ABSTRACT
Enantiopure poly(lactic acid) (PLA) can form stereocomplexes when enantiomeric PLA chains are mixed in equivalent amounts. Such materials provide interesting features that might be suitable for numerous applications. Despite several advantages, the main drawback of PLA is its narrow window of processing, thus limiting its use for industrial applications. Reported herein are achiral iron complexes, that are highly active, productive, and stereoselective under mild reaction conditions for the ring-opening polymerization of lactide. The corresponding catalytic systems enable the production of stereoblock polymers with high molecular weights, allowing the formation of thermally stable and industrially relevant stereocomplexes.
ABSTRACT
We report here a unique example of an in situ generated aluminum initiator stabilized by a C2-symmetric salen ligand which shows a hitherto unknown high activity for the ROP of rac-lactide at room temperature. Using a simple and robust catalyst system, which is prepared from a salen complex and an onium salt, this convenient route employs readily available reagents that afford polylactide in good yields with narrow polydispersity indices, without the need for time-consuming and expensive processes that are typically required for catalyst preparation and purification. In line with the experimental evidence, DFT studies reveal that initiation and propagation proceed via an external alkoxide attack on the coordinated monomer.
ABSTRACT
Most germane: Hexacoordinate germanium(IV) species exhibit unprecedented activities, yet controlled behavior, as initiators for the ring-opening polymerization of rac-lactide to form polylactide polymers.
Subject(s)
Germanium/chemistry , Tin/chemistry , Catalysis , Models, Molecular , Molecular Structure , Polyesters , Polymerization , Polymers , StereoisomerismABSTRACT
Organometallic complexes of the general formula [(η(6)-arene)Ru(NâN)Cl](+) and [(η(5)-Cp*)Rh(NâN)Cl](+) where NâN is a 2,2'-dipyridylamine (DPA) derivative carrying a thiol-targeted maleimide group, 2,2'-bispyridyl (bpy), 1,10-phenanthroline (phen) or ethylenediamine (en) and arene is benzene, 2-chloro-N-[2-(phenyl)ethyl]acetamide or p-cymene were identified as catalysts for the stereoselective reduction of the enzyme cofactors NAD(P)(+) into NAD(P)H with formate as a hydride donor. A thorough comparison of their effectiveness towards NAD(+) (expressed as TOF) revealed that the Rh(III) complexes were much more potent catalysts than the Ru(II) complexes. Within the Ru(II) complex series, both the NâN and arene ligands forming the coordination sphere had a noticeable influence on the activity of the complexes. Covalent anchoring of the maleimide-functionalized Ru(II) and Rh(III) complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins, some of them displaying formate dehydrogenase activity with potentially interesting kinetic parameters.
Subject(s)
2,2'-Dipyridyl/analogs & derivatives , Formate Dehydrogenases/chemistry , NADP/chemistry , Organometallic Compounds/chemistry , Papain/chemistry , Ruthenium/chemistry , 2,2'-Dipyridyl/chemistry , Benzene/chemistry , Cymenes , Ethylenediamines/chemistry , Monoterpenes/chemistry , Oxidation-Reduction , Phenanthrolines/chemistryABSTRACT
Covalent embedding of a (eta(6)-arene) ruthenium(II) complex into the protein papain gives rise to a metalloenzyme displaying a catalytic efficiency for a Lewis acid-mediated catalysed Diels-Alder reaction enhanced by two orders of magnitude in water.
Subject(s)
Acids/chemistry , Coordination Complexes/chemistry , Papain/chemistry , Ruthenium/chemistry , Water/chemistry , Catalysis , Crystallography, X-Ray , Kinetics , Molecular ConformationABSTRACT
The tridentate proligands (RNH-o-C6H4)2O (1a, R = C5H9; 1b, R = Cy) were found to readily react with LiAlH4 to yield the corresponding lithium aluminium dihydrido salt species [eta(2)-N,N-{(RN-o-C6H4)2O}AlH(mu-H)Li(THF)]2 (2a, R = C5H9; 2b, R = Cy) in 50% and 42% yield, respectively. The solid-state structure of both complexes 2a and 2b were determined by X-ray crystallographic studies. Compounds 2a and 2b readily react with one equivalent of benzaldehyde to afford the corresponding mono-benzyloxide species eta(2)-N,N-{RN-o-C6H4)2O}Al(H)(mu-OCH2Ph)Li(THF)2 (4a, R = C4H9; 4b, R = Cy), as confirmed by X-ray studies in the case of 4b. In a similar manner, when compounds 2a and 2b are reacted with two equivalents of benzaldehyde the bis-benzyloxide derivatives 5a and 5b eta(3)-N,N,O-{RN-o-C6H4)2O}Al( -OCH2Ph)2Li(THF)2 (5a, R = C4H9; 5b, R = Cy) may be prepared. While the lithium Al mono-alkoxide species 4a-b are inactive in the ring-opening polymerization (ROP) of lactide, their bis-alkoxide Al analogues 5a-b polymerize rac-lactide and (S)-lactide at room temperature, which is rather uncommon for Al-based alkoxide systems. Kinetic studies of the lactide ROP initiated by compound 5a suggest a strong preference for racemic enchainment during the ROP chain growth; the resulting PLAs are however moderately heterotactic due to detrimental transesterification processes occurring as the chain grows.
Subject(s)
Aluminum/chemistry , Anions/chemistry , Coordination Complexes/chemical synthesis , Dioxanes/chemistry , Ligands , Aluminum Compounds/chemistry , Coordination Complexes/chemistry , Crystallography, X-Ray , Lithium Compounds/chemistry , Molecular Conformation , Polymers/chemistryABSTRACT
Site-directed and covalent introduction of various transition metal-organic entities to the active site of the cysteine endoproteinase, papain, was achieved by treatment of this enzyme with a series of organometallic maleimide derivatives specially designed for the purpose. Kinetic studies made it clear that time-dependent irreversible inactivation of papain occurred in the presence of these organometallic maleimides as a result of Michael addition of the sulfhydryl of Cys25. The rate and mechanism of inactivation were highly dependent on the structure of the organometallic entity attached to the maleimide group. Combined ESI-MS and IR analysis indicated that all the resulting papain adducts contained one organometallic moiety per protein molecule. This confirmed that chemospecific introduction of the metal complexes was indeed achieved. Thus, three novel reagents for heavy-atom derivatization of protein crystals, which include ruthenium, rhenium and tungsten, are now available for the introduction of electron-dense scatterers for phasing of X-ray crystallographic data.
Subject(s)
Carica/enzymology , Cysteine/chemistry , Cysteine/metabolism , Metals/chemistry , Metals/metabolism , Papain/metabolism , Kinetics , Maleimides/chemistry , Maleimides/metabolism , Models, Molecular , Molecular Structure , Papain/chemistry , Papain/isolation & purification , Spectrometry, Mass, Electrospray Ionization , Spectrophotometry, InfraredABSTRACT
The crystal structure of the title compound, chloro(eta(5)-cyclopentadienyl)[(1R,2S)-2-[(diphenylphosphino)methylamino]-1-phenylpropyl diphenylphosphinite-kappa(2)P,P']ruthenium(II), [Ru(C(5)H(5))Cl(C(34)H(33)NOP(2))], is reported. The pseudo-octahedral complex is chiral and the configuration at the Ru atom is S. The seven-membered metallacycle adopts a boat-like conformation.
