Catalysis of Organic..

458Chiral BisphospholanesFirst, the preparation of required primary phosphines is rather tedious and expensive.Since for each bridge the individual synthesis of relevant primary phosphines isnecessary, the preparation of a family of primary diphosphines is - even if chemicallyfeasible not practicable. Secondly, most heterocycles bearing primary phosphinesare not stable due to the attack of the phosphine group on the functionalities of theheterocycle. Third, the nucleophilic substitution of cyclic sulfates by the metalphosphide in the presence of an excess of strong bases may seriously interfere withthe heterocyclic ring or other functional groups. Therefore a new and convergentsynthetic strategy was necessary which allows the coupling of the bridging scaffoldwith a phospholane moiety under neutral conditions without using primaryphosphines or phosphides as the building block.As a first approach we found that a suitable 2,5-dimethyl substituted P-silylatedphospholane can be easily derived from the corresponding chiral bis-mesylate byreaction with dichlorophenylphosphine (Figure 4) (15). This TMS-phospholaneproved to be a remarkablely stable reagent. It could be distilled under vacuum andstored under argon for a long time without any tendency of decomposition orepimerization. When this building block was reacted with 2,3-dichloromaleicanhydride in etheral solution at 0 C, in the absence of any base, a smooth and cleancoupling reaction took place. The optically pure ligand 1a could be precipitated in 53% yield as dark-red crystals from the reaction solution. The same methodology couldalso be successfully employed for the synthesis of related ligands such as 1b and 1cbased on maleimide and squaric acid, respectively. As another possibility ofvariation, a silylphospholane bearing ethyl groups in 2,5-positions was employed inthe coupling procedure. Due to this uniform methodology a broad variety of ligandsare accessible in a technically feasible and practicable manner.RR1. Li1. LiPhPCl 2RP PhP SiMe 32. TMSCl2. OMsRRR = Me, EtRORPOMsAR RPOROAOCl Clneutral conditions50 - 60%catASium (R) M1a, A = O1b, A = NMe1c, A = (CH 2) 0Figure 4. Modular synthesis of catASium fi M ligands