A general approach by our group for the development of new catalytic synthetic methods that occur with higher efficiency and selectivity, use simpler reagents, and proceed with lower energy demand involves new ancillary ligand design coupled with fundamental studies of how metal-ligand bonding dictates catalytic reactivity. In this context, the presentation will focus on our recent efforts to discover new phosphorus- and sulfur-based ligands and associated metal catalysts that manifest special properties from seemingly "weak" interactions, for instance dispersion. In one case, low-coordinate Pd complexes possessing polarizable diamondoid substituents are shown to enable a new transmetalation mechanism under exceptionally mild conditions, facilitate the first ever characterization and reactivity studies of monoligated Pd(0) â the true active catalyst in modern cross-coupling reactions, and enable direct visible light-induced bond weakening. Studies of oxidative dehydrogenative coupling reactions will also showcase evidence for a distinct CâH bond activation mechanism that we describe as electrophilic CMD or "eCMD", which has characteristics distinct from established pathways for CâH functionalization. Transition state analyses suggest this reaction pathway could be a general class of CâH activation that to date has been convoluted with the classic concerted metalation-deprotonation (CMD) model, and selection rules have been identified for predicting what catalyst structures manifest either CMD or eCMD, each of which occurs with characteristic substrate preferences and selectivity.
Bradley Carrow (Princeton University)