Separation of "light fluorous" reagents and catalysts by fluorous solid-phase extraction: synthesis and study of a family of triarylphosphines bearing linear and branched fluorous tags

A fluorous-tag-assisted fluorescent probe for simple and selective detection of hydrogen sulfide: application for turbid dyeing solutions

Accurate hydrogen sulfide (H2S) detection has attracted much attention because its toxicity may affect aquatic environments and human health. However, recognizing H2S levels by conventional fluorescent probes in turbid wastewater has been challenging because the opaque environment interferes with their photophysical properties. To overcome this limitation, a fluorous-tagging strategy can be used for the development of fluorescent sensors to detect H2S in turbid solutions. The use of fluorescent probe assisted with fluorous-tag allowed for easy isolation of the probe using polytetrafluoroethylene (PTFE) material, while disturbing species were eliminated through a simple aqueous wash. This approach enabled the fluorescent probe to effectively quantify H2S, even in opaque solutions containing organic dyes that could interfere with fluorescence emission.

Sequestration of Ruthenium Residues via Efficient Fluorous-enyne Termination

The creation of polymers without metal contamination remains a significant challenge for metathesis-based polymerization techniques and has complicated applications in biomedical and electronic applications. This communication reports a new approach for the removal of ruthenium byproducts through the design of an enyne terminator for metathesis polymerization that contains a fluorous tag. Upon reaction of a living polymer chain with the enyne, the ruthenium center is captured as a stable sulfur-chelated complex that can be efficiently removed after a single filtration through a fluorous cartridge. Levels of ruthenium residues as determined by ICP-MS were found to depend on the monomer structure, eluting solvent, and the degree of polymerization targeted. Ruthenium residues were minimized to low ppm levels (4-75 ppm) for most samples examined and also led to the improved thermal stability of the final materials. This represents the most efficient single method for removal of ruthenium residues from metathesis polymerization products.

Liquid-Phase Peptide Synthesis (LPPS): A Third Wave for the Preparation of Peptides

Since the last century, peptides have gained wide acceptance as drugs, with almost 100 already in the market and a large number in the pipeline. In this context, peptide synthesis has grown massively as a stringent field for pharmaceuticals around the globe. Three methodologies, namely, classical solution peptide synthesis (CSPS), solid-phase peptide synthesis (SPPS), and liquid-phase peptide synthesis (LPPS), have made significant contributions to the field. This review provides a comprehensive and integrated vision of LPPS as the third wave for peptide synthesis. LPPS combines the advantages of CSPS and SPPS, where peptide elongation is carried out in solution and the growing peptide chain is supported on a soluble tag, which confers characteristic properties. LPPS protocols allow the large-scale production of peptides and reduce the use of excess reagents and solvents, thus meeting the principles of green chemistry. In this review, tags associated with LPPS are broadly discussed under the following headings: polydisperse polyethylene glycol (PEG), membrane-enhanced peptide synthesis (MEPS), fluorous technology, ionic liquids (ILs), PolyCarbon, hydrophobic polymers, and group-assisted purification (GAP). It also highlights the signature accomplishments of LPPS tags and the limitations of the same.

Nitrogen-Doped Carbon-Incarcerated Zinc Electrodes as Heterogeneous Catalysts for Electrochemical Allylation of Carbonyl Compounds

Electrochemical allylation reactions of carbonyl compounds using cathodes prepared from nitrogen-doped carbon (NDC)-incarcerated zinc catalysts have been developed. A range of aldehydes and ketones afforded the desired allylic alcohols in high yields with <10 mol % zinc leaching, and the heterogeneous nature of the active species was suggested. Compared with bulk zinc electrodes, NDC-stabilized zinc nanoparticle species were compatible with a broader range of heteroaromatic substrates and enabled the use of an undivided cell.

Synthesis and application of non-bioaccumulable fluorinated surfactants: a review

Abstract

Due to negative effects of conventional fluorinated surfactants with long perfluorocarbon chain (CxF2x+ 1, x≥7) like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), these conventional long perfluorocarbon chain surfactants have been restricted in many industrial applications. Nowadays, their potential non-bioaccumulable alternatives have been developed to meet the requirements of environmental sustainable development. In this paper, the recent advances of potential non-bioaccumulable fluorinated surfactants with different fluorocarbon chain structures, including the short perfluorocarbon chain, the branched fluorocarbon chain, and the fluorocarbon chain with weak points, are reviewed from the aspects of synthesis processes, properties, and structure-activity relationships. And their applications in emulsion polymerization of fluorinated olefins, handling membrane proteins, and leather manufacture also are summarized. Furthermore, the challenges embedded in the current non-bioaccumulable fluorinated surfactants are also highlighted and discussed with the hope to provide a valuable reference for the prosperous development of fluorinated surfactants.

Graphical abstract

Recoverable Phospha-Michael Additions Catalyzed by a 4-N,N-Dimethylaminopyridinium Saccharinate Salt or a Fluorous Long-Chained Pyridine: Two Types of Reusable Base Catalysts

Phospha-Michael addition, which is the addition reaction of a phosphorus-based nucleophile to an acceptor-substituted unsaturated bond, certainly represents one of the most versatile and powerful tools for the formation of P-C bonds, since many different electrophiles and P nucleophiles can be combined with each other. This offers the possibility to access many diversely functionalized products. In this work, two kinds of basic pyridine-based organo-catalysts were used to efficiently catalyze phospha-Michael addition reactions, the 4-N,N-dimethylaminopyridinium saccharinate (DMAP·Hsac) salt and a fluorous long-chained pyridine (4-R_f-CH₂OCH₂-py, where R_f = C₁₁F₂₃). These catalysts have been synthesized and characterized by Lu’s group. The phospha-Michael addition of diisopropyl, dimethyl or triethyl phosphites to α, β-unsaturated malonates in the presence of those catalysts showed very good reactivity with high yield at 80–100 °C in 1–4.5 h with high catalytic recovery and reusability. With regard to significant catalytic recovery, sometimes more than eight cycles were observed for DMAP·Hsac adduct by using non-polar solvents (e.g., ether) to precipitate out the catalyst. In the case of the fluorous long-chained pyridine, the thermomorphic method was used to efficiently recover the catalyst for eight cycles in all the reactions. Thus, the easy separation of the catalysts from the products revealed the outstanding efficacy of our systems. To our knowledge, these are good examples of the application of recoverable organo-catalysts to the DMAP·Hsac adduct by using non-polar solvent and a fluorous long-chained pyridine under the thermomorphic mode in phospha-Michael addition reactions.

Short-Chained Platinum Complex Catalyzed Hydrosilylation under Thermomorphic Conditions: Heterogeneous Phase Separation at Ice Temperature

Homogeneous catalysts PtCl₂[5,5'-bis-(n-ClCF₂(CF₂)₃CH₂OCH₂)-2,2'-bpy] (2A) and PtCl₂[5,5'-bis-(n-HCF₂(CF₂)₃CH₂OCH₂)-2,2'-bpy] (2B), which contained short fluorous chains, were synthesized and used in catalysis of hydrosilylation of alkynes. In these reactions the thermomorphic mode was effectively used to recover these catalysts from the reaction mixture up to eight cycles by taking advantage of heterogeneous phase separation at ice temperature. This kind of catalysis had previously been observed in fluorous catalysts of platinum containing about 50% F-content, but in this work the percentage of F-content is decreased to only about 30%, by which we termed them as "very light fluorous". Our new type of catalyst with limited number of F-content is considered as the important discovery in the fluorous technology field as the reduced number of fluorine atoms will help to be able to comply the EPA 8-carbon rule. The metal leaching after the reaction has been examined by ICP-MS, and the testing results show the leaching of residual metal to be minimal. Additionally, comparing these results to our previous work, fluorous chain assisted selectivity has been observed when different fluorous chain lengths of the catalysts are used. It has been found that there exists fluorous chain assisted better selectivity towards β-(E) form in the Pt-catalyzed hydrosilylation of non-symmetric terminal alkyne when the Pt catalyst contains short fluorous chain (i.e., 4 Cs). Phenyl acetylenes showed the opposite regioselectivity due to pi-pi interaction while using the same catalyst via Markovnikov's addition to form terminal vinyl silane, which is then a major product for Pt-catalyzed hydrosilylation of terminal aryl acetylene with triethylsilane. Finally, the kinetic studies indicate that the insertion of alkyne into the Pt-H bond is the rate-determining step.

Synthesis and applications of fluorous phosphines

The synthesis, some of the properties and the applications of fluorous phosphines in biphasic, organometallic and organo-catalysis were reviewed.

Fluorous photoaffinity labeling to probe protein-small molecule interactions

Identifying cellular targets of bioactive small molecules is essential for their applications as chemical probes or drug candidates. Of equal importance is to determine their "off-target" interactions, which usually account for unwanted properties including toxicity. Among strategies to profile small molecule-interacting proteins, photoaffinity labeling has been widely used because of its distinct advantages such as sensitivity. When combined with mass spectrometry, this approach can provide additional structural and mechanistic information, such as drug-target stoichiometry and exact interacting amino acid residues. We have described a novel fluorous photoaffinity labeling approach, in which a fluorous tag is incorporated into the photoaffinity labeling reagent to enable the enrichment of the labeled species from complex mixtures for analysis. This new feature likely makes the fluorous photoaffinity labeling approach suitable to identify transient interactions, and low-abundant, low-affinity interacting proteins in a cellular environment.

Visible light-mediated atom transfer radical addition via oxidative and reductive quenching of photocatalysts

Herein, the development of visible light-mediated atom transfer radical addition (ATRA) of haloalkanes onto alkenes and alkynes using the reductive and oxidative quenching of [Ir{dF(CF(3))ppy}(2)(dtbbpy)]PF(6) and [Ru(bpy)(3)]Cl(2) is presented. Initial investigations indicated that the oxidative quenching of photocatalysts could effectively be utilized for ATRA, and since that report, the protocol has been expanded by broadening the scope of the reaction in terms of the photocatalysts, substrates, and solvents. In addition, further modifications of the reaction conditions allowed for the efficient ATRA of perfluoroalkyl iodides onto alkenes and alkynes utilizing the reductive quenching cycle of [Ru(bpy)(3)]Cl(2) with sodium ascorbate as the sacrificial electron donor. These results signify the complementary nature of the oxidative and reductive quenching pathways of photocatalysts and the ability to predictably direct reaction outcome through modification of the reaction conditions.

Design, synthesis, and incorporation of fluorous 5-methylcytosines into oligonucleotides

A palladium-catalyzed Negishi coupling reaction has been developed to synthesize fluorous 5-methylcytosines. These fluorous nucleosides are incorporated into the oligonucleotides that correspond to part of the promoter region of Oct4, a master gene that undergoes dynamic DNA demethylation during cellular reprogramming. The separation of the fluorous oligonucleotides from its nonfluorous analogues has been achieved through solid-phase extraction over fluorous silica, suggesting its potential use in probing DNA demethylation.

A fluorous phosphate protecting group with applications to carbohydrate synthesis

The first fluorous protecting group for phosphate is reported. This group can be used as a facile tag for purification and be removed under mild reducing conditions using zinc and ammonium formate. Synthesis of a disaccharide from Leishmania using this fluorous protecting group demonstrated the group's stability to the acidic conditions necessary for glycosylation as well as its orthogonality to several other common protecting groups.

Mass spectrometry of fluorocarbon-labeled glycosphingolipids

A method for generation of novel fluorocarbon derivatives of glycosphingolipids (GSLs) with high affinity for fluorocarbon phases has been developed, and their potential applications to mass spectrometry (MS)-based methodologies for glycosphingolipidomics have been investigated. Sphingolipid ceramide N-deacylase (SCDase) is used to remove the fatty acid from the ceramide moiety, after which a fluorocarbon-rich substituent (F-Tag) is incorporated at the free amine of the sphingoid. In initial trials, a neutral GSL, globotriaosylceramide (Gb(3)Cer), three purified bovine brain gangliosides, and four fungal glycosylinositol phosphorylceramides (GIPCs) were de-N-acylated, derivatized by prototype F-Tags, and recovered by solid phase extraction on fluorocarbon-derivatized silica (F-SPE). The efficacy of SCDase treatment of GIPCs was here demonstrated for the first time. Compatibility with subsequent per-N,O-methylation was established for the F-tagged Gb(3) Cer and purified gangliosides, and extensive mass spectra (MS(1) and MS(2)) consistent with all of the expected products were acquired. The potential use of F-tagged derivatives for a comprehensive MS based profiling application was then demonstrated on a crude ganglioside mixture extracted from bovine brain. Finally, a simple trial in microarray format demonstrated fixation of F-tagged G(M1) ganglioside to a fluorous glass surface, with the glycan intact and available for interaction with a fluorescent derivative of cholera toxin B chain. The methods described thus provide a new avenue for rapid GSL recovery or cleanup, potentially compatible with a variety of platforms for mass spectrometric profiling and structure analysis, as well as parallel analysis of functional interactions.

Synthesis and Solid State Structure of Fluorous Probe Molecules for Fluorous Separation Applications

A series of colored hydrocarbon and fluorocarbon tagged 1-fluoro-4-alkylamino-anthraquinones and 1,4-bis-alkylamino-anthraquinone probe molecules were synthesized from a (fluorinated) alkyl amine and 1,4-difluoroanthraquinone to aid in the development of fluorous separation applications. The anthraquinones displayed stacking of the anthraquinone tricycle and interdigitation of the (fluorinated) alkyl chains in the solid state. Furthermore, intramolecular N-H···O hydrogen bonds forced the hydrocarbon and fluorocarbon tags into a conformation pointing away from the anthraquinone tricycle, with the angle of the tricycle plane normal and the main (fluorinated) alkyl vector ranging from 1 to 39°. Separation of the probe molecules on fluorous silica gel showed that the degree of fluorination of the probe molecules plays only a minor role with most eluents (e.g., hexane-ethyl acetate and methyl nonafluorobutyl ethers-ethyl acetate). However, toluene as eluent caused a pronounced separation by degree of fluorination for fluorocarbon, but not hydrocarbon tagged probe molecules on both silica gel and fluorous silica gel. These studies suggest that hydrocarbon and fluorocarbon tagged anthraquinones are useful probe molecules for the development of laboratory scale fluorous separation applications.

Fluorous aryldiazirine photoaffinity labeling reagents

Two fluorous versions of trifluoromethyldiazirine derivatives have been designed and synthesized. The new photoaffinity labeling reagents have reactivity similar to that of their aryltrifluoromethyldiazirine parent when activated in MeOH, while the reaction products can be efficiently separated over fluorous silica gel. The alcohol group in the two reagents is further converted to activated carboxylic acid and amine, which enable coupling both reagents with small molecules and macromolecules under mild conditions.

Multicomponent reactions of convertible isonitriles

A new family of unsaturated isonitriles has been prepared by the base-promoted ring-opening of oxazoles, offering an alternative to the conventional formamide dehydration route. These compounds undergo the full complement of multicomponent reactions for which isonitriles are known and offer the desirable trait of giving amide products that readily participate in acyl substitution reactions (hence, they are convertible). Moreover, they do not have the objectionable odors for which isonitriles are typically known, making them more accessible as reagents for organic synthesis. One focus of the work is isonitriles bearing perfluorinated alkyl groups that enable the ready separation of such reagents from nonfluorinated reaction products using the "light" fluorous method of fluorous solid-phase extraction.

Mono- Vs. Di-flourous tagged Glucosamines for Iterative Oligosaccharide Synthesis

Fluorous-tagged protecting groups are attractive tools for elongating carbohydrate chains in oligosaccharide synthesis. To eliminate the accumulation of failed sequences during automated oligosaccharide synthesis conditions, an additional C(8)F(17) ester derived protecting group was attached to the glycosyl donor to better retain the desired doubly-tagged glycosylation product on fluorous solid phase extraction (FSPE) cartridges. Initial studies show that the double-fluorous-tagging strategy offers a robust enough separation using a commercial FSPE cartridge using simple gravity filtration to separate the desired product from the singly-fluorous-tagged starting materials and their decomposition products. In addition, removal of the fluorous acetate and its byproducts after sodium methoxide treatment and neutralization required only dissolution of the desired sugar in toluene and subsequent removal of the toluene layer from the denser fluorous byproducts.

Fluorous Chemistry in Pittsburgh: 1996-2008

The article summarizes a lecture presented at the American Chemical Society Symposium for the 2008 Award for Creative Work in Fluorine Chemistry on April 7, 2008. A high level, historical overview of work in the fluorous field at the University of Pittsburgh is presented.

Preparation of Allyltin Reagents Grafted on Solid Support: Clean and Easily Recyclable Reagents for Allylation of Aldehydes

The preparation of polymer-supported allyltin reagents was shown to be possible for both unfunctionalized and functionalized allyl units. These reagents were treated with aldehydes in the presence of cerium(III) or indium(III) salts to afford high yields of homoallylic alcohols, practically uncontaminated with organotin residues (less than 5 ppm). Some mechanism aspects are briefly discussed and the potential for regeneration and reuse of these supported reagents is pointed out.

Increasing Fluorous Partition Coefficients by Solvent Tuning

Low partition coefficients of fluorous components have been a persistent problem in liquid-liquid separations using perfluoroalkanes as the fluorous phase. Solvent tuning of both the nonfluorous and the fluorous phase dramatically enhances the partitioning of light or polar fluorous molecules into the fluorous liquid phase, while minimally effecting partition coefficients of nonfluorous molecules. These findings suggest an expanded scope for liquid-based separations in fluorous biphasic catalysis, fluorous-tagged reagents, fluorous-supported oligomer synthesis, and related areas. [reaction: see text]