Cyclopropane strained ring cations

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

This conclusion raises the question whether the radical cation of 107 may be localized on a single cyclopropane entity rather than involving both strained rings. The CIDNP method does not lend itself to decide this issue, because the two cyclopropane groups are magnetically equivalent in the substrate/product. The ESR method, on the other hand, is well suited to elucidate the precise nature of... 

The vinylcyclopropane radical cation, is another radical cation of structure type B, which is stabilized by conjugation. Its proposed structure was based exclusively on ab initio calculations (B3LYP/6-31G ) because the electron-transfer photochemistry of this species failed to provide clear-cut CIDNP effects [128], In this context it is worth noting that product studies cannot, in principle, establish the cyclopropane radical cation structure type. Irrespective of the structure, nucleophilic capture is expected to result in the cleavage of the strained ring. 

As has been discussed briefly in Section III above, the ring strain of the cyclopropenes is accommodated, in part, by somewhat unusual bonding to the substituents. Structural and spectroscopic studies suggest that the vinylic C-H bonds have ca. 44% s character and that this is approximately constant in bonding to other substituents. Consequently aspects of the chemistry of the cyclopropenes are expected to reflect this with enhanced acidity of the vinylic proton. Cleavage of an exocyclic bond at C(3) will result in the cyclopropenyl cation, radical or anion with consequent interaction with the n bond and perturbation of the energy of the system. By comparison addition to the n-system to yield a cyclopropane relieves ring strain by ca. llOkJmol" ... 

Facile cycloalkenylations of carbonyl groups have been carried out with cyclopropylphosphonium fluoroborates . Complex carbocyclic systems, such as the sesquiterpene a-cedrene, can be effectively constructed by cationic cyclization Epoxide cleavage which follows upon dissolving metal reduction of proximal cyclopropane rings makes possible the ready synthesis of functionalized strained ring compounds inaccessible by other methods... 

However, orbital factors may override thermodynamic control. For example, the regiochemistry of nucleophilic attack on the bridged norcaradiene radical cation 122 shows a significant deviation from thermodynamic control. Although attack on the cyclopropane ring should be favored by both release of ring strain and formation of delocalized free radicals (cf. Scheme 6.8), methanol attacks 122 " selectively at C2 (and C5), generating 123 and 124. There is little stereoselectivity Products derived from 123 and 124 were formed in comparable yields. ... 

The closed form 71-C has not been considered to be the energy minimum on the potential energy surface. Not only are the properties of the observed cation inconsistent with the presence of two cyclopropane rings, but this ion is expected to be highly strained. Distinction between 71-D and 71-0 has proved to be more difficult. 

Strained cyclic systems such as cyclopropane derivatives proved sensitive to silver ions. Indeed, silver usually induced ring opening of cyclopropanes, leading to argento cationic intermediates, which evolved further depending on substituents. Most, if not all, of these ring rearrangements resulted from initial formation of a a complex of the cyclopropane with silver ion. 

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. 

A similar system with additional strain on the cyclopropane ring is quadricyclane. Its reaction with benzeneselanyl chloride in apolar solvents gave quantitatively addition products with preference for the formation of stereoisomeric 3-chloro-2-phenylselanylbicyclo[2.2.1]hept-5-enes 4 and 5. In polar or protic solvents such as acetonitrile, methanol or acetic acid, the competing opening of only one of the cyclopropane rings became dominant. The benzeneselanyl cation cleaved the most strained C — C bond in cyclopropane with concomitant or subsequent reaction with a nucleophilic solvent molecule. ... 

Theoretical calculations, such as those performed in this work, lead to the structure presented in Fig. 16, in which the plane dehned by the carbons of the 2-propyl moiety is a perpendicular bisector of the cyclopropyl ring, as in the case of the 1-cyclopropylethyl cation. This is an important result, in full agreement with the NMR spectrum of the ion. Furthermore, rotation around the C(l)-C(a) bond involves a barrier of ca. 13.7 kcal mol These structural features, as well as those presented by 15 and 17, are consequences of the stabilizing interaction between the strained C-C bonds of the cyclopropane moiety and the formally empty p orbital of the alkyl moiety. 

The cation contains a three member ring. This ring is similar to cyclopropane, or ethylene oxide, and so are their properties. A combination of angle strain and positive charge on the carbons in the ring make the cyclic carbons susceptible to nucleophilic reagents. Therefore, the mechanism is ... 

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