Phase-Transfer Activation of Transition Metal Catalysts

With metal-based catalysts, it is quite common that a ligand (L) must first dissociate from a catalyst precursor (L′_nM£L) to activate the catalyst. The resulting coordinatively unsaturated active species (L′_nM) can either back react with the ligand in a k_-1 step, or combine with the substrate in a k₂ step. When dissociation is not rate determining and k_-1[L] is greater than or comparable to k₂[substrate], this slows the rate of reaction. By introducing a phase label onto the ligand L and providing a suitable orthogonal liquid or solid phase, dramatic rate accelerations can be achieved. This phenomenon is termed "phase-transfer activation". In this Concept, some historical antecedents are reviewed, followed by successful applications involving fluorous/organic and aqueous/organic liquid/liquid biphasic catalysis, and liquid/solid biphasic catalysis. Variants that include a chemical trap for the phase-labeled ligands are also described.