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'n M-L) to activate the catalyst. The resulting coordinatively unsaturated active species (L'n M) 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.
Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha (Qatar).|Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, PO Box 286, 1519 Budapest (Hungary).
Robert
R
Tuba
Department of Chemistry, Texas A&M University, PO Box 30012, College Station, Texas 77842-3012 (USA).
Zhenxing
Z
Xi
Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha (Qatar). bazzi@tamu.edu.
Hassan S
HS
Bazzi
Department of Chemistry, Texas A&M University, PO Box 30012, College Station, Texas 77842-3012 (USA). gladysz@mail.chem.tamu.edu.
John A
JA
Gladysz
biphasic catalysis; fluorous solvents; olefin metathesis; phase transfer; polymerization