Combining the Reactivity Properties of PCy3 and PtBu3 into a Single Ligand, P(iPr)(tBu)2. Reaction via Mono- or Bisphosphine Palladium(0) Centers and Palladium(I) Dimer Formation

Trialkylphosphine ligands are ubiquitous in catalysis. Via modulation of the steric bulk of these ligands, two central aspects that dictate reactivity and selectivity in catalysis can be controlled: i.e., the coordination sphere and favored oxidation state of the reactive metal center. Within this class of ligands, tricyclohexylphosphine (PCy3) and tri-tert-butylphosphine (PtBu3) are most widely used in catalysis. While the smaller PCy3 favors reactivity via Pd bisphosphine species with the test substrate 4-chlorophenyl triflate 1 and does not form dinuclear Pd­(I) complexes upon oxidation of Pd(0), PtBu3 reacts via a monophosphine-ligated Pd complex with 1 and forms dinuclear Pd­(I) complexes on oxidation. We herein report that the hybrid ligand P­(iPr)­(tBu)2, characterized by a cone angle that is between those of PCy3 and PtBu3, features all of these reactivity properties in a single scaffold. This is exemplified in chemoselectivity studies with test system 1, in which the site selectivity could be readily modulated. The novel Pd­(I) dimer {[P­(iPr)­(tBu)2]­PdI}2 has also been synthesized.