Diastereomeric isomerization of Fischer's base analogs of LTAM molecules in organic solvents: mechanistic investigation

Three possible mechanistic routes for the diastereomeric isomerization of the leuco-triarymethane (LTAM) molecules have been proposed, depending on the formal breaking of the carbon-carbon double bond via either heterolytic, homolytic or molecula splitting/reassembling.The substituents effects on the rates of the isomerization of the LTAM molecules are not consistent with the heterolytic process. From the negative result in the synthesis of an unsymmetric LTAM from the reaction of symmetric LTAM's and substituted Fischer bases, the process via the molecula splitting/reassembling can also be ruled out.The rates of the isomerizations of the LTAM molecules are decreased up to ten times with various weight percentages of the radical scavenger (RS), comparing to the rate without RS. The radical scavenging effects allow us to confirm that the homolytic process is the most promising mechanistic process for the isomerization of LTAM molecules among three proposed routes. On the basis of Walter's concept, central carbon-centered radicals of LTAM molecules are obviously of the Class S type and hence the isomerization rates are affected by EDG and EWG in the same directions.