Enantioselective organic syntheses using chiral transition metal complexes V . (2 S,3 S)-Bis(dibenzophospholyl)butane, a rigid ( S, S)-CHIRAPHOS analog

Abstract

The P–Ph cleavage of phenyldibenzophosphole ( 1) with lithium in THF gives lithium dibenzophospholide ( 2). Reaction of 2 with ethyleneglycol ditosylate produces the known chelate ligand 1,2-bis(dibenzophospholyl)ethane ( 3) in good yield. Similarly, 2 and (2 R,3 R)-butanediol ditosylate give the new chiral chelate ligand (2 S,3 S)-bis(dibenzophospholyl)butane ( 4). Ligand exchange of [CpRu(PPh 3) 2Cl] with 3 or 4 yields the halfsandwich complexes [CpRu(C 12H 8PC 2H 4PC 12H 8)Cl] ( 5) and [CpRu(( S, S)-C 12H 8PCHMeCHMePC 12H 8)Cl] ( 6). Complex 6 was characterized crystallographically (monoclinic, space group P2 1 (no. 4), a=820.6(4), b=1501.0(3), c=1172.8(6) pm, β=108.87(2)°, V=1.367(1)×10 9 pm 3, Z=2). The most conspicuous feature of the structure of 6 is the perfect coplanarity of the two dibenzophosphole moieties imposed by their steric interaction with the Cp ligand. Complex 6 and the thiophene complex [CpRu(( S, S)-C 12H 8PCHMeCHMePC 12H 8)(SC 4H 4)]BF 4 ( 7) derived therefrom are remarkably unreactive with regard to ligand substitutions. A possible explanation is the lack of intramolecular M⋯H–C stabilization en route to the transition state of ligand substitution. The enantiomeric purity of 6 and 7 could nevertheless be demonstrated by conversion to diastereomerically pure [CpRu(( S, S)-C 12H 8PCHMeCHMePC 12H 8)(( S)-CNCHMePh)]BF 4 ( 8).