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'nML) to activate the catalyst. The resulting coordinatively unsaturated active species (L'M-n) can either back react with the ligand in a k(-1) step, or combine with the substrate in a k(2) step. When dissociation is not rate determining and k(-1)[L] is greater than or comparable to k(2)[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.