Unlocking New Prenylation Modes: Azaindoles as a New Substrate Class for Indole Prenyltransferases

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Unlocking New Prenylation Modes: Azaindoles as a New Substrate Class for Indole Prenyltransferases

Enzyme catalysis: Five isomers of aza-tryptophan and the corresponding series of cyclic Aza-tryptophan-proline dipeptides were tested as unnatural substrates for three well-known indole-prenylating enzymes, FgaPT2, CdpNPT, and FtmPT1. Most substrates were found to produce prenylated products, which revealed isomer-dependent regioselectivity and a previously unreported class of cationic N-prenyl pyridinium products.


Abstract

Aza-substitution, the replacement of aromatic CH groups with nitrogen atoms, is an established medicinal chemistry strategy for increasing solubility, but current methods of accessing functionalized azaindoles are limited. In this work, indole-alkylating aromatic prenyltransferases (PTs) were explored as a strategy to directly functionalize azaindole-substituted analogs of natural products. For this, a series of aza-L-tryptophans (Aza-Trp) featuring N-substitution of every aromatic CH position of the indole ring and their corresponding cyclic Aza-L-Trp-L-proline dipeptides (Aza-CyWP), were synthesized as substrate mimetics for the indole-alkylating PTs FgaPT2, CdpNPT, and FtmPT1. We then demonstrated most of these substrate analogs were accepted by a PT, and the regioselectivity of each prenylation was heavily influenced by the position of the N-substitution. Remarkably, FgaPT2 was found to produce cationic N-prenylpyridinium products, representing not only a new substrate class for indole PTs but also a previously unobserved prenylation mode. The discovery that nitrogenous indole bioisosteres can be accepted by PTs thus provides access to previously unavailable chemical space in the search for bioactive indolediketopiperazine analogs.

Hydroelementation and Phosphinidene Transfer: Reactivity of Phosphagermenes and Phosphastannenes Towards Small Molecule Substrates

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Hydroelementation and Phosphinidene Transfer: Reactivity of Phosphagermenes and Phosphastannenes Towards Small Molecule Substrates

The synthesis of [(Me3Si)2CH]2E=PMes* (E=Ge, Sn) from the reaction of the tetrylenes with a phospha-Wittig reagent is described. Reactivity studies reveal that they are capable of undergoing hydroelementation reactions across the E=P bond and of acting as masked phosphinidene sources.


Abstract

We describe the facile synthesis of [(Me3Si)2CH]2E=PMes* (E=Ge, Sn) from the reaction of the tetrylenes with the phospha-Wittig reagent, Me3P−PMes*. Their reactivity towards a range of substrates with protic and hydridic E−H bonds (E=N, O, Si) is described. In addition to hydroelementation reactions of the E=P bonds, we show that these compounds, particularly [(Me3Si)2CH]2Sn=PMes*, also act as base-stabilized phosphinidenes, allowing phosphinidene transfer to other nucleophiles.

Stable Dication Diradicals of Triply Fused Metallo Chlorin‐Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity

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Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity

Two highly stable, novel, triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers are reported in which the bridge is completely fused between two porphyrin macrocycles. UV-vis, EPR, and ESI-MS investigations enabled us to identify some of the key reactive intermediates disclosing tentative mechanistic details of such an unusual transformation.


Abstract

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C−C and one C−N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals. In addition, the macrocyclic distortion also favours a rare intramolecular ferromagnetic interaction between the CuII and π-cation radical spins to form a triplet state. The structural and electronic perturbation in the unconjugated dication diradical possibly enables the bridging pyrrolic nitrogen to undergo a nucleophilic attack at the nearby β-carbon of the porphyrin π-cation radical with a computed free energy barrier of >20 kcal mol−1 which was supplied in the form of reflux condition to initiate such a rearrangement process. UV-vis, EPR and ESI-MS spectroscopies were used to monitor the rearrangement process in situ in order to identify the key reactive intermediates leading to such an unusual transformation.

Carbon Nitrides from Supramolecular Crystals: From Single Atoms to Heterojunctions and Advanced Photoelectrodes

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Carbon Nitrides from Supramolecular Crystals: From Single Atoms to Heterojunctions and Advanced Photoelectrodes

Supramolecular single crystals based on C−N monomers are used to prepared carbon nitride materials with tailored morphology and porosity, single atom catalysts or heterojunctions. This approach can be applied to fabricate efficient water splitting photoelectrodes.


Abstract

Carbon nitride materials (CN) have become one of the most studied photocatalysts within the last 15 years. While CN absorbs visible light, its low porosity and fast electron-hole recombination hinder its photoelectric performance and have motivated the research in the modification of its physical and chemical properties (such as energy band structure, porosity, or chemical composition) by different means. In this Concept we review the utilization of supramolecular crystals as CN precursors to tailor its properties. We elaborate on the features needed in a supramolecular crystal to serve as CN precursor, we delve on the influence of metal-free crystals in the morphology and porosity of the resulting materials and then discuss the formation of single atoms and heterojunctions when employing a metal-organic crystal. We finally discuss the performance of CN photoanodes derived from crystals and highlight the current standing challenges in the field.

Intramolecular Through‐Space Double‐Electron Transfer Between A Pair of Redox‐Active Guanidine Units Aligned by Dithiolate Bridges

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Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Transfer in pairs: Intramolecular through-space electron transfer between an oxidized, dicationic and a reduced, neutral triguanidine unit is observed if the two units are preorientated by a dithiolate bridge.


Abstract

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Yttrium Complexes with Group 13 Heterobenzene‐Type Ligands

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Yttrium Complexes with Group 13 Heterobenzene-Type Ligands

The gallabenzene-type yttrium complex [(1-Me-3,5-tBu2−C5H3Ga)(μ-Me)Y(2,4-dtbp)] (A) is readily formed from one-pot-reactions using mixtures [YMe3] n /GaMe3/K(2,4-dtbp) (2,4-dtbp=2,4-di-tert-butyl-pentadienyl), while the remaining pentadienyl ligand gets easily displaced by pentamethylcyclopentadienyl affording B, showcasing the strong interaction of the heterobenzene ligand with the rare-earth-metal center. Distinct ligand bonding is revealed by 89Y NMR chemical shifts.


Abstract

The yttrium gallabenzene complex [(1-Me-3,5-tBu2−C5H3Ga)(μ-Me)Y(2,4-dtbp)] is accessible from Y(GaMe4)3 and K(2,4-dtbp) via a tandem salt metathesis/methane elimination (2,4-dtbp=2,4-di-tert-butyl-pentadienyl). The pentadienyl ligand in [(1-Me-3,5-tBu2−C5H3E)(μ-Me)Y(2,4-dtbp)] (E=Al, Ga) is easily displaced by salt metathesis with KC5Me5 and KTpMe,Me (TpMe,Me=tris(pyrazolyl-Me2-3,5)borato) affording [(1-Me-3,5-tBu2−C5H3E)(μ-Me)Y(TpMe,Me)] and [(1-Me-3,5-tBu2−C5H3E)(μ-Me)Y(C5Me5)]. The yttrium center in [(1-Me-3,5-tBu2−C5H3E)(μ-Me)Y(2,4-dtbp)] readily forms adducts with neutral Lewis bases like 4-DMAP (4-dimethylaminopyridine), PMe3, DMPE (1,2-bis(dimethylphosphino)ethane), and DME (1,2-dimethoxyethane). In stark contrast, addition of TMEDA (N,N,N’,N’-tetramethylethylenediamine) results in methyl/pentadienyl exchange between aluminum and yttrium resulting in [(1-(2,4-dtbp)-1-Me-3,5-tBu2−C5H3Al)Y(Me)(tmeda)]. The bonding features of the newly synthesized complexes are analyzed by single-crystal X-ray diffraction (SCXRD) and heteronuclear (89Y, 31P) NMR spectroscopy.

Solid‐State [4+4] Cycloaddition and Cycloreversion with Use of Unpaired Hydrogen‐Bond Donors to Achieve Solvatomorphism and Stabilization

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Solid-State [4+4] Cycloaddition and Cycloreversion with Use of Unpaired Hydrogen-Bond Donors to Achieve Solvatomorphism and Stabilization

The solid-state [4+4] cycloaddition of a commercially available anthracene derivative affords facile synthesis of a cycloadduct. The cycloaddition is reversible in the solid-state using heat or mechanical force. The cycloadduct is highly solvatomorphic due to presence of unused, strong hydrogen-bond donors and is applied to the thermal stabilization of monomers.


Abstract

The crystal structure of a commercially available anthracene derivative, anthracene-9-thiocarboxamide, is reported here for the first time. The compound undergoes a [4+4] cycloaddition in the solid state to afford facile synthesis of the cycloadduct (CA). The cycloaddition is also reversible in the solid state using heat or mechanical force. Due to the presence of unpaired, strong hydrogen-bond donor atoms on the CA, significant solvatomorphism is achieved, and components of the solvatomorphs self-assemble into four different classes of supramolecular structures. The CA readily crystallizes with a variety of structurally-diverse solvents including those containing oxygen-, nitrogen-, or pi-acceptors. Some of the solvents the CA crystallized with include thiophene, benzene, and the three xylene isomers; thus, the CA was employed in industrially-relevant solvent separation. However, in competition studies, the CA did not exhibit selectivity. Lastly, it is demonstrated that the CA crystallizes with vinyl-containing monomers and is currently the only compound that crystallizes with both widely used monomers 4-vinylpyridine and styrene. Solid-state complexation of the CA with the monomers affords over a 50 °C increase in the monomer's thermal stabilities. The strategy of designing molecules with unused donors can be applied to achieve separations or volatile liquid stabilization.

A Combination of B‐ and N‐Doped π‐Systems Enabling Systematic Tuning of Electronic Structures and Properties

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A Combination of B- and N-Doped π-Systems Enabling Systematic Tuning of Electronic Structures and Properties

B/N-codoped polycyclic aromatic hydrocarbons (PAHs) were constructed via combining the B- and N-doped π-systems. Two B/N-codoped PAHs were synthesized through the Mallory photoreaction, and their electronic structures and optical properties could be effectively modulated via static and dynamic control of intramolecular charge transfer states.


Abstract

Doping heteroatoms into polycyclic aromatic hydrocarbons (PAHs) may alter their structures and thereby physical properties. This study reports the construction of B/N-codoped PAHs via combining the B- and N-doped π-systems. Two π-extended B/N-codoped PAHs were synthesized through the Mallory photoreaction. Both feature a C48BN2 π-skeleton, which is assembled by linearly fusing three substructures including B-doped and sp2-hybridized N-doped π-moieties and one pyrene unit. In comparison to the pristine B-doped analog, their intramolecular charge transfer (ICT) states are distinctly modulated by the fused N-doped π-system and the further incorporated cyano group, leading to their tunable optical properties, as revealed by detailed theoretical and experimental analysis. Furthermore, these three molecules have sufficient Lewis acidity and can coordinate with Lewis base to form Lewis acid-base adducts, and notably, such intermolecular complexation can further dynamically modulate their ICT transitions and thereby photophysical properties, such as producing blue, green and red fluorescence.

Effect of Modified Biochar Prepared by Co‐pyrolysis of MgO on Phosphate Adsorption Performance and Seed Germination

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Modified biochar was prepared directly by co-pyrolysis of MgO and rice straw in this study. The effects of single factors, such as pyrolysis temperature, dosage, pH, and coexisting ions, on phosphate adsorption performance, were investigated, and the effects of modified biochar leachate on the germination of corn and rice seeds after phosphate adsorption were examined. The results showed that phosphate adsorption by the modified biochar first increased and then decreased as the pyrolysis temperature increased, with modified biochar prepared at 800 °C showing the greatest adsorption, and the best phosphate adsorption effect of modified biochar was achieved at a dosage of 0.10 g and the solution pH=3. The adsorption kinetics study revealed that the process of phosphate adsorption by the modified biochar was more in line with the pseudo-second-order model and dominated by chemisorption, whereas the adsorption isotherm results indicated that the process of adsorption was more in line with the Langmuir model and was dominated by monomolecular layer adsorption, with a maximum adsorption of 217.54 mg/g. Subsequent seed germination tests showed that although the phosphate-adsorbed modified biochar leachate had no significant effect on the germination rate of corn seeds, it improved the germination rate of rice seeds.