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 similar to 2:1 ratio was reacted with N,N-dimethyl-(S)-1-[(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)-1-{(R)-2-(diphenylphosphino)ferrocenyl}ethyl][3.3.1]bicyclononane, in 68% isolated yield. (S)-(R)-5a crystallizes in the orthorhombic space group P2(1)2(1)2(1), Z = 4, a = 7.393(3) Angstrom, b = 19.261(5) Angstrom, and c = 19.546(8) Angstrom. 5a was used in the asymmetric Pd-catalyzed alkylation of 1,3-diphenyl-3-acetoxypropene with dimethyl malonate. Enantioselectivities up to 85% ee were obtained. The cationic Pd-allyl complexes [Pd(eta(3)-C3H5)(5a)]O3SCF3 (6) and [Pd(eta(3)-PhCHCHCHPh)(5a)]O3SCF3 (7) were prepared and characterized by X-ray diffraction, Complex 6 crystallizes in the monoclinic space group P2(1), Z = 2, a = 9.162(4) Angstrom, b = 16.069(5) Angstrom, c = 11.816(5) Angstrom, and beta = 96.86(3)degrees. Crystalline 7 was obtained as a CH2Cl2 monosolvate and belongs to the triclinic system: space group P1, Z = 1, a = 11.07(2) Angstrom, b = 11.216(14) Angstrom, c = 11.888(16) Angstrom, alpha = 62.37(9)degrees, beta = 65.96(11)degrees, and gamma = 70.29(11)degrees. The ligand assumes very different conformations in its complexes, as compared to the free state. Multidimensional (31)p, C-13, and H-1 NMR studies reveal that 7 exists in solution as a mixture of four isomers. Aspects of the selective equilibria were elucidated using P-31- and H-1-exchange spectroscopy.