Antiaromatic, oxirenes

The antiaromatic oxirenes 11 are extremely elusive but their potential involvement in photochemical Wolff rearrangements by equilibration with the -oxocarbenes 12 has generated both theoretical and experimental interest and the subject has been reviewed <2004MR0291>. Experimental evidence for participation of oxirenes <2004MR0291> is supported by DFT calculations that suggest that oxirene corresponds to an energy minimum <2003JMT(629)263, CHEC-III(1.03.7)215>. 

Transition metal carbenes constitute a very important class of molecules that have found a multitude of applications. With reference to manganese, a number of new carbene complexes have been reported recently. The ) -alkyne complex (18) was found to undergo oxidation by dimethyldioxirane to afford the a-keto carbene (19). The fascinating aspect of this reaction is the likely existence of an oxirene intermediate (20). Stable oxirene complexes have never been reported, imdoubtedly because of the extreme instability of the antiaromatic oxirene ring. The possibility of trapping an oxirene by epoxidation of a coordinated alkyne is intriguing. Scheme 10 summarizes the chemistry involved. 

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. 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

We calculated the ASE as product energies minus reactant energies, as we did for benzene, and it came out negative, which means that the aromatic stabilization energy here is really destabilizing oxirene is antiaromatic [173]. 

Background information on the chemistry of oxirenes is presented in Chapter 1.03, and this area has been reviewed by Zeller <2002SOS(9)19>. The potential antiaromatic character of oxirenes, the strain of the unsaturated three-membered ring, and the possible involvement in important reactions (e.g., oxidation of alkynes, Wolff rearrangement)... 

Energetically, oxirene is found to be less stable than the oxiranylidene isomer (with 4-31G). Its low stability is also indicated by the very negative bond separation energy (Table 1) which reflects the unfavorable (antiaromatic) r-structure as well as the ff-bond strain. The total energy of oxirene is predicted at the 6-31G level to lie 89.3 kcal/mol above that of ketene.5 ) Clark, 80,38) in an earlier study of oxirene with a smaller basis also obtained a large value for this exothermicity (102.6 kcal/mol). Our calculated dipole moment (2.96D with 6-31G ) is slightly higher than the value (2.SOD) obtained by Clark.oo)... 

The bond separation energy is quite negative (—37.9 kcal/mol with 6-31G ), but less so than other antiaromatic molecules such as oxirene. This may partly be due to the distortion of the triangle noted above and also to a stabilizing interaction between the polar bonds. The total energy of oxazirine is predicted (by 6-31G ) to lie 97.3 kcal/mol above that of the most stable HCNO isomer, isocyanic acid. )... 

Considerable research effort has been directed towards the generation, detection, and isolation of oxirenes, a highly strained class of antiaromatic heterocycles <83CRV519>. Although the unsaturated oxiranes are called oxirenes, much confusion is generated by the fact that Chemical Abstracts denotes oxiranes fused onto a polycyclic aromatic compound as oxirenes also (e.g., compound (16), Figure 9 <91JHC473 . 

Due to their strain and putative electronic destabilization, such three-membered heterocycles possessing a cyclic array of 47r-electrons offer a considerable challenge to synthesis. Such molecules are expected to be both unimolecularly and bimolecularly reactive, if they exist at all as energy minima. Since no isolable tellurirenes have been reported, only some reactions of selenirenes are described in this section. Selenirene and its kindred systems, oxirene, azirine, and thiirene, are of interest because of their theoretical significance as prototypes of antiaromatic species. 

Oxirene is formally a 4 71 antiaromatic system, and furthermore contains considerable ring strain. The ring strain in cyclopropene is ca. 53 kcal/mol Oxirenes are therefore expected to be short-lived high-energy intermediates. There has been much discussion among the theoreticians whether oxirene is more or less stable than the isomeric oxocarbene. The conflicting results of calculations are indicated in Fig. 1. 

Nitrogen pentafluoride represents an interesting contrast to oxirene. Oxirene is, on paper, a reasonable molecule there is no obvious reason why, however unstable it might be because of antiaromaticity [4] or strain, it should not be able to exist. On the other hand, NFs defies the hallowed octet rule why should it be more reasonable than, say, CHfi Yet a comprehensive computational study of this molecule left tittle doubt that it is a (relative) minimum on its potential energy surface [5]. The full armamentarium of post-HF methods, CASSDF, MRCl, CCSDT, CCSD(T), MP2 (section 5.4) and DFT (chapter 7) was employed here, and all agreed that Dan (section 2.6) NF5 is a minimum. 

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. 

For oxirene, the "SE" here is contaminated with antiaromatic resonance energy O... 

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