61st Annual Report on Research 2016

The research project currently supported by the Petroleum Research Fund aims to incorporate of Lewis acidic anion co-catalysts for use in developing more selective and useful homogeneous catalytic reactions. To achieve our objective of employing Lewis acidic co-catalysts in reactions, our group has spent the last year pursuing projects along two thrusts: Deciphering the effects of neutral Lewis acids in known catalyst systems, and preparing ion pairing catalyst scaffolds capable of pairing Lewis acids with cationic metal complexes.

In the first approach, we are seeking to quantitate the effects of distal interactions - relatively weak forces outside the primary, inner sphere of a transition metal complex - between exogenous additives (Lewis acids, salts, and other reaction components) and metal complexes. The first independent research paper from our group, submitted just prior to the beginning of the present funding period, demonstrated a quantitative ¹H NMR technique for measuring Lewis acid binding enthalpy to a catalytically-relevant Pt acyl complex (Figure 1). Follow-up work has led us to the discovery of a more stable Pd acyl complex for gaining further insights into the interactions of acyls and Lewis acids. The Pd acyl complex is an olefin isomerization catalyst and is accelerated by the addition of B(C₆F₅)₃. A full mechanistic study is now underway to better understand the effects of the exogenous B(C₆F₅)₃ additive on catalysis.

It is essential that all of the potential effects of an additive are evaluated before they can be employed in a rational manner within a catalytic context. In some cases, common additives with one expected influence may have an additional unexpected effect. In recently submitted work we have found that silver salts, commonly added to activate Au catalysts through halide abstraction, can influence the selectivity of a alkyne hydration reaction featuring a zwitterionic catalyst lacking an inner-sphere halide. Since silver additives are used widely we are now working to understand the specific influences of Ag upon this catalytic reaction. In separate work (Figure 2) we have reinvestigated the effect of exogenous Lewis acid additives upon the Pd-catalyzed N-arylation of secondary amides. Direct coordination by Lewis acid to N-heteroaryl intermediates had been previously invoked to rationalize a rate acceleration, yet we now find that substrates lacking a N-heteroaryl intermediate still show rate enhancements - suggesting a further, previously undetected influence on catalytic rates.

Our group will continue to find routes towards rational catalyst design through understanding (and exploiting) interactions in the secondary coordination sphere - both through examining the effects of exogenous additives and by designing structures capable of enforcing intermolecular association. In this latter approach, we are currently preparing bimetallic ion pairs with the intent to exert the influence of exogenous additives on a catalytic reaction while leveraging the power of effective molarity to enforce high proximity between catalyst and additive. In preliminary work we have successfully prepared novel anionic constructs based on an imidazolide core, such as 2-phenyl-[N,N'-bis(tris(pentafluorophenyl)borato)]imidazolide,  constructed from tris(pentaflurophenyl)boron and 2-phenyl imidazole (Figure 3). Our immediate future work involves derivatization of these anions so that they can accommodate pendant Lewis acid functionality.