Norbornadiene valence isomerization

Sensitized isomerization of norbornadiene Valence isomerization of norbomadiene to quadricyclene has been suggested as a possible means of photochemical conversion and storage of solar energy. The isomerization of... 

Nakabayashi, K. and Takamuki, S., Triplet sensitized quadricyclane-norbornadiene valence isomerization of methyl 5-(4-biphenyl)tetracyclo[3.2.0.2 0. ]heptane-l-carboxylate, Bull. Chem. Soc. Jpn., 65, 3177-3179, 1992. 

Cassar and Halpern (85) provided evidence that the Rh -catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism ... 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbornadiene to quadricyclene conversion.

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Both the bidentate and monodentate processes probably play a role in the catalysis of symmetry-restricted reactions. For some reactions, hke the cyclobutanation of olefin Hgands suggested in olefin metathesis bidentate coordination is essential to the critical transformation. In other processes, such as the valence isomerization of quadricyclene 9) to norbornadiene (10) 10), both paths are available. 

Valence Isomerism.—Photochemical energy storage via the photosensitized valence isomerization of norbornadiene to quadricyclene has continued to attract attention. The effects of excitation wavelength and suppression of side reactions have been studied and the efficiency of various photosensitizers has been compared. It has been concluded that such systems are technically feasible but economically non-competitive at present. Similar valence... 

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). 

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."... 

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes ", the photocatalyzed valence isomerization of norbornadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially useful However, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

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-... 

Recent work (20, 21) in our laboratory has focused upon the use of transition metal compounds to sensitize the energy-storing valence isomerization of norbornadiene, NBD, to quadricyclene, Q (Reaction 3). In particular we have found that a catalytic amount of CuCl functions as an effective and quite specific sensitizer for this transformation. Conversions of greater than 90% have been achieved since Cu(I) is ineffective as a catalyst for the energy-releasing reverse reaction. Spectral and photochemical evidence support a mechanism which features a 1 1 ClCu—NBD complex as the photoactive species. As illustrated in Figure 3, an obvious consequence of complexation is a shift of the absorption spectrum of the system into a region accessible to the 313-nm irradiation used. Possible pathways by which the photo-excited complex relaxes to Q have been discussed (12). 

The donor acceptor norbornadiene derivative (116) exhibits high quantum yields for valence isomerization using blue light/ Intramolecular electron transfer is involved in ring opening of the quadricyclane derivative (117) to the corresponding norbornadiene. The study has indicated that electron transfer occurs from the quadricyclane moiety to the BF2 chromophore. Further work on such systems has shown that intramolecular triplet energy transfer from the carbazole to the norbornadiene moiety also occurs in the molecule (118). ... 

This photo-isomerization reaction is referred to as a valence isomerization. It is a reaction in which electron reshuffling occurs and the nuclei move to make or break new n and ct bonds. A number of polymers were, therefore, prepared with the norbomandiene moieties either in the backbone or as pendant groups. Among them are polyesters that were synthesized with donor-acceptor norbornadiene residues in the main chain [184] by polyaddition of 5-(4-methoxyphenyl)-l,4,6,7,7-pentamethyl-2,5-norbomadiene-2,3-dicarboxylic acid or 5,6-bis(4-methoxyphenyl)-7,7- dimethyl-2,5-norbomadiene-2. 

The hydrogenolysis of 3-acetoxynortricyclene (481) using Hj-Pt in the presence of traces of perchloric add afforded 7-acetoxynorbornane (68 %) and 2-exo-norbomyl acetate (32%) the formation of (482) using Dj-Pt indicates >95% stereospecific deuterium incorporation into the 2,6-endo-positions. Similar deuterolysis of 7-acetoxyquadricyclane gave the 5yn- ndo,anti-exo-tetradeuterio-derivative (483), which is also formed from 7-acetoxynorbornadiene in fact valence isomerization of the quadricyclane to the norbornadiene is a prelude to the deuterium incorporation. 

Juris, A., Sandrini, D., and Rancati, E., New coordination compounds as tentative photosensitizers for the valence isomerization of norbornadiene to quadricyclane, Chim. Ind., 62, 837-842, 1980. 

Schwendiman, D.P. and Kutal, C., Transition metal photoassisted valence isomerization of norbornadiene. An attractive energy-storage reaction, Inorg. Chem., 16, 719-721, 1977. 

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. 

Wu, Q.H., Zhang, B.W., Ming, Y.F., and Cao, Y., Inter- and intramolecular electron transfer isomerization of norbornadiene derivatives,/. Photochem. Photobiol. A Chem., 61, 53-63, 1991. Okada, K., Sakai, H., Oda, M., and Kikuchi, K., Intramolecular X -energy transfer from anthryl group studied by stepwise two-color two-photon excitation a cis to trans isomerization and a valence isomerization, Chem. Lett., 977-978, 1995. 

Maruyama, K., Terada, K., and Yamamoto, Y., Highly efficient valence isomerization between norbornadiene and quadricyclane derivatives under sunUght, Chem. Lett., 839-842, 1981. 

Nishikubo, T, Kawashima, T., Inomata, K., and Kameyama, A., Design of multifunctional polymeric photosensitizers containing pendant (nitroaryl)oxy groups and quaternary onium salts for photochemical valence isomerization of potassium 3-phenyl-2,5-norbornadiene-2-carboxylate, Macromolecules, 25, 2312-2318, 1992. 

Wang, X.S., Zhang, B.W., and Cao, Y, Valence isomerization of norbornadiene in polymer systems for solar energy storage, /. Photochem. Photobiol. A Chem., 96,193-198,1996. 

Photochemical valence isomerization between norbornadiene 83 and quadricyclane 84, which includes [2-1-2]-cycloaddition, has long been of interest because of its potential use in solar energy conversion and storage. Photoenergy can be stored as strain energy ( 20 kcal/mol) in a molecule of 84 however, the problem is that the photoreaction of 83 does not ordinarily occur on irradiation with sunhght due to... 

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