Soluble Polymer-Bound Ligand-Accelerated Catalysis: Asymmetric Dihydroxylation

Cu(ii)@Luviset clear as recyclable catalyst for the formation of C–C bond in homo-coupling of terminal alkynes

A recyclable catalytic system of Cu(ii)@Luviset clear was developed for the homo-coupling of terminal alkyne with excellent product yield.

Recent Advances in Chemistry and Applications of Substituted Poly(ethylene glycol)s

Poly(ethylene glycol)s are well defined and easily accessible macromolecules with suitable properties for applications in chemistry, biotechnologies and medicine. The terminal hydroxy groups of poly(ethylene glycol)s can easily be converted into reactive functional groups by a number of routine reactions of organic chemistry. The chemical reagents or catalysts anchored to poly(ethylene glycol) chains were used in a number of syntheses including the enantioselective ones. Poly(ethylene glycol)s serve as carriers in combinatorial syntheses in the liquid phase. Coupling of poly(ethylene glycol)s with other polymers was used to prepare series of block copolymers having numerous applications. From the point of view of medical applications it is significant that substituted poly(ethylene glycol)s are non-toxic and resistant to recognition by the immunity system. That is why they are often used as carriers of many low-molecular-weight as well as high-molecular-weight medical drugs (drug delivery systems). In the conjugates with drugs their biological activity increases and their toxicity decreases. A review with 41 references.

New Types of Soluble Polymer-Supported Bisphosphine Ligands with a Cyclobutane Backbone for Pd-Catalyzed Enantioselective Allylic Substitution Reactions

A highly efficient and practical optical resolution of anti head-to-head racemic coumarin dimer 7 has been achieved by molecular complexation with TADDOL, (-)-8, through a hydrogen bonding interaction to afford the corresponding two enantiomers, (-)- and (+)-7, in 70 and 75 % yields, respectively, with >99 % ee. Starting from enantiopure (-)-7, a new type of C2-symmetric bisphosphine ligand (S,S,S,S)-3 with a cyclobutane backbone has been synthesized in good yield by facile transformations. The asymmetric induction efficiency of these chiral bisphosphine ligands in Pd-catalyzed asymmetric allylic substitution reactions was evaluated. Under the experimental conditions, the allylic substitution products could be obtained in excellent yields (up to 99 %) and enantioselectivities (up to 98.9 % ee). By taking advantage of the high enantioselectivity of this catalytic reaction and the easily derivable carboxylate groups on the cyclobutane backbone of ligand (S,S,S,S)-3, a new type of analogous ligand (S,S,S,S)-4 as well as the MeO-PEG-supported soluble ligand (S,S,S,S)-5 (PEG=polyethylene glycol) have also been synthesized and utilized in asymmetric allylic substitution reactions. In particular, the MeO-PEG supported (S,S,S,S)-5 b had a synergistic effect on the enantioselectivity of the reaction compared with its nonsupported precursor (S,S,S,S)-4 c, affording the corresponding allylation products 14 a and 14 b with excellent enantioselectivities (94.6 and 97.2 % ee, respectively). Moreover, the Pd complex of (S,S,S,S)-5 b could easily be recovered and recycled several times without significant loss of enantioselectivity and activity in the allylic substitution reactions.

Polymer-supported Cinchona alkaloid-derived ammonium salts as recoverable phase-transfer catalysts for the asymmetric synthesis of alpha-amino acids

Alkaloids such as cinchonidine, quinine and N-methylephedrine have been N-alkylated using polymeric benzyl halides or co-polymerized and then N-alkylated, thus affording a series of polymer-supported chiral ammonium salts which have been employed as phase-transfer catalysts in the asymmetric benzylation of an N-(diphenylmethylene)glycine ester. These new polymeric catalysts can be easily recovered by simple filtration after the reaction and reused. The best ee's were achieved when Merrifield resin-anchored cinchonidinium ammonium salts were employed.

A liquid-phase peptide synthesis in cyclohexane-based biphasic thermomorphic systems

Combinations of typical organic solvents composed of cyclohexane and qualified aprotic polar organic solvents were found to realize an effective, biphasic thermomorphic system in arbitrary ratios of upper and lower phases that enable a practical application of a liquid-phase peptide synthesis.

Catalytic asymmetric dihydroxylation of olefins with reusable OsO(4)(2-) on ion-exchangers: the scope and reactivity using various cooxidants

Exchanger-OsO(4) catalysts are prepared by an ion-exchange technique using layered double hydroxides and quaternary ammonium salts covalently bound to resin and silica as ion-exchangers. The ion-exchangers with different characteristics and opposite ion selectivities are specially chosen to produce the best heterogeneous catalyst that can operate using the various cooxidants in the asymmetric dihydroxylation reaction. LDH-OsO(4) catalysts composed of different compositions are evaluated for the asymmetric dihydroxylation of trans-stilbene. Resin-OsO(4) and SiO(2)-OsO(4) designed to overcome the problems associated with LDH-OsO(4) indeed show consistent activity and enantioselectivity in asymmetric dihydroxylation of olefins using K(3)Fe(CN)(6) and molecular oxygen as cooxidants. Compared to the Kobayashi heterogeneous systems, resin-OsO(4) is a very efficient catalyst for the dihydroxylation of a wide variety of aromatic, aliphatic, acyclic, cyclic, mono-, di-, and trisubstituted olefins to afford chiral vicinal diols with high yields and enantioselectivities irrespective of the cooxidant used. Resin-OsO(4) is recovered quantitatively by a simple filtration and reused for a number of cycles with consistent activity. The high binding ability of the heterogeneous osmium catalyst enables the use of an equimolar ratio of ligand to osmium to give excellent enantioselectives in asymmetric dihydroxylation in contrast to the homogeneous osmium system in which excess molar quantities of the expensive chiral ligand to osmium are invariably used. The complexation of the chiral ligand (DHQD)(2)PHAL, having very large dimension, a prerequisite to obtain higher ee, is possible only with the OsO(4)(2-) located on the surface of the supports.