Cyclopropenones aromaticity

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

The structural features and the spectroscopic characteristics of the thiirene dioxide system (22) are of special theoretical interest since, on the basis of analogy with cyclopropenone (23), it is a possible nonbenzenoid aromatic system with all the physical and chemical implications involved. Aromatic and/or conjugative effects, if any, require transmission through the d-orbitals of the sulfur atom. 

On the basis of a naive analogy with cyclopropenones, the ground-state aromatic stabilization of which has been recently reconfirmed, some kind of aromaticity can, in principle, be assigned to these systems when Z = SO or S02, assuming a possibility for transmission of electronic effects via -conjugation. 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

The isolation of cyclopropenones and their undoubtedly increased stability compared to the less-strained saturated cyclopropanones might well be attributed to the validity of the above symbolism of aromatic cyclopropenium contribution to the ground state of 7. It should nevertheless be clear, that the information available on the electronic structure of cyclopropenones demands certain refinements of this very useful qualitative concept. 

The synthesis of cyclopropenone imines 3 has been accomplished by several methods. Thus aromatic amines, e.g. p-nitraniline, can be reacted either with diphenyl cyclopropenone in HCl/ethanol or with the ethoxy cation 75 forming the immonium cation 150, which is deprotonated by tertiary bases to the N-(p-nitro-phenyl)-imine /5/llsl ... 

Thus aromaticity in cyclopropenone cannot be detected by the magnetic criterion although it is suggested by other chemical and physical properties. UV, IR, and -NMR data of cyclopropenone are summarized in Table 6, but are discussed in later chapters. 

Dimerization of cyclopropenones has also been found to occur under reductive conditions. Tetraphenyl resorcinol is formed in addition to a small amount of tetra-phenyl p-benzoquinone on treatment of diphenyl cyclopropenone with aluminum amalgam200 its formation can be rationalized via dimerization of the cyclopropenone ketyl 266 and subsequent aromatization, possibly according to a prismane mechanism. 

It is interesting to note that according to Ref.136) regardless of the model used, the CNDO/2 treatment predicts a larger ir-polarization for cyclopropenone than for tropone. This means that the electrostatic work to achieve a cyclopropenium oxide structure is considerably less than for cycloheptatrienylium oxide. From this reason, cyclopropenone seems to be a closer approximation to an aromatic system than tropone which can be described better as a polyolefin. 

Three-membered ring systems have also provided examples of aromatic and antiaromatic behavior. Despite the very substantial angle strain, Breslow and his collaborators have succeeded in preparing a number of cyclopropenyl cations (51).30 Cyclopropenone (52) has been isolated and is stable in pure form below... 

Aromatic fused anhydrides are well known to act as a source of arynes by pyrolytic loss of CO2 and CO. In some cases the cyclopropenone resulting from loss of only CO2 can be trapped and this is the case for 294, where FVP at 500 °C allows trapping of both the thienocyclopropenone 295 stabilized as the dipolar form shown, and the thiophyne 296147. 

Cyclopropenones (62) are an important class of nonbenzenoid aromatics, and so have an interesting conceptual blend of stabilizing and destabilizing molecular features. Yet the only cyclopropenone derivative for which there is available gas-phase enthalpy of formation data is the additionally conjugated diphenyl species 62, X - = Ph, namely 316.7... 

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