Overview. A main focus of our lab is the mechanism-driven design of homogeneous transition metal catalysts for the synthesis of materials and biologically-relevant small molecules. Our recent efforts have centered around complexity building from feed stock chemicals and integrating greener reagents to improve the sustainability of synthetic methods. Embracing the power of ancillary ligands to modulate the lifetime, reactivity, and selectivity of metal catalysts, our core approach involves an iterative cycle of mechanistic studies to establish new catalyst structure-function relationships that inform rational ligand evolution.
Cross-dehydrogenative coupling (CDC) reactions directly form a C−C bond through cleavage of two C−H bonds and bypass the functionalized substrates required for traditional cross-coupling reactions. The oxidizing conditions of CDC reactions and necessary selection of one C−H bond among many in typical organic substrates nevertheless present significant challenges toward efficient CDC catalysis. We have found that simple thioethers can enable significant rate acceleration and switchable site selectivity in the dehydrogenative Heck reaction (DHR), one of the most common CDC-type reactions. Combined experimental and computational mechanistic studies suggest the thioether triggers a concerted, electrophilic mechanism of C−H bond cleavage, which is distinct from step-wise electrophilic metalation (EM) and classic concerted metalation-deprotonation (CMD) pathways generally proposed in CDC reactions with electron-rich (hetero)arenes. This observation of ligand-switchable mechanisms of C−H activation provides new opportunities for enabling catalyst control in non-directed C−H functionalization. Ongoing work in our lab suggests thioether-Pd catalysts retain their high reactivity in many areas of CDC chemistry.
NEW ORGANOMETALLIC STRUCTURES & MECHANISM
Metals with low electron counts and vacant orbitals are often exceptionally reactive, but such species are typically stable only at very low steady-state concentration. In this context, we have developed a general synthetic route to large tertiary phosphines possessing diamondoids – the smallest subunits of the diamond lattice. Because large hydrocarbyl groups can manifest significant van der Waals forces, these new ancillary ligands are attractive candidates for studying the influence of such “dispersion energy donors” on organometallic reaction mechanisms. Recent indications suggest pronounced effects might indeed be possible. We have found that highly reactive, coordinatively-unsaturated organo-Pd complexes stabilized by PAd3 (Ad = 1-adamantyl) can trigger direct B to Pd transmetalation even in the absence of a strong base that is classically necessary in many catalytic coupling reactions. This mechanism switch is notable because base incompatibility is a widespread problem, such as with important classes of organoboron reagents (e.g., heteroaromatic and polyfluoro), protecting group-free synthesis, and biorthogonal chemistry. Leveraging dispersion forces in molecular catalyst design should be a generally useful concept for accessing unexpected reactivity together with kinetic stability in transition and main group element chemistry.
The plastics economy of the future will need to address issues of pollution, sustainability, and advanced material properties not currently accessible with 20th century thermoplastic materials. We are pursuing late metal catalysts that can enchain historically incompatible mixtures of non-polar with polar C1 & C2 monomers to create functional polymers with enhanced properties that can help to address these issues.
Conjugated polymers and oligomers are integral constituents in many optoelectronic materials, such as organic solar cells (OSCs), organic field effect transistor (OFETs), organic light-emitting diodes (OLEDs), and electrochromic materials. The use of stoichiometric metal reagents (i.e., Li, Mg, Cu, Ag, Sn, Hg) in the preparation of these materials remains widespread and considerably compromises the ‘greenness’ of organic materials. The ligands and catalysts we are developing have shown significant potential for enabling scalable, metal additive-free syntheses of a range of privileged and novel organic electronic motifs.