PALLADIUM-MEDIATED AND COBALT-MEDIATED CYCLIZATIONS OF HALO-POLYENES - A COMPARATIVE-STUDY

Cobalt(I)-mediated mono- and bi-cyclisations of halo-polyenes are comp ared and contrasted with corresponding reactions using the Heck reacti on. The procedures are found to complement one another, with each havi ng advantages and disadvantages, in the cases studied. Thus, treatment of the vinyl iodide 1 under Heck conditions led to the novel 1,3-dien e 3 in 93% yield, whereas reaction between 1 and cobalt(I) salophen re sulted in the formation of the organocobalt complex 4 (similar to 50%) as black crystals. Subsequent irradiation of 4 then led to the 6,5-ri ng fused bicycle 5 in 85% yield. In similar fashion, a Heck reaction w ith the analogous vinyl iodide 8 produced the product 9 of 6-ring cycl isation followed by beta-hydride elimination, whereas the correspondin g cobalt-mediated reaction with 8 led to the new organocobalt 10 albei t in only low yield (similar to 20%). In reactions akin to those studi ed by Overman et al., Heck reaction with the dienyl aryl iodide 14 was shown to lead to a mixture of the bicycle 16 and the tricycle 17 in a 1:1 ratio. By contrast treatment of 14 with cobalt(I) salophen produc ed largely the tetralin 18. The iodo-diene 33 gave rise to the spirocy cle 35 under Heck reaction conditions, and the related spirocycle 42 w as produced from the bromo-acetal 40 via 41, using cobalt(I) mediated reactions.