COBALT-CATALYZED [2-PI+2-PI+2-PI] (HOMO DIELS-ALDER) AND [2-PI+2-PI+4-PI] CYCLOADDITIONS OF BICYCLO[2.2.1]HEPTA-2,5-DIENES

The scope of the cobalt-catalyzed [2 pi + 2 pi + 2 pi] (home Diels-Alder, HDA) and [2 pi + 2 pi + 4 pi] cycloaddition reactions with norbornadienes has been investigated. Cobalt acetylacetonate, Co(acac)(3) or Co(acac)(2), upon reduction by diethylaluminum chloride (Et(2)AlCl) in the presence of 1,2-bis(diphenylphosphino)ethane (dppe), is very effective in promoting the HDA reaction between norbornadiene and a variety of unactivated acetylenes to yield deltacyclenes. Azeotropic drying of the cobalt compound before use is found to increase the reactivity of the catalyst. Moderate to excellent enantioselectivity of these [2 pi + 2 pi + 2 pi] (up to 91% ee) and [2 pi + 2 pi + 4 pi] (up to 79% ee) cycloadditions can be achieved by the use of a chiral phosphine. 7-Substituted norbornadienes are also found to be reactive in the cobalt-catalyzed HDA reactions, affording the corresponding deltacyclenes in good yields. However, low anti/syn selectivities are observed, in contrast with the corresponding nickel-catalyzed HDA reaction with electron-deficient dienophiles. 2-Substituted norbornadienes are found to be less reactive in the cobalt-catalyzed HDA reactions, and the regio- and stereoselectivities are only moderate. The intramolecular versions of these [2 pi + 2 pi + 2 pi] and [2 pi + 2 pi + 4 pi] cycloadditions have also been investigated and provide efficient methods for the construction of highly strained pentacyclic frameworks from norbornadiene.