New Modular Di- and Tridentate Phospholane Ligands

A well-known piece of phosphorus chemistry is the functionalization of a P-H bond by direct reaction with a twofold excess of formaldehyde in acidic conditions, which leads to the formation of an air- and moisture-stable phosphonium salt [60, 61]. Subsequent treatment of a phosphonium salt with a base was followed by the addition of a nucleophile, typically an amine (including ammonia, primary, or secondary amines) [62]. In this way, achiral mono- [63], di-[60,64], and tridentate [65] ligands have been synthesized by combining two modules, namely the phosphine unit and the amine group. 

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. 

To investigate the potential of the new ligands, the enantioselective hydrogenation of a series of prochiral olefins, serving as benchmark substrates using isolated Rh complexes as catalysts, were studied. The substrates included methyl esters of three dehydro-a-acetamido acids, namely methyl (Z)-a-acetamidocinnamate 

Cyclohydroaminations of y-Allenyl Sulfonamides with Mono-, Bis-, and Trisphospholane Gold(l) Catalysts 

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