Camphor-derived, chelating auxiliaries for the highly diastereoselective intermolecular Pauson-Khand reaction:: Experimental and computational studies

A family of enantiomerically pure (2R)-10-(alkylthio)isoborneols [methylthio (1), neopentylthio (2), phenylthio (3)], specifically designed as chiral auxiliaries suitable for chirality transfer to cobalt in Pauson-Khand reactions, has been synthesized. The dicobalt hexacarbonyl complexes of the alkoxyacetylenes derived from these alcohols (10a-12a) can be converted to the rather stable, internally chelated, pentacarbonyl complexes 10b-12b by treatment with NMO. The intermolecular Pauson-Khand reactions of 10b-12b with strained olefins take place with synthetically useful rates at low temperatures (down to -20 degrees C), with high yields and diastereoselectivities: norbornene (77%; 92.8), norbornadiene (82%; 96:4), bicyclo[3.2.0]hept-6-ene (91%; 93:7). The major diastereomer of the adduct of 10b with norbornadiene, 14, has been used as the starting point for a synthesis of (S)-(-)-4-alkyl-2-cyclopentenones through a sequence consisting of completely diastereoselective conjugate addition, reductive cleavage with recovery (>95%) of the chiral auxiliary, and retro Diels-Alder reaction. The stereochemical course of the reaction of 10b with norbornadiene has been analyzed and rationalized by theoretical means by using a combined semiempirical [PM3-(tm)]/density functional theory [VWN-Perdew-Wang 91] approach.