Catalytic Asymmetric Intramolecular Pauson Khand and Pauson Khand Type - - - Reactions

One of the limitations of the PKR is the poor regioselectivity observed in intermolecular reactions where unsymmetrical alkynes and alkenes can give rise to mixtures of four constitutional isomers. Inherent regiocontrol of the intramolecular PKR of enynes 3 to form bicyclic cyclopentenones 4 (Scheme 1) makes this variant appealing and it has been actively investigated since its inception in 1981. The original protocol for the PKR required superstoichiometric amounts of a metal complex, which limits its utility. The first catalytic PKR was reported in 1994 and shortly thereafter the first catalytic PKR where a metal other than cobalt, titanium was used. Further development has produced catalytic, intramolecular PKRs that employ ruthenium and rhodium(I) complexes as catalysts. Although the catalytic, asymmetric, and intramolecular PKRs were developed separately, the demand for a combined, catalytic, asymmetric variant was clear. This demand was met with the report of the first catalytic, asymmetric intramolecular PKR. Here, a chiral titanocene complex is employed in the enantioselective intramolecular construction of a variety of bicyclo[3.3.0]octenones 4 (Z = CR2, O, NR) from 1,6-enynes 3 (Z = CR2, O, NR).