Rationalization of Product Selectivities in Asymmetric Horner-Wadsworth-Emmons Reactions by Use of a New Method for Transition-State Modeling

A new method for creating a transition-state force field, based on quantum chemical normal-mode analysis, is described. The force field was used to rationalize the experimentally observed product selectivities in asymmetric Horner-Wadsworth-Emmons reactions between some chiral phosphonates and chiral, racemic aldehydes. It was shown that if the transition states for the addition step (TS1) and for the subsequent ring closure to an oxaphosphetane (TS2) are both considered in the modeling, the product selectivity can be rationalized with good accuracy.