Homoaromatic stabilization

Predict whether the following systems would be expected to show strong (aromatic or homoaromatic) stabilization, weak stabilization by conjugation (non-aromatic) or destabilization (antiaromatic) relative to localized model structures. Explain the basis for your prediction. 

Another measure of the homoaromatic stabilization of the homotropylium systems can be gained from the heats of protonation of the ketones [17]—[20] (Childs et al., 1983). The difference in heats of protonation between [18] and [19] (A//I8 i9) is significantly larger than the corresponding difference between [17] and [18] (AHl7 ls). This increase in stabilization is associated with the homoaromaticity of [21]. Similarly, there is a large discontinuity between A//17 18 and A//18 2o which is associated with the aromaticity of the tropylium ion [22]. 

There is, no doubt, a decrease in the homoaromatic stabilization when conjugation is interrupted in two (bishomoaromatic) or more places (Paquette et al., 1977a Paquette, 1978). In spite of this, several bishomoaromatic cations have been well characterized. The simplest bishomoaromatic the 4-cyclopentyl cation [29] is, as yet, unknown (see Olah et al., 1972, and references cited therein). The cation [29] was not detected by NMR studies, nor was it invoked as an intermediate in the solvolysis of appropriately substituted cyclopentenes. 

Presumably, in these systems the energy cost of ring puckering is greater than the stabilization due to bishomoaromaticity (Bartlett and Rice, 1963). However, if the ring is already forced into a puckered conformation (e.g. by bridging between the 3 and 5 positions) then homoaromatic stabilization becomes effective. Solvolytic studies on the bridged systems [30] and [31]... 

The trishomoaromatic 1,3-dehydro-5-adamantyl cation [52] has been invoked to explain the extremely high reactivity of the bromide [53] (Scott and Pincock, 1973). Recent calculations suggest that [52] does indeed enjoy homoaromatic stabilization to the extent of 25.4 kcal mol1 relative to the 1-adamantyl cation and 1,3-dehydroadamantane (Bremer et al., 1987). 

Thiin dioxides form anions which appear to have some homoaromatic stability. The anions react with aldehydes (Scheme 54). 

Alternatively, we can deduce some homoaromatic stabilization of bicyclo[4.1.0]hepta-2,4-diene and homo antiaromatic destabilization of bicyclo[2.1.0]pent-2-ene from enthalpies associated with hydrogenation to form the saturated bicycloalkanes. The bicy-cloheptadiene hydrogenation enthalpy (i.e. 39 —> 18, n = 6) is ca 200 kJ mol-1, to be compared with (229.6 1.3) kJ mol1 for the hydrogenation of the corresponding monocyclic... 

What is found for the homoaromatic stabilization for bicyclo[4.1,0]hepta-2,4-diene (39) The natural comparison in the current section is between the enthalpies of formation of this species and that of bicyclo[4.2.0]octa-2,4-diene (56, n = 4). We find103 a difference of ca 13 kJ mol1. By contrast, the difference of the enthalpies of formation of bicy-clo[4.1. OJheptane (18) and bicyclo[4.2.0]octane (53, n = 4) is 27.5 kJ mol1. This suggests ca 15 kJ mol-1 homoaromatic stabilization for bicyclo[4.1.0]hepta-2,4-diene, in qualitative agreement with the earlier presented value" of 30 kJ mol 1. 

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. 

Homoaromatic stabilization energies from calculations with a... 

Evaluation of homoaromatic stabilization energies by using isodesmic reactions... 

One can avoid these problems by using Coupled Cluster (CC) theory107, which contains infinite-order effects and therefore does not lead to the oscillatory behaviour of properties calculated with MPn108. Homoaromatic stabilization energies have been calculated for smaller molecules with CCSD(T) or QCISD(T)54 56. These are CC methods, which cover S and D excitations and, in addition, include T effects in a perturbational way109110. They represent some of the most accurate single determinant ab initio methods available today that can be applied in a routine way. 

The description of no-bond homoaromatic systems as frozen transition states is in line with the observation that their PES is rather flat in the direction of the interaction distance. This means that (a) homoaromatic stabilization energies are small (see Table 2 and the discussion presented above) and (b) relatively small energy increases lead to relatively large... 

A more direct approach to measurement of homoaromatic stabilization was taken by Childs and colleagues in which the heats of protonation of the series of ketones 26-31 in FS03H were measured102. Of particular interest are the differences in heats of protonation for the various ketones and these data are summarized in Scheme 7. 

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