Carbonylative dimerisation of norbornene by cobalt carbonyl: an overlooked by-product of the Pauson–Khand reaction

Regiochemistry in Cobalt-Mediated Intermolecular Pauson–Khand Reactions of Unsymmetrical Internal Heteroaromatic Alkynes with Norbornene

“…For the imidazole and benzimidazole alkynes 3d and 3e no reaction was observed (entries 4 and 5); note that the (μ 2alkynyl)Co 2 (CO) 6 complexes of 3d and 3e can be formed and characterized. However, a small amount of the known lactone 5 (Figure 1), which is derived from the carbonylative dimerization of two molecules of norbornene, 29 was formed under the reaction conditions in a yield of 2À7% with respect to the quantity of norbornene added. In a control reaction, heating Co 2 (CO) 8 and norbornene at 90 °C for 20 min, under focused microwave heating, gave 5 in 19% yield.…”

Dicobaltbisnorbornadienetetracarbonyl [Co2(Nbd)2(CO)4] Reacts with Oxygen-Containing Alkynes and Gives Crossover Products in the Presence of other Alkenes

“…(3) The reaction of norbornene with cobalt carbonyl species which are not coordinated to other organic fragments leads not to stable alkene complexes but to a carbonylative dimerisation, 7 which was not observed in this reaction.…”

“…3 This dimeric, carbonyl-bridged species was reported 1 to be inactive in the PK reaction and represented a dead-end in terms of wasting both metal and alkene. As, in recent years, both cost and environmental considerations have led to much investigation of catalytic variations on the PK reaction, 4,5 and in view of our interest in the fundamental organometallic chemistry of such systems 6 and the reactions of unsaturated bicylics with cobalt carbonyl 7 an investigation was undertaken into the reported unreactivity of this species. Given that any species which could be formed by the displacement of either norbornadiene, 2 or carbonyls 3, by an alkyne would be similar to the proposed intermediates III in the PK reaction 8 (Scheme 2) it might be expected that such a reaction would lead to cyclopentenone products.…”