Cyclobutenyl cation, homoaromaticity

The cyclobutenyl cation is the homoaromatic analog of the very stable cycloprope-nium cation. This ion can be prepared from 3-acetoxycyclobutene with the use of superacid conditions ... 

The cyclobutenyl cation (92) and the homotropylium cation, CgHcf 93 are both prototypes of homoaromatic systems. 

We reiterate that both homoaromaticity and aromaticity are more pronounced in ions than in related neutrals. In the tour-de-force of computational theory, S. Sieber, P. v. R. Schleyer, A. H. Otto, J. Gauss, F. Reichel and D. Cremer [7. Phys. Org. Chem., 6,445 (1993)] document considerable homoaromatic stabilization of the cyclobutenyl cation. Yet the difference of the enthalpy of formation they calculate from their quantum chemical cations, 1021 kj mol-1, is only 54 kJmol-1 lower than that archivally recommended for the cyclopropenium ion [S. G. Lias,... 

J. E. Bartmess, J. F. Liebman, J. L. Holmes, R. D. Levin and W. G. Mallard, J. Phys. Chem. Ref. Data, 17 (1988), Supplement 1], This last number allows us to conclude that the aromatic stabilization of cyclopropenium ion exceeds the homoaromatic stabilization of cyclobutenyl cation by at least (160 - 54) = 106 kJ mol-1. Regrettably, inadequate time and space does not allow us to discuss the energetics of ions containing three-membered rings in the current chapter. 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

The question of two rings going-on one is also seminal to the study of homoaromaticity. For n odd, the set of [(CH) CH2] ions demonstrate a delicate balance between mono and bicyclic structures (see Ref 133 and numerous references cited therein to both the experimental and theoretical literature). In the case of n = 3, one can imagine a planar cyclobutenyl cation (63) and a markedly non-planar, highly puckered bicyclobutyl cation (64). Both calculational theory on the parent and experiment on derivatives show the latter geometry to be preferred. However, as in the case of the other purported bicyclobutane derivatives characterized by the 2,4-carbons trigonally coordinated that were discussed earlier in this section, formal theory shows there is no 1,3-bond. The ion is not homoaromatic and there is no cyclopropane ring. In the case of n = 5,... 

Since the first preparation of cyclobutenyl cations in 1964 (97), the importance of 1,3 overlap in connection with the question of the homoaromatic nature of these cations has attracted considerable attention (98). In the case of the pentamethyl-substituted cation 67-l,3-(CD3)2 labeled by trideuteriomethyl... 

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... 

Roberts and coworkers, investigating the ionization reactions of cyclobutene derivatives, found that the resulting cyclobutenyl ions were unusually stable10-113. They suggested that rather than regarding these ions as simple allyl cations, their properties were consistent with a C(1 ),C(3) interaction and cyclic delocalization of the 7t-electrons. As such, these 27r-electron systems were considered to be the homoaromatic counterparts of the well established, aromatic cyclopropenium ions39. 

The structures and electron delocalization in these boron-substituted derivatives of the cyclobutenyl/homocyclopropenium cations are fully consistent with their designation as homoaromatic systems. 

Most of the work reported in this area is limited to carbocyclic systems. The recent developments with the boron analogues of the cyclobutenyl/homocyclopropenium and norbomenyl/norbornadienyl cations point to the potential importance of cyclopropyl homoconjugation and homoaromaticity in a much wider sphere of organic systems. This will likely be an area where there will be considerable further work. 

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