Preparation of NewN-Heterocyclic Carbene Metal-Alkyne Complexes and Application to a Stereocontrolled Pauson-Khand Reaction

Abstract

The first reported bimetallic metal-alkyne complexes containing a singly bonded N-heterocyclic carbene ligand have been synthesised and utilised in a stereocontrolled Pauson-Khand reaction.Bimetallic metal-alkyne complexes of type 1 have been known for many years. 2 As well as displaying interesting structural chemistry, they have been utilised in the synthetically useful Pauson-Khand reaction 3,4 (PKR) and within Nicholas carbocation chemistry 5 since the early 1970's. In addition to the parent hexacarbonyl complexes, several related species with ligands replacing one or more of the carbon monoxides have been reported, the most common of which is the triphenylphosphine complex 2. In the substituted complexes, the use of an unsymmetrical alkyne renders the complex chiral, as the four corners of the tetrahedral bimetallic core are different. 6 Alternatively, chirality may be achieved via the replacement of one of the cobalt vertices by a different metal core, forming a heterobimetallic complex, such as 3 ( Figure 1). These heterobimetallic complexes have been successfully employed in organic synthesis including asymmetric versions of the Pauson-Khand reaction, 7,8 stereocontrolled Nicholas reactions 9 and recently as an alkyne based chiral auxiliary. 10 We also have an interest in imidazolium derived carbene ligands, and were intrigued to see whether we could use these to desymmeterise the metal-alkyne complexes. Of particular interest were the reported enhanced thermal and kinetic stabilities, and reactivities of many catalytic organometallic complexes when N-heterocyclic imidazolium ligands were used as phosphine mimics. 11 In addition to possible synthetic advantages, we were also intrigued by the electronic properties of these compounds, as imidazolium carbenes have been shown to be strong donor ligands with little or no acceptor properties 11a and comparison with the phosphine analogues would be of interest. Biscobalt-carbene complexes were prepared in the late 1970's and contained dinuclear bridging carbenes. 12,13 Very recently, Gibson has shown that a carbene substituted bis-cobalt-heptacarbonyl complex can be used in a catalytic variant of the Pauson-Khand reaction. 14 Here, we show that the carbene substituted alkyne complexes are isolable, and we have confirmed the structure of one of these by X-ray analysis.The precursor carbene salt, 1,3-bis(2,6-diisopropylphenyl) imidazolium tetrafluoroborate (IPr), was made according to literature precedent. Using Nolan's method 15 for in situ generation of the carbene, we then added the bis-cobalt-alkyne complexes. Stirring at 65 °C in hexane for 45 minutes to one hour, followed by purification by column chromatography, produced the desired complexes Figure 1 Metal-alkyne complexesR 2 R 1 Scheme 1 Preparation of IPr substituted metal-alkyne complexes R 2 R 1 Co(CO) 3 (OC) 3 Co R 2 R 1 Co(CO) 2 IPr (OC) 3 Co IPrHBF 4 K Amylate, hexane 65 °C, 1 h a R 1 = Ph, R 2 = Ph, 67% b R 1 = Ph, R 2 = H, 59% c R 1 = H, R 2 = CH 2 OH, 71% d R 1 = Me, R 2 = CHO, 54% e R 1 = H, R 2...