M. Lautens et al., COBALT-CATALYZED [2-PI-PI+2-PI] (HOMO DIELS-ALDER) AND [2-PI+2-PI+4-PI] CYCLOADDITIONS OF BICYCLO[2.2.1]HEPTA-2,5-DIENES(2), Journal of the American Chemical Society, 117(26), 1995, pp. 6863-6879
The scope of the cobalt-catalyzed [2 pi + 2 pi + 2 pi] (home Diels-Ald
er, HDA) and [2 pi + 2 pi + 4 pi] cycloaddition reactions with norborn
adienes 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 effe
ctive in promoting the HDA reaction between norbornadiene and a variet
y 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% e
e) 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 con
trast with the corresponding nickel-catalyzed HDA reaction with electr
on-deficient dienophiles. 2-Substituted norbornadienes are found to be
less reactive in the cobalt-catalyzed HDA reactions, and the regio- a
nd 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 cons
truction of highly strained pentacyclic frameworks from norbornadiene.