Tridentate phospholane

The one-pot synthesis of a large family of hi- and tridentate phospholanes represented in Scheme 15.16 employed the air-stable chiral phospholanium chloride salts and primary amines or NH Cl as starting materials. These were transformed into the C2-symmetric dimethylphospholane and diphenylphospholane ligands which reacted with [Rh(COD)2]Bp4 to yield the rhodium complexes [Rh(L)(COD)]Bp4 (L = DMP or DPP bisphospholane ligands) [67]. 

Scheme 15.16 Synthesis of mono-, di-, and tridentate phospholane ligands using amines (or NH4CI) and phospholanium salts as key intermediates [67].

The first reported application of phospholane-based ligands for enantiomeric hydrogenation was described by Brunner and Sievi in 1987 [6], Unfortunately, these trans-3,4-disubstituted phospholanes (1-3) were derived from tartaric acid, and proved to be relatively unselective for the rhodium-catalyzed hydrogenation of (Z)-a-(N-acetamido)cinnamic acid (6.6-16.8% ee). This was, presumably, due to the remoteness of the chiral centers from the metal coordination sphere failing to impart a significant influence. This was also found to be the case with several other bi- and tridentate analogues [7]. 

We therefore focused on the development of a modular synthesis of bi- and tridentate chiral phospholane ligands and selected the 2,5-dimethyl (DMP) and 2,5-diphenyl (DPP) substituted phospholanes as stereodirecting groups, as these two privileged chiral units had shown remarkable reactivity and enantioselectivi-ties with special impact in catalytic asymmetric hydrogenations [66]. 

The corresponding trisphospholane complexes were also obtained in high yields. In the derivative containing the tridentate 2,5-dimethylphospholane, the tripod ligand adopted a symmetrical tridentate binding form (Pigure 15.6). A different coordination behavior was found in the case of the diphenyl-substituted (DPP) complex in which the coordination of only two of the three phospholane moieties to the metal center was observed. 

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