Other Phospholane-Containing Ligands

In addition to direct DuPhos and BPE analogues, several other ligands containing five-membered phosphacycles have been reported (Fig. 24.6). As early as 1991, non-C2-symme trie phospholane-containing phosphines 37-39 were reported by Brunner and Limmer [7]. These were prepared by base-induced addition of the secondary phospholane to the appropriate diphenylphosphino-substi-tuted olefin. As for the symmetrical 3,4-disubstituted bisphospholanes, enantios-electivities for the Rh-catalyzed reduction of a-acetamidocinnamate were poor. 

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

51 has recently appeared in separate patents from both Kobayashi and Schmid [76]. Given the name cis and Lrans-PM P5 by Schmid et al., the Rh complexes have been reported to be active catalysts for the reduction of several standard substrates, a-enol acetates and //-keloacid derivatives, with variable enantioselec-tivities [76 b]. In general, the cis isomer is more selective than the tmns. 

The group of Salzer has recently reported phospholanes 54-56 based on chiral half-sandwich complexes [79]. These were obtained by treatment of the appropriately substituted complex with a secondary phospholane, itself accessed via the cyclic sulfate or dimesylate and PH3. These were tested against a range of substrates with C=C, C=0 and C=N bonds, with variable results [80]. 

In recent years, the research group of Imamoto has been very active in the area of C2-symmetric P-stereogenic phosphine ligands [87]. Two such ligands, 

---