Fundamental Reactions for Gold Catalysis

Abstract

Successful design of catalytic processes profits form comprehensive understanding of the kinetics and thermodynamics of fundamental reactions. Potential elementary reactions of gold(I) catalysed olefin hydroamination as well as transmetalation of alkylboronic acids to gold were studied for this purpose. Substrate binding energies between 186 and 208 kJ/mol were measured for [(Me3P)Au]+ adducts of trans-oct-4-ene, oct-1-ene, oct-1-yne and oct-4-yne in the gas phase. Gold(I) adducts of hydroamination products, i.e. of enamines and amines, lost [LAuH] in collision induced dissociation (CID) experiments, possibly by beta-H elimination. In order to study the addition step, several methods for preparation of beta-aminoethyl gold complexes were investigated. Of these, only metalation of 2-phenylethan-2-amines with Schlosser’s base (t-BuLi/t-BuOK), followed by reaction with [LAuCl] was successful. Unfortunately, the scope was narrow and the structure of the products could not be proven ultimately. Nevertheless, when such products were reacted with methyl triflate at a short contact time, methylated ions were observed by mass spectrometry. In CID elimination of trimethylamine under formation of an olefin complex ion [LAu(olefin)]+ occurred, which subsequently lost the olefin to form [LAu]+. How intensity ratios between [LAu(olefin)]+ and [LAu]+ fragment ions depended on structure was rationalised by calculation of the relative size of the dissociation energies for amine and olefins loss. Systematic trends for amine elimination were found by density functional theory for a broader range structures. Dissociation energies varied substantially. Based on calculations, it was excluded that protodeauration occurred intramolecularly or via protonation of the gold. Gold hydroxo and gold alkoxo complexes were found to condense with alkylboronic acids to form boronato gold complexes. Crystallisation led to the first X-ray diffraction structure of an alkylboronato complex. Upon heating, transmetalation occurred for a range of solvents, ligands and primary boronic acids. In a kinetic study by nuclear magnetic resonance, sigmoid concentration-time profiles were observed for the boronato complex and the transmetalated product. An intermediate and a side product that were bisligated were present. Addition of trimethylboroxine, accelerated the reaction, but led to more side product and made the mechanism more complex.