Substitution Effects, Selectivity, and Mechanism

Best efficiencies under the standard quasi-catalytic PK conditions are gained with electron-poor alkynes, strained olefins (e. g., norbomene), and open-chain olefins with low substitution (e. g., ethylene). Ethylene has the advantage that it can be applied in large excess by pressurizing the reaction system. Equations (13) and (14) are standard examples. 

There is a plausible mechanistic proposal for the standard PKR on the basis of Co2(CO)g as mediator and/or catalyst. Clearly the well-known //-alkyne dicobalt 

The main side reactions are cyclotrimerization (of the alkyne), co-cyclotrimer-ization (alkyne/alkyne/alkene), and formation of cyclopentadienones 5 and 6. In addition, spirofuranones such as 7 and 8 can form, with the latter being the Diels-Alder product of 7 with ethylene. It is generally observed that high ethylene and CO pressures disfavor the formation of side products. Further details on the side-product formation were given in [4d]. 

---