Optical Resolution of Pyramidal Compounds

The resolution of an optically active (tetrahedral) phosphine oxide was first achieved by Meisenheimer and Lichtenstadt [18] in 1911, but in the subsequent 50 years only a handful of phosphorus compounds were resolved. In 1961 it was convincingly demonstrated by Horner [19] that pyramidal phosphorus compounds had sufficient configurational stability to permit separation of their enantiomeric forms. Prior to this date there had, by analogy with nitrogen compounds, been some doubt as to whether pyramidal phosphorus compounds would, in fact, be sufficiently stable to make their resolution possible. 

The resolution of pyramidal nitrogen compounds can rarely be achieved because of rapid interconversion of the two possible enantiomorphic forms by a process of inversion of the pyramid. 

The temperatures at which the half-life times for thermal racemisation would be 2 h, have been calculated as -168°C for NMe3, +1°C for PMe3 and +107°C for AsMe3. The energy barrier to inversion of simple tertiary phosphines is about 30-35 kcals/mol. 

With a fixed pnictide atom the inversion times generally increase as the total mass of the substituent groups is increased. Inversion ceases completely, of course, if the pyramida are locked in one position by chemical bonding as in cage molecules such as P4O6 (4.41e). 

Using a method of electrolytic reduction, Horner [3] in 1961 obtained optically active phosphines from the corresponding phosphonium salts. These reactions proceed with retention of configuration (13.57) and the products racemise only slowly in boiling toluene. 

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