Quadricyclane, valence isomerization

Photolysis (> 290 nm) induced quantitative conversion of (20c and d) to the crystalline 3-azaquadricyclanes (22c and d) that isomerize in solution at 20° to the azepines (23c and d) at measurable rates. The azepines rapidly dimerize. The thermally forbidden 47 reversal of 22 to 20 was not observed.80 Prinzbach and Eberbach81 have carried out numerous cycloadditions and acid-catalyzed rearrangements of 22c and d and of 23c and d. The 3-azaquadricyclanes 22 have diene properties but, as valence isomerization to azepines is so easy (above), cycloadditions were achieved only with the very reactive dienophiles DMAD and EP. Reactions with 22c and d at -15° to - 30° over 24 to 48 hours using a large excess of ester gave 24 (R = H or E). If equimolecular proportions of the quadricyclane and ester are used, the products (24) compete as dienophile for example, 24d (R = H) added to 20d to give 25.81... 

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],... 

A number of complexes, notably of Rh1, Pd°, and Ni° will induce valence isomerization of strained hydrocarbons. A typical example is the interconversion of norborna-diene and quadricyclane. 

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."... 

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-... 

CIDNP experiments have also been performed to gain insight into the mechanisms of PET induced [2 + 4] cyclizations of quinones with allenes [119], as well as of the photocyloadditions of chloranil with furan derivatives [120] and with N-vinylpyrrolin-2-one [121]. The sensitized valence isomerizations of norbomadiene and quadricyclane have received some... 

Nickel (0) -Catalyzed Valence Isomerization of Quadricyclane to Nor-bornadiene Catalytic Trapping of the Organometallic Intermediate... 

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). ... 

The goal to utilize photochemical valence isomerization between norbomadiene and quadricyclane for solar energy collection and storage was reported by others [185]. Nagai et al. synthesized five different polymers with trifluoromethyl-substituted norbomadiene moieties in the side chains and in the main chain [185] ... 

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. 

According to our definitions, the well studied valence isomerization of quadricyclane to norbomadiene in the presence of appropriate transition metal complexes is also regarded as a photoinduced catalytic reaction. This reaction was recently discussed by Kutal[35], [36] as photo-generated catalysis. The observed quantum yield exceeds unity (( )s = 1.6) because a chain reaction is involved. Photochemically formed [Ru(bpy)3] , e.g., initiates the chain reaction by oxidation of quadricyclane to the corresponding cation radical which acts as chain carrier. The quadricyclane cation radical undergoes an isomerization to the more stable norbomadiene cation radical. The oxidation of quadricyclane by the latter one leads to the formation of norbomadiene as the product upon regeneration of the chain carrier. 

The photochemical valence isomerization of N residues in poly(ester-amide)s 102 occurs smoothly to give the corresponding quadricyclane-containing polymer 103 upon irradiation with sunlight in films or in THF solution.The back-photochemical reaction proceeds also in the polymeric films 103 under irradiation with 272 nm fight. The value of the stored thermal energy in such systems is about 84 kj mok. ... 

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. 

Onishi, M., Hiraki, K., Itoh, H., Eguchi, H., and Abe, S., Synthesis of new ferrocenylphosphinecop-per(I) complexes and their application to the valence isomerization of norbomadiene to quadricyclane, Inorg. Chim. Acta, 145,105-109, 1988. 

Grutsch, P.A. and Kutal, C., Charge-transfer sensitization of the valence isomerization of norbor-nadiene to quadricyclane by an orthometallated transition-metal complex,/. Am. Chem. Soc., 108, 3108-3110, 1986. 

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. 

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. 

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. 

Yang, L., Zhang, M., Liu, Y, Liu, Z., and Chow, Y.L., Triplex promoted intersystem crossing of ion-radical pairs in the photosensitized valence isomerization of quadricyclane chemically induced dynamic nuclear polarization (CIDNP) evidence,/. Chem. Soc., Chem. Commun., 1055-1056,1995. Kajitani, M., Fujita, X, Hisamatsu, N., Hatano, H., Akiyama, X, and Sugimori, A., Photochemical reactions of several adducts of metalladithiolenes and metalladithiazoles. Dissociation and rearrangement, Coord. Chem. Rev., 135, 175-180, 1994. 

Maruyama, K., Tamiaki, H., and Yanai, T, Valence isomerization between water-soluble norbor-nadiene and quadricyclane derivative. Bull. Chem. Soc. Jpn., 58, 781-782,1985. 

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

Hirao, K., Ando, A., Hamada, T., and Yonemitsu, O., Valence isomerization between coloured acylnorbornadienes and quadricyclanes as a promising model for visible (solar) light energy storage,/. Chem. Soc., Chem. Commun., 300-302,1984. 

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... 

Sulfur dioxide cycloadds to quadricyclane 3 to give the unstable /9-sultine 4 as the major product.2 It also causes isomerization of 3 to I (a valence lautomef). 

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