Asymmetric Synthesis of an Atropisomeric ß-Carboline via Regioselective Intermolecular Rh(I)-Catalyzed [2 + 2 + 2] Cyclotrimerization.

The rational design of atropisomeric small molecules is becoming increasingly common in chemical synthesis as a result of the unique advantages this property provides in drug discovery, asymmetric catalysis, and chiroptical activity. In this study, we designed a synthesis of a configurationally stable ß-carboline in six steps. Our synthesis made use of an innovative Grignard addition/elimination reaction that formed an yne-ynamide precursor that then reacted with ethyl cyanoformate in a rhodium(I)-catalyzed [2+2+2] cyclotrimerization reaction to give the atropisomeric ß-carboline in excellent yield, good enantioselectivity, and excellent regioselectivity. Extensive optimization of this transformation is described. Racemization kinetics experiments were also conducted on the individual atropisomers and their absolute configurations were determined by circular dichroism.

Mechanochemical Cyclotrimerization: A Versatile Tool to Covalent Organic Frameworks with Tunable Stacking Mode.

We introduce the first mechanochemical cyclotrimerization of nitriles, a facile strategy for synthesizing triazine-containing molecules and materials, overcoming challenges related to carbonization and solubility. Conducting this solid-state approach in a mixer ball mill with 4-Methylbenzonitrile, we synthesize Tris(4-methylphenyl)-1,3,5-triazine quantitatively in as little as 90 minutes. Just as fast, this mechanochemical method facilitates the synthesis of the covalent triazine framework CTF-1 using 1,4 Dicyanobenzene. Material characterization confirms its porous (650 m2 g-1) and crystalline nature. Adjusting the induced mechanical energy allows control over the obtained stacking conformation of the resulting CTFs - from a staggered AB arrangement to an eclipsed AA stacking conformation. Finally, a substrate scope demonstrates the versatility of this approach, successfully yielding various CTFs.

Dissymmetric Triaryltriazines: Small Mass Columnar Glasses.

Columnar liquid crystals with very small molecular masses that form anisotropic glasses well above room temperature are obtained by mixed dissymmetric substitution of sym-triazine with ester-bearing phenyl and phenanthryl or tetrahelicenyl moieties. The combination of low molecular symmetry with configurational flexibility and short polar ester moieties stabilizes the mesophase over large temperature ranges and induces pronounced calorimetric glass transitions within the anisotropic fluid despite the smallness of the molecules. In contrast to more symmetrical homologs, no ester tails longer than ethyl are necessary to induce the liquid crystalline state, allowing for the near-absence of any insulating and weight-increasing alkyl periphery. Films drop-cast from solution show in all cases emission spectra that do not show significant change of fluorescence emission upon annealing, indicating that the columnar hexagonal mesoscopic order is obtained directly upon deposition from solution and is resistant to crystallization upon annealing.

Columnar Liquid Crystalline Glasses by Combining Configurational Flexibility with Moderate Deviation from Planarity: Extended Triaryltriazines.

Triply phenanthryl- and tetrahelicenyl-substituted triazine-hexaalkyl esters with short alkyl chains show glass transitions conveniently above room temperature within the hexagonal columnar liquid crystalline state, resulting in a solid columnar order at room temperature. As the hexagonal columnar mesophase is easily aligned with the director perpendicular to a solid substrate, such glassy columnar liquid matrices are aimed at for the orientation of guest emitters, to obtain anisotropic emission. A condition for face-on alignment on substrates are attainable melting and clearing temperatures, which is achieved with the moderately nonplanar tetrahelicenyl derivatives in spite of their short alkyl periphery. An unusual phase transition between two columnar mesophases of same hexagonal symmetry, but very different long-distance regularity of the column lattice, is found in one phenanthryl homolog.

Cyclotrimerization Approach to Symmetric [9]Helical Indenofluorenes: Diverting Cyclization Pathways.

Catalytic cyclotrimerization routes to symmetrical [9]helical indenofluorene were explored by using different transition-metal complexes and thermal conditions. Depending on the reaction conditions, the cyclotrimerizations were accompanied by dehydro-Diels-Alder reaction giving rise to another type of aromatic compounds. Structures of both symmetrical [9]helical cyclotrimerization product as well as the dehydro-Diels-Alder product were confirmed by single-crystal X-ray diffraction analyses. Limits of enantioselective cyclotrimerization were assessed as well. DFT calculations shed light on the reaction course and the origin of diminished enantioselectivity.

Synthesis of two-dimensional triazine covalent organic frameworks at ambient conditions to detect and remove water pollutants.

Synthesis of fully triazine frameworks (C3N3) by metal catalyzed reactions at high temperatures results in carbonized and less-defined structures. Moreover, metal impurities affect the physicochemical, optical and electrical properties of the synthesized frameworks, dramatically. In this work, two-dimensional C3N3 (2DC3N3) has been synthesized by in situ catalyst-free copolymerization of sodium cyanide and cyanuric chloride, as cheap and commercially available precursors, at ambient conditions on gram scale. Reaction between sodium cyanide and cyanuric chloride resulted in electron-poor polyfunctional intermediates, which converted to 2DC3N3 with several hundred micrometers lateral size at ambient conditions upon [2 + 2+2] cyclotrimerization. 2DC3N3 sheets, in bulk and individually, showed strong fluorescence with 63% quantum yield and sensitive to small objects such as dyes and metal ions. The sensitivity of 2DC3N3 emission to foreign objects was used to detect low concentration of water impurities. Due to the high negative surface charge (-37.7 mV) and dispersion in aqueous solutions, they demonstrated a high potential to remove positively charged dyes from water, exemplified by excellent removal efficiency (>99%) for methylene blue. Taking advantage of the straightforward production and strong interactions with dyes and metal ions, 2DC3N3 was integrated in filters and used for the fast detection and efficient removal of water impurities.

Possible Roles of Transition Metal Cations in the Formation of Interstellar Benzene via Catalytic Acetylene Cyclotrimerization.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous interstellar molecules. However, the formation mechanisms of PAHs and even the simplest cyclic aromatic hydrocarbon, benzene, are not yet fully understood. Recently, we reported the statistical and dynamical properties in the reaction mechanism of Fe+-catalyzed acetylene cyclotrimerization, whereby three acetylene molecules are directly converted to benzene. In this study, we extended our previous work and explored the possible role of the complex of other 3d transition metal cations, TM+ (TM = Sc, Ti, Mn, Co, and Ni), as a catalyst in acetylene cyclotrimerization. Potential energy profiles for bare TM+-catalyst (TM = Sc and Ti), for TM+NC--catalyst (TM = Sc, Ti, Mn, Co, and Ni), and for TM+-(H2O)8-catalyst (TM = Sc and Ti) systems were obtained using quantum chemistry calculations, including the density functional theory levels. The calculation results show that the scandium and titanium cations act as efficient catalysts in acetylene cyclotrimerization and that reactants, which contain an isolated acetylene and (C2H2)2 bound to a bare (ligated) TM cation (TM = Sc and Ti), can be converted into a benzene-metal-cation product complex without an entrance barrier. We found that the number of electrons in the 3d orbitals of the transition metal cation significantly contributes to the catalytic efficiency in the acetylene cyclotrimerization process. On-the-fly Born-Oppenheimer molecular dynamics (BOMD) simulations of the Ti+-NC- and Ti+-(H2O)8 complexes were also performed to comprehensively understand the nuclear dynamics of the reactions. The computational results suggest that interstellar benzene can be produced via acetylene cyclotrimerization reactions catalyzed by transition metal cation complexes.

1,4,6,10-Tetraazaadamantanes (TAADs) with N-amino groups: synthesis and formation of boron chelates and host-guest complexes.

A synthetic route to 1,4,6,10-tetraazaadamantanes (TAADs) bearing free and protected amino groups at the bridge N-atoms has been developed via intramolecular cyclotrimerization of C=N units in the corresponding tris(hydrazonoalkyl)amines. In a similar fashion, unsymmetrically substituted TAADs having both amino and hydroxy groups at the bridge N-atoms were prepared via a hitherto unknown co-trimerization of oxime and hydrazone groups. The use of N-TAAD derivatives as potential ligands and receptors was showcased through forming boron chelates and host-guest complexes with water and simple alcohols.

Computational prediction on the catalytic activity of heterobimetallic complex featuring MM' triple bond in acetylene cyclotrimerization: Mechanistic study.

A detailed reaction mechanism of acetylene cyclotrimerization catalyzed by V(i PrNPMe2 )3 Fe-PMe3 (denote as CAT), a heterobimetallic complex featuring V-Fe triple bond, was computationally investigated using density functional theory. The calculated results show that the first acetylene firstly attaches to the V atom of CAT to get a four-membered ring structure through [2 + 2] cycloaddition reaction. For the second acetylene addition, there are two cyclotrimerization mechanisms, outer sphere mechanism and inner mechanism. The inner sphere reaction pathway is the main reaction pathway. By replacing the V with Nb and Ta, Fe with Ru and Os, a series of new catalysts are screened computationally. The calculated results show that, all of the nine heterobimetallic complexes show high activity at mild condition. The energy barrier of the rate determining step is related to the natural population analysis (NPA) charge of M' and the Wiberg bond index (WBI) of M-M' bond. The more negative NPA charge of M' and the smaller WBI of M-M' bond, the lower energy barrier is.

Case Study of N-i Pr versus N-Mes Substituted NHC Ligands in Nickel Chemistry: The Coordination and Cyclotrimerization of Alkynes at [Ni(NHC)2 ].

A case study on the effect of the employment of two different NHC ligands in complexes [Ni(NHC)2 ] (NHC=i Pr2 ImMe 1Me , Mes2 Im 2) and their behavior towards alkynes is reported. The reaction of a mixture of [Ni2 (i Pr2 ImMe )4 (µ-(η2 : η2 )-COD)] B/ [Ni(i Pr2 ImMe )2 (η4 -COD)] B' or [Ni(Mes2 Im)2 ] 2, respectively, with alkynes afforded complexes [Ni(NHC)2 (η2 -alkyne)] (NHC=i Pr2 ImMe : alkyne=MeC≡CMe 3, H7 C3 C≡CC3 H7 4, PhC≡CPh 5, MeOOCC≡CCOOMe 6, Me3 SiC≡CSiMe3 7, PhC≡CMe 8, HC≡CC3 H7 9, HC≡CPh 10, HC≡C(p-Tol) 11, HC≡C(4-t Bu-C6 H4 ) 12, HC≡CCOOMe 13; NHC=Mes2 Im: alkyne=MeC≡CMe 14, MeOOCC≡CCOOMe 15, PhC≡CMe 16, HC≡C(4-t Bu-C6 H4 ) 17, HC≡CCOOMe 18). Unusual rearrangement products 11 a and 12 a were identified for the complexes of the terminal alkynes HC≡C(p-Tol) and HC≡C(4-t Bu-C6 H4 ), 11 and 12, which were formed by addition of a C-H bond of one of the NHC N-i Pr methyl groups to the C≡C triple bond of the coordinated alkyne. Complex 2 catalyzes the cyclotrimerization of 2-butyne, 4-octyne, diphenylacetylene, dimethyl acetylendicarboxylate, 1-pentyne, phenylacetylene and methyl propiolate at ambient conditions, whereas 1Me is not a good catalyst. The reaction of 2 with 2-butyne was monitored in some detail, which led to a mechanistic proposal for the cyclotrimerization at [Ni(NHC)2 ]. DFT calculations reveal that the differences between 1M e and 2 for alkyne cyclotrimerization lie in the energy profile of the initiation steps, which is very shallow for 2, and each step is associated with only a moderate energy change. The higher stability of 3 compared to 14 is attributed to a better electron transfer from the NHC to the metal to the alkyne ligand for the N-alkyl substituted NHC, to enhanced Ni-alkyne backbonding due to a smaller CNHC -Ni-CNHC bite angle, and to less steric repulsion of the smaller NHC i Pr2 ImMe .

Rhodium-Catalyzed Enantioselective Synthesis of Highly Fluorescent and CPL-Active Dispiroindeno[2,1-c]fluorenes.

The enantioselective synthesis of chiral [7]-helical dispirodihydro[2,1-c]indenofluorenes (DSF-IFs) was achieved for the first time in good yields with high er values (er up to 99 : 1). The crucial step of the whole reaction sequence was the enantioselective intramolecular [2+2+2] cycloaddition of tethered triynediols to indenofluorenediols, which was catalyzed by a Rh/SEGPHOS® complex. Further transformations led to the corresponding DSF-IFs. The prepared helically chiral DSF-IFs combine circularly polarized luminescence (CPL) activity (glum =∼10-3 ) with exceptionally high fluorescence quantum yields (up to Φlum =0.97).

Nematic Triphenyltriazine Triesters and the Induction of the Columnar Mesophase by Fluorine Substitution.

Whereas their para homologs are not mesogenic, the disk-shaped triphenyltriazine meta-trialkylesters obtained via trimerization of 3-cyanobenzoic alkylester, which are configurationally more flexible, exhibit a monotropic nematic mesophase. Introduction of fluorine atoms into the alkyl chains or into the phenyl moieties leads to the appearance of an enantiotropic columnar mesophase. If fluorine is introduced both in the chains and in the phenyl moieties, only a monotropic mesophase remains. Fluorination of either the alkyl chains or the aromatic core, but not both, appears thus as a simple means of inducing or stabilizing columnar self-assembly in disk-shaped systems. As the homeotropically alignable columnar mesophase can thus be made to persist at room temperature, as energies higher than 3 eV of the first excited triplet state are computed in agreement with the value reported for the parent arene, and as they are not fluorescent themselves, these compounds are of promise as aligning host matrices for blue-emitting TADF devices with improved light outcoupling. Dilution of a columnar with a nonmesogenic homolog induces the nematic state, indicating that the nanoscopic make-up of both mesophases is closely related.

Benzo-fused Tri[8]annulenes as Molecular Models of Cubic Graphite.

Cyclotrimerization of 9,10-dibromo-9,10-dihydrodibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrene with potassium tert-butoxide in the presence of a transition-metal catalyst afforded two polycyclic aromatic hydrocarbon stereoisomers consisting of three cyclooctatetraene (COT) moieties connected via a central benzene ring. Both isomeric tri[8]annulenes were obtained selectively through the choice of the catalyst: The α,α,α-form (Ru catalyst) displayed a threefold symmetrywith the COT subunits forming the side walls of a (chiral) molecular cup. In the thermodynamically more stable α,α,ß-isomer (Pd catalyst), one of the three boat-shaped COTs was flipped over and faced the opposite molecular hemisphere with respect to the central benzene ring as evidenced by crystal structure analysis. Both title compounds are small segments of "cubic graphite", an elusive carbon allotrope.

Reactions of a Dilithiomethane with CO and N2 O: An Avenue to an Anionic Ketene and a Hexafunctionalized Benzene.

Synthesis of value-added products from simple C1 feedstocks is an attractive alternative avenue to traditional fossil fuels. Hexa-substituted benzene derivatives are highly useful molecules but are often challenging to prepare. Herein, we report that the lithium complex [(Ph2 P(S))2 CLi2 (THF)]2 1 reacts with CO lead to C-C bond formation and migration of a Ph2 P(S)-fragment affording 2. Subsequent reaction with N2 O results in oxidative cleavage of a P-C bond affording [Ph2 P(S)OLi(THF)2 ]2 4 and the anionic ketene-derivative Ph2 P(S)CCOLi(THF)2 5. Heating 5 prompts cyclotrimerization giving the hexa-substituted benzene derivative [Ph2 P(S)CCOLi(THF)2 ]3 6 regioselectively. This transition metal-free protocol to a hexa-substituted benzene is viable on a gram scale and permits the incorporation of 13 C labels. The mechanisms of these reactions are detailed via extensive DFT computations.

Synthesis of π-Conjugated Benzocyclotrimers.

Polycyclic aromatic hydrocarbons (PAHs), especially three branchphene benzocyclotrimers represent a series of molecules with intriguing physical and chemical properties. Benzocyclotrimers are also important precursors to construct fullerenes and graphenes. In this article, we review the recent progress in the preparation methods of π-conjugated benzocyclotrimers. In particular, cyclotrimerization reactions to construct varying shaped and edged benzocyclotrimers are illustrated. Various typical characterization methods for these materials, such as variable-temperature 1 H-NMR, single crystal X-ray analysis, density functional theory (DFT) calculations and atomic force microscope (AFM) measurements are included for discussion.

Development of New Synthetic Strategies, Tactics and their Applications.

This account covers our work related to the development of various synthetic methodologies since 1994. We summarized our strategies and their application to design various functional molecules. In this regard, we also report the utility of our methodologies in others research work. These methods we report here are simple and efficient for the synthesis of a wide variety of intricate molecules such as heterocycles, polycycles, unusual α-amino acids, star-shaped molecules, and modified peptides. For this purpose, we used various transition metal-based reagents and catalysts. Various popular reactions such as metathesis, Suzuki coupling, [2+2+2] cyclotrimerization were used to assemble these targets. Moreover, rongalite has been used to expand the Diels-Alder chemistry.

Synthesis of Aromatic Rings Embedded in Polycyclic Scaffolds by Triyne Cycloaddition: Competition between Carbonylative and Non-Carbonylative Pathways.

Cycloadditions have emerged as some of the most useful reactions for the formation of polycyclic compounds. The carbonylative cycloaddition of triynes can lead to carbonylative and non-carbonylative competitive pathways, each leading to the formation of an aromatic ring. We report herein the one-pot synthesis of fully- and unsymmetrically-substituted tetracyclic 6,5,7,5-troponic and 6,5,6,5-benzenoid scaffolds using pre-organized triynes showing the competition between these two pathways.

Synthesis and Reactivity of an Early-Transition-Metal Alkynyl Cubane Mn4 C4 Cluster.

While the coordination chemistry of monometallic complexes and the surface properties of extended metal particles are well understood, the control of metal nanocluster formation has remained challenging. The isolation of discrete metal clusters provides an especially rare snapshot at the nanoscale of cluster growth. The synthesis and full characterization of the first early-transition-metal alkynyl cubane and the first µ3 -alkynyl Mn3 motif are reported.

Straightforward Synthesis and Properties of Highly Fluorescent [5]- and [7]-Helical Dispiroindeno[2,1-c]fluorenes.

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.

Convergent Total Syntheses of (-)-Rubriflordilactone†B and (-)-pseudo-Rubriflordilactone†B.

A highly convergent strategy for the synthesis of the natural product (-)-rubriflordilactone B, and the proposed structure of (-)-pseudo-rubriflordilactone B, is described. Late stage coupling of diynes containing the respective natural product FG rings with a common AB ring aldehyde precedes rhodium-catalyzed [2+2+2] alkyne cyclotrimerization to form the natural product skeleton, with the syntheses completed in just one further operation. This work resolves the uncertainty surrounding the identity of pseudo-rubriflordilactone B and provides a robust platform for further synthetic and biological investigations.