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The Bicycle Rearrangement. Relationship to the Di-?-methane Rearrangement and Control by Bifunnel Distortion. Mechanistic and Exploratory Organic Photochemistry
Zimmerman, Howard E.,Factor, Ronda E.
, p. 3538 - 3548 (1980)
The generality of the bicycle rearrangement was extended and the mechanism was further investigated by study of the photochemistry of 3,4-benzo-2-methylene-6,6-dimethylbicyclohex-3-ene, 2,2-dimethyl-1-methylene-1,2-dihydronaphthalene, 4,4-dimethyl-1-methylene-1,4-dihydronaphthalene, 2,2-dimethylspiro, and 1,1-dimethyl-2-methylene-1,2-dihydronaphthalene.Direct irradiation of the benzobicyclohexene led to the spiroindene as the major product with a quantum yield of 0.040.The minor photoproduct was the 2,2-dimethyldihydronaphthalene, formed with a quantum yield of 0.0033.Both products arise from a mechanism in which the isopropylidene moiety bicycles along the 1-methylenindene ? system.The 2,2-dimethyldihydronaphthalene derives from an intermediate cyclopropyldicarbinyl diradical, arising in the bicycling process, opening its three ring.Irradiation of the 2,2-dimethyldihydronaphthalene led exclusively to the benzobicyclohexene without formation of the spiroindene isomer; the efficiency here was 0.086.This reaction is only formally the reverse of the benzobicyclohexene photolysis and utilizes a different state of the cyclopropyldicarbinyl diradical as an intermediate.The lack of formation of spiroindene product from the 2,2-dimethyldihydronaphthalene is discussed in terms of a distorted bifunnel effect.The photolysis of the 4,4-dimethyldihydronaphthalene led to the benzobicyclohexene with an efficiency of 0.22; here, again, a bicycle mechanism is used.The spiroindene isomer and the 1,1-dimethyldihydronaphthalene were unreactive.Also, the triplets throughout were unreactive.Singlet excited state rate constants were derived for each of the reactions.Correlation diagrams, consisting of a triptych with benzobicyclohexene, dihydronaphthalene, and spiro compound at the three branches, reveal a HOMO-LUMO crossing on the benzobicyclohexene branch.The positioning of the crossing, again, accounts for the unidirectionality of the reactions.At the SCF-CI level a distorted bifunnel was encountered and the hypersurfaces concur in predicting the photochemistry.The distorted funnel concept was developed along with other related photochemical theory.
