Catalysis of Organic..

Pugin et al. 297complexes with one and two rhodium ions, were performed at both 1:1 and 2:1rhodium-to-ligand ratios.As seen in Table 1, the mono- and bis-rhodium complexes of tetraphosphine 2provide similar enantioselectivities in the chiral hydrogenation of both substrates asthe rhodium complex of the diphosphine (Josiphos) ligand 1 does. The bis-rhodiumcomplex of 6 provides higher conversion but similar enantioselectivity as therhodium complex of the diphosphine (Bophoz) ligand 5 in the chiral hydrogenationof MAC.In contrast, the mono- and bis-rhodium complexes of tetraphosphine 4 providehigher conversion and enantioselectivity in the chiral hydrogenation of MACcompared to the rhodium complex of diphosphine (Josiphos) ligand 3. For DMI, themono-rhodium complex of 4 provides much higher conversion and enantioselectivitycompared to the rhodium complex of 3, while the bis-rhodium complex of 4 isintermediate in both conversion and enantioselectivity to 3•Rh and 4•1Rh. Finally,both the mono- and bis-rhodium complexes of 6 provide similar conversion withlower enantioselectivities in the chiral hydrogenation of DMI compared to 5•Rh.From these initial survey results, we postulate that the dominant species formedby the rhodium complexes of tetraphosphine ligand 2 are the same as the rhodiumcomplex formed by Josiphos ligand 1 and the two catalytic sites in ligand 2 actessentially independently.However, because the rhodium complexes of tetraphosphine ligand 4 behavedifferently than the rhodium complex of Josiphos ligand 3 for both substrates, wepostulate that either a) different rhodium complexes are being formed (differentbinding modes as discussed above, see Fig. 2) by 3 and 4, or b) the substituents onthe second cyclopentadiene (cp) ring influence the catalytically active site in therhodium complexes of 4.The unique performance of 4•Rh and 4•2Rh relative to 3•Rh led us to initiateinvestigations to better understand rhodium complexation by these ligands. As a firststep we prepared catalyst precursors by mixing ligands 3 and 4 with [Rh(NBD) 2 ]BF 4 .We obtained 31 P NMR spectra of 3, 3 + 1Rh, 4, 4 + 1Rh, and 4 + 2Rh which areshown in Figure 4.The spectrum of 3 exhibits two phosphorus resonances at -26.6 (sidearmPMOD 2 ) and 50.1 ppm (PPh 2 attached to ferrocene), each showing 31 P coupling of50 Hz, while the spectum of 4 exhibits a corresponding pair of resonances at -27.2and 52.0 ppm and a 31 P coupling of 63 Hz.Addition of one equivalent of [Rh(NBD) 2 ]BF 4 to 3 or two equivalents of[Rh(NBD) 2 ]BF 4 to 4 produces similar spectra with two strong resonances at 22.5 and76.3 ppm for 3•Rh and 22.2 and 78.2 ppm for 4•2Rh, each resonance appearing asfour lines due to combined 31 P (30 Hz) and 103 Rh (155 Hz) coupling. Both spectrashow minor resonances around -25 to -35 ppm, indicative of non-complexedphosphorus, while 4•2Rh also exhibits a weak resonance at 97.8 ppm split by Rh butnot by 31 P.The major resonances suggest that complexation of two Rhodium ions by thetetraphosphine ligand 4 occurs in the same manner as binding of one rhodium by thediphosphine Josiphos ligand 3 (see Fig. 2). While the minor resonances observed