Sebastian Beil

Nature Chemistry, Published online: 21 October 2019; doi:10.1038/s41557-019-0346-2

Asymmetric Sonogashira C(sp3)–C(sp) couplings provide complementary approaches to established C(sp3)–C(sp2/sp3) couplings for chiral C–C bond formation; however, relatively few reactions have been developed. Now, a versatile, enantioconvergent Sonogashira coupling via a radical intermediate has been developed. The approach uses a copper catalyst featuring a multidentate electron-rich cinchona alkaloid-derived ligand.

Lock on: A pair of radial [5]catenane isomers with different sequences of the peripheral macrocycles, β‐cyclodextrin and cucurbit[6]uril units, was obtained in high yield. Given the marked difference in binding strength and interlocking sequence of the peripheral macrocycles, interesting sequence‐dependent properties, characteristic of mechanically bonded macrocycles, were realized.

Abstract

A pair of radial [5]catenanes, with either an isomeric cyclic ‐AABB‐ or ‐ABAB‐ type sequence of the interlocked β‐cyclodextrin (β‐CD) and cucurbit[6]uril (CB[6]) units, has been efficiently synthesized. Because of a marked difference in the binding strength and interlocking sequence of the peripheral macrocycles, interesting sequence‐dependent properties, characteristic of mechanically bonded macrocycles, were realized. Variable‐temperature ¹H NMR studies showed that the ‐ABAB‐ isomer has a more independent β‐CD dynamic, whereas the β‐CD motions in the ‐AABB‐ isomer are coupled. Dynamics of the pH‐insensitive β‐CD can also be further modulated upon base‐triggered mobilization of the CB[6]. These unique properties of the mechanical bond expressed in a sequence‐specific fashion and the transmission of the control on the macrocycle dynamics from one interlocked component to another, highlight the potential of similar complex hetero[n]catenanes in the design of advanced, multicomponent molecular machines.

Switchability: Nonsymmetrical ditopic ligands (L) drive the formation of a dynamic Pd₂L₄ open cage that is capable of turn‐on/off anion binding through stoichiometry‐driven structural transitions of the host cage.

Abstract

This work demonstrates a new nonconventional ligand design, imidazole/pyridine‐based nonsymmetrical ditopic ligands (1 and 1 ^S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd²⁺ at a 1:4 molar ratio, 1 and 1 ^S initially form mononuclear PdL₄ complexes (Pd²⁺(1)₄ and Pd²⁺(1 ^S )₄) without formation of a cage. The PdL₄ complexes undergo a stoichiometrically controlled structural transition to Pd₂L₄ open cages ((Pd²⁺)₂(1)₄ and (Pd²⁺)₂(1 ^S )₄) capable of anion binding, leading to turn‐on anion binding. The structural transitions between the Pd₂L₄ open cage and the PdL₄ complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1 ^S to the (Pd²⁺)₂(1 ^S )₄ open cage holding a guest anion ((Pd²⁺)₂(1 ^S )₄⋅G⁻) enables the structural transition to the Pd²⁺(1 ^S )₄ complex, which does not have a cage and thus causes the release of the guest anion (Pd²⁺(1 ^S )₄+G⁻).

Supramolecular macrocyclic hosts have been of great interest in smart materials for years, whereas their triplet emission and regulation at crystal level is still rarely excavated. Herein, ultralong and universal room temperature phosphorescence (RTP) phenomenon is evacuated for traditional crown ethers. Supramolecular strategy involving chain length adjustment and morphological locking through complexation with K+ was explored as general methods to tune phosphorescence lifetime in solid state. Significantly, B15C5 exhibits 10‐folds increasing of lifetime after forming complex accompany with invisible to visible phosphorescence. A deep encryption based on this activated RTP strategy was facile fabricated. This work open a new world for supramolecular macrocycles and their intrinsic guest responsiveness offers new avenue for versatile smart luminescent materials.

Host‐enhanced intermolecular charge transfer enabled the fabrication of a supramolecular radical dimer with strong absorption in the NIR‐II biowindow and high stability. The dimer exhibited high‐efficiency photothermal conversion and strong inhibition on cancer cells through NIR‐II photothermal therapy.

Abstract

Photothermal therapy at the NIR‐II biowindow (1000–1350 nm) is drawing increasing interest because of its large penetration depth and maximum permissible exposure. Now, the supramolecular radical dimer, fabricated by N,N′‐dimethylated dipyridinium thiazolo[5,4‐d]thiazole radical cation (MPT^.+) and cucurbit[8]uril (CB[8]), achieves strong absorption at NIR‐II biowindow. The supramolecular radical dimer (2MPT^.+‐CB[8]) showed highly efficient photothermal conversion and improved stability, thus contributing to the strong inhibition on HegG2 cancer cell under 1064 nm irradiation even penetrating through chicken breast tissue. This work provides a novel approach to construct NIR‐II chromophore by tailor‐made assembly of organic radicals. It is anticipated that this study provides a new strategy to achieve NIR‐II photothermal therapy and holds promises in luminescence materials, optoelectronic materials, and also biosensing.

[5]‐ and [7]‐Helical compounds possessing the dispiroindeno[2,1‐c]fluorene framework were prepared in 5–6 steps with high overall yields by using catalytic intramolecular [2+2+2]‐cyclotrimerization as the crucial step. They exhibit fluorescence in the region of 351–428 nm with high quantum yields (up to 0.88).

Abstract

This work presents a general approach for synthesis of substituted [5]‐helical dispiroindeno[2,1‐c]fluorenes based on Rh‐catalyzed intramolecular cyclotrimerization of triynes. This approach was further extended for the first synthesis of configurationally stable [7]‐helical dispiroindeno[2,1‐c]fluorenes. A series of variously substituted derivatives was prepared and their photophysical and electrochemical properties were evaluated. Their fluorescence emission maxima were in the region of 351–428 nm and quantum yields up to 88 % are the highest measured among the full‐carbon helical compounds.

Separation into rich and poor: The formation of supramolecular nanofibrils from amphiphilic amino acids and short peptides follows a nucleation–elongation mechanism mediated by liquid–liquid phase separation. The initial formation of solute‐poor and solute‐rich liquid droplets is entropy driven, while the transition of liquid droplets to nanofibrils is dominated by enthalpic interactions.

Abstract

The transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid–liquid phase separation into solute‐rich and solute‐poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute‐rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

Tilt a whirl: A new class of chiral [1]ferrocenophanes is described which feature a record‐high Cp ring tilt. From one of the monomers a poly(ferrocenylborane) was obtained that, according to DFT calculations, exhibits a left‐handed helical structure. This is the first evidence of a polyferrocene with a chiral secondary structure.

Abstract

Silicon‐bridged [1]ferrocenophanes are a versatile class of monomers to obtain well‐defined metallopolymers, however, their boron‐bridged analogues are far less utilized despite being significantly higher strained. We assumed that the reactivity of known bora[1]ferrocenophanes towards ring‐opening polymerization is hampered by π‐donating R₂N groups at the bridging boron atom and therefore prepared the first bora[1]ferrocenophanes lacking such electronic stabilization. The new, isolated ferrocenophane with a 2,4,6‐triisopropylphenyl group attached to the bridging boron atom exhibits the most tilted Cp rings among all isolated strained sandwich compounds [α(DFT)=33.3°] with a measured record value of the bathochromic shift (λ _max=516 nm). Attempts to purify the mesityl analogue by vacuum sublimation transformed this monomer to a purple‐colored polymer that resulted in Cotton effects in circular dichroism spectroscopy. DFT calculations revealed a left‐handed helical structure for this polymer. This is the first evidence for a polyferrocene with a chiral secondary structure.

Soft matter, hard shell: pH‐responsive hydrogels that can chelate Fe³⁺ were prepared from guanosine‐5′‐hydroxamic acid (HA). At high pH, the hydrogel binds thiazole orange, signaled by enhanced fluorescence. The HA units can also act as a supramolecular siderophore to form red complexes with Fe³⁺, which allowed the patterning of the hydrogel surface with FeCl₃.

Abstract

Guanosine‐5′‐hydroxamic acid (3) forms hydrogels when mixed with guanosine (1) and KCl. The 5′‐hydroxamic acid (HA) unit is pH‐responsive and also chelates Fe³⁺. When gels are prepared under basic conditions, the 5′‐HA groups are deprotonated and the anionic hydrogel binds cationic thiazole orange (TO), signaled by enhanced fluorescence. The HA nucleoside 3, when immobilized in the G‐quartet gel, acts as a supramolecular siderophore to form red complexes with Fe³⁺. We patterned the hydrogel's surface with FeCl₃, by hand and by using a 3D printer. Patterns form instantly, are visible by eye, and can be erased using vitamin C. This hydrogel, combining self‐assembled G‐quartet and siderophore–Fe³⁺ motifs, is strong, can be molded into different shapes, and is stable on the bench or under salt water.

The triphenylene and fluoranthene molecules—potential precursors to two‐ and three‐dimensional graphitic nano sheets in the interstellar medium—can be formed through rapid molecular mass growth processes by the reactions of phenyl radical with biphenyl and naphthalene at high temperatures.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.

Based on a truxene a chiral monkey saddle shaped polyaromatic hydrocarbon (PAH) with three eight‐membered rings was accessible in only three synthetic steps. The enantiomers were separated by chiral HPLC and investigated photophysically.

Abstract

A contorted polycyclic aromatic hydrocarbon (PAH) in the shape of a monkey saddle has been synthesized in three steps from a readily available truxene precursor. The monkey saddle PAH is consisting of three five‐, seven six‐, and three eight‐membered rings and has been unambiguously characterized by single‐crystal X‐ray diffraction. Owing to the three biaryl axes the monkey saddle PAH is inherently chiral. The inversion of the two enantiomeric structures into each other preferably occurs through a twisting of peripheral rings rather than by a fully planar intermediate, as has been calculated by DFT methods. Enantiomers were separated by chiral HPLC and inversion barriers determined by variable temperature circular dichroism spectroscopy, supporting the twisting mechanism.

All in a twirl: The gram‐scale synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The optically pure [13]helicenes can be synthetically functionalized both at the termini (see picture: blue) and the periphery (red).

Abstract

Fully aromatic helicenes with more than one pitch‐length are illustrious synthetic targets with potential applications in advanced optical devices and nano‐electronics. The task of extending the length of fully conjugated helicenes past one pitch length is challenging. Now, the synthesis of a series of azaoxa[7]‐, [10]‐, and [13]helicenes is described. The synthesis is based on iterative oxidative furan formation between 3,6‐dihydroxycarbazoles and/or 2‐naphthols. The flexibility of the presented method allows the convenient and scalable synthesis of symmetric, unsymmetrical, and asymmetric homo‐chiral structures. The [13]helicenes can be synthetically functionalized both at the termini and the periphery. The full range of helicenes were characterized using NMR and optical spectroscopy (UV/Vis, fluorescence, and CD) along with single‐crystal X‐ray crystallography. The enantiomers of the [13]helicenes are the longest optically pure helicenes isolated to date.

Nature, Published online: 08 October 2019; doi:10.1038/d41586-019-03020-6

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Nature Catalysis, Published online: 07 October 2019; doi:10.1038/s41929-019-0357-9

Selective functionalization of C(sp3)–H bonds is difficult in alkanes and other hydrocarbons, and especially so for enantioselective reactions. Here the authors report a photocatalyst and chiral metal catalyst to allow the radical, asymmetric addition of alkyl, allylic and benzylic groups to imines.

Multifunctional organic emitter: One simple organic molecule exhibits aggregation‐induced emission (AIE), thermally activated delayed fluorescence (TADF), room‐temperature phosphorescence (RTP), and mechanoluminescence (ML) simultaneously.

Abstract

Aggregation‐induced emission (AIE), thermally activated delayed fluorescence (TADF), room‐temperature phosphorescence (RTP), and mechanoluminescence (ML) have attracted widespread interest. However, a multifunctional organic emitter exhibiting simultaneous AIE, TADF, RTP, and ML has not been reported. Now, two multifunctional blue emitters with very simple structures, mono‐DMACDPS and Me‐DMACDPS, exhibit typical AIE, TADF, and RTP properties but different behavior in mechanoluminescence. Crystal structure analysis reveals that large dipole moment and multiple intermolecular interactions with tight packing mode endow mono‐DMACDPS with strong ML. Combined with the data of crystal analysis and theoretical calculation, the separated monomer and dimer in the crystal lead to the typical TADF and RTP properties, respectively. Simple‐structure mono‐DMACDPS is the first example realizing TADF, RTP, AIE, and ML simultaneously.

Nature Catalysis, Published online: 07 October 2019; doi:10.1038/s41929-019-0361-0

While the oxidative addition of Pd to carbon–halide bonds is often regarded as being essentially irreversible, this is sometimes not the case. This Perspective looks at the conditions leading to reductive elimination of Pd from carbon–halide bonds, and the synthetic opportunities that this offers are discussed.

Reversible covalent bonds play a significant role in achieving the high‐yielding synthesis of mechanically interlocked molecules. Still, only a handful of such bonds have been successfully employed in synthetic procedures. Herein, we introduce a novel approach for the fast and simple preparation of mechanically interlocked molecules, combining the dynamic bond character of bis(acyloxy)iodate(I) anions with macrocyclic bambusuril anion receptors. The proof of principle was demonstrated on rotaxane synthesis, with near‐quantitative yields observed in both the classical and “in situ” approach. The rotaxane formation was confirmed in the solid state and solution by the X‐ray and NMR studies, respectively. Our novel approach could be utilized in the fields of dynamic combinatorial chemistry, supramolecular polymers, or molecular machines, as well inspire further research on molecules that exhibit dynamic behavior, but due to their high reactivity, have not been considered as constituents of more elaborate supramolecular structures.

Theories about the origin of life require chemical pathways that allow formation of life’s key building blocks under prebiotically plausible conditions. Complex molecules like RNA must have originated from small molecules whose reactivity was guided by physico-chemical processes. RNA is constructed from purine and pyrimidine nucleosides, both of which are required for accurate information transfer, and thus Darwinian evolution. Separate pathways to purines and pyrimidines have been reported, but their concurrent syntheses remain a challenge. We report the synthesis of the pyrimidine nucleosides from small molecules and ribose, driven solely by wet-dry cycles. In the presence of phosphate-containing minerals, 5'-mono- and diphosphates also form selectively in one-pot reactions. The pathway is compatible with purine synthesis, allowing the concurrent formation of all Watson-Crick bases.