Quadricyclane triplet state

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],... 

Photolyses of 3-azinortricyclane (45) were conducted within CyDs and FAU zeolites to alter the selectivity of the carbene intermediate, 3-nortricyclanylidene (46). It was hypothesized that constriction of carbene 46 within each host would suppress the formation of 4-ethynylcyclopentene (47) and concomitantly enhance that of quadricyclane (48). But only 47 was observed. Since CyD innermolecular products were also formed, control experiments in relevant solvents were performed. Both triplet-state participation and the Bronsted basicity of carbene 46 were hinted at. 

A detailed study of the photochemical cyclization of norbornadiene to quadricyclane using aryl phosphine copper(I) halides has been reported. The quadricyclane (144) is formed on irradiation of the norbornadiene (145) at 366 nm. The quantum yield (0.68) for the process is high. The influence of triplet sensitizers ( metal complexes, biacetyl and acetophenone) were found to be less efficient ( ct> = 0.1). The authors suggest that the lowest singlet state of norbornadiene is much more reactive than the triplet state. ... 

One typical example is given by G. Kaupp with the [2+2] cycloaddition of diphenyl-acetylene to 1,4-dioxene leading to a bicyclic cyclobutene in good yield (cf. iv of this chapter). Another example is presented by T. Miyashi starting from naphthoquinonor-bomadiene. In an intramolecular reaction a quadricyclane derivative is quantitatively formed upon irradiation at X > 400 nm. It is assumed that this process involves the diketo-norbomadiene in its triplet state. Although the spin multiplicity of intermediates have not... 

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-... 

Ab initio calculations [11] have shown that the photochemical cyclization of norbornadiene VIII to quadricyclan IX will occur most readily in the triplet-excited state, which could not be predicted while staying within the framework of orbital concepts. The processes of making of the bonds C1C3 and C2C4 in the reaction VIIIX are asynchronous, as may be seen from the structure VIII which corresponds to the most stable form of the lowest triplet state common to VII and IX. The length of the bond in VIII calculated in the MP2/6-31G approximation is 1.5 A, while the distance C2C4 amounts to 2.365 A. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

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