Stoichiometric studies Pauson-Khand reaction

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, /3-unsaturated cyclopentanones (equation 23). With unstrained alkenes the reaction works best in a stoichiometric setting, but with strained alkenes like norbomadiene the reaction can be made catalytic. These reactions have been fairly extensively studied, and the reaction proceeds through an alkyne-bridged Co2(CO)6 dimer species. Unsymmetrical alkynes lead to mixtures of the various substituted a, /3-unsaturated cyclopentanone products. 

Enynes in which three or four atoms separate the double and triple bonds cyclize upon complexation to Co2(CO)g and subsequent heating to give bicyclic enones (equation 52). With the exception of slightly elevated temperatures the conditions required are no different than those of the stoichiometric procedure described earlier for reactive substrates in intermolecular Pauson-Khand reactions. The intramolecular cycloaddition cannot in general be carried out under catalytic conditions. Hex-l-en-5-yne, which would give a four-membered ring upon intramolecular cycloaddition, instead undergoes alkyne trimerization exclusively.3 The most extensively studied systems are those derived from hept-l-en-6-yne, as the products, bicyclo[3.3.0]oct-l-en-3-ones, are useful in the synthesis of numerous cyclopentane-based polycy-clics. 

In the course of studying the stoichiometric, carbonylative synthesis of y-buty-rolactones from enones and a titanocene complex [37], it was observed that certain aryl enones could be cyclocarbonylated in a catalytic manner, Eq. (12) [38]. This success in effecting a titanocene-catalyzed cyclocarbonylation led to an examination of the analogous reaction with enyne substrates. Indeed, it was found that under similar conditions, a titanocene-catalyzed Pauson-Khand type cyclization was possible, Eq. (13) [36]. 

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