Propenium cation

The simplest of all of the homoaromatics is the homocyclopropenium cation [2], This species was first invoked as an unusually stable intermediate by Applequist and Roberts (1956) to explain the reactions of methylene cyclobutenes with electrophiles. Since that time a wealth of studies has provided data supporting the special stability of the monohomocyclo-propenium cation (Story and Clark, 1972 Paquette, 1978). The parent cation and several simple derivatives have been examined by a variety of techniques including the convincing, direct H and 13C NMR observation... 

Ga3R3]2- (R = bulky aryl group) [24], isoelectronic with the aromatic cyclo-propenium cation. In addition, tin clusters exhibit a variety of interesting cage structures (see Chapter 2.5.4) [25]. 

Me2N)3C3]2[Pt2Cle] 0.058 Pt2Clg ions sandwiched between two parallel cyclo-propenium cations. Pt—Pt 3.481 A 109... 

Berndt et al. demonstrated that 1,2-diboretanes with electron-deficient boron atoms prefer the non-classical 125a to the classical 125b structure. The nonclassical structure is characterized by two 3c/2e interactions, one for the hydrogen-bridged B—H—B and one for the homoaromatic B—C—B three-membered ring (isoelectronic with the homocyclo-propenium cation). Bemdt et al. prepared three derivatives (126) of 1,2-diboretane. They obtained an... 

Attention was drawn [2] to the contrast between triphenylcyclo-propenium bromide which exists as an ionic salt and triphenylmethyl bromide in which the bromine atom is covalently bound. Yet strain factors, and the increased electronegativity of the ring carbon atoms in the cyclopropene derivative, due to the different hybridisation in the three-membered ring, should favour ionic dissociation of the triphenylmethyl compound rather than the triphenylcyclo-propenium derivative. The stable existence of this cyclopropenium cation was thus evidence for the extra stability conferred on this ion by its delocalised system of two 7r-electrons. 

The crystal structures of various Ni, Pd, and Pt triphenylcyclo-propenium complexes of type [M(C3Ph3)(PPh3)2] have been determined. While the Pt complex with PFe as counterion adopts structure 14, the Pd cation with PFe and CIO4 have structures 15 and 16, respectively, and the Ni complex adopts structure 17. An analysis of the details of these solid state structures in conjunction with MO calculations has led to the conclusion that these chart out the likely pathway for ring-whizzing as shown in Scheme 4. ... 

The reactions of cyclopropenone with a wide variety of reagents have been reported by Breslow, Oda, and Pecoraro. " Bromine gives an acyclic product after initial electrophilic attack at the carbonyl oxygen rather than at the double bond. As expected, triethyloxonium tetrafluoroborate gives ethoxycyclo-propenium tetrafluoroborate, and trifluoroacetic acid opens the C-1—C-2 bond, but this could conceivably occur after initial addition to the C-2—C-3 double bond. A cyclopropenyl cation is also a proposed intermediate in the... 

Olah GA, Staral JS, Spear RJ, Liang G (1975) Novel aromatic systems. 11. Cyclobutenyl cations and the question of their homoaromaticity. Preparation and study of the homocyclo-propenium ion, the simplest homoaromatic system. J Am Chem Soc 97 5489... 

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