Role of sulfur chirality in the chemical processes of biology

Chiral structures profoundly influence chemical and biological processes. While chiral carbon biomolecules have received much attention, chirality is also possible in certain sulfur compounds; just as with carbon, there can be differences in the physiological behavior of chiral sulfur compounds. For instance, one drug enantiomer, Nexium (esomeprazole, a chiral sulfoxide), is used for its superior clinical properties as a proton pump inhibitor over the racemic mixture, Prilosec (Losec, omeprazole). This critical review introduces sulfur stereochemistry and nomenclature, and provides a comprehensive approach to chiral sulfur compounds and their enzymatic reactions in general and secondary metabolism. The major structural types of biological interest are sulfonium salts, sulfoxides, and sulfoximines. (103 references).     

Formation of Coaxial Nanocables with Amplified Supramolecular Chirality through an Interaction between Carbon Nanotubes and a Chiral π‐Gelator

In an attempt to gather experimental evidence for the influence of carbon allotropes on supramolecular chirality, we found that carbon nanotubes (CNTs) facilitate amplification of the molecular chirality of a π‐gelator ( MC‐OPV ) to supramolecular helicity at a concentration much lower than that required for intermolecular interaction. For example, at a concentration 1.8×10^?4 m , MC‐OPV did not exhibit a CD signal; however, the addition of 0–0.6 mg of SWNTs resulted in amplified chirality as evident from the CD spectrum. Surprisingly, AFM analysis revealed the formation of thick helical fibers with a width of more than 100 nm. High‐resolution TEM analysis and solid‐state UV/Vis/NIR spectroscopy revealed that the thick helical fibers were cylindrical cables composed of individually wrapped and coaxially aligned SWNTs. Such an impressive effect of CNTs on supramolecular chirality and cylindrical‐cable formation has not been reported previously.     

A Carbon Dioxide Bubble‐Induced Vortex Triggers Co‐Assembly of Nanotubes with Controlled Chirality

It is challenging to prepare co‐organized nanotube systems with controlled nanoscale chirality in an aqueous liquid flow field. Such systems are responsive to a bubbled external gas. A liquid vortex induced by bubbling carbon dioxide (CO₂) gas was used to stimulate the formation of nanotubes with controlled chirality; two kinds of achiral cationic building blocks were co‐assembled in aqueous solution. CO₂‐triggered nanotube formation occurs by formation of metastable intermediate structures (short helical ribbons and short tubules) and by transition from short tubules to long tubules in response to chirality matching self‐assembly. Interestingly, the chirality sign of these assemblies can be selected for by the circulation direction of the CO₂ bubble‐induced vortex during the co‐assembly process.     

A Facile Synthesis of High‐Surface‐Area Sulfur–Carbon Composites for Li/S Batteries

Small‐grained elemental sulfur is precipitated from sodium thiosulfate (Na₂S₂O₃) in a carbon‐containing oxalic acid (HOOC?COOH) solution through a novel spray precipitation method. Surface area analysis, elemental mapping, and transmission electron micrographs revealed that the spray‐precipitated sulfur particles feature 11 times higher surface area compared to conventional precipitated sulfur, with homogeneous distribution in the carbon. Moreover, the scanning electron micrographs show that these high‐surface‐area sulfur particles are firmly adhered to and covered by carbon. This precipitated S–C composite exhibits high discharge capacity with about 75 % capacity retention. The initial discharge capacity was further improved to 1444 mA h g^?1 by inserting a free‐standing single‐walled carbon nanotube layer in between the cathode and the separator. Moreover, with the help of the fixed capacity charging technique, 91.6 % capacity retention was achieved.     

A Carbon‐Sulfur Hybrid with Pomegranate‐like Structure for Lithium‐Sulfur Batteries

A carbon‐sulfur hybrid with pomegranate‐like core–shell structure, which demonstrates a high rate performance and relatively high cyclic stability, is obtained through carbonization of a carbon precursor in the presence of a sulfur precursor (FeS₂) and a following oxidation of FeS₂ to sulfur by HNO₃. Such a structure effectively protects the sulfur and leaves enough buffer space after Fe³⁺ removal and, at the same time, has an interconnected conductive network. The capacity of the obtained hybrid is 450 mA h g^?1 under the current density of 5 C. This work provides a simple strategy to design and prepare various high‐performance carbon‐sulfur hybrids for lithium‐sulfur batteries.     

Radially Inwardly Aligned Hierarchical Porous Carbon for Ultra‐Long‐Life Lithium–Sulfur Batteries

Rational design of hollow micro‐ and/or nano‐structured cathodes as sulfur hosts has potential for high‐performance lithium‐sulfur batteries. However, their further commercial application is hindered because infusing sulfur into hollow hosts is hard to control and the interactions between high loading sulfur and electrolyte are poor. Herein, we designed hierarchical porous hollow carbon nanospheres with radially inwardly aligned supporting ribs to mitigate these problems. Such a structure could aid the sulfur infusion and maximize sulfur utilization owing to the well‐ordered pore channels. This highly organized internal carbon skeleton can also enhance the electronic conductivity. The hollow carbon nanospheres with further nitrogen‐doping as the sulfur host material exhibit good capacity and excellent cycling performance (0.044 % capacity degradation per each cycle for 1000 cycles).     

Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries

Well‐confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium–sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as‐prepared composite material consists of graphene‐wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g^?1 at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano‐network for use in lithium–sulfur rechargeable batteries.     

Controllable Sulfur,Nitrogen Co‐doped Porous Carbon for Ethylbenzene Oxidation: The Role of Nano‐CaCO3

Heteroatom‐doped porous carbon materials have exhibited promising applications in various fields. In this work, sulfur, nitrogen co‐doped carbon materials (SNCs) with abundant pore structure were prepared by pyrolysis of sulfur, nitrogen‐containing porous organic polymers (POPs) mixed with nano‐CaCO₃ at high temperature. Among the resultant materials, SNC‐Ca‐850 possesses a relatively high level of doped heteroatoms and exhibits an excellent catalytic performance for the selective oxidation of benzylic C?H bonds. It is noteworthy that nano‐CaCO₃ increases the doped sulfur content in the synthesized carbon materials to a large extent and impacts the existence modes of sulfur. In addition, it enhances the porous structure and specific surface area of the resultant SNCs significantly. This work provides a viable strategy to promote the doping of sulfur into carbon materials during the pyrolysis process.     

基于模板法构建手性小分子有机纳米管

Organic nanotubes from two kinds of chiral molecules, R-di-2-naphthylprolinol （DNP） with an asymmetric carbon atom and R-（＋）-1,1＇-bi-2-naphthol dimethyl ether （BNDE） with the conformational asymmetry, were prepared by the immersing technique using porous alumina membranes as the template. It was found that the nanotubes from DNP with an asymmetric carbon atom presented the same chirality as the solution with slight red shift of the CD signals upon the formation of the nanotubes, while no well-defined chirality could be identified for the nanotubes from BNDE with the conformational asymmetry.     

Divergent Control of Point and Axial Stereogenicity: Catalytic Enantioselective C−N Bond‐Forming Cross‐Coupling and Catalyst‐Controlled Atroposelective Cyclodehydration

Catalyst control over reactions that produce multiple stereoisomers is a challenge in synthesis. Control over reactions that involve stereogenic elements remote from one another is particularly uncommon. Additionally, catalytic reactions that address both stereogenic carbon centers and an element of axial chirality are also rare. Reported herein is a catalytic approach to each stereoisomer of a scaffold containing a stereogenic center remote from an axis of chirality. Newly developed peptidyl copper complexes catalyze an unprecedented remote desymmetrization involving enantioselective C?N bond‐forming cross‐coupling. Then, chiral phosphoric acid catalysts set an axis of chirality through an unprecedented atroposelective cyclodehydration to form a heterocycle with high diastereoselectivity. The application of chiral copper complexes and phosphoric acids provides access to each stereoisomer of a framework with two different elements of stereogenicity.     

Reduction of β-ketosulfoxides : a highly efficient asymmetric synthesis of both enantiomers of methyl carbinols from the corresponding esters

It is shown that reductions of β-ketosulfoxides of identical chirality (R) at sulfur, lead to reduction products of opposite stereochemistry according to the reducing agent used. The high enantiomeric excesses obtained (80 to 100%) provide a general route to both enantiomers of methylcarbinols from the corresponding esters.     

Chirality Transfer in Gold(I)‐Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study

Gold(I)‐catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ‐substituted secondary allylic ethers. Our investigations include a full substrate‐scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality‐transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π‐bound Au–allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that erode the efficiency of chirality transfer become competitive.     

Asymmetric Synthesis of Chiral Sulfoximines through the S‐Alkylation of Sulfinamides

Innovation in drug discovery critically depends on the development of new bioisosteric groups. Chiral sulfoximines, which contain a tetrasubstituted sulfur atom that bears one nitrogen, one oxygen, and two different carbon substituents, represent an emerging chiral bioisostere in medicinal chemistry. Chiral sulfoximines are conventionally prepared by a stereospecific nitrene transfer reaction to chiral sulfoxides; however, the number of readily available chiral sulfoxides remains limited. Herein, we report the asymmetric synthesis of a class of hitherto difficult‐to‐access chiral sulfoximines with two structurally similar alkyl chains. Our synthetic approach is based on the sulfur‐selective alkylation of easily accessible chiral sulfinamides with commercially available reagents under simple and safe conditions. This stereospecific S‐alkylation offers a general and scalable approach to the asymmetric synthesis of chiral sulfoximines, which represent important substructures in bioactive molecules.     

Optically Active Helical Vinylterphenyl Polymers: Chiral Teleinduction in Radical Polymerization and Tunable Stereomutation

Helical vinyl aromatic polymers are emerging as interesting chiral materials due to their dynamic tailorability, synthetic simplicity, and outstanding chemical and physical stabilities. This Personal Account discusses long‐range chirality transfer in the radical polymerization of vinylterphenyl monomers and tunable stereomutation of the resultant polymers. It begins with a general introduction to the design, synthesis, and characterization of helical poly{(+)‐2,5‐bis[4′‐((S)‐2‐methylbutyloxy)phenyl]styrene}, the first one of this series of polymers. Then, long‐range chirality transfer during radical polymerization of terphenyl‐based vinyl monomers is explained. After that, the chiroptical property control of the resultant polymers by means of the transition from kinetically controlled conformation to thermodynamically controlled conformation and external stimulus is described. This Personal Account concludes by discussing the advantages and disadvantages of the strategy of using vinylterphenyls to obtain optically active helical polymers and providing a short outlook, especially emphasizing the importance of tacticity on the chiroptical properties of polymers.     

Phase‐Transfer‐Catalyzed Asymmetric Synthesis of Axially Chiral Anilides

Catalytic asymmetric synthesis of axially chiral o‐iodoanilides and o‐tert‐butylanilides as useful chiral building blocks was achieved by means of binaphthyl‐modified chiral quaternary ammonium‐salt‐catalyzed N‐alkylations under phase‐transfer conditions. The synthetic utility of axially chiral products was demonstrated in various transformations. For example, axially chiral N‐allyl‐o‐iodoanilide was transformed to 3‐methylindoline by means of radical cyclization with high chirality transfer from axial chirality to C‐centered chirality. Furthermore, stereochemical information on axial chirality in o‐tert‐butylanilides could be used as a template to control the stereochemistry of subsequent transformations. The transition‐state structure of the present phase‐transfer reaction was discussed on the basis of the X‐ray crystal structure of ammonium anilide, which was prepared from binaphthyl‐modified chiral ammonium bromide and o‐iodoanilide. The chiral tetraalkylammonium bromide as a phase‐transfer catalyst recognized the steric difference between the ortho substituents on anilide to obtain high enantioselectivity. The size and structural effects of the ortho substituents on anilide were investigated, and a wide variety of axially chiral anilides that possess various functional groups could be synthesized with high enantioselectivities. This method is the only general way to access a variety of axially chiral anilides in a highly enantioselective fashion reported to date.     

Spontaneous transmission of chirality through multiple length scales

The hierarchical transfer of chirality in nature, from the nano‐, to meso‐, to macroscopic length scales, is very complex, and as of yet, not well understood. The advent of scanning probes has allowed chirality to be monitored at the single molecule or monolayer level and has opened up the possibility to track enantiospecific interactions and chiral self‐assembly with molecular‐scale detail. This paper describes the self‐assembly of a simple, model molecule (naphtho[2,3‐a]pyrene) that is achiral in the gas phase, but becomes chiral when adsorbed on a surface. This polyaromatic hydrocarbon forms a stable and reversibly ordered system on Cu(111) in which the transmission of chirality from single surface‐bound molecules to complex 2D chiral architectures can be monitored as a function of molecular packing density and surface temperature. In addition to the point chirality of the surface‐bound molecule, the unit cell of the molecular domains was also found to be chiral due to the incommensurate alignment of the molecular rows with respect to the underlying metal lattice. These molecular domains always aggregated in groups of three, all of the same chirality, but with different rotational orientations, forming homochiral “tri‐lobe” ensembles. At a larger length scale, these tri‐lobe ensembles associated with nearest‐neighbor tri‐lobe units of opposite chirality at lower packing densities before forming an extended array of homochiral tri‐lobe ensembles at higher converges. This system displayed chirality at a variety of size scales from the molecular (≈1 nm) and domain (≈5 nm) to the tri‐lobe ensemble (≈10 nm) and extended array (>25 nm) levels. The chirality of the tri‐lobe ensembles dictated how the overall surface packing occurred and both homo‐ and heterochiral arrays could be reproducibly and reversibly formed and interchanged as a function of surface coverage. Finally, these chirally templated surfaces displayed remarkable enantiospecificity for naphtho[2,3‐a]pyrene molecules adsorbed in the second layer. Given its simplicity, reversibility, and rich degree of order, this system represents an ideal test bed for the investigation of symmetry breaking and the hierarchical transmission of chirality through multiple length scales.     

Capillary electrophoresis of covalently functionalized single‐chirality carbon nanotubes

We demonstrate the separation of chirality‐enriched single‐walled carbon nanotubes (SWCNTs) by degree of surface functionalization using high‐performance CE. Controlled amounts of negatively charged and positively charged functional groups were attached to the sidewall of chirality‐enriched SWCNTs through covalent functionalization using 4‐carboxybenzenediazonium tetrafluoroborate or 4‐diazo‐N,N‐diethylaniline tetrafluoroborate, respectively. Surfactant‐ and pH‐dependent studies confirmed that under conditions that minimized ionic screening effects, separation of these functionalized SWCNTs was strongly dependent on the surface charge density introduced through covalent surface chemistry. For both heterogeneous mixtures and single‐chirality‐enriched samples, covalently functionalized SWCNTs showed substantially increased peak width in electropherogram spectra compared to nonfunctionalized SWCNTs, which can be attributed to a distribution of surface charges along the functionalized nanotubes. Successful separation of functionalized single‐chirality SWCNTs by functional density was confirmed with UV‐Vis‐NIR absorption and Raman scattering spectroscopies of fraction collected samples. These results suggest a high degree of structural heterogeneity in covalently functionalized SWCNTs, even for chirality‐enriched samples, and show the feasibility of applying CE for high‐performance separation of nanomaterials based on differences in surface functional density.     

2-Pyridylsulfinamides as effective catalysts in the asymmetric alkylation of aldehydes with diethylzinc

Chiral 2-pyridylsulfinamides were shown to be effective catalysts in the alkylation of aryl and alkyl aldehydes with diethylzinc providing the corresponding alcohols in excellent enantioselectivity. Sulfinamide catalysts possessing solitary chirality at the sulfur center produced the product phenethyl alcohol in good enantioselectivity. Diastereomeric sulfinamides possessing chirality at the carbon-bearing nitrogen and at the sulfur of the sulfinamide increased the enantioselectivity of the product alcohols up to >99%. However, there is no effect of the match-mismatch pair of sulfinamide diastereomers on the outcome of the chiral induction of the product phenethyl alcohols. It was conclusively proved that chirality at the sulfur center is mandatory for obtaining good enantioselectivity in the reaction.     

Carbene‐Catalyzed Desymmetrization and Direct Construction of Arenes with All‐Carbon Quaternary Chiral Center

Multisubstituted arenes such as indanes with attached all‐carbon quaternary centers are unique scaffolds in synthetic functional molecules and sophisticated natural products. A key challenge in preparing such molecules lies in the enantioselective installation of the quaternary carbon centers. Conventional methods in this direction include asymmetric substitution reactions and substrate‐controlled cyclization reactions. These reactions lead to poor stereoselectivities and/or require long and tedious synthetic steps. Disclosed here is a one‐step organic catalytic strategy for enantioselective access to this class of molecules. The reaction involves an N‐heterocyclic carbene catalyzed process for direct benzene construction, indane formation, remote‐carbon desymmetrization, and excellent chirality control. This approach will enable the concise synthesis of arene‐containing molecules, including those with complex structures and challenging chiral centers.     

Synthesis of Chiral Sulfur‐Containing Polymers: Asymmetric Copolymerization of meso‐Epoxides and Carbonyl Sulfide

Synthesis of chiral sulfur‐containing polymers was realized for the first time by the asymmetric alternating copolymerization of achiral meso‐epoxides with carbonyl sulfide (COS) using catalyst systems based on enantiopure binaphthol‐linked dinuclear Co^III complexes under mild reaction conditions. The resultant poly(monothiocarbonate)s have main‐chain chirality and more than 99 % isotacticity. Notably, the stereoregular copolymers are typical semicrystalline thermoplastics with high melting temperatures up to 232 °C. Additionally, these sulfur‐containing polymers have good optical properties with refractive indices of up to 1.56 and Abbe's numbers of up to 43.