An Enantiospecific Pauson-Khand Reaction

Abstract

The use of optically pure heterobimetallic alkyne complexes for the synthesis of enantiopure cyclopentenones is described. The inherent chirality of the complexes has been used to effect high levels of stereocontrol in transformations giving optically enriched organic end products.

Reversing the Stereoselectivity of the Intermolecular Pauson−Khand Reaction: Formation of endo-Fused Norbornadiene Adducts

“…After chromatographic purification, orange-colored complexes 1 − 6 were isolated in yields comparable to those reported in the literature for other alkynes. 4a, For N -(phenylpropionyl)-4-benzyloxazolidinone (second entry of Table ) only, the formation of the heterobimetallic complex could not be observed, probably because of the increased steric hindrance of the substrate . The diastereoselectivities of the complexation reactions of the two chiral oxazolidinone derivatives (first and third entries of Table ) were very similar, and the diastereomeric complexes ( 1a / 1b and 2a / 2b , respectively) were easily separated by column chromatography in both cases.…”

“…Recently, Christie et al have reported that, upon replacing one of the Co(CO) 3 groups of an alkyne−dicobalt(hexacarbonyl) complex by an isoelectronic MoCp(CO) 2 moiety, the resulting heterobimetallic complexes still are active partners in intermolecular Pauson−Khand cycloadditions with norbornadiene, giving rise to the expected exo -fused adducts. Moreover, the chirality of the metal−alkyne tetrahedral core introduced by this substitution can be used to induce high levels of stereocontrol in the reaction.…”

“…Not unexpectedly, the two diastereomers of a given heterobimetallic complex lead to different diastereomer ratios of the adduct. Thus, while complex 1a (first entry of Table ) affords a 4:1 mixture of diastereomers, complex 1b (second entry of Table ) gives rise to a 1:7.3 diastereomer mixture, in which the minor isomer obtained with 1a now largely predominates.…”