Aromaticity cyclopropenes

Now let s look at some other compounds and determine whether they are aromatic. Cyclopropene is not aromatic because it does not have an uninterrupted ring of p orbital-bearing atoms. One of its ring atoms is sp hybridized, and only sp and sp hybridized carbons have p orbitals. Therefore, cyclopropene does not fulfill the first criterion for aromaticity. 

Very recently, it has been shown that on the basis of the energetic criterion of antiaromaticity and the proton affinity of 3-cyclopropenyl anion (13) this ion does not merit being differentiated from other aUylic anions and is therefore best thought of as non-aromatic. Cyclopropene is the smallest cycloafkene, and its conjugate base at C3 is considered to be a special anion that is destabihzed due to the presence of 4jt electrons in this fuUy conjugated monocycHc species. Its acidity, however, follows the same correlation as for cyclobutene, cyclopentene, cyclohexene, and propene. No additional parameter beyond the central C—C—C bond angle is needed to explain or account for the weak acidity of cyclopropene. 

Benzocyclopropene is an intriguing example in which the electronic structure of benzene is greatly perturbed by the fusion of the smallest alicyclic ring, cyclopropene, to the aromatic system. Benzocyclopropene thus arouses theoretical interest and the high strain energy (approximately 68 kcal./mole)3 associated with the compound suggests unusual chemical reactivity. A review article has recently appeared.4... 

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

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 chemistry of cycloproparenes is characterized by the interplay of two contradictory effects, namely aromaticity which is generally known to stabilize compounds, and strain which destabilizes them. The fusion of a cyclopropene to a benzene ring results in geometrical distortions and strain which has consequences on the properties of the resulting cycloproparene. It perturbs the aromatic Ji-electron... 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

In contrast, isomers of 115 have so far not been isolated. An early attempt to generate cyclopropa[a,e]naphthalene (118) failed. More recently, the generation of dicyclopropa[a,c]naphthalene (119) was attempted by reaction of 120 with base. When the aromatization was carried out in the presence of DPIBF (44), stereoi-someric bis-adducts of cyclopropenes were isolated. However, the adducts provide no evidence for the formation of 119 as a reactive intermediate, since they are formed by sequential elimination-cycloaddition via 121. Cyclopropene interception of 121 is faster than further elimination to 119. The failure of the reaction to produce 119 has been attributed to the high strain energy of the product, which is estimated some 2 8 kcal/mol higher than that expected for two isolated cyclopropene units. ... 

The consequences of the distortion of the aromatic benzene ring by fusion to a cyclopropene, as it occurs in cycloproparenes, has been the subject of much discussion and speculation. Much of this debate concerned the question of bond fixation in strained aromatics which has a long historical background. In 1930 Mills and Nixon observed different reactivities towards electrophilic substitution of the a and positions in tetralin (251) and indane (252). This observation was... 

A compared to the same bond in cyclopropene. These features of the geometry of the borirene ring may be regarded, in accordance with the structural criteria, as evidence for the aromaticity of borirene. 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

Stabilization of the cyclopropene formed during the dehydrohalogenation may also be achieved by letting its double bond become part of an aromatic system. By this approach several highly strained hydrocarbons like cyclopropabenzene, cyclopropanaphthalene etc. have been synthesized. 

Dehydrochlorination of pentachlorocyclopropane, formed from trichloroethylene and sodium trichloroacetate as a source of dichlorocarbene, yields tetrachloro-cyclopropene [150], a particularly versatile reagent for various applications. It is a reasonably reactive dienophile [151], a reagent applicable to heterocyclic synthesis [152], and an electrophile for aromatic substitutions [153] and additions to alkenes [154] in the presence of Lewis acids. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

Reaction of cyclopropenes with cyclopropenium ions leads to a three carbon ring expansion to aromatic derivatives, presumably through ring opening of an initially formed cyclopropenyl-cyclopropylcation 276) ... 

The cyclopropenyl cation is the simplest aromatic system, and thus of some theoretical interest. In addition, the chemistry of cyclopropane derivatives is full of Interesting rearrangements to other novel structures,9 reflecting the great strain energy Of the cyclopropene ring. 

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