Anion effects in Homogeneous Palladium and Gold Catalysis

Abstract

The overarching theme of this work is to use weak interactions to improve transition metal catalysis by studying the ways transition metal cations interact with both traditional and novel weakly-coordinating anions. Understanding of these interactions will better allow synthetic chemists to choose appropriate counterions for catalytic transformations, as well as giving mechanistic insight for organometallic reactivity. Additionally, new anions can be synthesized with specific functionalities to improve reactions of interest. A series of ion pairs featuring the [Pd(IPr)(C(O)C6H¬9N)]+ cation were synthesized with various weakly-coordinating anions. Solid- and solution-state interactions with the anions were investigated using X-ray crystallography, NMR and IR spectroscopy, and computation. Proton NMR and percent buried volume calculations inform on the steric properties of the anions, whereas IR and DFT report on the electronics of anion binding. A qualitative scale of anion coordinating ability was created. In the process of this study, a new anion [IMP-H] – was also synthesized based on a phenylimidazole core. A variety of these functionalized [IMP-R] – anions were synthesized, with functionalities such as electron withdrawing groups, tertiary amides, and benzylic alcohols and amines. The coordinating abilities of these anions were analyzed with the techniques described above, with fairly large differences observed between the various functionalities. Further functionalization of these anions may be beneficial from a catalytic standpoint, as coordinating ability of anions has been shown to have dramatic effects on catalytic activity. By designing particular functionalities on anions, efficiency of catalytic processes may be markedly improved. To investigate the effects of the functionalized anions on catalysis, the anions were paired with gold cations such as [(IPr)Au(NCMe)]+, [(JohnPhos)Au(NCMe)]+, and [(tBuXPhos)Au(NCMe)]+. These ion pairs were used as catalysts for several reactions known to show differences in reactivity based on the anion present. While the IPr- and JohnPhos-containing ion pairs both resulted in the formation of gold nanoparticles under various reaction conditions, the tBuXPhos complexes did not, and were therefore more amenable to study. Mild anion effects were seen in the gold-catalyzed hydroalkoxylation of 3-hexyne as well as gold-catalyzed intermolecular [2+2] cycloaddition of α-methylstyrene and phenylacetylene; solvent polarity was also found to have a major effect on the reaction. Other reactions investigated include alkyne hydration, isomerization of 5-hexyn-1-ol to form cyclic vinyl ethers, and propargylation of substituted benzaldehydes.