Quadricyclane strained-ring

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. 

Examples of well-known photochemical reactions which involve electron transfer include the primary step in plant and bacterial photosynthesis [2], the photoreduction of ketones by amines [3], a series of sensitized isomerizations of olefins and small ring compounds such as cyclopropanes or of strained polycyclics such as quadricyclane to norbornadiene or Dewar benzenes to benzenes [4], and the reactions of electron-rich substrates in the presence of oxygen which proceed via superoxide [5]. These reactions and others have proved valuable for synthetic applications in addition to their fundamental interest to photochemists. 

Quadricyclane effectively contains two cyclopropyl rings and it can be converted to bicyclo [2.2. l]hepta-2,5-diene when treated with numerous transition metal complexes (equation 9). The reaction is of current interest as an energy storage system since bicyclo[2.2.1]hepta-2,5-diene can be photolyzed to quadricyclane. The most hkely mechanism for the decychzation reaction is initial interaction between the strained cyclopropyl rings and the transition metals d 2 orbitals followed by formation of the metallocycle (1). The isomerizations of heavily substituted quadricyclanes (equation 10) are slow. ... 

Silver(I) ions generally catalyze the rearrangement of strained polycyclic systems which contain cyclopropane groups. It was shown, however, that silver(I) cleaved one or two of the strained cyclopropane bonds in quadricyclane (13) in an oxidation reaction. A complex mixture was obtained on treatment of quadricyclane (13) with silver(I) trifluoroacetate. The main addition products contained two oxygen functions and one intact cyclopropane 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. ... 

Like protons, transition-metal ions are strongly acidic and they can, in principle, add to both the C—H and C—C bonds of alkanes. As already noted in the section on proton affinities (Table 1) strained cycloalkanes are intrinsically more basic than open-chain alkanes, and the reaction of cyclopropane with Pt((II) to form a platinacyclo-butane (equation 14) was the first reaction of a formally saturated hydrocarbon with a transition-metal ion . The driving force in this reaction is relief of the strain associated with the small ring. The resulting metallacyclobutane is essentially free of ring strain. Many low-valent transition-metal complexes have been found to react with cycloalkanes. Metal ions convet the strained hydrocarbons quadricyclane , cubane , bicyclo-[2.1.0]pentanes , bicyclo[3.1.0]hexanes , bicyclo[4.1.0]heptanes and bicyclo-butanes into less strained isomers (usually cyclohexanes). 

In the presence of electron-rich low-valent transition metal systems, strained cyclic hydrocarbons can undergo ring opening and react with CO2 to afford carboxylated products. The carboxylation reaction proceeds stoichiometrically with Ni(0) systems [150, 151]. For instance, Ni(bpy)(cod) (bpy = 2,2 -bipyridine cod = 1,4-cyclooctadiene) promoted the stoichiometric carboxylation of quadricyclane, probably through the reaction sequence illustrated in Scheme 4.29... 

---