Achiral Starting Phosphines

Quenching of the same lithiated species with CO2, followed by reduction of the carboxyUc acid functionality obtained with BH3-THF complex, yielded the next higher analogues 78 to these alcohols [94]. Subsequent treatment of the depro-tonated alcohols with TsCl or MsCl afforded (l )-l-boranato[alkyl(methyl)plios-phino] ethanol-2-tosylates or the mesylate phosphine-boranes in over 90% ee and excellent overall yields. 

---

Starting from achiral phospholes, phosphine sulfides 123 were obtained as a racemate. Hydrolysis and coupling to (5)-valinol furnished 124 and 124, which were easily separated by column chromatography in 93% combined yield. From these precursors, the desired enantiopure ligands 125 could be prepared by conventional methods. 

To date, the most popular and successful designs for PN-ligands have incorporated an achiral phosphine with a substituted non-aromatic heterocyde as the primary chiral unit, largely due to the ease by which the P and N modules may be altered, and the ready availability of enantiopure, amino add-derived starting materials.1281 Considerable variation of the predse steric and electronic environment imposed on a chelated metal centre has been achieved by the synthesis of a substantial library of... 

---