the assistance of the two -ir electrons of the double bond to corrfer antiaromaticity [6,7] on the ring arrd thus to electronically destabilize it. Let us corrsider these features in turn. 

Strain is a ubiquitous and enormously important concept in cherrristry, although there is no uniquely correct quantitative definition of it. A qualitative - and useful -feel for the idea is readily acquired by attempting to corrstract strained molecules like oxirene or cyclopropane with ball- and stick molectrlar models if the sticks are 

Levjars, Modeling Marvels, DOI 10.1007/978-l-4020-6973 3, Springer Science+Business Media B.V. 2008 

Orbital overlap in cyclopropane, using pure p atomic orbitals 

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Including oxiranes and oxirenes under the same rubric is somewhat akin to attempting to present an integrated discussion of the chemistries of cyclohexane and benzene the... 

Oxirene (2) is one of a number of heterocycles in which the CH2 group of cyclopropene has been replaced by a group or element associated with Groups V or VI of the periodic table. Replacement of the CHj group of cyclopropene by an NH group gives l//-azirine... 

We may now be on the verge of the third phase of oxirene chemistry in which modern matrix isolation techniques (80CSR1) will permit the spectroscopic observation of this system, theory will serve as a guide to the synthesis of relatively stable oxirenes (c/. a fairly stable... 

It has very recently been suggested that this species is not oxirene, but the authors do not make it clear how their intermediate differs from an oxirene canonical form (82JA216). 

The simplest, qualitative, theoretical understanding of the nature of oxirene is provided by Breslow s concept of antiaromaticity. Whatever criticisms may be levelled at this notion (78JA6920), it does correctly predict that oxirene should be unusually unstable. 

Neither ground-state ethynol (hydroxyacetylene) (80) nor carbenaoxirane (81) appears to be a viable point of ingress to the oxirene-methanoylcarbene system, as both can isomerize to ketene by lower-energy pathways. The limited experimental information available on carbenaoxirane (Section 5.05.6.3.4(f/)) indicates that it is indeed largely isolated from the oxirene-methanoylcarbene manifold (but note the photolysis of ketene in Section 5.5.6.3.4(ff)) appropriate labelling experiments with (the unknown) ethynol have not been performed. 

Reactions Possibly Involving an Oxirene, or with the a priori Potential of Generating an Oxirene... 

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