Norbornadienes Pauson-Khand reaction

The intermolecular Pauson-Khand reaction of the resulting S/P-cobalt complexes with norbornadiene was studied under thermal and A -oxide activation conditions. Thus, heating the diastereomerically pure complex (R = Ph, R = Cy) with ten equivalents of norbornadiene at 50 °C in toluene afforded the corresponding exo-cyclopentenone in a quantitative yield and with an enantio-selectivity of 99% ee. Under similar conditions, the analogous trimethylsilyl complex (R = TMS, R = Cy) afforded the expected product in a high yield but with a lower enantioselectivity of 57% ee. In order to increase this enantio-selectivity, these authors performed this reaction at room temperature in dichloromethane as the solvent and in the presence of NMO, which allowed an enantioselectivity of 97% ee to be reached. These authors assumed that the thermal activation promoted the isomerisation of the S/P ligand leading to a nonstereoselective process. 

Non-oxidative promoters, in Pauson-Khand reaction with dicobalt octacarbonyl, 11, 338 Norbornadienes... 

Pauson and Khand discovered the very important class of alkyne/alkene/CO cyclization catalytic reactions catalyzed, once again, by Co2(CO)g (see Pauson-Khand Reaction) This reaction produces a,/ -unsaturated cyclopentanones (equation 23). With unstrained alkenes the reaction works best in a stoichiometric setting, but with strained alkenes like norbornadiene the reaction can be made catalytic. These reactions have been fairly extensively studied, and the reaction proceeds through an alkyne-bridged C02 (CO)g dimer species. Unsymmetrical alkynes lead to mixtures of the various substituted a, /3-unsaturated cyclopentanone products. 

The first report of a catalytic intermolecular cyclization was made by Pauson and Khand in 1974 [22], but the scope was limited to gaseous acetylene as the alkyne partner, strained olefins such as norbornene and norbornadiene as the alkene component, and TON s (turnover numbers) were modest (8-11). Several subsequent reports detailed the production of cyclopentenones from a substoi-chiometric amount of Co2(CO)g, but none were as efficient as Pauson s initial work [23,24]. Using ethylene as the alkene component, Rautenstrauch demonstrated the first efficient catalytic Pauson-Khand cyclization with a TON of 220, Eq. (5) [25]. A more general catalyst system employing (indenyl)Co(cod) was recently reported by Chung and Jeong, Eq. (6) [26]. The reaction was quite effec-... 

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