Pergamon Tetrahedron Letters 41 (2000) 3235–3239 TETRAHEDRON LETTERS Preparation and reaction of desymmetrised cobalt alkyne complexes David R. Carbery, a William J. Kerr, a,* David M. Lindsay, a James S. Scott a and Stephen P. Watson b a Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, UK b Glaxo Wellcome Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK Received 20 January 2000; accepted 28 February 2000 Abstract Prochiral alkyne hexacarbonyl dicobalt complexes are desymmetrised directly with brucine N-oxide in the presence of a phosphine or phosphite ligand to produce the corresponding phosphorus-containing pentacarbonyl complex with appreciable enantiomeric excess. In Pauson–Khand reactions, it is found that the enantiomeric integ- rity of the desymmetrised complex is conserved in the cyclopentenone product. Moreover, the major enantiomer obtained in these reactions is opposite to that from a direct brucine N-oxide promoted Pauson–Khand reaction, allowing the preparation of either enantiomeric cyclopentenone in enriched form from a single source of chirality. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: amine N-oxides; asymmetric synthesis; cobalt and compounds; desymmetrisation; Pauson–Khand reactions. In the preceding paper, 1 we have described the further development of our initial discovery 2 that chiral amine N-oxides can induce asymmetry in the Pauson–Khand (P–K) reaction. In this work, we showed that a chiral amine N-oxide could effect selective decarbonylation of one cobalt vertex of the dicobalt alkyne cluster, leading, in the presence of alkene, to enantiomerically enriched cyclopentenones of up to 78% ee. Herein, we wish to report the application of this selective decarbonylation concept to the preparation of chiral phosphinated cobalt alkyne complexes and show how this class of desymmetrised dicobalt species has, subsequently, been used in the P–K reaction to, again, provide access to optically enriched cyclopentenones. The replacement of carbon monoxide by phosphorus-based ligands in alkynehexacarbonyldicobalt complexes is well documented. 3 Dissociation of a carbon monoxide ligand from the hexacarbonyl complex generates a vacant co-ordination site on one of the cobalt atoms which, in turn, may be occupied by the phosphorus ligand. This initial CO dissociation is also proposed as the first step of the P–K mechanism 4 and, paralleling the traditional methods for carrying out the P–K reaction, early reports of the preparation of phosphorus-substituted complexes also relied on thermal conditions to effect dissociation * Corresponding author. Fax: +44-(0)141-548-4246; e-mail: cbas69@strath.ac.uk (W. J. Kerr) 0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(00)00356-7 tetl 16636