Total Synthesis Confirms Laetirobin as a Formal Diels–Alder Adduct

Laetirobin, isolated from a parasitic fungus host–plant relationship, was synthesized in six practical steps with an overall yield of 12 % from commercially available 2,4‐dihydroxyacetophenone. Because the product is a pseudosymmetric tetramer of benzo[b]furans, each step of the synthesis was designed to involve tandem operations. Highlights include: 1) the double Sonogashira reaction of a bis(alkyne), 2) the practical copper(I)‐mediated formation of a bis(benzo[b]furan), and 3) the biomimetic [4+2] dimerization and unexpected cationic [5+2] annulation of gem‐diaryl alkene precursors. Preliminary structure–activity relationship data between the isomeric [4+2] and [5+2] tetramers revealed only the natural product to possess promising anticancer potential. Specifically, laetirobin is capable of blocking tumor cell division (mitosis) and invoking programmed cell death (apoptosis).     

碱性介质中分光光度法研究二羟基二（过碘酸）合银（III）配离子氧化一些二元醇的反应动力学及机理

The kinetics of oxidation of ethylene glycol and 1,3-butylene glycol by dihydroxydiperiodatoargentate（Ⅲ） in alkaline medium have been studied by spectrophotometry in the range of 298.2-318.2 K. It is shown that the reaction was first order with respect to each reductant and Ag（Ⅲ）, and kobs increased with an increase of [OH^-]. A plausible mechanism of reaction involving a pre-equilibrium of adduct formation between complex and reductants was proposed, which could be applied to explain all experimental phenomena, and the activation parameters of the ratedetermining step have been also calculated.     

Facile and Efficient Fabrication of Photoresponsive Microgels via Thiol–Michael Addition

A photoresponsive microgel is designed by the combination of a noncovalent assembly strategy with a covalent cross‐linking method. End‐functionalized poly(ethylene glycol) with azobenzene [(PEG‐(Azo)₂)] was mixed with acrylate‐modified β‐CD (β‐CD‐MAA) to form photoresponsive inclusion complex through host–guest interaction. The above photoresponsive complex was cross‐linked by thiol‐functionalized PEG (PEG‐dithiol) via Michael addition click reaction. The photoreversibility of resulted microgel was studied by TEM, UV–Vis spectroscopy, and ¹H NMR measurements. The characterization results indicated that the reversible size changes of the microgel could be achieved by alternative UV–Vis irradiations with good repeatability.     

Guest Binding via N−H⋅⋅⋅π Bonding and Kinetic Entrapment by an Inorganic Macrocycle

Modern supramolecular chemistry is overwhelmingly based on non‐covalent interactions involving organic architectures. However, the question of what happens when you depart from this area to the supramolecular chemistry of structures based on non‐carbon frameworks remains largely unanswered, and is an area that potentially provides new directions in molecular activation, host–guest chemistry, and biomimetic chemistry. In this work, we explore the unusual host–guest chemistry of the pentameric macrocycle [{P(μ‐N^tBu}₂NH]₅ with a range of anionic and neutral guests. The polar coordination site of this host promotes new modes of guest encapsulation via hydrogen bonding with the π systems of the unsaturated C≡C and C≡N bonds of acetylenes and nitriles as well as with the PCO^? anion. Halide guests can be kinetically locked within the structure by oxidation of the phosphorus periphery by oxidation to P^V. Our study underscores the future promise of p‐block macrocyclic chemistry.     

Theoretical prediction of the host–guest interactions between novel photoresponsive nanorings and C60: A strategy for facile encapsulation and release of fullerene

A series of photoresponsive‐group‐containing nanorings hosts with 12～14 Å in diameter is designed by introducing different number of azo groups as the structural composition units. And the host–guest interactions between fullerene C₆₀ and those nanoring hosts were investigated theoretically at M06‐2X/6‐31G(d)//M06‐L/MIDI! and wB97X‐D/6‐31G(d) levels. Analysis on geometrical characteristics and host–guest binding energies revealed that the designed nanoring molecule (labeled as 7 ) which is composed by seven azo groups and seven phenyls is the most feasible host for encapsulation of C₆₀ guest among all candidates. Moreover, inferring from the simulated UV‐vis‐NIR spectroscopy, the C₆₀ guest could be facilely released from the cavity of the host 7 via configuration transformation between trans‐form and cis‐form of the host under the 563 nm photoirradiation. Additionally, the frontier orbital features, weak interaction regions, infrared, and NMR spectra of the C₆₀@7 host–guest complex have also been investigated theoretically. © 2015 Wiley Periodicals, Inc.     

Ab initio X‐ray structural characterization of an inclusion compound with a compositionally disordered chiral guest: no prior knowledge of the crystal composition

The crystal structure and absolute configuration of a molecular host/guest/impurity inclusion complex were established unequivocally in spite of our having no prior knowledge of its chemical composition. The host (4R,5R)‐4,5‐bis(hydroxydiphenylmethyl)‐2,2‐dimethyl‐1,3‐dioxolane, (I), displays expected conformational features. The crystal‐disordered chiral guest 4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one, (II), is present in the crystal 85.1 (4)% of the time. It shares a common site with 4a‐hydroperoxymethyl‐4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one, (III), present 14.9 (4)% of the time, which is the product of autoxidation of (II). This minor peroxide impurity was isolated, and the results of nuclear magnetic resonance, mass spectrometry, and X‐ray fluorescence studies are consistent with the proposed structure of (III). The complete structure was therefore determined to be (4R,5R)‐4,5‐bis(hydroxydiphenylmethyl)‐2,2‐dimethyl‐1,3‐dioxolane–4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one–4a‐hydroperoxymethyl‐4,4a,5,6,7,8‐hexahydronaphthalen‐2(3H)‐one (1/0.85/0.15), C₃₁H₃₀O₄·0.85C₁₀H₁₄O·0.15C₁₀H₁₄O₃, (IV). There are host–host, host–guest, and host–impurity hydrogen‐bonding interactions of types S and D in the solid state. We believe that the crystals of (IV) were originally prepared to establish the chirality of the guest (II) by means of X‐ray diffraction analysis of host/guest crystals obtained in the course of chiral resolution during cocrystallization of (II) with (I). In spite of the absence of `heavy' elements, the absolute configurations of all anomeric centres in the structure are assigned as R based on resonant scattering effects.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.     

A Supramolecular Photosynthetic Model Made of a Multiporphyrinic Array Constructed around a C60 Core and a C60–Imidazole Derivative

The photophysical properties of a supramolecular fullerene–porphyrin ensemble resulting from the self‐assembly of a pyrrolidinofullerene–imidazole derivative ( F1 ) with a multimetalloporphyrin array constructed around a hexasubstituted fullerene core ( F(ZnP)₁₂ ) have been investigated. The fullerene hexa‐adduct core of the host system does not play any active role in the cascade of photoinduced events of the supramolecular ensemble, indeed no intercomponent photoinduced processes could be observed in host F(ZnP)₁₂ . In contrast, upon axial coordination with the monosubstituted fullerene guest F1 , a quantitative quenching of the fluorescence signal of the metalloporphyrins was observed for the supramolecular complex [F(ZnP)₁₂(F1) _n ] both in polar and nonpolar solvents. In toluene, the supramolecular ensemble exhibits a charge transfer emission centered around 930 nm, suggesting the occurrence of intramolecular face‐to‐face interactions of F1 with neighboring metalloporphyrin moieties within the self‐assembled photoactive array. This mechanism is supported by the fact that a one order of magnitude increase in the binding constant was observed for the supramolecular complex [F(ZnP)₁₂(F1) _n ] when compared with a reference system lacking the pyrrolidinofullerene unit. In benzonitrile, a long‐lived charge‐separated state (τ=0.3 μs) has been detected for the supramolecular adduct.     

Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self‐Assembled Calixarene–Salen Host

A calix[4]arene host equipped with two bis‐[Zn(salphen)] complexes self‐assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix–salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K_2,1 of 1.59×10¹¹ M ^?2 for the assembled host–guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host–guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.     

Programmable Polymer‐Based Supramolecular Temperature Sensor with a Memory Function

A new class of polymeric thermometers with a memory function is reported that is based on the supramolecular host–guest interactions of poly(N‐isopropylacrylamide) (PNIPAM) with side‐chain naphthalene guest moieties and the tetracationic macrocycle cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the host. This supramolecular thermometer exhibits a memory function for the thermal history of the solution, which arises from the large hysteresis of the thermoresponsive LCST phase transition (LCST=lower critical solution temperature). This hysteresis is based on the formation of a metastable soluble state that consists of the PNIPAM–CBPQT⁴⁺ host–guest complex. When heated above the transition temperature, the polymer collapses, and the host–guest interactions are disrupted, making the polymer more hydrophobic and less soluble in water. Aside from providing fundamental insights into the kinetic control of supramolecular assemblies, the developed thermometer with a memory function might find use in applications spanning the physical and biological sciences.     

A Reduced‐Symmetry Heterobimetallic [PdPtL4]4+ Cage: Assembly,Guest Binding,and Stimulus‐Induced Switching

A strategy is presented that enables the quantitative assembly of a heterobimetallic [PdPtL₄]⁴⁺ cage. The presence of two different metal ions (Pd^II and Pt^II) with differing labilities enables the cage to be opened and closed selectively at one end upon treatment with suitable stimuli. Combining an inert Pt^II tetrapyridylaldehyde complex with a suitably substituted pyridylamine and Pd^II ions led to the assembly of the cage. ¹H and DOSY NMR spectroscopy and ESI mass spectrometry data were consistent with the quantitative formation of the cage, and the heterobimetallic structure was confirmed using single‐crystal X‐ray crystallography. The structure of the host–guest adduct with a 2,6‐diaminoanthraquinone guest molecule was determined. Addition of N,N′‐dimethylaminopyridine (DMAP) resulted in the formation of the open‐cage [PtL₄]²⁺ compound and [Pd(DMAP)₄]²⁺ complex. This process could then be reversed, with the reformation of the cage, upon addition of p‐toluenesulfonic acid (TsOH).     

Complexation Between (O‐Methyl)6‐2,6‐Helic[6]arene and Tertiary Ammonium Salts: Acid/Base‐ or Chloride‐Ion‐Responsive Host–Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O ‐methyl)₆‐2,6‐helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base‐ and chloride‐ion‐responsive host–guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6‐helic[6]arene‐based [2]rotaxane was also synthesized from the condensation between the host–guest complex and isocyanate.     

Hydrogen-bonded network and enforced supramolecular cavities in molecular crystals: An orthogonal aromatic-triad strategy. Guest binding,molecular recognition,and molecular alignment properties of a bisresorcinol derivative of anthracene in the crystalline state

Abstract

Crystallization of an orthogonal resorcinol-anthracene-resorcinol compound 1a (host) from an ester solvent such as alkyl benzoate (guest) affords a 1:2 host-guest adduct 1a·2(ester). An essential aspect of the crystal structures of ethyl, propyl, and isobutyl benzoate adducts (space group, P2₁/n) and also that of methyl benzoate adduct (C2/c) is an extensive hydrogen-bonded network of host 1a, leading to a molecular sheet composed of hydrogen-bonded polyresorcinol chains and anthracene columns. This network generates well-defined, cyclophane-like supramolecular cavities, which incorporate two alkyl benzoate molecules in a highly selective manner via a combination of essential host-guest hydrogen-bonding and what may be called the cavity-packing effect. The selectivity factor between methyl benzoate (the lowest-affinity guest) and isobutyl benzoate (the highest-affinity guest) is 1:70 under competitive conditions. The actual geometry of the cavity is somehow dependent on and hence induced-fit adjustable to the guest structures by manipulating the intramolecular (anthracene-resorcinol dihedral angle) and intermolecular conformation (tilt angle between two hydrogen-bonded resorcinol rings) of compound 1a as well as the sheet-to-sheet distance. The adducts 1a·2(guest) can also be obtained by solid-state guest-exchange or guest-binding, respectively, using a preformed adduct or guest-free apohost dipped in an appropriate guest solvent. The methyl benzoate adducts obtained in these ways exhibit the same X-ray powder diffraction pattern as the genuine single-crystal obtained by direct crystallization of host 1a from methyl benzoate. Thus, even internal supramolecular cavities maintained by the hydrogen-bonded network are readily accessible to molecules in bulk solution. In addition, they undergo an induced-fit adjustment to a guest molecule newly added by the guest-exchange or the guest-binding process, during which the crystallinity is maintained. The potential use of symmetrical and divergent multiple hydrogen-bonding sites with an orthogonal aromatic spacer (orthogonal aromatic-triad strategy) is discussed in terms of a tool to construct a new class of porous organic crystals that show novel molecular recognition, crystalline-state guest-binding, and crystalline-phase molecular alignment properties.     

A Supramolecularly Activated Radical Cation for Accelerated Catalytic Oxidation

Tuning the activity of radicals is crucial for radical reactions and radical‐based materials. Herein, we report a supramolecular strategy to accelerate the Fenton reaction through the construction of supramolecularly activated radical cations. As a proof of the concept, cucurbit[7]uril (CB[7]) was introduced, through host–guest interactions, onto each side of a derivative of 1,4‐diketopyrrolo[3,4‐c]pyrrole (DPP), a model dye for Fenton oxidation. The DPP radical cation, the key intermediate in the oxidation process, was activated by the electrostatically negative carbonyl groups of CB[7]. The activation induced a drastic decrease in the apparent activation energy and greatly increased the reaction rate. This facile supramolecular strategy is a promising method for promoting radical reactions. It may also open up a new route for the catalytic oxidation of organic pollutants for water purification and widen the realm of supramolecular catalysis.     

Thermodynamics of Hydrophobic Interactions: Entropic Recognition of a Hydrophobic Moiety by Poly(Ethylene Oxide)–Zinc Porphyrin Conjugates

The recognition of 4‐alkylpyridines by water‐soluble poly(ethylene oxide)–zinc porphyrin conjugates was studied with a focus on the thermodynamic parameters of binding. Microcalorimetric studies indicated that binding of the alkyl group of the guest in water is driven by the entropic term (δΔH⁰=ΔH⁰(4‐pentylpyridine)? ΔH⁰(4‐methylpyridine)=+1.7 kJ mol^?1, δTΔS⁰=TΔS⁰(4‐pentylpyridine)? TΔS⁰(4‐methylpyridine)=+11.8 kJ mol^?1 at 298 K), thus showing the significance of water reorganization during host–guest interaction. The enthalpy–entropy compensation temperature of binding of 4‐alkylpyridines was as low as 38 K; only below this temperature could the enthalpic term be a driving force. The binding affinity was modulated by the addition of cations and by varying the degree of polymerization of poly(ethylene oxide), which suggests that guest binding is coupled with polymer conformation.     

Bright Fluorescence and Host–Guest Sensing with a Nanoscale M4L6 Tetrahedron Accessed by Self‐Assembly of Zinc–Imine Chelate Vertices and Perylene Bisimide Edges

A highly luminescent Zn₄L₆ tetrahedron is reported with 3.8 nm perylene bisimide edges and hexadentate Zn^II–imine chelate vertices. Replacing Fe^II and monoamines commonly utilized in subcomponent self‐assembly with Zn^II and tris(2‐aminoethyl)amine provides access to a metallosupramolecular host with the rare combination of structural integrity at concentrations <10^?7 mol L^?1 and an exceptionally high fluorescence quantum yield of Φ_em=0.67. Encapsulation of multiple perylene or coronene guest molecules is accompanied by strong luminescence quenching. We anticipate this self‐assembly strategy may be generalized to improve access to brightly fluorescent coordination cages tailored for host–guest light‐harvesting, photocatalysis, and sensing.     

Recognition Properties and Self‐assembly of Planar [M(2‐pyridyl‐1,2,3‐triazole)2]2+ Metallo‐ligands

Molecular recognition continues to be an area of keen interest for supramolecular chemists. The investigated [M( L )₂]²⁺ metallo‐ligands (M=Pd^II, Pt^II, L =2‐(1‐(pyridine‐4‐methyl)‐1 H‐1,2,3‐triazol‐4‐yl)pyridine) form a planar cationic panel with vacant pyridyl binding sites. They interact with planar neutral aromatic guests through π–π and/or metallophilic interactions. In some cases, the metallo‐ligands also interacted in the solid state with Ag^I either through coordination to the pendant pyridyl arms, or through metal–metal interactions, forming coordination polymers. We have therefore developed a system that reliably recognises a planar electron‐rich guest in solution and in the solid state, and shows the potential to link the resultant host–guest adducts into extended solid‐state structures. The facile synthesis and ready functionalisation of 2‐pyridyl‐1,2,3‐triazole ligands through copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) “click” chemistry should allow for ready tuning of the electronic properties of adducts formed from these systems.     

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a – d with the Lewis acidic species B(C₆F₅)₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a – d , which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a . Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a . Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d . The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C₆F₅)₃. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C₆F₅)₃ with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C₆F₅)₃. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct.     

Asymmetric Allylation of Ketones and Subsequent Tandem Reactions Catalyzed by a Novel Polymer‐Supported Titanium–BINOLate Complex

By using a novel, simple, and convenient synthetic route, enantiopure 6‐ethynyl‐BINOL (BINOL=1,1‐binaphthol) was synthesized and anchored to an azidomethylpolystyrene resin through a copper‐catalyzed alkyne–azide cycloaddition (CuAAC) reaction. The polystyrene (PS)‐supported BINOL ligand was converted into its diisopropoxytitanium derivative in situ and used as a heterogeneous catalyst in the asymmetric allylation of ketones. The catalyst showed good activity and excellent enantioselectivity, typically matching the results obtained in the corresponding homogeneous reaction. The allylation reaction mixture could be submitted to epoxidation by simple treatment with tert‐butyl hydroperoxide (TBHP), and the tandem asymmetric allylation epoxidation process led to a highly enantioenriched epoxy alcohol with two adjacent quaternary centers as a single diastereomer. A tandem asymmetric allylation/Pauson–Khand reaction was also performed, involving simple treatment of the allylation reaction mixture with Co₂(CO)₈/N‐methyl morpholine N‐oxide. This cascade process resulted in the formation of two diastereomeric tricyclic enones in high yields and enantioselectivities.     

Dendronized polystyrene via orthogonal double‐click reactions

Dendronized copolymers bearing two different dendrons as side chains have been synthesized using a modular orthogonal “double‐click” reaction based strategy. The orthogonality of the Huisgen‐type azide‐alkyne cycloaddition and the Diels–Alder reaction was utilized to attach different dendrons to the polymer backbone via the “graft‐to” strategy. First through third generations of polyaryl ether dendrons appended with an alkyne group and polyester dendrons possessing a furan‐protected maleimide group at their focal point were reacted with a styrene based copolymer containing azide and anthracene moieties as side chains. The efficiency and selectivity of the orthogonal dendronization of the copolymers were examined via various analytical methods such as ¹H NMR spectroscopy, FTIR and gel permeation chromatography. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 5029–5037