Theoretical studies on the mechanism of transition metal catalyzed Tishchenko reaction -

Abstract

In the conventional Tishchenko reaction an aluminum alkoxide reagent is used to catalyze the transformation of two aldehydes into an ester. This reaction can be useful in organic synthesis, but large catalyst loads are often required, and the turnover numbers are usually low. In recent years there have been several new studies in which the Tishchenko reaction was catalyzed by transition metal complexes. The mechanisms in the new systems have not been determined but they have been generally speculated to be the same as in the aluminum-alkoxide systems. We used density function theory (DFT) electronic structure methods to investigate the mechanism of the Tishchenko reaction catalyzed by some of the more efficient transition metal complexes. The main target of our investigations is the octahedral catalyst [Rh(PhBP3)(H)2(NCMe)] (P = PPh2) disclosed by Kristina Tejel and coworkers. The authors proposed activity in this system to start by substitution of the acetonitrile by an aldehyde. Hydride transfer from the metal to the coordinated aldehyde was proposed to generate a metal alkoxide that undergoes further reaction with a new aldehyde following the conventional Tishchenko mechanism described above. This is an inner sphere mechanism meaning that the substrates of interest (the two aldehydes) both coordinate to the metal at some stage of the reaction. However, one can envision an alternative simpler mechanism in the given system based on an outer sphere mechanism. Such mechanism would start in direct transfer of a hydride from the intact octahedral complex to the aldehyde and rearrangement into an octahedral rhodium-alkoxide. The alkoxide can then transfer to the second aldehyde to make a hemiacetaloxide anion that transfers a hydride back to the metal in a outer sphere mode to give an ester and regenerate the catalyst. The full energy profile for both the inner-sphere and the outer-sphere mechanisms were computed. The inner sphere mechanism turned out to be favored since the hydride transfer transition state and a