Auxiliary Groups Research Articles

Quantum Chemical Designing of Efficient TC4-Based Sensitizers by Modification of Auxiliary Donor and π-Spacer

Abstract In the present study, ten metal-free dyes for dye-sensitized solar cells (DSSCs) have been designed. These dyes have designed by structural modification of TC4. Density functional theory was used for structure optimization and determination of photophysical properties. These dyes contain triphenylamine as electron donor and cyanoacrylic acid as electron acceptor. Five π-spacers are used to connect the donor and acceptor. Two auxiliary donors are also used to assist the donor. Results of this study indicate that stronger electron-donating auxiliary groups and longer π-conjugation enhance, while electron-withdrawing fluorine at a π-spacer decreases the performance of dyes in DSSC. As compared to auxiliary electron donors, π-spacers have a strong impact on the performance of the dyes in the DSSCs. Major absorption peaks of all dyes were in the visible region. These absorption peaks are associated with the π–π* transitions of the entire molecule. From calculations it is clear that all dyes will be good photosensitizers in DSSC. The present calculations will provide new ways for experimentalists to synthesize high-performance dyes.

Modular approach to obtaining organic glasses from low-molecular weight dyes using 1,1,1-triphenylpentane auxiliary groups: Nonlinear optical properties

A new modular synthetic approach to obtain organic glasses from low-molecular weight azobenzenes and stilbenes has been accomplished using 1,1,1-triphenylpentane crystallization preventing auxiliary groups. Six new structures show excellent solubility in non-polar solvents and thin films with good optical qualities have been obtained using a spin-coating technique. The glass transition temperatures of the new amorphous materials were in the range of 73–108 °C. The nonlinear optical activity in thin amorphous films was measured after a corona poling procedure.

Improving nuclease activity of copper(II)–terpyridine complex through solubilizing and charge effects of glycine

Copper complexes are potential metallonucleases that may find application in biotechnology and molecular biology. In this study, a ternary copper–terpyridine complex [Cu(ttpy)(Gly)(NO3)](NO3)·H2O (1) (ttpy=4′-p-tolyl-2,2′:6,2″-terpyridine) is synthesized and characterized by X-ray crystallography and ESI-MS as an artificial nuclease. Glycine (Gly) is introduced into the complex to enhance the water-solubility and electrostatic affinity for the nucleic acid target. The interaction between complex 1 and DNA has been studied by spectroscopy and gel electrophoresis, using a structural analog [Cu(ttpy)(NO3)2] (2) as the reference. Complex 1 demonstrates an increased DNA binding ability and oxidative cleavage activity towards supercoiled pBR322 DNA as compared with complex 2. The enhanced water-solubility and positive charge of complex 1 may facilitate its access to DNA and formation of hydrogen bonds with the sugar-phosphate backbone. The results indicate that carefully positioned auxiliary groups in a copper complex can significantly affect the substrate binding or activation ability and consequently the nuclease efficiency of the complex.

Molecularly designed architectures – the metalloligand way

Designed materials offer noteworthy applications which are often architecture dependent. Despite knowing such a fact, one of the major challenges faced by the scientific community is to find ways to predict and, if possible, control the resultant architecture of a network. If such an exercise is fruitful, it creates enormous opportunities to synthesize exotic materials with tailor-made applications. Any network is composed of individual molecules and the transition from a single molecule to a network can be achieved through several routes taking advantage of synthetic chemistry. There exists a molecular building block at the heart of such a transition which mediates such a process from a single molecule to a network. Although a large number of building blocks have created assorted materials, utilization of a well-defined coordination complex as the building block (i.e., metalloligand) is unique for the construction of a designed architecture. A coordination complex as the building block offers structural rigidity that places the auxiliary functional groups to a pre-organized conformation. Such auxiliary functional groups could then coordinate a secondary metal ion or be involved in the self-assembly via weak interactions, such as hydrogen bonds. This review focuses on the recent progress achieved through assorted molecular building blocks towards generating ordered networks. Broadly, two classes of metalloligands will be discussed: those offering hydrogen bond sensitive functional groups and those tendering coordination bond responsive groups. Nevertheless, the result is the construction of networks of a highly-ordered nature in both cases. The present review is expected to provide new strategies for constructing functional materials through metalloligands for challenging and practical applications.

Stereoselective synthesis of enantiomerically pure inherently chiral p-tert-butylcalix[4]arene carboxylic acids

Both enantiomers of inherently chiral dipropoxy-p-tert-butylcalix[4]arene carboxylic acids with AABH substitution patterns have been prepared by the stereoselective reactions of 25,26-dipropoxy-p-tert-butylcalix[4]arene with enantiomerically pure (S)- or (R)-N-(1-phenylethyl)bromoacetamide, separation of the diastereomers by recrystallization, and removal of the chiral auxiliary groups. The absolute configurations of the obtained compounds have been established by X-ray analysis.

Synthesis and stereochemical configuration of inherently chiral p-tert-butylcalix[4]arene carboxylic acids and their derivatives

Both enantiomers of inherently chiral p-tert-butylcalix[4]arene carboxylic acids with ABHH substitution patterns have been prepared by stereoselective reaction of monopropoxy-p-tert-butylcalix[4]arene with an (R)-N-(1-phenylethyl)bromoacetamide, separation of the diastereomers by column chromatography, and removal of the chiral auxiliary groups. The absolute configuration of obtained compounds has been established by X-ray analysis.

Two-photon absorption triggered drug delivery from a polymer for intraocular lenses in presence of an UV-absorber

Abstract Two-photon-absorption (TPA) triggered photochemistry is a versatile tool to photochemically release compounds in an uncaging reaction where the desired reaction should occur behind an UV absorbing barrier, e.g. as in intraocular lenses (IOLs). Nonlinear effects of the UV-absorbing barrier, in this case the cornea, are negligible as long as the laser light passes through it at an intensity low enough that nonlinear effects do not occur. Only in the focus of the beam, i.e. inside the IOL, the required intensities are reached and TPA triggered photo cleavage occurs. The situation becomes complicated as soon as UV-absorbers are admixed to the polymer material. We show that typical concentrations of an UV-absorber only slightly affect the TPA-triggered uncaging reaction rate. As a testbed we used a newly synthetized acrylic polymer derivatized with an o -nitrobenzyl linker group carrying 5-fluorouracil as the model drug to be released. No photochemical decomposition of the UV absorber was observed. The release rate of 5-fluorouracil in presence of the UV absorber was reduced by about 6% only. Further the polymer presented here, is the first to release unmodified 5-fluorouracil without any auxiliary groups attached. This is important for potential applications in humans.

Synthesis and crystal structures of the first cobalt complexes of macrocyclic oxamido

One novel macrocyclic oxamido-cobalt (II) complex CoL (H2L=2,3-dioxo-5,6;13,14-dibenzo-7,12-dimethyl-1,4,8,11-tetraazacyclotetradeca-7,11-diene) (1) has been synthesized solvothermally and characterized by X-ray diffraction analysis. By using the mononuclear cobalt complex as a building block and N3− as auxiliary groups, a trinuclear complex (CoL)2Mn(N3)4·2H2O (2) and two one-dimensional complexes (CoL)2Cd(N3)4·2H2O (3), (CoL)2Co(N3)4·2H2O (4) have been prepared by stepwise synthesis and characterized. The structure of complex 1 consists of a neutral cobalt (II) complex of the [14] N4 macrocyclic oxamido dianion. In complex 2, the center manganese (II) ion is in a slightly distorted octahedral environment and N3− groups only act as terminal ligands. Complex 3 and 4 are similar in structure and both show a one-dimensional chain structure via oxamido and N3− ligands. N3− ligands show two kinds of coordination mode. In complex 2–4, cobalt ion in the ring is trivalence.

Catalytic functionalization of unactivated primary C–H bonds directed by an alcohol

New synthetic methods for the catalytic functionalization of C-H bonds have the potential to revolutionize the synthesis of complex molecules. However, the realization of this synthetic potential requires the ability to functionalize selectively one C-H bond in a compound containing many such bonds and an array of functional groups. The site-selective functionalization of aliphatic C-H bonds is one of the greatest challenges that must be met for C-H bond functionalization to be used widely in complex-molecule synthesis, and processes catalysed by transition-metals provide the opportunity to control selectivity. Current methods for catalytic, aliphatic C-H bond functionalization typically rely on the presence of one inherently reactive C-H bond, or on installation and subsequent removal of directing groups that are not components of the desired molecule. To overcome these limitations, we sought catalysts and reagents that would facilitate aliphatic C-H bond functionalization at a single site, with chemoselectivity derived from the properties of the catalyst and site-selectivity directed by common functional groups contained in both the reactant and the desired product. Here we show that the combination of an iridium-phenanthroline catalyst and a dihydridosilane reagent leads to the site-selective γ-functionalization of primary C-H bonds controlled by a hydroxyl group, the most common functional group in natural products. The scope of the reaction encompasses alcohols and ketones bearing many substitution patterns and auxiliary functional groups; this broad scope suggests that this methodology will be suitable for the site-selective and diastereoselective functionalization of complex natural products.

Do Glycosyl Sulfonium Ions Engage in Neighbouring‐Group Participation? A Study of Oxathiane Glycosyl Donors and the Basis for their Stereoselectivity

Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.

Tunable Charge Tags for Electron-Based Methods of Peptide Sequencing: Design and Applications

Charge tags using basic auxiliary functional groups 6-aminoquinolinylcarboxamido, 4-aminopyrimidyl-1-methylcarboxamido, 2-aminobenzoimidazolyl-1-methylcarboxamido, and the fixed-charge 4-(dimethylamino)pyridyl-1-carboxamido moiety are evaluated as to their properties in electron transfer dissociation mass spectra of arginine C-terminated peptides. The neutral tags have proton affinities that are competitive with those of amino acid residues in peptides. Charge reduction by electron transfer from fluoranthene anion-radicals results in peptide backbone dissociations that improve sequence coverage by providing extensive series of N-terminal c-type fragments without impeding the formation of C-terminal z fragments. Comparison of ETD mass spectra of free and tagged peptides allows one to resolve ambiguities in fragment ion assignment through mass shifts of c ions. Simple chemical procedures are reported for N-terminal tagging of Arg-containing tryptic peptides.

Studies Directed Towards the Preparation of Probes for the Photoaffinity Labelling of Gibberellin Receptors

Model studies for the preparation of photoaffinity probes designed to explore the nature of gibberellin receptor sites have provided a wide range of gibberellin derivatives that should afford useful scaffolds incorporating auxiliary groups attached to C-2 and C-12. Methodology features the stereocontrolled opening of 2β,3β-epoxy gibberellins by attack on the lower face at C-2, while functionalization of C-12 was effected by the rhodium acetate-catalyzed CH insertion reaction of a 17-diazo ketone. Compounds were screened for bioactivity in growth and barley endosperm-based bioassays.

Molecular Engineering of Benzothiazolium Salts with Large Quadratic Hyperpolarizabilities: Can Auxiliary Electron-Withdrawing Groups Enhance Nonlinear Optical Responses?

A series of push−pull chromophores comprising a dimethylamino or diphenylamino electron-donating functionality and a cationic benzothiazolium acceptor with an additional electron-withdrawing group (EWG = NO2 or CN) at various positions of the heterocyclic benzene ring have been synthesized and comprehensively investigated for their linear and quadratic nonlinear optical (NLO) properties by means of UV−visible spectroscopy and hyper-Rayleigh scattering, as well as by quantum-chemical calculations at different levels of theory (B3LYP, CAM-B3LYP, MP2, and RI-CC2). In general, all chromophores under study display large static quadratic hyperpolarizabilities β0, comparable to or in most cases even larger than their conventional stilbazolium-containing analogues, which makes these systems attractive for practical NLO applications. The introduction of an auxiliary EWG into the heterocyclic benzene ring causes a substantial red shift of the intramolecular charge-transfer band. Still, at the same time, this modifi...

ChemInform Abstract: Amino Acid Esters as Chiral Auxiliary Groups in the Lewis Acid Catalyzed Reactions of Electron-Rich Siloxydienes with Imines.

ChemInform Abstract: Amino Acid Esters as Chiral Auxiliary Groups in the Lewis Acid Catalyzed Reactions of Electron-Rich Siloxydienes with Imines.

Reductive alkylation of p-benzoquinone using mixed organoboranes

Abstract Mixed organoboranes based on diphenyl- or dimethylalkylboranes transfer the alkyl group in the reductive alkylation of p-benzoquinone to form the alkylhydroquinones in very high yields. The auxiliary groups do not transfer or have a low migratory aptitude. Primary and secondary alkyl groups are transferred with retention of regio- and stereochemistry to the hydroquinone. O-Alkylation is the major product with tertiary and secondary groups with steric bulk in proximity to the site of attachment. The presence of metal salts, such as magnesium, results in reduction to the unsubstituted hydroquinone. This reaction makes the first practical route to alkylhydroquinones via organoboranes.

Noncovalent Synthesis of Shape-Persistent Cyclic Hexamers from Ditopic Hydrazide-Based Supramolecular Synthons and Asymmetric Induction of Supramolecular Chirality

With properly encoded recognition sites and a well-defined algorithm for intermolecular and intramolecular interactions, the 120 degrees spacer linked ditopic hydrazide-based supramolecular synthons were found to self-assemble into shape-persistent cyclic hexamers in apolar solvents. The two hydrazide motifs displayed separate sets of signals in the NMR spectra because of the interlocked conformation. While in hydrogen bonding competitive solvent such as DMSO-d(6) the spectra were in agreement with the structures with C(2) axes. Moreover, the terminative groups were found to affect the stability of the assemblies substantially. Consequently, the monomer with ureido-hydrazide terminative groups could form the most stable cyclic hexamer in solution for the attractive spectator secondary electrostatic interactions. Owing to the dynamically and kinetically stable nature of these kinds of assemblies, the assembly from the monomer with chiral auxiliary terminative groups also displayed supramolecular chirality in solution, which was confirmed by concentration-dependent CD spectra.

Synthesis, optical resolution and absolute configuration of inherently chiral calixarene carboxylic acids

Both enantiomers of inherently chiral calixarene carboxylic acids with ABCD substitution patterns have been prepared by the benzoylation of 25-propoxy-27-( R)- N-(α-phenylethyl)amidomethyloxycalix[4]arene followed by resolution of the diastereomers formed, monobromination of them and removal of the benzoyl and α-phenylethylamide auxiliary groups. The absolute configuration of the calixarenes obtained has been established by X-ray analysis. Preliminary study of the chiral recognition properties of calixarene carboxylic acid was performed.

Cage Chirality of PC Cage Compounds: Highly Diastereoselective Formation of Diastereomeric P5‐Deltacyclenes, Separation of Diastereomers, and Removal of the Chiral Auxiliary

An effective cyclic addition reaction of diastereomeric (R*)diphenyltin-3,5-di(tert-butyl)-1,2,4-triphosphole derivatives 6 a-c (R* = (-)-cis-myrtanyl (a), (-)-trans-myrtanyl (b), m-(2-bornyl-2-ene)phenyl (c) with two equivalents of tert-butylphosphaalkyne 1 leads to 1:1 mixtures of diastereomeric stannylated pentaphosphadeltacyclene derivatives 7 a-c with seven stereogenic centers in the cage unit. The (-)-cis-myrtanyl derivative 7 a could be separated into its diastereomers; destannylation of diastereomer 7 a(RR) led to the P-H cage 8 as a pure enantiomer. Circular dichroism (CD) spectroscopy of the pure diastereomer 7 a(RR) and the enantiomer 8 give evidence for identical stereoisomers of the P(5)-deltacyclene cage units and prove a strong dominance of the chiral cages over the chiral auxiliary groups with respect to their chiroptical properties. Absolute X-ray structure investigations of the majority of the compounds presented in the paper reveal the details of the stereochemistry of the asymmetric P-C cage units. In this paper we demonstrated for the first time a general preparative route to stereochemically fully defined asymmetric P-C cage compounds by separation of diastereomers and replacement of the chiral auxiliary group.

Adverbial descriptions in Northern Sotho

Owing to a virtual lack of a ‘basic adverbs’ word class in Northern Sotho, the objective of this article is to investigate some language strategies which are exploited to express adverbial meanings in this language. In popular Northern Sotho grammar books, it is clearly stated that adverbs in Northern Sotho are usually derived from other parts of speech. Important language strategies, such as the use of auxiliary word groups and ideophones in expressing adverbial meanings are, to a great extent, neglected in most popular Northern Sotho grammars. Owing to the fact that in most cases, no one-to-one lexical equivalence exists between Northern Sotho and for instance English adverbs, the compilers of Northern Sotho-English dictionaries find it difficult to accommodate the different strategies in these dictionaries. In this article, various ways of conveying adverbial expressions in Northern Sotho are illustrated, with the focus mainly on auxiliary word groups and ideophones. The aim of this article is not to do a grammatical analysis of the adverb and ideophone word classes as such, but rather to focus on the descriptive function of these words in certain Northern Sotho contexts.

Modulating Affinities of Di-2-picolylamine (DPA)-Substituted Quinoline Sensors for Zinc Ions by Varying Pendant Ligands

We have developed a series of di-2-picolylamine (DPA)-substituted quinoline sensors, HQ1- 4, bearing a pendant ligand at the 8 position of quinoline. UV-vis spectra of HQ1- 4 showed similar variations to that of HQ5 but with different varying extents upon the titration of zinc ions. Fluorescence intensities of HQ1, HQ3, and HQ4 were enhanced 4-6 times upon the addition of 1 equiv of zinc ions under an aqueous buffer. Somewhat unexpectedly, HQ2 is nonfluorescent in the presence of metal ions, including zinc ions. The affinities of HQ sensors are distributed in a broad range from nanomolarity to femtomolarity by varying the pendant ligands near the coordination unit. More importantly, these new sensors exhibited very high selectivity for Zn(2+) over Na(+), K(+), Mg(2+), and Ca(2+) at the millimolar level and over other transition metal ions at the micromolar level, except for Cd(2+). These findings indicated that the incorporations of the pendant groups exerted no effect on the spectroscopic properties and selectivity of the parent fluorescent sensor, with the exception of HQ2. Finally, X-ray crystal structures of ZnHQ's revealed that the auxiliary pendant groups at the 8 position participated in zinc coordination and were able to tune the affinities of HQ sensors.