Direct Resolutions with Non-metallic Auxiliaries

In the case of phosphonium salts (entries 3-6) once the individual epimers had been separated, stereoselective alkaline hydrolysis or electroreductive cleavage of one of the groups afforded the phosphine oxide or the free phosphine respectively. 

Resolution of phosphine oxides involves protonation of the weakly basic phosphoryl group by the chiral auxiliary. This method showed limited success for simple monophosphine oxides (entries 1, 2 and 10) but is better suited to resolve compounds bearing carboxylic acid groups (entries 11 and 12) or diphosphine oxides (entries 16-20). It is still sporadically used nowadays, as for the compounds of entries 19 and 20. 

The secondary phosphine oxide shown in entries 7 and 8 is the most important representative of the family of F-stereogenic secondary phosphine oxides (SPOs), which can be used directly as P-donor ligands to transition metals. Furthermore, this particular compound has been also used to prepare many mono- and diphosphine oxides by stereoselective alkylation and other reactions.  

To resolve phosphine boranes, an additional protic group is required (entries 13-15). Enantiomerically pure phosphinous acid boranes of entries 14 and 15 are versatile compounds that were transformed, often stereoselectively, to many other P-stereogenic compounds.  

Finally, the resolutions of the phosphines in entries 21 23 were carried out whilst the ligand was coordinated to Rh or Pd. 

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