Quadricyclane radical cation

Haselbach et al. [9] also classified radical electrocyclic reactions in the three types shown in Fig. 2, but were the first to point out that formally state-forbidden radical ion reactions can be extremely facile because state crossings can occur at very low activation energies. The principles outlined were used to analyze the rearrangement of the quadricyclane radical cation, 1, to the norbor-nadiene radical cation, 2, a reaction that occurs at extremely low temperatures in Freon matrices [10]. 

The Radical Cations of the System Quadricyclane — Norbornadiene No discussion about cyclopropane radical cations would be complete without reference to the quadricyclane radical cation (Q-+) and its valence isomer, the... 

Table 6. Hyperfine coupling constants (Gauss) of bicyclo[2.2.1]heptadiene and quadricyclane radical cations.

The nucleophilic addition of methanol to quadricyclane radical cation, 90 ", produced two methanol adducts, 102, having a 3-ejco-methoxy group, and 103, bearing a 7-fl h -methoxy group. The stereochemistry of the methoxy groups in these structures identified the direction of nucleophilic attack upon 90 + as exclusively from the exo position [242], It can be viewed as a backside attack of the nucleophile on the weakened cyclopropane bond with inversion of configuration. 7-Methylenequadricyclane also was attacked exclusively from the exo face [243]. 

Several theoretical studies have been performed on thermal reactions. Ab initio electronic-structure calculations at the 6-3IG level have been performed to determine the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The thermal interconversion of the norbornadiene and quadricyclane radical cations has been studied, with geometries optimized at the UMP2/6-31G and UHF/6-31G levels. Two transition structures are proposed with C and C2 symmetry, respectively. They conclude by suggesting the true transition state is the Ci symmetric one whereas the second is a second-order saddle point. The thermal rearrangement of diformyl peroxide has been studied using the UHF/AMl MO method to determine the mechanism of the isomerization and the MP2 method to improve the barrier energies of the reaction. A stepwise radical reaction is proposed. ... 

Inadomi, Y., Morihashi, K., and Kikuchi, O., Theoretical study of the thermal interconversion mechanism between the norbornadiene and quadricyclane radical cations, /. Mol. Struct. (Theochem), 434, 59-66, 1998. 

Haselbach, E., Bally, T., Lanyiova, Z., and Baertschi, R, The type C valence-isomeric system quadricyclane radical cation/norbornadiene radical cation, Helv. Chim. Acta, 62, 583-588,1979. 

This same CIDNP study also ruled out the existence of the bishomo-aromatic system [158c] by finding two distinct species, [158a] and [158b], for the intramolecular cycloreversion of the radical cation of quadricyclane to that of norbornadiene. 

One of the interesting molecules that has been studied in considerable detail is nor-bomadiene (80). Much of this interest has been associated with the interactions between the double bonds of the system. Thus irradiation affords quadricyclane (81). This area of study will be discussed later in this chapter. The radical cation 82 can also be formed from both norbomadiene and quadricyclane by irradiation in acetonitrile/methanol solution with the DCB/phenanthrene sensitizer system. Several products (Scheme 2) are formed in low yield and it should be noted that there is little difference in the yields of products obtained from either starting material. However, it is evident that attack by methanol occurs from the exo face32. 

This is the case for the quadricyclane - to norbornadiene" reaction. Although the C2K reaction path provides an attractive interpretational tool for understanding the progress of this reaction, its highest point represents a conical intersection at which the two relevant states have the same energy at the same geometry. This point cannot be a transition state, so that lowering the symmetry in any direction leads to a stabilization. The result is an asynchronous reaction path in which one of the two cyclopropane bonds is broken first to form the biradical-like transition state la. The second bond can then break to form the norbornadiene radical cation 2. 

Optical spectroscopy has merits in identifying radical cations, particularly when their spectra are known independently. For example, the radiolysis of quadricyclane led to the observation of the known spectrum of norbornadiene radical cation. In another study, irradiation of cyclooctatetraene radical cation caused the color of the sample to change from bright red to royal blue, suggesting the conversion to a different species, the previously identified semibullvalene radical cation. Further irradiation of the latter led to a characteristic banded (vibrationally resolved) spectrum the nature of this spectrum suggested that the rearranged species may be a linear conjugated radical cation and helped in its identification as 1,4-dihydropen-talene radical cation. ... 

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. 

Figure 6.13. Schematic energy diagram for quadricyclane (15) and norbomadiene (16) and their radical cations. The respective minima on the two surfaces are in a unique relationship with characteristic changes in bond lengths and angles. Experimental and calculated hyperfine coupling constants (in parentheses G B3LYP/6-31G //MP2/6-31G ) are shown below.

The process is induced photochemically and involves the single-electron transfer oxidation of cubane then completed with a backward electron transfer to the transient radical cations. A Li+ salt with a weakly coordinating anion is able to induce pericyclic transformations, including the rearrangement of cubane to cuneane, quadricyclane to norbomadiene, and basketene to Nenitzescu s hydrocarbon 392... 

Fig. 21. Schematic energy diagram for norbornadiene, quadricyclane, and their radical cations. Each minimum on the radical cation potential energy surface corresponds to a minimum on the energy surface of the neutral diamagnetic precursor, with characteristic changes in bond lengths (ab initio, 6-31 G ) and angles [322]...

Table 4. 1H Hyperfine coupling constants [G] for radical cations of quadricyclane, norbomadiene, and their 7-methylene derivatives...

Fig. 22. 1H CIDNP spectra observed during the photoreaction of chloranil with norborna-diene (top) and quadricyclane (bottom). The insets in each spectrum show the hyperfine coupling constants calculated for the two radical cations [322], For the comparison of the calculated couplings with observed signal intensities, it should be noted that there are twice as many nuclei in the position N0 and Q0 as in the positions Nbh, Nb and Qbb, Qb...

The anionic and cationic ends of the NC s TICT state were found to be quenched by electron donors and acceptors respectively. The quenching rate constants were analyzed with the help of the Marcus model and found consistent with the expected value for electron transfer mechanism. This was confirmed by photochemical reactions running through a radical cation or a radical anion intermediate. The isomerization of quadricyclane to norborna-diene was used as a check for the reactivity of the radical-cation end of the NC s... 

Because of the low reduction potential of TCB, the energy content of the radical ion pair resulting from the reaction of triplet excited sensitizer 3TCB and many olefins is insufficient lead to the triplet state of the olefins. In this case the isomerization takes place via a chain radical-cation mechanism [194, 195], The same electron acceptor also induces the electron transfer-sensitized valence isomerization of quadricyclane to norbomadiene [196, 197],... 

Quadricyclanes (160) also undergo a valence isomerization to norborna-dienes if irradiated in the presence of electron acceptors such as fumaro-nitrile (Jones and Becker, 1982). Two distinct radical cation structures are observed for the hydrocarbon, corresponding roughly to the bonding patterns of norbornadiene and quadricylane, respectively (Roth et al., 1981). 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

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