Enrfo-Dicyclopentadiene

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

The importance of precoordination is also evident in the CuOTf-promoted Itt + Iit photocycloaddition of enrfo-dicyclopentadiene. This diene forms an isolable 2 1 complex with CuOTf involving e.ro-monodentate coordination with the 8,9-C=C bond of two molecules of diene. Consequently, intermolecular lir + lit photocycloaddition involving exo addition to the 8,9-C=C bond is strongly favored over intramolecular reaction between the 8,9-and 3,4-C=C bonds (eq 11). This contrasts with the intramolecular photocycloaddition that is promoted by high energy triplet... 

That CuOTf-catalyzed 2tt + 2tt photocycloadditions are not restricted to cyclic alkenes was first demonstrated in mixed cycloadditions involving allyl alcohol. To suppress homodimerization of enrfo-dicyclopentadiene (i.e. eq 11) the diene to Cu ratio is maintained at < 1 1 and allyl alcohol is used as solvent. Under these conditions, a high yield of mixed cycloadduct is generated (eq 19). ... 

Mono-substituted norbornenes as a rule contain a substituent in the 5-position and consist of endo- and exo-isomers. Since they are obtained by Diels-Alder condensation, the enrfo-isomer is dominant in reaction isomer mixtures. 5-Trimethylsilylnorbornene (NBSiMes), obtained by condensation of cyclopen-tadiene and vinyltrichlorosilane accompanied by methylation, contains predominantly endo-isomer (70-75%) [177]. The interaction of dicyclopentadiene with vinyltrimethylsilane at higher temperature (200-210°C) led to equimolar mixture of endo- and exo-5-trimethylsilylnorbornene [187], It was shown that silyl-substituted norbornenes are extremely active in ROMP (Table 2), exo-isomers being more active than endo-forms [153, 188]. 

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