Isomerization norbornadiene-quadricyclane

A variety of bis(dithiolene) complexes has been reported as catalysts for the isomerization between quadricyclane and norbornadiene (Eq. 9) (86). 

Cyclopentadienyl dithiolene complexes such as CpCo(S2C2R2) and C p2Ti-(S2C2R2) have been shown to catalyze the isomerization of quadricyclane to norbornadiene (Eq. 9) (133). The catalytic activity is closely related to the reduction potential of the complexes Complexes with higher Zs]/2 are more active. This result suggests that the reaction involves a certain degree of charge transfer between quadricyclane and the dithiolene complex. 

Norbornadiene undergoes a photochemical valence isomerization to quadricyclane with unit quantum yield when photosensitized with acetophenone. The proposed reaction scheme (14) includes two triplet state intermediates norbornadiene triplet (14b) and the 3,5-nortricyclanediyl biradical (14c). Based on the full Breit—Pauli with a rather modest qual-... 

Isomerization of quadricyclane Quadricyclane (1) is isomerized mainly Id norbornadiene (2) in the presence of a nickel(O) catalyst bis(acrylonitrile)-lilckel(O) or bis(l,5-cyolooctadiene)nickel(0). Evidence that a complex such an (a) is involved in the product-determining step is presented. For example. Isomerization in the presence of acrylonitrile results in exo- and endo-isomers of the adduct (3). 

Catalysts for the Isomerization of Quadricyclane to Norbornadiene in a Photochemical Energy Storage System... 

No detailed mechanistic interpretation of the catalysis of the conversion of quadricyclane to norbornadiene by these triphenylcyclopro-penylnickel derivatives is warranted at the present time. However, these reactions appear to be quite similar to isomerizations of quadricyclane to norbornadiene catalyzed by other nickel complexes (14). 

The Ni -catalysed (2+2) cycloaddition of activated olefins such as acrylonitrile or acrylic acid esters to quadricyclane which gives endo- and exo-substituted products is complicated particularly in the case of 1,2-disubstituted olefins by a competing isomerization of quadricyclane to norbornadiene. Norbomadiene gives identical cycloadducts under these conditions with a similar isomer ratio. Kinetic studies of the cycloaddition reactions of quadricyclane gave a plot of log [quadricyclane]t o/ [quadricyclane]i=t against time which exhibited a marked deviation from linearity due to the retarding effect of norbomadiene which is also formed and in turn coordinates to the Ni catalyst. Scheme 11 accounts for these observations. 

Norbornadiene 24 rearranged to quadricyclane 25 upon UV irradiation in the presence of a sensitizer. Thermal isomerization of quadricyclane to norbornadiene proceeded at 140 °C with = 14 h. The reaction rate was dramatically increased by a transition metal-olefin complex (Scheme 2.25). For instance, t 2 at -26 Cis... 

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. 

Kutal, C., Kelley, C.K., and Ferraudi, G., Catalyzed valence isomerization of quadricyclane to norbornadiene via photochemical generation of a strong ground-state oxidant, Inorg. Chem., 26, 3258-3261, 1987. 

The results for quadricyclane (QC) shown in Fig. 4.6 are qualitatively similar to those for norbornadiene, except that the onset of decomposition for quadricyclane is about 100 K lower. QC is stable on the available time scale at temperatures up to 523 K, above which decomposition sets in rapidly. The QC is completely decomposed within 2 ms (Table 4.1) by about 750 K. As with norbornadiene, the dominant low-temperature decomposition channel is fragmentation to C5H6 + C2H2, while isomerization to more stable isomers (TOL) becomes increasingly important at higher temperatures. There is some evidence for production of norbornadiene in the 700-800 K temperature range, but at higher... 

In 5-47 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 hexamethylprismane1002 are thermodynamically much less stable (because much more strained) than their corresponding isomeric dienes, norbornadiene and hexamethylbicy-clo[2.2,0]hexadiene (105).1,103 Yet the former two compounds can be kept indefinitely at... 

The first report of triplet photochemistry of three-membered rings concerned the reversibility of the photosensitized isomerization of norbornadiene to quadricyclane.270 Since no spectroscopic triplet levels of cyclopropane are known, it was felt that this reaction represents an extreme sort of nonvertical energy transfer with actual bond breaking in the acceptor. 

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. 

Fig. 12. The role of stabilization by the cluster in the unimolecular isomerization of norbornadiene to quadricyclane in a Neizs cluster at an impact velocity of 6 km/s. Two curves are the bond distances of the C = C double bonds that, during bond switching become single bonds. The two other curves are C to C distances for two carbon atoms that aie not regarded as chemically bound in norbornadiene but that form a C-C single bond in quadricyclane. Note the high vibrational energy content of the new bonds and how the molecule is about to dissociate and is prevented from doing so by a deactivating collision.

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). 

Of the photochemical conversions of 3,6-bridged cyclohexa-1,4-dienes to 3,6-bridged syn-tricyclo[3.1.0.0 ]hexanes, the light-induced isomerization of norbornadienes to quadricyclanes (see Houben-Weyl, Vol.4/5a, pp 232-237) has been most intensively studied. The parent hydrocarbon, quadricyclane (1, tetracyclo[3.2.0.0 . 0 ]heptane) has become available on a preparative scale by acetophenone sensitized isomerization of norbornadiene (bicyclo [2.2.1]hepta-2,5-diene). ... 

Electron-withdrawing substituents on the norbornadiene C-C double bond facilitate the valence isomerization and also stabilize the quadricyclane formed by photoisomerization, as demonstrated by the selective formation of the methoxycarbonyl-substituted quadricyclane 4 (mp 50-52 C) from both 2 and 3 on direct irradiation. ... 

Obviously, substitution of the norbornadiene double bond with two electron-withdrawing groups lowers the activation energy barrier for valence isomerization even more. A summary of examples for the preparation of 1,5-disubstituted quadricyclanes 4 is given."... 

The quadricyclane-l-carboxylate /norbornadiene-2-carboxylate 49/48 seems to be the only asymmetrie isomerization studied sueeessfully [106,117]. We tried the isomerization of 3-(l-methylpropyl)azobenzene with epl, but no CD effeet appeared. 

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