Bicyclo heptadienes formation

Formation of the tricyclo[3.3.0.0.]decane 209 by the reaction of [3.2.0]bicyclo-heptadiene 205 with propyne complex (206) is an example [81], The Pauson Khand reaction is explained by the following simplified mechanism. At first the oxidative cyclization of 205 and 206 generates the cobaltacyclopentene 207, to which insertion of CO gives 208. Finally, reductive elimination of208 affords the cyclopentenone 209. 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

Irradiation of hydroxy-substituted 1,6-heptadienes in the presence of CuOTf results in formation of bicyclo[3.2.0]heptanes with complete regioselectivity and impressive stereoselectivity [27]. For example, when the S isomer of 6-methyl-l,6-heptadien-3-ol was irradiated in the presence of CuOTf the endo-bicyclo photoadduct shown in Sch. 14 was obtained in 98% e.e. Interestingly, the endoselectivity of this reaction increases in polar solvents. The authors argue that the copper intramolecular coordination between the hydroxyl group and the alkenes is favored in increasingly polar media. 

The two pathways, electrocyclic closure to the bicyclo[3.2.0]heptadiene and the [l,7]-shift of hydrogen, are in competition. This makes the experimental work more difficult, because the four possible isomers of the cycloheptatriene interconvert during the photolysis. Nevertheless the pattern of reactivity is clear, and the observation is that the better the donor substituent, the more it favours the formation of the bicyclo[3.2.0]heptadiene at the expense of the [l,7]-shift. This too is reasonable, since the hydride migration towards the donor-substituted atom 8.80 leads to a less stable cation, and ought to be slow. 

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