Norbomadiene—quadricyclane

This reaction proceeds readily with metal ions such as Fe1 or Cu in the gas phase.143 The forward reaction is catalyzed, for example, by PdCl2(norbomadiene) while the norbomadiene — quadricyclane photoisomerization is sensitized by an Irm ortho-metallated complex, [(bipy)2Ir(2,2 -bipyrid-3-yl-C3,N1)]2+. 

The polymer cobalt containing prophyrins (32), (33) were used in the photochemical energy storage system norbomadiene-quadricyclane (Eq. IS) This system for storage of solar energy is useful because of the high AH value between the two valence isomers on one side and the low price for the starting material (norbornadiene) on the other side. 

Bren, V.A., Dubonosov, A.D., Minkin, V.I., and Chernoivanov, VA., Norbomadiene-quadricyclane — an effective molecular system for the storage of solar energy, Russ. Chem. Rev., 60,451—469,1991. 

Nishino, H. and Inoue, Y, Chiral norbomadiene-quadricyclane derivative and reversible optical recording material using it, Jpn. Kokai Tokkyo Koho JP 2000086588, 2000. 

In 15-58, we used the principle of conservation of orbital symmetry to explain why certain reactions take place readily and others do not. The orbital symmetry principle can also explain why certain molecules are stable though highly strained. For example, quadricyclane and hexamethylprismane are thermodynamically much less stable (because much more strained) than their corresponding isomeric dienes, norbomadiene and hexamethylbicyclo[2.2.0]hexadiene (108). Yet the... 

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. 

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.

Solar energy can be stored by isomerizing organic substance photochemically to an energy-rich unstable isomer. The photoisomerization of norbomadiene (NBD) to quadricyclane (QC) (Eq. (26)) is the most wellknown system to store solar energy one this principle 100a). 

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... 

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

Cyclopentadienyl dithiolene complexes such as CpCo(S2C2R2) and Cp2Ti-(S2C2R2) have been shown to catalyze the isomerization of quadricyclane to norbomadiene (Eq. 9) (133). The catalytic activity is closely related to the reduction potential of the complexes Complexes with higher E /2 are more active. This result suggests that the reaction involves a certain degree of charge transfer between quadricyclane and the dithiolene complex. 

The quadricyclane-1-carboxylate /norbomadiene-2-carboxylate 49/48 seems to be the only asymmetric isomerization studied successfully [106,117]. We tried the isomerization of 3-(l-methylpropyl)azobenzene with cpl, but no CD effect appeared. 

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],... 

Photocyclizations are usually observed with symmetric systems. Indeed, when vinyl-vinyl bridging is equally possible for the four alkenic carbons, a [2 + 2] photocyclization can occur. As an example, norbomadiene prefers to give quadricyclane via photocyclization rather than the tricyclic di-ir-methane product. The latter, however, has been generat from the appropiate azoalkane as shown in... 

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