Lewis Basic Groups Research Articles

Bicatalytic Allylation–Cross-Metathesis Reactions as γ-Carbonyl Cation Equivalents

The products corresponding to the reactions of arenes and γ-carbonyl cations may be obtained by a one-pot, bicatalytic process involving InCl3-catalyzed arene allylation and cross metathesis with electron-deficient alkenes. The process is successful with electronically neutral and electron-rich arenes, and modestly Lewis basic donor groups are tolerated with an increase in InCl3 loading from 10 mol% to 15 mol%, and in one case, 20 mol%.

Access to "Friedel-Crafts-restricted" tert-alkyl aromatics by activation/methylation of tertiary benzylic alcohols.

Herein we describe a two-step protocol to prepare m-tert-alkylbenzenes. The appropriate tertiary benzylic alcohols are activated with SOCl(2) or concentrated HCl and then treated with trimethylaluminum, affording the desired products in 68-97% yields (22 examples). This reaction sequence is successful in the presence of a variety of functional groups, including acid-sensitive and Lewis-basic groups. In addition to t-Bu groups, 1,1-dimethylpropyl and 1-ethyl-1-methylpropyl groups can also be installed using this method.

Effects of polyelectrolyte hybridization on the crystal structure, physicochemical properties, and electrochemical activity of layered manganese oxide

The effects of polyelectrolyte intercalation on the crystal structure, physicochemical properties, and electrode performance of manganese oxide were investigated with intercalative nanohybrids composed of layered manganate nanosheets and polyelectrolytes such as polyethylenimine (PEI), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethyl ammonium) chloride (PDDA). The intercalative hybridization between layered manganate nanosheets and polyelectrolytes was confirmed by X-ray diffraction, field emission-scanning electron microscopy, and FT-IR spectroscopy. N 2 adsorption–desorption isotherm analysis clearly demonstrated that the PEI-layered MnO 2 nanohybrid showed a larger surface area than the other nanohybrids. According to Mn K-edge X-ray absorption spectroscopy, the PEI-layered MnO 2 nanohybrid possessed a lower Mn oxidation state compared with the other nanohybrids, reflecting the electron transfer from Lewis basic amine groups of the PEI to the manganate layers. All the present nanohybrids exhibited pseudocapacitance behavior, suggesting their applicability as electrode for supercapacitor. The PEI-layered MnO 2 nanohybrid showed larger capacitances than the PDDA- and PAH-intercalates. The observed superior electrode performance of the former could be understood by a larger surface area and a lower Mn oxidation state of this material.

Donor−Acceptor Assisted Alkyne Hydration: A Luminescent Boron-Stabilized Enol

An alkyne molecule 2-(2'-BMes(2)-phenylethynyl)pyridine (1) that contains a Lewis acidic BMes(2) group and a Lewis basic pyridyl group has been found to undergo facile hydration under ambient conditions in the presence of CuI, forming a highly emissive and stable enol ester (2). The lack of any hydration reactivity by a control compound 2-(2'-BMes(2)-phenylethynyl)benzene (3) under the same conditions supports that the cooperativity between the Lewis donor and acceptor groups plays a key role in the formation of 2.

Intactness and spatial proximity of acid–base groups in bifunctional SBA-15 as revealed by solid-state NMR

The intactness and spatial proximity of acid and base groups in bifunctional mesoporous SBA-15 has been studied by various NMR techniques. The NMR results show that Brønsted acid and Lewis base groups coexist peacefully on the surface of the same support and maintain their acidity and basicity, respectively. Meanwhile, the acid groups and the base groups are in close proximity with suitable distance.

Versatile nuclearity in copper complexes with ortho functionalized 1,3-bis(aryl)triazenido ligands

The synthesis, characterization and crystal structures of three new copper complexes derived from 1,3-bis(aryl)triazenido ligands bearing either a methoxycarbonyl, methylthio or a hydroxymethyl group in the ortho position of one of the aromatic rings are reported. In addition to the coordination of the triazenido fragment, the Lewis basic groups coordinate to the copper centers to form complexes with different nuclearity: {1-[2-(methoxycarbonyl)phenyl]-3-[4-methylphenyl]}triazene and {1-[2-(methylthio)phenyl]-3-[4-methylphenyl]}triazene form stable dinuclear and tetranuclear Cu(I) complexes, respectively. Reaction of {1-[2-(hydroxymethyl)phenyl]-3-[4-methylphenyl]}triazene with either Cu(I) or Cu(II) results in a novel Cu(II) hexanuclear macrocyclic complex.

Porphyrin decorated CdSe quantum dots for direct fluorescent sensing of metal ions

In this study, we report on a novel hybrid structure for the direct sensing of metal ions based on the use of CdSe quantum dots functionalized with tetrapyridyl-substituted porphyrin, a cation-selective carrier. It is shown that the porphyrin ion carrier coordinates to Cd atoms of the CdSe quantum dots through the Lewis basic pyridyl groups. The porphyrin-quantum dot ligation allows for the direct communication between the porphyrin and the underlying CdSe quantum structure. The quantum yield of CdSe is maintained despite the robust capping generated when the unmetallated porphyrins coordinate onto the quantum dots. Upon coordination with zinc ions, this porphyrin capping is shown to strongly contribute to the increase in the fluorescence efficiency of CdSe, via an activating interaction with the quantum dots surface. The results provide the grounds for the development of highly sensitive and selective nano-optode sensing systems.

Peptide Length, Steric Effects, and Ion Solvation Govern Zwitterion Stabilization in Barium-Chelated Di- and Tripeptides

Infrared multiple-photon dissociation (IRMPD) spectroscopy has given infrared spectra of complexes of di- and tripeptides (AlaAla, AlaAlaAla, AlaPhe, PheAla) with singly and doubly charged metal ions (K(+), Ca(2+), Sr(2+), and Ba(2+)). The switch between charge-solvated (CS) and salt-bridged zwitterion (SB) conformations is displayed through highly diagnostic features in the mid-infrared. Systematic trends are found correlating with the length of the peptide chain (tripeptides favoring CS conformations), metal ion size (larger metals favoring SB conformations), metal ion charge (doubly charged ions favoring SB conformations), and sterically available Lewis-basic side-chain interactions with the metal ion (for example a cation-pi interaction with Ba(2+) stabilizes CS for PheAla but not for AlaPhe). The principle is that CS conformations are favored for small metal ions with high charge density and extensive microsolvation of the charge by Lewis-basic groups, especially amide carbonyls; SB conformations are favored by metal ions of high charge but low charge density, which are better stabilized by salt-bridge Coulomb interactions.

Cathodic electrodeposition of ZnO and CuSCN thin films in the presence of glutathione

The effect of a tripeptide molecule glutathione (γ-L-glutamyl-L-cysteniylglycine, QCG) is studied as a structure-directing agents (SDA) for cathodic electrodeposition of ZnO and CuSCN from aqueous solutions, which are known to be wide bandgap semiconductors of n and p types, respectively. From experiences, hard Lewis basic –COOH group act as an anchor to the hard Lewis acidic Zn(II) sites of ZnO, whereas soft Lewis basic groups such as –SH and –NH2 are expected to interact with soft Lewis acidic Cu(I) sites of CuSCN. QCG have all these anchoring groups within its structure and was found to affect the growth of both ZnO and CuSCN, leading to alteration of crystallographic orientations and evolution of unique nanostructures.

Catalysis of 6π Electrocyclizations

Filling the gap: Lewis acid catalysis of a 6π electrocyclization has been achieved for the first time (see scheme). DFT calculations were performed on the coordination of a Lewis acid to a Lewis basic group in the 2-position of a triene, and experimentally, the rates of the electrocyclization increase in the presence of Me2AlCl.

Alkali Metal Complexes of the Dipeptides PheAla and AlaPhe: IRMPD Spectroscopy

Complexes of PheAla and AlaPhe with alkali metal ions Na(+) and K(+) are generated by electrospray ionization, isolated in the Fourier-transform ion cyclotron resonance (FT-ICR) ion trapping mass spectrometer, and investigated by infrared multiple-photon dissociation (IRMPD) using light from the FELIX free electron laser over the mid-infrared range from 500 to 1900 cm(-1). Insight into structural features of the complexes is gained by comparing the obtained spectra with predicted spectra and relative free energies obtained from DFT calculations for candidate conformers. Combining spectroscopic and energetic results establishes that the metal ion is always chelated by the amide carbonyl oxygen, whilst the C-terminal hydroxyl does not complex the metal ion and is in the endo conformation. It is also likely that the aromatic ring of Phe always chelates the metal ion in a cation-pi binding configuration. Along with the amide CO and ring chelation sites, a third Lewis-basic group almost certainly chelates the metal ion, giving a threefold chelation geometry. This third site may be either the C-terminal carbonyl oxygen, or the N-terminal amino nitrogen. From the spectroscopic and computational evidence, a slight preference is given to the carbonyl group, in an RO(a)O(t) chelation pattern, but coordination by the amino group is almost equally likely (particularly for K(+)PheAla) in an RO(a)N(t) chelation pattern, and either of these conformations, or a mixture of them, would be consistent with the present evidence. (R represents the pi ring site, O(a) the amide oxygen, O(t) the terminal carbonyl oxygen, and N(t) the terminal nitrogen.) The spectroscopic findings are in better agreement with the MPW1PW91 DFT functional calculations of the thermochemistry compared with the B3LYP functional, which seems to underestimate the importance of the cation-pi interaction.

β-Phosphinoethylboranes as Ambiphilic Ligands in Nickel−Methyl Complexes

The ambiphilic β-phosphinoethylboranes Ph2PCH2CH2BR2 (BR2 = BCy2 (1a), BBN (1b)), which feature the ethano spacer CH2CH2 between the Lewis acidic boryl and Lewis basic phosphino groups, were synthesized in nearly quantitative yields via the hydroboration of vinyldiphenylphosphine. Compounds 1a,b were fully characterized by elemental analysis and by NMR and IR spectroscopy. X-ray crystallographic studies of compound 1b revealed infinite helical chains of the molecules connected through P···B donor–acceptor interactions. The ability of these ambiphilic ligands to concurrently act as donors and acceptors was highlighted by their reactions with (dmpe)NiMe2. The zwitterionic complexes (dmpe)NiMe(Ph2PCH2CH2BCy2Me) (2a) and (dmpe)NiMe(Ph2PCH2CH2[BBN]Me) (2b) were generated via the abstraction of one of the methyl groups, forming a borate, and intramolecular coordination of the phosphine moiety to the resulting cationic metal center. Compound 2b was characterized by X-ray crystallography. Furthermore, B(C6F5)3 abstracts the methyl group of a coordinated borate ligand to generate a free, three-coordinate borane center in [(dmpe)NiMe(1a)]+[MeB(C6F5)3]– (3).

Comparison of liquid chromatographic behaviors on N-methylimidazolium functionalized ZrO 2/SiO 2-4 and N-methylimidazolium functionalized SiO 2 stationary phases

A new stationary phase N-methylimidazolium functionalized ZrO 2/SiO 2-4 (Zr/SilprMim) has been prepared. The chromatographic property of this stationary phase is investigated by ion chromatography (IC) with inorganic and organic anions, and normal phase HPLC with basic compounds and hydroxybenzenes. The effects of pH and the strength of Lewis base of eluent on separation of anions are studied. This new stationary phase is also compared with a N-methylimidazolium functionalized SiO 2 stationary phase (SilprMim). The results show that this new stationary phase can be used in analysis of inorganic anions with great prospects. At the same time, successful separations of some organic anions can be obtained by using phosphate buffer solution as mobile phase. This new stationary phase also has a distinct advantage in the separation of basic compounds in normal phase. But due to the presence of Lewis acid sites on the surface of ZrO 2/SiO 2-4, Lewis bases such as hydroxybenzenes adsorb very strongly on this new stationary phase, and Lewis acid sites can be masked or modified by the eluent that contain Lewis base groups.

Influence of Organic Ligands on the Reduction of Polyhalogenated Alkanes by Iron(II)

Experimental work demonstrates that polyhalogenated alkanes (PHAs) are rapidly reduced in aqueous solutions containing Fe(II) complexes with organic ligands that possess either catechol or organothiol Lewis base groups in their structure and are representative of extracellular ligands and metal-complexing moieties within humic substances (tiron, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid, thioglycolic acid, and 2,3-dimercaptosuccinic acid). In solutions containing Fe(II)-tiron complexes, 1,1,1-trichloroethane (1,1,1-TCA) is reduced quantitativelyto acetaldehyde, a product previously reported for reactions with Cr(II), but not with Fe-based reductants. Observed pseudo-first-order rate constants for 1,1,1-TCA reduction by Fe(II)-organic complexes (k'(obs)) generally increase with increasing pH and ligand concentration when Fe(II) concentration is fixed. For the Fe(II)-tiron system, k'(obs) is linearly correlated with the concentration of the 1:2 Fe(II)-tiron complex (FeL2(6-)), and kinetic trends can be described by k'(obs) = k(FeL2)6- [FeL2(6-)], where k(FeL2)6- is the bimolecular rate constant for PHA reaction with the 1:2 Fe(II)-tiron complex. Comparing reaction rates for 14 polyhalogenated ethanes and methanes reveals linear free energy relationships (LFERs) with molecular descriptors for PHA reduction (D(R-X'), deltaG(0'), and E(LUMO)), with the strongest correlation being obtained using carbon-halogen bond dissociation energies, D(R-X'). The collective experimental results are consistent with a dissociative one-electron transfer process occurring during the rate-limiting step.

Effect of Grafted Lewis Base Groups on the Phase Behavior of Model Poly(dimethyl siloxanes) in CO2

The impact of various Lewis bases on the miscibility of siloxane polymers in CO2 was investigated using both ab initio calculations and experimental phase behavior studies. A series of side-chain functional silicones were synthesized containing various Lewis bases in the side chain, and their phase behavior was compared in CO2 at 295 K. Calculations showed that interactions between CO2 and ethers (either a dialkyl ether or the ether oxygen in an ester group) should be as favorable as interactions between CO2 and a carbonyl oxygen. Indeed, phase behavior results seemed to support this, as ether-functional silicones exhibited miscibility pressures as low or lower than acetate-functional analogues. Further, a keto-functional material was not nearly as CO2-philic as the acetate functional analogue. In general, the location of the phase boundary in CO2 is governed by a balance between forces working to increase miscibility pressures, such as increased cohesive energy density of the polymer or factors suppressi...

Metathesis of Olefin-Substituted Pyridines: The Metalated NCN-Pincer Complex in a Dual Role as Protecting Group and Scaffold Part of this work was presented at the 14th International Symposium on Olefin Metathesis, Boston, August 2001

Pincer-palladium(II) and -platinum(II) cat- ions, YCY-M (YCY (2,6-(YCH2)2C6H3) ;Y NMe2, SPh; M Pd II ,P t II ), bound to diolefin-substituted pyridines (3,5- or 2,6-substitution) were successfully synthesized, and subsequently used in olefin meta- thesis (RCM) as a model study for template-directed synthesis of macrocycles. Especially a 3,5-disubstitut- ed pyridine bound to a NCN-Pt II -center (5a) gave a fast metathesis reaction, while the same reaction with the Pd II analogue (4a) was much slower and less selective (isomerization products were formed). Fur- thermore, it was found that 2,6-diolefin-substituted pyridines (4b, 5b, 5c) gave slow metathesis reactions, which is mainly ascribed tosteric hindrance during the ring-closing step. In all cases where prolonged reaction times were required an isomerization pro- cess, most likely assisted by cationic pincer-M II species, was observed as a competing reaction. 1 HN MR spectroscopy experiments revealed that pyridines are stronger bound to a cationic NCN-Pt II -center than toits Pd II -analogue. This aspect is of crucial importance when these pincer-pyridine complexes are applied in metathesis, since free pyridine in solution deactivates the Ru-metathesis catalyst. For the tem- plated construction of macrocycles, a strong M-N(py) bond is also important since it determines the selectivity for the desired product. In addition, these results open a new research field in which organo- metallic (pincer) complexes are used as protecting groups for strong Lewis-basic groups in catalysis. From failed attempts to prepare macrocycles using hexakis(SCS-Pd II -(1a)) complex 14, and from the results obtained with the monometallic pincer com- plexes in RCM, it can be concluded that the most suitable candidate for constructing macrocycles should comprise 2,6-diolefin-substituted pyridines bound to a multi-(NCN-Pt II )-template. In such a system, intrapyridine metathesis (steric hindrance) as well as isomerization reactions (strong M-N(py) bond) are suppressed.

Metal complexes of azacrown ethers in molecular recognition and catalysis

Reversible coordination of ligands or substrates with Lewis-basic groups to Lewis-acidic metal ion binding sites can provide the binding enthalpy and selectivity necessary for molecular recognition or catalysis. Metal complexes of functionalised azamacrocycles have proven to be particularly useful for this purpose. This review summarises the properties and applications of some recently published compounds of this kind, giving typical examples, but no comprehensive coverage of the field. With focus on molecular recognition, the binding of inorganic and organic anions to metal complex binding sites, changes of anion properties induced by the binding and the interaction of metal complexes with the polyanion DNA are discussed. The second part of the review focuses on catalysis mediated by azamacrocyclic Lewis-acidic metal binding sites leading to ester hydrolysis, redox catalysis or polymerisation.

Polyanionic electrolytes with high alkali ion conductivity

We describe the synthesis of chain polymers that contain Lewis base or Lewisacid groups in the backbone that can then be reacted with acidic or basicgroups to form polyanions with weak binding sites for alkali cations. Wereview both `base-in-chain' and `acid-in-chain' approaches to this type ofpolyanion. When the anionic groups are optimally separated, the conductivitiesof the polymers are found to be high relative to those of most single ionicconducting polymers and even most non-imide salt-in-polymer electrolytes. Theconcentration of ionic groups, can be varied using polyethylene glycol links(spacers) of variable length. The conductivity is found to maximize near thevalue which maximizes the conductivity in simple salt-in-fully amorphouspolyether polymers (the number of the ethyleneoxy repeating unit n = 12).Finally a novel process, which achieves the same objective by a differentroute, will be described. This class of weakly coordinating polyanionicelectrolyte, in which the coulomb traps for Li+ are little if at allstronger than the polyether chelation traps, is seen as a promisingdevelopment in the solid polymer electrolyte field. Except for the case of`weak trap' acid-in-chain polymers, all these electrolytes are unicationic intheir conduction.

Nickel Catalyzed Hydrovinylation of Arylethylenes: General Method of Synthesis of α-Arylpropionic Acids Intermediates

The hydrovinylation of arylethylenes catalyzed by [Ni(MeCN) 6][BF 4] 2/AlEt 2Cl/PPh 3 was modulated to obtain 3-aryl-1-butenes in high yields and selectivities. A wide variety of arylethylenes containing electron-donating or electron-withdrawing groups and Lewis basic group, can be hydrovinylated at room temperature and under mild conditions by changing the relative ratios of the three-component catalyst. Similar activities and selectivities were observed for o-, m- and p-substituted styrenes. Also, the hydrovinylation of olefins with substituents in the vinyl fragment, such as α-methylstyrene and indene, can be accomplished.

Reactivity of ether- and amine-complexed dimers and tetramers of alkyllithiums towards triphenylmethane.

Kinetics of Lewis base (LB) complexed primary and secondary sigma-alkyllithiums (RLi) with triphenylmethane (TPMH) are reported. RLis in which one or two LB groups (-OMe, -NMe, -NMeR) are part of the molecule form, in benzene, intramolecularly complexed tetramers, for example, 2(4), or dimers, for example, 4(2). They are used as models for their intermolecular congeners R4Li4 x 4LB and R2Li2 x 4LB (LB = NR'3, OR'2). Nonunity reaction orders in [RLi] are in line with reactions via as yet unidentified 1:1 complexes formed in an equilibrium (K(stat. corr.) approximately = 1) between aggregated RLi and TPMH. In some cases, a tetramer/dimer equilibrium mixture undergoes complexation/reaction. Reaction rates correlate linearly with calculated concentrations of the complexes. Relative rates of complexes range from 1 [prim-R4Li4 x 3LB x TPMH (presumed)] to 4250 [sec-R2Li2 x 3LB x TPMH (presumed)]. A major role in the reactivity enhancement owing to LB-induced conversion of tetramers into dimers is ascribed to increased LB participation in LB-richer dimer transition states. Amine and ether complexes have practically equal reactivities. Lithiation of TPMH by dimeric RCH2Li is retarded by a factor of 24000 if a silyl group is linked to the alpha-carbon.