Thermally latent polyaddition and curing of Di‐ and tri‐functional hemiacetal esters with diepoxide by salen‐zinc complex with tunable catalytic activity and model and networking reactions

Salen‐zinc complexes (Zn/ 1 _R ) thermal‐latently catalyzed the polyaddition of a diepoxide ( 2 ) with a difunctional hemiacetal ester ( 3 ), which proceeded at moderate temperatures (100–150 °C) for curing of mixtures containing monomers and initiators. The catalytic activities of Zn/ 1 _R depended on the Lewis acidities of the complexes controlled by the electronic character of the salen ligands. For example, Zn/ 1 _3,5‐Cl bearing four electron‐withdrawing chlorine atoms initiated the polyaddition at the lowest temperature (100 °C), and Zn/ 1 _OMe bearing two electron‐donating methoxy groups initiated the polyaddition at 120 °C. The Lewis acidities of the complexes were evaluated by NMR and IR spectroscopies and computational calculation. The polyadditions with the salen‐zinc complexes proceeded quantitatively at 150 °C, and the use of a tri‐functional hemiacetal ester ( 7 ) with 2 afforded the corresponding networked polymer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1427–1439, 2008     

Synthesis and characterization of biologically active new Schiff bases containing 3‐functionalized 1,2,4‐triazoles and their zinc(II) complexes: crystal structure of 4‐bromo‐2‐[(E)‐(1H‐1,2,4‐triazol‐3‐ylimino)‐ methyl]phenol

Biologically active triazole Schiff bases ( L ¹  L ³ ) derived from the reaction of 3‐amino‐1,2,4‐triazole with chloro‐, bromo‐ and nitro‐ substituted salicylaldehydes and their Zn(II) complexes (1–3) have been synthesized and characterized by their physical, spectral and analytical data. Triazole Schiff bases potentially act as tridentate ligands and coordinate with the Zn(II) metal atom through salicylidene‐O, azomethine‐N and triazole‐N. The complexes have the general formula [M(L‐H)₂], where M = zinc(II) and L = ( L ¹ – L ³ ), and observe an octahedral geometry. The Schiff bases and their Zn(II) complexes have been screened for in‐vitro antibacterial, antifungal and brine shrimp bioassay. The biological activity data show the Zn(II) complexes to be more potent antibacterial and antifungal than the parent simple Schiff bases. Copyright © 2011 John Wiley & Sons, Ltd.     

C‐Selective and Diastereoselective Alkyl Addition to β,γ‐Alkynyl‐α‐imino Esters with Zinc(II)ate Complexes

Since umpolung α‐imino esters contain three electrophilic centers, regioselective alkyl addition with traditional organometallic reagents has been a serious problem in the practical synthesis of versatile chiral α‐amino acid derivatives. An unusual C‐alkyl addition to α‐imino esters using a Grignard reagent (RMgX)‐derived zinc(II)ate was developed. Zinc(II)ate complexes consist of a Lewis acidic [MgX]⁺ moiety, a nucleophilic [R₃Zn]^? moiety, and 2 [MgX₂]. Therefore, the ionically separated [R₃Zn]^? selectively attacks the imino carbon atom ,which is most strongly activated by chelation of [MgX]⁺. In particular, chiral β,γ‐alkynyl‐α‐imino esters can strongly promote highly regio‐ and diastereoselective C‐alkylation because of structural considerations, and the corresponding optically active α‐quaternary amino acid derivatives are obtained within 5 minutes in high to excellent yields.     

Planar Chiral Organoborane Lewis Acids Derived from Naphthylferrocene

Enantiomerically pure metalated 2‐(1‐naphthyl)ferrocene (NpFc) derivatives NpFcM (M=SnMe₃, HgCl) were prepared and characterized by multinuclear NMR and UV/Vis spectroscopy, cyclic voltammetry, and elemental analysis. Optical rotation measurements were performed and the absolute configuration of the new planar chiral ferrocene species was confirmed by single‐crystal X‐ray diffraction analysis. The mercuriated species NpFcHgCl proved suitable as a reagent for the preparation of the chiral organoborane Lewis acid NpFcBCl₂, which can in turn be converted to other ferrocenylboranes by replacement of Cl with nucleophiles. The highly Lewis acidic perfluoroarylborane derivatives NpFcB(C₆F₅)Cl and NpFcB(C₆F₅)₂ were successfully prepared by treatment with CuC₆F₅. The structures were studied by single‐crystal X‐ray diffraction and variable‐temperature ¹⁹F NMR spectroscopy, which suggested that π stacking of a C₆F₅ group on boron with the adjacent naphthyl group is energetically favorable. UV/Vis absorption spectroscopy and cyclic voltammetry measurements were performed to examine the electronic properties of these novel redox‐active chiral Lewis acids.     

Chiral proton donor reagents: tin tetrachloride--coordinated optically active binaphthol derivatives

The Lewis acid–assisted chiral Brønsted acids (chiral LBAs), which are prepared from tin tetrachloride and optically active binaphthol derivatives, are highly effective chiral proton donor reagents for enantioselective protonation and biomimetic polyene cyclization. These chiral LBAs can directly protonate various silyl enol ethers and ketene disilyl acetals to give the corresponding α‐aryl or α‐halo ketones and α‐arylcarboxylic acids, respectively, with high enantiomeric excess (up to 98% ee). A catalytic version of enantioselective protonation was also achieved using stoichiometric amounts of 2,6‐dimethylphenol and catalytic amounts of monomethyl ether of optically active binaphthol in the presence of tin tetrachloride. The biomimetic cyclization of simple isoprenoids to polycyclic isoprenoids using chiral LBA is also described. This is the first example of a chiral Brønsted acid–induced enantioselective ene cyclization in synthetic chemistry. Geranyl phenyl ethers, o‐geranylphenols, and homogeranylphenol derivatives were directly cyclized in the presence of (R)‐binaphthol derivatives and tin tetrachloride (up to 90% ee). Compounds bearing a farnesyl group could also be cyclized under the same conditions to give the natural products (?)‐ambrox^® and (?)‐chromazonarol, and (?)‐tetracyclic polyprenoids of sedimentary origin. These chiral LBAs recognize the prochiral face of a trisubstituted terminal olefin and site selectively generate carbocations on the substrates. © 2002 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 2: 177–188,2002: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.10020     

Thermally latent reaction of hemiacetal ester with epoxide catalyzed by recyclable polymeric catalyst consisting of salen‐zinc complex and polyurethane main chain

This article describes the synthesis of and catalysis with a polymeric catalyst (Zn/ 1_NHCOO ) carrying salen‐zinc complex structure in the main chain prepared from polyaddition of zinc/bis(4‐hydroxy)salicylidene‐1,2‐diiminoethane and 4,4′‐diphenylmethane diisocyanate. Poly(Zn/ 1_NHCOO ) promoted the reaction of glycidyl phenyl ether (2) with 1‐propoxyethyl‐2‐ethylhexanoate (3) only at moderately elevated temperatures. Poly(Zn/ 1_NHCOO ) can be recycled by simple filtration from the reaction mixtures, and the recycled polymer is as active as the freshly prepared one. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3673–3681, 2008     

Design of Stereoelectronically Promoted Super Lewis Acids and Unprecedented Chemistry of Their Complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state‐of‐the‐art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride‐ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid–fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent‐carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H₂, N₂, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four‐coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.     

Enantioselective Cascade Michael Addition/Cyclization Reactions of 3‐Nitro‐2H‐Chromenes with 3‐Isothiocyanato Oxindoles: Efficient Synthesis of Functionalized Polycyclic Spirooxindoles

An unprecedented Zn(OTf)₂‐catalyzed asymmetric Michael addition/cyclization cascade of 3‐nitro‐2H‐chromenes with 3‐isothiocyanato oxindoles has been disclosed. This transformation provides an efficient access to various synthetically important polycyclic spirooxindoles in a highly stereoselective manner under mild conditions (72–99 % yields, up to >95:5 d.r. and >99 % ee). The reaction leads to the formation of three consecutive stereocenters, including 1,3‐nonadjacent tetrasubstituted carbon stereocenters, in a single operation. A bifunctional activation model of the chiral Zn(OTf)₂/bis(oxazoline) complex was proposed based on control experiments, wherein the Zn^II moiety serves as a Lewis acid and the N atom of the free NH group acts as a Lewis base by a hydrogen‐bonding interaction.     

Octahedral Chiral‐at‐Metal Iridium Catalysts: Versatile Chiral Lewis Acids for Asymmetric Conjugate Additions

Octahedral iridium(III) complexes containing two bidentate cyclometalating 5‐tert‐butyl‐2‐phenylbenzoxazole ( IrO ) or 5‐tert‐butyl‐2‐phenylbenzothiazole ( IrS ) ligands in addition to two labile acetonitrile ligands are demonstrated to constitute a highly versatile class of asymmetric Lewis acid catalysts. These complexes feature the metal center as the exclusive source of chirality and serve as effective asymmetric catalysts (0.5–5.0 mol % catalyst loading) for a variety of reactions with α,β‐unsaturated carbonyl compounds, namely Friedel–Crafts alkylations (94–99 % ee), Michael additions with CH‐acidic compounds (81–97 % ee), and a variety of cycloadditions (92–99 % ee with high d.r.). Mechanistic investigations and crystal structures of an iridium‐coordinated substrates and iridium‐coordinated products are consistent with a mechanistic picture in which the α,β‐unsaturated carbonyl compounds are activated by two‐point binding (bidentate coordination) to the chiral Lewis acid.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

Synthesis,characterization, and antimicrobial activity of palladium(II) and platinum(II) complexes with 2‐substituted benzoxazole ligands

The synthesis and antimicrobial activity of palladium(II) and platinum(II) complexes derived from heterocyclic bidentate ligands, namely 2‐(2′‐aminophenyl)benzoxazole [L¹H₂], 2‐(2′‐hydroxyphenyl)benzoxazole [L²H], and 2‐(2′‐mercaptophenyl)benzoxazole [L³H], are reported here. These complexes have been characterized by elemental analyses, molecular weight determinations, conductance measurements, infrared, ¹H NMR, and electronic spectral studies. The resulting colored complexes are monomeric in nature. On the basis of above‐described studies, square‐planar geometry has been suggested for the resulting complexes. The ligands and their metal complexes were tested against certain microorganisms to assess their antimicrobial properties. The results indicate that the metal complexes are found more active than the parent ligands. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:44–50, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20578     

Modifying the structure of dinuclear ruthenium complexes with antitumor activity

In order to modulate the structure of a recently developed series of antitumor‐active, dinuclear Ru(II)–arene compounds, complexes 1c – 4c were synthesized. The complexes were modified with respect to their pyridinone moieties and the spacer linking the two metal centers. More particularly, the series of dinuclear ruthenium(II) complexes was extended to compounds with longer spacers, i.e. tetradecane and 3,7,10‐trioxotridecane, and the pyridinone ring was modified by replacing the methyl group by an ethyl group and by shifting the position of the methyl group. The organometallic ruthenium compounds were obtained from the reaction between [RuCl₂(η⁶‐p‐isopropyltoluene)]₂ and ligands 1b – 4b with yields ranging from 41 to 67%. All compounds were characterized by standard methods: MS, ¹H and ¹³C NMR spectroscopy and elemental analysis. Copyright © 2008 John Wiley & Sons, Ltd.     

Ruthenium(II) complexes derived from C2‐symmetric ferrocene‐based chiral bis(phosphinite) ligands: synthesis and catalytic activity towards the asymmetric reduction of acetophenones

Chiral secondary alcohols are very important building blocks and valuable synthetic intermediates both in organic synthesis and in the pharmaceutical industry for producing biologically active complex molecules. A series of new chiral Ru–phosphinite complexes ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ) were prepared from chiral C₂‐symmetric ferrocenyl phosphinites and corresponding chloro complex, [Ru(η⁶‐p‐cymene)(μ‐Cl)Cl]₂. The complexes were characterized using conventional spectroscopic methods. The binuclear complexes were tested as pre‐catalysts and were found to be good pre‐catalysts for the asymmetric transfer hydrogenation of substituted acetophenones in basic 2‐propanol at 82°C, providing the corresponding optically active alcohols with almost quantitative conversion and modest to high enantioselectivities (46–97%). Amongst the all complexes, complex 6 gave the highest ee of 97% in the reduction of 2‐methoxyacetophenone to (S)‐1‐(2‐methoxyphenyl)ethanol at 82°C. Copyright © 2015 John Wiley & Sons, Ltd.     

Catalytic hydrolysis of p‐nitrophenyl picolinate by copper(II) and zinc(II) complexes of N‐(2‐deoxy‐β‐D‐glucopyranosyl‐2‐salicylaldimino)

D‐glucosamine Schiff base N‐(2‐deoxy‐β‐D‐glucopyranosyl‐2‐salicylaldimino) and its Cu(II) and Zn(II) complexes were synthesized and characterized. The hydrolysis of p‐nitrophenyl picolinate (PNPP) catalyzed by ligand and complexes was investigated kinetically by observing the rates of the release of p‐nitrophenol in the aqueous buffers at 25°C and different pHs. The scheme for reaction acting mode involving a ternary complex composed of ligand, metal ion, and substrate was established and the reaction mechanisms were discussed by metal–hydroxyl and Lewis acid mechanisms. The experimental results indicated that the complexes, especially the Cu(II) complex, efficiently catalyzed the hydrolysis of PNPP. The catalytic reactivity of the Zn(II) complex was much smaller than the Cu(II) complex. The rate constant k_N showing the catalytic reactivity of the Cu(II) complex was determined to be 0.299 s^?1 (at pH 8.02) in the buffer. The pK_a of hydroxyl group of the ternary complex was determined to be 7.86 for the Cu(II) complex. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 345–350, 2002     

Spectroscopic characterization,thermogravimetry, density functional theory and biological studies of some mixed‐ligand complexes of meloxicam and 2,2′‐bipyridine with some transition metals

The mixed‐ligand Mn(II), Fe(III), Ni(II), Cu(II), Zn(II) and Zr(IV) complexes of meloxicam (H₂mel) and 2,2′‐bipyridine (Bipy) were prepared and characterized. For all complexes, the analytical and spectroscopic results revealed that H₂mel acts in a monobasic bidentate manner through the oxygen of the amide and nitrogen of the thiazole groups, whereas Bipy coordinates through the two nitrogen atoms with slightly distorted octahedral geometry. Thermodynamic parameters (E, ΔS^*, ΔH^* and ΔG^*) were calculated using Coats–Redfern and Horowitz–Metzger methods. The geometries of H₂mel and the complexes were carefully studied using density functional theory to predict the properties of materials performed using the hybrid density functional method B3LYP. All studied complexes are soft with respect to H₂mel where η varies from 0.096 for Zn(II) complex to 0.067 for Fe(III) complex and σ varies from 10.42 to 14.93 eV, while η and σ for H₂mel are 0.14 and 7.14 eV, respectively. The antibacterial activities of the ligands and metal complexes were investigated and the data show that the complexes are active against some bacterial species compared with H₂mel.     

Synthesis and analytical characterization of novel pyridyl‐substituted 1,1′‐ethenedithiolato complexes

Some 1,1′‐ethenedithiolato complexes of nickel(II), palladium(II), and platinum(II) with different phosphine ligands, such as PPh₃, PEt₃, and dppe were prepared. Starting from 2‐, 3‐ as well as 4‐pyridyl methyl ketone, the complexes 1–15 were obtained in an one‐pot synthesis through reaction with carbon disulfide, using potassium‐tert‐butylate as a base. They were characterized by ¹H, ¹³C, and ³¹P NMR, mass spectra, infrared spectra, and UV–VIS spectra. The molecular structures of the (Ph₃P)₂Pd^II complex 9 containing the 3‐pyridyl‐ethenedithiolato ligand and of the (Et₃P)₂Pt^II complex 12 containing the 4‐pyridyl‐ethenedithiolato ligand were determined. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:369–378, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20103     

Synthesis of terpyridine ligands and their complexation with Zn2+ and Ru2+ for optoelectronic applications

A series of soluble conjugated ligands TT , FT , PT , and NT that contained terpyridyl terminal units were synthesized by Heck coupling. These ligands absorbed at 341–434 nm and emitted blue–green light, with maximum at 440–522 nm and photoluminescence quantum yields of 0.15–0.71 in solution. The double‐layer electroluminescent devices with the configuration of ITO/PEDOT/ligand FT , PT , or NT /Ca/Al exhibited a brightness of 43–63 cd/m². These ligands were further reacted with zinc(II) and ruthenium(II) ions and subsequent anion exchange to afford linear and supramolecular complexes. The photophysics of these complexes were investigated. A significant redshift of the emission maximum of the Zn complexes relative to the corresponding ligands was observed. The Ru complexes were used to fabricate photovoltaic devices with the structure ITO/PEDOT/Ru complex/P3HT:PCBM (1:1 wt/wt)/Ca/Al. The power conversion efficiency of these polymer–fullerene bulk heterojunction photovoltaic devices was up to 0.71%. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7702–7712, 2008     

Some divalent metal(II) complexes of salicylaldehyde‐derived Schiff bases: Synthesis,spectroscopic characterization,antimicrobial and in vitro anticancer studies

Mononuclear transition metal(II) complexes of the type M(L)₂?2H₂O (where M = Co, Ni, Cu, Zn) have been synthesized from uninegative Schiff base ligands (HL₁–HL₄) designed by condensation of 4‐fluorobenzylamine with 2‐hydroxy‐1‐naphthaldehyde/3,5‐dichlorosalicylaldehyde/3,5‐dibromosalicylaldehyde/3‐bromo‐5‐chlorosalicylaldehyde. The compounds were successfully characterized using spectroscopic and physiochemical methods together with elemental analysis. Spectroscopic elucidation indicates a monobasic bidentate nature of ligands coordinated via deprotonated phenolic oxygen and azomethine nitrogen atom which suggests an octahedral geometry around the central metal ions. The complexes and ligands were screened for their in vitro antimicrobial activity against bacterial and fungal strains, the zinc(II) complexes being more active against the tested microbial strains. Further, the metal complexes were found to be more active than the uncomplexed ligands due to chelation process and, moreover, the complexes were more active against fungal strains than bacterial strains. Cytotoxic activities of all compounds were evaluated towards human alveolar adenocarcinoma epithelial cell line (A549), human breast adenocarcinoma cell line (MCF7), human prostate cancer cell line (DU145) and one normal human lung cell line (MRC‐5) using MTT colorimetric assay with doxorubicin as a standard. The zinc complexes were most active against the cancer cell lines and also found to be less toxic against MRC‐5 normal cell line than standard doxorubicin.     

3,3'-Diphosphoryl-1,1'-bi-2-naphthol-Zn(II) complexes as conjugate acid-base catalysts for enantioselective dialkylzinc addition to aldehydes

A highly enantioselective dialkylzinc (R(2)(2)Zn) addition to a series of aromatic, aliphatic, and heteroaromatic aldehydes (5) was developed based on conjugate Lewis acid-Lewis base catalysis. Bifunctional BINOL ligands bearing phosphine oxides [P(=O)R(2)] (7), phosphonates [P(=O)(OR)(2)] (8 and 9), or phosphoramides [P(=O)(NR(2))(2)] (10) at the 3,3'-positions were prepared by using a phospho-Fries rearrangement as a key step. The coordination of a NaphO-Zn(II)-R(2) center as a Lewis acid to a carbonyl group in a substrate and the activation of R(2)(2)Zn(II) with a phosphoryl group (P=O) as a Lewis base in the 3,3'-diphosphoryl-BINOL-Zn(II) catalyst could promote carbon-carbon bond formation with high enantioselectivities (up to >99% ee). Mechanistic studies were performed by X-ray analyses of a free ligand (7) and a tetranuclear Zn(II) cluster (21), a 31P NMR experiment on Zn(II) complexes, an absence of nonlinear effect between the ligand (7) and Et-adduct of benzaldehyde, and stoichiometric reactions with some chiral or achiral Zn(II) complexes to propose a transition-state assembly including monomeric active intermediates.     

Chiral Recognition in π Complexes of Alkenes,Aldehydes, and Ketones with Transition Metal Lewis Acids; Development of a General Model for Enantioface Binding Selectivities

Prochiral alkenes, aldehydes, and ketones constitute the most frequently used starting materials for enantioselective organic syntheses. Protocols often involve chiral binding agents or Lewis acids that can give two diastereomeric adducts, the ratios of which are measures of chiral recognition. With π adducts, the diastereomers differ in the enantioface of the C?C or O?C group bound to the Lewis acid. This review provides the first comprehensive analysis of such equilibria and related binding phenomena with chiral transition metal Lewis acids. An extensive body of data from the authors' laboratory for complexes of the pyramidal rhenium fragment [(η⁵?C₅H₅)Re(No)(PPh₃)]⁺ ( I ) affords particular insight. Literature data for other complexes are also summarized. A general model for chiral recognition based upon the relative steric properties of four quadrants is presented. This enables binding selectivities to be individually and rationally optimized for different classes of ligands. Electronic effects are also identified and correlated with specific structural properties. Relationships between binding equilibria, reactivity, and product configurations are discussed.