X-ray crystallographic analysis of red crystals formed on mixing octacarbonyldicobalt(0) and BINAP, combined with NMR evidence obtained from a catalytic asymmetric Pauson-Khand reaction, suggests that this complex is a precatalyst to this reaction and leads to a new hypothesis for the role of axially chiral diphosphanes in the catalytic asymmetric Pauson-Khand reaction.The Pauson-Khand reaction (PKR), the [2 + 2 + 1] cyclocarbonylation of an alkyne and an alkene to form a cyclopentenone, is of considerable synthetic interest not only because cyclopentenones are useful building blocks for more elaborate structures but also because they are important biologically active compounds in their own right. For example, the cyclopentenone prostanoids have recently started to attract much interest, and this area of research is now providing promising candidates for, inter alia, antiinflammatory and antiviral pharmaceuticals. 1 While most applications of the PKR to date have used stoichiometric amounts of cobalt, there are now several catalytic versions of the reaction available that use a range of metals. 2 Initial exploratory attempts to introduce asymmetry into the reaction using titanium, 3 rhodium, 4 and iridium 5 catalysts have been encouraging, providing good enantioselectivities and turnover numbers.The feasibility of an asymmetric cobalt-catalyzed PKR has been demonstrated. 6 Using octacarbonyldicobalt(0) (20 mol %) as the cobalt source, a range of chiral diphosphanes (20 mol %) were tested on standard intramolecular substrates. While very modest enantioselectivities were observed for DIOP, DuPHOS, and planar chiral ferrocene diphosphanes, high selectivities were observed with the axially chiral ligand BINAP. For example, cyclocarbonylation of enyne 1a (eq 1) gave the product cyclopentenone 2a in 53% yield and 90% ee in 14 h. It was proposed that the phosphorus atoms of the BINAP ligand bridge the two cobalt atoms, while cyclopentenone formation occurs with participation of both cobalt centers 6 according to the generally accepted Pauson-Khand mechanism. 7 As a result of our interest in the cobalt-catalyzed PKR, 8 we initiated a study of its asymmetric version. After detailing our modified conditions for the cobaltcatalyzed asymmetric PKR based on axially chiral diphosphanes, we wish to report herein (a) the isolation of a hexacarbonyldicobalt(0) complex in which BINAP binds to just one of the two cobalts and (b) evidence that suggests that this complex is a precatalyst to asymmetric catalytic PKRs.We initially examined the cyclocarbonylation of enyne 1a in the presence of (S)-BINAP. Optimization of this reaction led to the use of 3.75 mol % of Co 4 (CO) 12 as the cobalt source, 9 which in our hands proved more robust and reliable than Co 2 (CO) 8 , together with 7.5 mol % of (S)-BINAP. After Co 4 (CO) 12 and (S)-BINAP were premixed, operating at 75°C under 1.05 atm of carbon monoxide for 5 h gave a 70% yield of cyclopentenone 2a and 89% ee (Table 1, entry 1). A survey of five other diphosphanes (Table 1, entries ...
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