Successful application of a “forgotten” phosphine in asymmetric catalysis: A 9-phosphabicyclo[3.3.1]non-9-yl ferrocene derivative as chiral ligand

The technical mixture phobane”, containing the two isomers 9-phospha-9H-bicyclo[3.3.1]-nonane (3a) and 9-phospha-9H-bicyclo[4.2.1]nonane (3b) in a ~2:1 ratio was reacted with N,N-dimethyl-(S)-l-[(R)-2-(diphenylphosphino)ferrocenyl]ethylamine (4) in acetic acid. The clean amine substitution product is the new chiral biphosphine 5. When only 2 equiv of 3 were reacted with 4, a 4:1 mixture of the two isomeric products 5a and 5b were obtained. However, the use of a 10-fold excess of 3 afforded the pure [3.3.1]-isomer 5a, 9-phospha-9-[(S)-l-{(R)-2-(diphenylphosphino)ferrocenyl}ethyl][3.3.1]bicyclononane, in 68% isolated yield. (S)-(R)-5a crystallizes in the orthorhombic space group P2₁2₁2₁, Z = 4, α = 7.393(3) Å, b = 19.261(5) Å, and c = 19.546(8) Å. 5a was used in the asymmetric Pd-catalyzed alkylation of l,3-diphenyl-3-acetoxypropene with dimethyl malonate. Enantioselectivities up to 85% ee were obtained. The cationic Pd—allyl complexes [Pd(η³-C₃H₅)(5a)]O₃SCF₃(6) and [Pd(η³-PhCHCHCHPh)(5a)]O₃SCF₃ (7) were prepared and characterized by X-ray diffraction. Complex 6 crystallizes in the monoclinic space group P2₁, Z = 2, α = 9.162(4) Å, b = 16.069-(5) Å, c = 11.816(5) Å, and β = 96.86(3)°. Crystalline 7 was obtained as a CH₂Cl₂ monosolvate and belongs to the triclinic system: space group P1, Z = 1, a = 11.07(2) Å, b = 11.216(14) Å, c = 11.888(16) Å, a = 62.37(9)°, β = 65.96(11)°, and γ = 70.29(11)°. The ligand assumes very different conformations in its complexes, as compared to the free state. Multidimensional ³¹P, ¹³C, and ¹H NMR studies reveal that 7 exists in solution as a mixture of four isomers. Aspects of the selective equilibria were elucidated using ³¹P- and ¹H-exchange spectroscopy.