Rhodium-Catalyzed Pauson–Khand Reaction Using a Small-Bite-Angle P-Stereogenic C₁-Diphosphine Ligand

Abstract

The asymmetric Pauson−Khand reaction catalyzed by [Rh(COD)(MaxPHOS)]BF 4 is described. Several 1,6-enynes have been chosen as model substrates affording moderate yields and selectivities of up to 86% ee. Besides binap-type ligands, we have demonstrated that the Pstereogenic C 1 -symmetry small-bite-angle ligand MaxPHOS is a viable ligand in this process. The formation of [2+2+2] cycloaddition compounds has shown to be a competitive process. A mechanism is proposed to account for the observed results. The intermediate rhodium dicarbonyl complex 6 was synthesized, and its solid-state structure was elucidated by X-ray crystallography. ■ INTRODUCTIONThe Pauson−Khand reaction (PKR) is one of the most efficient methods for the construction of cyclopentanic compounds. 1,2 Both the intra-and the intermolecular version of this process have been utilized in the synthesis of natural products and bioactive molecules. 3,4 A great deal of effort has been devoted to the development of enantioselective versions of the PKR using cobalt, 5 titanium, 6 rhodium, 7 and iridium. 8,9 In 2000 Jeong and co-workers published the first catalyzed enantioselective intermolecular PKR under a CO atmosphere in the presence of a rhodium source and the chiral atropoisomeric ligand BINAP (Scheme 1). 7 Since this early report, mostly C 2 -symmetric 1,1-binaphthyl diphosphines have been used in Rhcatalyzed intermolecular PKR. 7,10−17 Ratovelomanana-Vidal has shown that electronic and steric effects of the diphosphine ligands are crucial to achieve good conversions and high enantioselectivity. 10,11,13 Interestingly, other C 2 -diphosphines such as DIOP, CHIRAPHOS, or Me-DuPHOS have not shown catalytic activity in this reaction. 7 To the best of our knowledge, no examples of electron-rich diphosphines bearing a stereogenic phosphorus atom have been applied in the PKR.Our group has a long-standing experience in the development of asymmetric approaches for the intermolecular PKR using chiral auxiliaries and ligands. 3,4,18−21 We have recently developed novel methodologies for the synthesis of Pstereogenic phosphine ligands. In this field, we have prepared the small-bite-angle MaxPHOS ligand (Scheme 2). 22−24 We have shown that the MaxPHOS-Rh complex 2 is a highly active and efficient catalyst in asymmetric hydrogenation. 25 Due to NH/PH tautomerism, protonation of MaxPHOS leads to the

Recent Advances in the Pauson–Khand Reaction

“…[25] The P-stereogenic MaxPHOS-Rh complex (Scheme 16, top) had previously been successfully employed in asymmetric hydrogenation chemistry. [26] As shown in Scheme 16 (bottom), the reaction was higher yielding and more selective with the N-tosyl tethered enyne.…”

Betti base derived P-stereogenic phosphine-diamidophosphite ligands with a single atom spacer and their application in asymmetric catalysis

“…In fact, there are only a handful of examples of chiral bisphosphine ligands with a single atom (carbon, oxygen or nitrogen) linker, such as bis-(diphenylphosphino)methane, 13,14 trichickenfootphos (TCFP), 15 MaxPHOS 16–19 which all found application in asymmetric catalysis (Fig. 1a).…”

Synthesis, Coordination Study, and Catalytic Pauson–Khand Reactions of QuinoxP*(CO)4-μ-Alkyne Dicobalt Complexes

“…Although several efficient systems have been developed for the asymmetric intramolecular reaction, , the intermolecular PKR has proven to be much more challenging. In general, the poor reactivity of bisphosphine complexes in the Pauson–Khand reaction is one of the principal reasons for the scarce presence of these complexes in the literature, with only few examples that show significant activity in the intermolecular PKR , using electron-poor bisphosphines.…”