Major–minor concept revisited: causes for the reversal of thermodynamically determined intermediate ratios under reaction conditions

Abstract

The present work deals with the kinetic reasons for the inversion of the intermediates during catalytic selective reactions. By using a model system, typical ratios between diastereomer “catalyst–substrate” complexes are simulated by stable diolefin complexes [Rh(PP)(diolefin)]BF₄ of NBD (norborna-2,5-diene) and COD ((Z,Z)-cycloocta-1,5-diene). The detailed kinetic study demonstrates figuratively how the thermodynamically dominant species during the catalysis is not necessarily the major “catalyst–substrate” complex. By determination of all rate constants involved, the reasons for the unusual experimental behaviour of intermediate reversal under reaction conditions are depicted. The experimentally detected effect on a model reaction is of great importance for the explanation of the direction of, e.g., stereoselection and considerably extends the mechanistic understanding of selective reactions.