Synthesis of tertiary alkylphosphonate oligonucleotides through light-driven radical-polar crossover reactions.

Chemical modification of nucleotides can improve the metabolic stability and target specificity of oligonucleotide therapeutics, and alkylphosphonates have been employed as charge-neutral replacements for naturally-occurring phosphodiester backbones in these compounds. However, at present, the alkyl moieties that can be attached to phosphorus atoms in these compounds are limited to methyl groups or primary/secondary alkyls, and such alkylphosphonate moieties can degrade during oligonucleotide synthesis. The present work demonstrates the tertiary alkylation of the phosphorus atoms of phosphites bearing two 2'-deoxynuclosides. This process utilizes a carbocation generated via a light-driven radical-polar crossover mechanism. This protocol provides tertiary alkylphosphonate structures that are difficult to synthesize using existing methods. The conversion of these species to oligonucleotides having charge-neutral alkylphosphonate linkages through a phosphoramidite-based approach was also confirmed in this study.

Synthesis of Polycyclic Arenes Composed of Four-, Five-, Six-, and Eight-Membered Rings via an Unexpected Four-Membered Ring Formation Reaction.

The synthesis of polycyclic arenes composed of four-, five-, six-, and eight-membered rings via an unexpected four-membered ring formation reaction is reported. The carbonylation of an octalithiated tetraphenylene yielded a tricarbonylated arene containing a four-membered ring or a tetracarbonylated one, depending on the carbonyl reagents. The structures were determined by X-ray crystallography, and their electronic properties were examined by using absorption spectroscopy and cyclic voltammetry. The antiaromaticity of the four-, five-, and eight-membered rings of these compounds was studied by theoretical calculations.

Half-substituted fluorocycloparaphenylenes with high symmetry: synthesis, properties and derivatization to densely substituted carbon nanorings.

Fluorinated cycloparaphenylenes (FCPPs) have attracted attention as electron-accepting CPPs as well as strained fluoroarenes. Herein, we report the synthesis and properties of novel FCPPs; F16[8]CPP and F12[6]CPP. Furthermore, the derivatization of F16[8]CPP afforded a new carbon nanoring where sixteen pyrrole rings are densely substituted on the CPP framework.

Synthesis of penta- and hexa(3,4-thienylene): size-dependent structural properties of cyclic oligothiophenes.

Penta- and hexa(3,4-thienylene)s were synthesized as a potential precursor for thiophene-containing polyarenes, and the structures were determined via X-ray crystallography. The interconversion of thiophene rings is fast in penta(3,4-thienylene), and slow in hexa(3,4-thienylene) reflecting the activation energy for enantiomerization. Size-dependent bathochromic shifts were observed in UV-vis absorption spectra.

An Electron-Deficient CpE Iridium(III) Catalyst: Synthesis, Characterization, and Application to Ether-Directed C-H Amidation.

The synthesis, characterization, and catalytic performance of an iridium(III) catalyst with an electron-deficient cyclopentadienyl ligand ([CpE IrI2 ]2 ) are reported. The [CpE IrI2 ]2 catalyst was synthesized by complexation of a precursor of the CpE ligand with [Ir(cod)OAc]2 , followed by oxidation, desilylation, and removal of the COD ligand. The electron-deficient [CpE IrI2 ]2 catalyst enabled C-H amidation reactions assisted by a weakly coordinating ether directing group. Experimental mechanistic studies and DFT calculations suggested that the high catalytic performance of [CpE IrI2 ]2 is due to its electron-deficient nature, which accelerates both C-H activation and IrV -nitrenoid formation.

N-doped nonalternant aromatic belt via a six-fold annulative double N-arylation.

The design and synthesis of nitrogen (N)-doped molecular nanocarbons are of importance since N-doped nanocarbons have received significant attention in materials science. Herein, we report the synthesis and X-ray crystal structure of a nitrogen-inserted nonalternant aromatic belt. The palladium-catalyzed six-fold annulative double N-arylation provided an aromatic belt bearing six nitrogen atoms in one step from cyclo[6]paraphenylene-Z-ethenylene, the precursor of the (6,6)carbon nanobelt. The C 3i-symmetric structure of the aromatic belt in the solid state was revealed using X-ray crystallography. The multistep (electro)chemical oxidation behavior of the belt, which was facilitated by the six p-methoxyaniline moieties, was studied, and a stable dication species was successfully identified by X-ray crystallography. The present study not only shows the unique structure and properties of the N-doped nonalternant aromatic belt but also expands the scope of accessibility of synthetically difficult belt molecules by the conventional intramolecular contraction pathway.

Perfluorocycloparaphenylenes.

Perfluorinated aromatic compounds, the so-called perfluoroarenes, are widely used in materials science owing to their high electron affinity and characteristic intermolecular interactions. However, methods to synthesize highly strained perfluoroarenes are limited, which greatly limits their structural diversity. Herein, we report the synthesis and isolation of perfluorocycloparaphenylenes (PFCPPs) as a class of ring-shaped perfluoroarenes. Using macrocyclic nickel complexes, we succeeded in synthesizing PF[n]CPPs (n = 10, 12, 14, 16) in one-pot without noble metals. The molecular structures of PF[n]CPPs (n = 10, 12, 14) were determined by X-ray crystallography to confirm their tubular alignment. Photophysical and electrochemical measurements revealed that PF[n]CPPs (n = 10, 12, 14) exhibited wide HOMO-LUMO gaps, high reduction potentials, and strong phosphorescence at low temperature. PFCPPs are not only useful as electron-accepting organic materials but can also be used for accelerating the creation of topologically unique molecular nanocarbon materials.

A photochromic carbazolyl-imidazolyl radical complex.

A carbazole-incorporated photochromic radical complex is synthesized. The long-wavelength photosensitivity of the photochromic reaction of the molecule is enhanced up to ∼580 nm by substituting a triphenylamine group into the 3-position of the carbazole moiety. These photochromic reactions are investigated by subpicosecond-to-microsecond transient absorption measurements.

Stepwise Generation of Mono-, Di-, and Triply-Reduced Warped Nanographenes: Charge-Dependent Aromaticity, Surface Nonequivalence, Swing Distortion, and Metal Binding Sites.

The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π-surface and defined C80 H30 composition with Cs metal used as the reducing and complexing agent allowed the isolation of three different reduced states with one, two, and three electrons added to its π-conjugated system. This provided a unique series of nanosized carbanions with increasing negative charge for in-depth structural analysis of consequences of controlled electron charging of non-planar nanographenes, using X-ray crystallographic and computational tools. The 3D molecular electrostatic potential (MEP) maps identified the negative charge localization at the central part of the WNG surface where selective coordination of Cs+ ions is confirmed crystallographically. In-depth theoretical investigation revealed a complex response of the WNG to the stepwise electron acquisition. The extended and contorted π-surface of the WNG undergoes subtle swinging distortions that are accompanied by notable changes in the electronic structure and site-dependent aromaticity of the resulting carbanions.

Double-Helix Supramolecular Nanofibers Assembled from Negatively Curved Nanographenes.

The layered structures of graphite and related nanographene molecules play key roles in their physical and electronic functions. However, the stacking modes of negatively curved nanographenes remain unclear, owing to the lack of suitable nanographene molecules. Herein, we report the synthesis and one-dimensional supramolecular self-assembly of negatively curved nanographenes without any assembly-assisting substituents. This curved nanographene self-assembles in various organic solvents and acts as an efficient gelator. The formation of nanofibers was confirmed by microscopic measurements, and an unprecedented double-helix assembly by continuous π-π stacking was uncovered by three-dimensional electron crystallography. This work not only reports the discovery of an all-sp2-carbon supramolecular π-organogelator with negative curvature but also demonstrates the power of three-dimensional electron crystallography for the structural determination of submicrometer-sized molecular alignment.

Synthesis of a zigzag carbon nanobelt.

The structure-selective precise synthesis of carbon nanotubes (CNTs) has been long sought in materials science. The aromatic molecules corresponding to segment structures of CNTs, that is, carbon nanobelts (CNBs), are of interest as templates for CNT growth. Among the three types of CNB (armchair, chiral and zigzag CNBs), zigzag CNBs have been considered the most difficult type to synthesize. Here we report the synthesis, isolation and structural characterization of a zigzag CNB. The synthesis involves an iterative Diels-Alder reaction sequence followed by reductive aromatization of oxygen-bridged moieties. As predicted by theoretical calculations, this CNB was isolated as a stable compound. The structure of the zigzag CNB was fully characterized by X-ray crystallography and its wide energy gap with blue fluorescence properties was revealed by photophysical measurements. With synthetic strategies towards all three types of CNB in hand, the road to the precise synthesis of CNTs can now proceed to the next stage.

Exciton Spatial Dynamics and Self-Trapping in Carbon Nanocages.

Three-dimensional cage-shaped molecules formed from chainlike structures hold potential as unique optoelectronic materials and host compounds. Their optical, structural, and dynamical features are tunable by changes in shape and size. We perform a comparison of these properties for three sizes of strained conjugated [n.n.n]carbon nanocages composed of three paraphenylene chains (bridges) of length n = 4, 5, or 6. The exciton intramolecular redistribution occurring during nonradiative relaxation has been explored using nonadiabatic excited-state molecular dynamics. Our results provide atomistic insight into the conformational features associated with the observed red- and blue-shift trends in the absorption and fluorescence spectra, respectively, with increasing nanocage size. Their internal conversion processes involve intramolecular energy transfer that leads to exciton self-trapping on a few phenylene units at the center of a single bridge. The dependence of these dynamical features on the size of the nanocage can be used to tune their host-guest chemical properties and their use for organic electronics and catenane-like applications.

A theoretical study on the strain energy of helicene-containing carbon nanobelts.

In this study, the theoretical analysis of the strain energy of helicene-containing carbon nanobelts is reported. It was found that the combined method of linear regression analysis and suitable homodesmotic reactions can successfully estimate the strain energies of various helicene-containing carbon nanobelts including previously synthesized chiral (18,12) carbon nanobelts.

Synthesis of cycloiptycenes from carbon nanobelts.

The synthesis of each of the cycloiptycene derivatives was achieved in one step from the (6,6)carbon nanobelt. It was revealed that the carbon nanobelt reacted as a diene in the Diels-Alder reaction with arynes and alkynes. The structures of all products were identified by X-ray crystallography to confirm that the Diels-Alder reactions took place at the six central benzene rings of the carbon nanobelt. DFT calculations indicated that the release of strain energy is the driving force to promote the Diels-Alder reaction. By using this method, we have successfully synthesized cyclotetracosiptycene, the largest iptycene ever synthesized.

A Nonalternant Aromatic Belt: Methylene-Bridged [6]Cycloparaphenylene Synthesized from Pillar[6]arene.

The synthesis, structure, and properties of methylene-bridged [6]cycloparaphenylene ([6]CPP), a nonalternant aromatic belt, are described. This belt-shaped methylene-bridged [6]CPP, in which each phenylene unit is tethered to its neighbors by methylene bridges, was constructed through 6-fold intramolecular nickel-mediated aryl-aryl coupling of triflate-functionalized pillar[6]arene in 18% isolated yield. As compared to the analogous [6]CPP, the methylene bridges coplanarize neighboring paraphenylene units and enhance the degree of π-conjugation, which results in a significant decrease in energy gap. Moreover, the incorporation of small molecules in the defined pocket of methylene-bridged [6]CPP makes it an attractive supramolecular architecture. Methylene-bridged [6]CPP is characterized by high internal strain energy reaching 110.2 kcal mol-1, attributed to its restricted structure. This work not only exhibits an efficient strategy to construct a new family of aromatic belt, but also showcases their properties, which combine the merits of CPPs and pillararenes.

Synthetic Strategies of Carbon Nanobelts and Related Belt-Shaped Polycyclic Aromatic Hydrocarbons.

The development of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons has gained momentum in recent years. This Minireview focuses on the synthetic strategies used in constructing these aesthetically appealing molecular nanocarbons. Examples of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons reported in recent years as well as some representative synthetic attempts in earlier times are discussed.

Six-fold C-H borylation of hexa-peri-hexabenzocoronene.

Hexa-peri-hexabenzocoronene (HBC) is known to be a poorly soluble polycyclic aromatic hydrocarbon for which direct functionalization methods have been very limited. Herein, the synthesis of hexaborylated HBC from unsubstituted HBC is described. Iridium-catalyzed six-fold C-H borylation of HBC was successfully achieved by screening solvents. The crystal structure of hexaborylated HBC was confirmed via X-ray crystallography. Optoelectronic properties of the thus-obtained hexaborylated HBC were analyzed with the support of density functional theory calculations. The spectra revealed a bathochromic shift of absorption bands compared with unsubstituted HBC under the effect of the σ-donation of boryl groups.

Synthesis and Structure of [9]Cycloparaphenylene Catenane: An All-Benzene Catenane Consisting of Small Rings.

A catenane consisting of two [9]cycloparaphenylenes ([9]CPPs) has been synthesized. Density functional theory calculations suggested that [n]CPPs (n = 5, 6) are highly strained upon the formation of catenanes compared with the corresponding uncatenated CPPs, whereas [n]CPP catenanes (n ≥ 7) are not strained. The synthesis of ([9]CPP)([9]CPP)catenane was accomplished via the following route: (i) a spirosilylation, (ii) a nickel(0)-mediated macrocyclization, (iii) a desilylation, and (iv) reductive aromatization reactions. An X-ray diffraction analysis revealed a catenated structure of ([9]CPP)([9]CPP)catenane.

Two-step synthesis of a red-emissive warped nanographene derivative via a ten-fold C-H borylation.

The regioselective ten-fold borylation of warped nanographene (WNG: C80H30) was achieved by modifying the reaction conditions of a previously reported Ir-catalyzed C-H borylation, affording decaborylated WNG in high yield (75%) from pristine WNG. The solid-state structure of decaborylated WNG was confirmed by X-ray crystallography. Corresponding decaarylated WNGs containing electron-withdrawing and -donating groups were synthesized from decaborylated WNG using Suzuki-Miyaura cross-coupling reactions to afford the red-emissive warped nanographene derivative.

Ni-Catalyzed α-Selective C-H Borylations of Naphthalene-Based Aromatic Compounds.

The ability of α-borylated naphthalene-based aromatic compounds is important because it provides ready access to interesting novel extended π-systems. In this report, we disclose the Ni-catalyzed α-selective C-H borylations of naphthalene-based aromatic compounds. The reaction proceeds with peri-xanthenoxanthene and other aromatic compounds to regioselectively afford the α-borylated products without directing groups. The selectivities of these transformations are different from those of Ir-catalyzed C-H borylation reactions and can be used in a complementary manner.