Prof. Yasuhiro Uozumi (Division of Complex Catalysis, IMS) and his collaborators (Dr. Go Hamasaka and Mr. Tsubasa Muto) have developed a new aquacatalytic system based on self-assembly of amphiphilic pincer palladium complex.
Biological membrances are known to play a key role in controlling life-related molecular functions (e.g., active/passive transport, diffusion, filtration, selective permeation, etc.). Lipid bilayer vesicles (i.e., liposomes) are small assemblages of amphiphilic molecules bearing both hydrophilic and hydrophobic groups that offer promising prospects for the understanding of biological membrances. If less catalytically active small molecules can self-assemble to form bilayer vesicles and, in so doing, gain unique catalytic functions for a given molecular transformation, this process could conceivably be used in a catalysis-driven system.
The pincer palladium complex having pairs of hydrophobic dodecyl chains and hydrophilic tri(ethylene glycol) (TEG) chains, located opposite to one another on the rigid planer backbone, was designed and prepared for use in the self-assembled formation of vesicles exhibiting catalytic activity in water. Treatment of the complex in water at 60 °C for 4 hours gave the vesicle (Figure 1). The structure of vesicle was confirmed by various analyses (DLS, SEM, AFM, TEM, and CLSM).
Figure 1. Formation of vesicle by self-assembly of the amphiphilic pincer palladium complex.
With the desired vesicles of the palladium-pincer complex in hand, we next explored their catalytic potential for arylating oxirane ring-opening reactions of vinyl epoxide with phenylboronic acid (Figure 2). A mixture of vinyl epoxide and phenylboronic acid (1.2 equiv) was stirred in water with 2 mol% palladium within the vesicle for 12 hours. The reaction mixture was extracted with tert-butyl methyl ether to give an 84% yield of the arylated liner product (l) along with its branched regioisomer (b). However, under similar conditions, the reaction with nano-suspension of the monomer complex did not proceed as efficiency. Neither vesicle nor monomer catalyzed the arylation in organic solvents, where the vesicle disassembled to the catalytically less active monomer. Thus, the vesicle formation is essential for the amphiphilic pincer complex to acquire the necessary catalytic property in water.
Figure 2. Palladium-catalyzed oxirane ring opening with PhB(OH)2.
Paper Information
Journal: Angew. Chem. Int. Ed., 2011, 50, 4876-4878.
Title: Molecular-Architecture-Based Administration of Catalysis in Water: Self-Assembly of an Amphiphilic Pincer Complex
Authors: Go Hamasaka, Tsubasa Muto, and Yasuhiro Uozumi
UOZUMI Group Information
http://www.ims.ac.jp/english/know_en/bio/uozumi/uozumi_en.html
