3 years ago

Mechanism and Selectivity of Cooperatively Catalyzed Meyer–Schuster Rearrangement/Tsuji–Trost Allylic Substitution. Evaluation of Synergistic Catalysis by Means of Combined DFT and Kinetics Simulations

Mechanism and Selectivity of Cooperatively Catalyzed Meyer–Schuster Rearrangement/Tsuji–Trost Allylic Substitution. Evaluation of Synergistic Catalysis by Means of Combined DFT and Kinetics Simulations
Marcin Kalek, Fahmi Himo
The reaction between propargylic alcohols and allylic carbonates, engaging vanadium and palladium catalysts, is an exemplary case of a cooperatively catalyzed process. This combined Meyer–Schuster rearrangement/Tsuji–Trost allylic substitution clearly illustrates the enormous advantages offered by the simultaneous use of two catalysts, but also the inherent challenges regarding selectivity associated with such a reaction design. These challenges originate from the fact that the desired product of the combined process is formed by a bimolecular coupling of the two substrates activated by the respective catalysts. However, these two processes may also occur in a detached way via the reactions of the catalytic intermediates with the starting propargylic alcohol present in the reaction mixture, leading to the formation of two side-products. Herein, we investigate the overall mechanism of this reaction using density functional theory (DFT) methodology. The mechanistic details of the catalytic cycles for all the individual processes are established. In particular, it is shown that the diphosphine ligand, dppm, used in the reaction promotes the formation of dinuclear palladium complexes, wherein only a single metal center is directly involved in the catalysis. Due to the complexity of the combined reaction network, kinetics simulation techniques are employed in order to analyze the overall selectivity. The simulations directly link the results of the DFT calculations with the experimental data and confirm that the computed free energy profiles indeed reproduce the observed selectivities. In addition, a sensitivity analysis is carried out to assess the importance of the individual steps on the product distribution. The observed behavior of the kinetic network is rationalized, and trends in the reaction outcome upon changing the initial conditions, such as the catalysts amounts and ratio, are discussed. The results provide a general framework for understanding the factors governing the selectivity of the cooperatively catalyzed reactions.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b01931

DOI: 10.1021/jacs.7b01931

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