Chiral Nonracemic α-Alkylidene and α-Silylidene Cyclopentenones from Chiral Allenes Using an Intramolecular Allenic Pauson−Khand-Type Cycloaddition

We have successfully effected a transfer of chirality from a chiral nonracemic allene to an α-alkylidene and an α-silylidene cyclopentenone. The molybdenum-mediated examples possessing a silyl group on the terminus of the allene show good facial selectivities, but isomerization of the (E)-silylidene cyclopentenone to the (Z)-silylidene cyclopentenone occurs upon purification of these products. Alternatively, an alkyl group on the terminus of the allene undergoes cycloaddition with moderate selectivities but gives products that undergo an isomerization of the (Z)-alkylidene cyclopentenone to the (E)-alkylidene cyclopentenone when exposed to acidic conditions. Thus, erosion of the enantiomeric excesses is observed for one of the two products as a result of this isomerization. The allenic Pauson−Khand-type cycloaddition has also been effected by first isolation the (η6-arene)molybdenum tricarbonyl complex, demonstrating for the first time that this is most likely the active complex in the molybdenum-mediated reactions. Attempts to increase the facial selectivity by increasing the size of the arene moiety resulted in a marginal increase in the selectivity at the expense of the yield. Based upon these results, we have concluded that altering the approach for the preparation of nonracemic α-alkylidene cyclopentenones is necessary in order to obtain synthetically useful levels of stereocontrol.