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PR1R2R3 phosphines are chiral, but the enantiomers can be separated if the interconvertion is sufficiently slow. The mechanism goes through a triangular-planar transition state. NR1R2R3 amines are also chiral but only those carrying bulky groups, that raise the energy barrier for the interconvertion, can be isolated as enantioners.
This isomerization occurs via rotation of the three groups located in a triangular face . Note that not all rotations give rise to a cis-trans isomerization. This is the most probable mechanism proposed for octahedral compounds of type MX4L2 (M = Sn, Ti, Zr).
The isomerization of P(OMe)3 involves an intramolecular methyl migration towards atom phosphorus, generating the P=O double bond. Phosphorus environment goes from pyramidal to tetrahedral. This reaction can be catalyzed reaction by alkyl halides (see reaction below).
Isomerization of phosphites P(OR)3 is spontaneous, but is catalyzed by alkyl (or aryl) halides. In this case, the transformation of P(OR)3, trivalent and pyramidal, into OPR(OR)2, pentavalent and tetrahedral involves a phosphonium species [PR(OR)3] X (quaternization of phosphorus), followed by removal of XR. Obviously only if R = R ' the process will be an isomerization.The transformation P(OR)3 + XR´ ↔ OPR´(OR)2 + XR is known as Michaelis-Arbuzov reaction.