Metal‐Catalyzed C−H Bond Activation of 5‐Membered Carbocyclic Rings: A Powerful Access to Azulene,Acenaphthylene and Fulvene Derivatives

Azulene, acenaphthylene and fulvene derivatives exhibit important physical properties useful in materials chemistry as well as valuable biological properties. Since about two decades ago, the metal‐catalyzed functionalization of such compounds, via C?H bond activation of their 5‐membered carbocyclic ring, proved to be a very convenient method for the synthesis of a wide variety of azulene, acenaphthylene and fulvene derivatives. For such reactions, there is no need to prefunctionalize the 5‐membered carbocyclic rings. In this review, the progress in the synthesis of azulene, acenaphthylene and fulvene derivatives via metal‐catalyzed C?H bond activation of their 5‐membered carbocyclic ring are summarized.     

Formation of Azulene‐Embedded Nanographene: Naphthalene to Azulene Rearrangement During the Scholl Reaction

Incorporation of a non‐hexagonal ring into a nanographene framework can lead to new electronic properties. During the attempted synthesis of naphthalene‐bridged double [6]helicene and heptagon‐containing nanographene by the Scholl reaction, an unexpected azulene‐embedded nanographene and its triflyloxylated product were obtained, as confirmed by X‐ray crystallographic analysis and 2D NMR spectroscopy. A 5/7/7/5 ring‐fused substructure containing two formal azulene units is formed, but only one of them shows an azulene‐like electronic structure. The formation of this unique structure is explained by arenium ion mediated 1,2‐phenyl migration and a naphthalene to azulene rearrangement reaction according to an in‐silico study. This report represents the first experimental example of the thermodynamically unfavorable naphthalene to azulene rearrangement and may lead to new azulene‐based molecular materials.     

Azulen als Brückenligand in Zweikernigen Paramagnetischen Nickelkomplexen — Synthese und Eigenschaften

Azulene as Bridging Ligand in Dinuclear Paramagnetic Complexes of Nickel — Synthesis and Spectroscopic Properties The reactivity of dianions of several alkyl substituted azulene derivates towards half sandwich complexes [(η⁵‐C₅R_1‐5)Ni (η²‐acetylacetonat)] (R = H, Me, Ethyl) has been studied and leads to the formation of paramagnetic dinickel complexes of the nonbenzenoid hydrocarbon azulene. From these studies it is found that the five membered ring system of the azulene ligand is coordinated in a nickelocene like fashion, the seven membered ring system in a halfopen metallocene type fashion. Therein the seven membered azulene ring unit is complexed in a pentadienyllike fashion. This corresponds to cyclovoltammetric studies in which a correlation with the potentials for alkylated nickelocene derivatives is found.     

Application of the ‘Direct Amide Cyclization’ to Peptides Containing an Anthranilic Acid Residue

The 2,2‐disubstituted 2H‐azirin‐3‐amines 7a – 7c were used as amino acid synthons to prepare linear peptides derived from anthranilic acid. These linear peptides, which contain α,α‐disubstituted α‐amino acids, were synthesized by using the ‘azirine/oxazolone method’ (Schemes 2–5) and were subjected to the acid‐catalyzed ‘direct amide cyclization’. Unfortunately, all attempts to isolate the ten‐membered cyclotripeptides, by starting with different precursors, failed. Instead of the expected cyclotripeptides, the corresponding 1,3a,7‐triazabenz[e]azulene‐3,6‐diones 12 and 17 (Schemes 2 and 3) were obtained in up to 44% yield. Most likely, these products were formed by a transannular ring contraction of the desired products, followed by dehydration. In the case of the linear precursor 24 , the 20‐membered cyclodimer 25 was formed in 50% yield, together with traces of the 1,3a,7‐triazabenz[e]azulene‐3,6‐dione 27 and the completely unexpected quinazolinone 26 (Scheme 5). In the cases of the linear precursors 30 and 34 , no ring closure was observed under the conditions of the ‘direct amide cyclization’, which indicates the limitations of this method.     

Cationic ring‐opening copolymerization behavior of trioxane and seven‐membered cyclic carbonate

Cationic ring‐opening copolymerization behavior of trioxane (TOX) and a seven‐membered cyclic carbonate, 1,3‐dioxepan‐2‐one (7CC) is described. When TOX and 7CC were cationically copolymerized under various feed ratios using trifluoromethane sulfonic acid (TfOH) as an initiator in nitrobenzene at 30 °C, 7CC was consumed faster than TOX and the decarboxylation was accompanied to afford the corresponding polyacetal–polycarbonate type copolymers containing poly(oxytetramethylene) units. The copolymer composition could be controlled by the feed ratio of 7CC, whose increase resulted in the high copolymer composition of the 7CC unit. The solubility of the copolymers increased as the increase of the 7CC content. Thermogravimetric, size‐exclusion chromatographic, and X‐ray analyses of the copolymers suggest that the sequences of the copolymer chains consist of the segments containing the units originated from 7CC and those with TOX unit‐rich compositions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 733–739, 2008     

Free radical ring‐opening polymerization of cyclic allylic sulfides: Liquid monomers with low polymerization volume shrinkage

The free radical polymerization of four methylated cyclic allylic sulfides was examined with reference to their polymerization volume shrinkage and the effect of ring size on reactivity. The compounds examined were 2‐methyl‐5‐methylene‐1,3‐dithiane ( 5 ) (solid), 2‐methyl‐6‐methylene‐1,4‐dithiepane ( 6 ) (liquid), 6‐methyl‐3‐methylene‐1,5‐dithiacyclooctane ( 7 ) (liquid), and 6,8‐dimethyl‐3‐methylene‐1,5‐dithiacyclooctane ( 8 ) (liquid). The monomers were stable materials not requiring any special handling or storage conditions. They were polymerized in bulk using thermal azobisisobutyronitrile (AIBN, VAZO88) and photochemical initiators (Ciba DAROCUR 1173) and in benzene solutions (AIBN, 70 °C). The six‐membered ring monomer 5 was unreactive whereas seven‐membered ring monomer 6 polymerized to high conversion in bulk. In addition, 6 did not polymerize in benzene solution at 70 °C at [ 6 ] = 1.25M. Eight‐membered ring monomers 7 and 8 polymerized in bulk to complete conversion with thermal and photochemical initiators to give lightly crosslinked materials. Near complete conversion to soluble polymers could be obtained in solution polymerizations in benzene. Soluble polymers were also obtained in photochemical initiated bulk polymerizations by lowering initiator concentrations or length of irradiation. The methyl substituent had no effect on which allylic carbon–sulfur bond fragmented in the ring‐opening step. The polymerization volume shrinkages of monomers 7 and 8 were 1.5 and 2.4% respectively and together with monomer 4 (1.5–2.0% shrinkage) are the best available liquid free radical ring‐opening monomers that can be polymerized in bulk at room temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 202–215, 2001     

NIR‐Absorbing π‐Extended Azulene: Non‐Alternant Isomer of Terrylene Bisimide

The first planar π‐extended azulene that retains aromaticity of odd‐membered rings was synthesized by [3+3] peri‐annulation of two naphthalene imides at both long‐edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π‐extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD‐DFT, as well as nucleus‐independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X‐ray crystallography confirmed the planarity of the reported π‐scaffolds and aromaticity of their azulene moiety.     

Modulating the Properties of Azulene‐containing Polymers Through Functionalization at the 2‐Position of Azulene

Poly(2‐arylazulene‐alt‐fluorene) and poly(2‐arylazulene‐alt‐thiophene) are synthesized via Suzuki and Stille cross‐coupling polymerization, respectively, using 1,3‐dibromo‐2‐arylazulenes as monomers, which are prepared by a novel directed C?H activation method of 2‐carboxylic azulene and subsequent bromination reaction. Our study shows that functionalization at the 2‐position of azulene monomers influences polymer properties. For instance, different from electron‐withdrawing groups that discourage the protonation of azulene, electron‐donating aryl groups, however, enhances the sensitivity of response to acid. Protonation of the polymers leads to significant shifts in absorption spectra accompanying with obvious color changes from green to brown in majority cases because of the formation of poly(azulenium cation). The electrochromic properties of polymers are examined, exhibiting that nature of aryl group at the 2‐position of azulene influences the stability of their electrochromic devices.     

Synthesis and polymerization of styrene monomers bearing spiroorthoester structure and volume change during crosslinking by double ring‐opening of the pendant spiroorthoesters of the obtained polymers

Novel styrene monomers bearing a five or seven‐membered spiroorthoester structure (SOE5, SOE7) were synthesized and their radical polymerizations as well as volume change during crosslinking of the obtained polymers were investigated. SOE5 and SOE7 were prepared from 4‐vinylbenzyl glycidyl ether and γ‐butyrolactone or ε‐caprolactone using boron trifluoride diethyl ether complex as a catalyst, respectively. Radical polymerizations of these monomers using 2,2′‐azobisisobutyronitrile (AIBN) gave the corresponding styrene‐based polymers with keeping the spiroorthoester structures unchanged. These polymers could be transformed to networked polymers by heating with a sulfonium antimonate, a thermally latent cationic polymerization initiator. Copolymerization of SOE5 or SOE7 with styrene at various compositions was carried out to efficiently obtain the corresponding copolymers, respectively. These polymers and copolymers showed little volume shrinkage or slight volume expansion during the crosslinking. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1790–1795     

Radical polymerization behavior of a vinyl monomer bearing five‐membered cyclic carbonate structure and reactions of the obtained polymers with amines

Radical polymerization behavior of a vinyl substituted cyclic carbonate, 4‐phenyl‐5‐vinyl‐1,3‐dioxoran‐2‐one ( 1 ), is described. Radical polymerization of 1 proceeded through selective vinyl polymerization to produce polymers bearing carbonate groups in the side chain, in contrast to that of an oxirane analogue of 1 , 1‐phenyl‐2‐vinyl oxirane that proceeds via the selective ring‐opening fashion. Although the homopolymerization of 1 produce polymers in relatively lower yield, copolymerizations effectively provided cyclic carbonate‐containing copolymers. Nucleophilic addition of primary amines to the resulting homopolymers and copolymers produced the corresponding multifunctional polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 584–592, 2005     

Mapping the characteristics of the radical ring‐opening polymerization of a cyclic ketene acetal towards the creation of a functionalized polyester

Radical ring‐opening polymerization of cyclic ketene acetals is a means to achieve novel types of aliphatic polyesters. 2‐methylene‐1,3‐dioxe‐5‐pene is a seven‐membered cyclic ketene acetal containing an unsaturation in the 5‐position in the ring structure. The double bond functionality enables further reactions subsequent to polymerization. The monomer 2‐methylene‐1,3‐dioxe‐5‐pene was synthesized and polymerized in bulk by free radical polymerization at different temperatures, to determine the structure of the products and propose a reaction mechanism. The reaction mechanism is dependent on the reaction temperature. At higher temperatures, ring‐opening takes place to a great extent followed by a new cyclization process to form the stable five‐membered cyclic ester 3‐vinyl‐1,4‐butyrolactone as the main reaction product. Thereby, propagation is suppressed and only small amounts of other oligomeric products are formed. At lower temperatures, the cyclic ester formation is reduced and oligomeric products containing both ring‐opened and ring‐retained repeating units are produced at higher yield. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4587–4601, 2009     

Application of Privileged Molecular Framework of 7‐Fluoro‐1,4‐benzodiazepin‐2‐one‐5‐methylcarboxylate to the Synthesis of Its 1‐ and 5‐Disubstituted Analogs of Medicinal Interest

Synthetic potential of the privileged molecular framework of 7‐fluoro‐1,4‐benzodiazepin‐2‐one containing a methyl carboxylate substituent at its 5‐position was exploited to develop efficient protocols to the synthesis of several novel 5‐(1′,3′,4′)oxadiazole ring incorporated analogs of medicinal interest, appended with the Mannich's base motifs at the nitrogen atom in its seven‐membered ring.     

(RS)‐6‐Ethyl 2‐carboxy‐1,2,3,4‐tetra­hydro­azulene‐6‐carboxylate

The title compound, C₁₄H₁₆O₄, was obtained during the synthesis of 2,6‐disubstituted azulene derivatives. In the partially reduced azulene skeleton, the absence of a H atom at the ester substitutent position of the seven‐membered ring, as well as lengthened double bonds, indicate a conjugative stabilized system with two overlaid tautomers.     

Mikrobielle Herstellung von 1, 3‐Propandiol. Fermentative Biotechnologie

The microbial production of 1,3‐propanediol is a success story for modern biotechnology. Once a specialty chemical, 1,3‐propanediol has risen to a bulk chemical within a few years. The interest in 1,3‐propanediol as a new commodity chemical is due to its use as a starting material for novel polymers with excellent physical and chemical properties. With the introduction of a new biotechnological production process, 1,3‐propanediol can be made at a competitive price from renewable resources with the aid of genetically modified E. coli bacteria. The development of the recombinant E. coli strain took more than 7 years and 36 genes had to be altered in order to enable the production of 1,3‐propanediol. In addition, the transformation of glycerol to 1,3‐propanediol could be the solution for the gycerol problem of biodiesel production.     

Naphthalimide‐Based Triptycenes: Synthesis and Optoelectronic Properties

In the past decades, the naphthalimide structure has found application in many areas of chemistry due to its unique photophysical properties. Naphthalimide has two isomers, 1,8‐naphthalimide containing a six‐membered imide ring and 2,3‐naphthalimide containing a five‐membered imide ring. The former has been widely investigated while studies on the latter are considerably more rare. On the other hand, naphthalimide can also be regarded as a building block to construct polycyclic aromatic hydrocarbons (PAHs), which have found wide applications in optical materials. Here we report the synthesis and optoelectronic properties of three 2,3‐naphthalimide‐based triptycenes. These three triptycene derivatives enrich the family of triptycenes.     

Exploring Strain‐Promoted 1,3‐Dipolar Cycloadditions of End Functionalized Polymers

Strain‐promoted 1,3‐dipolar cycloaddition of cyclooctynes with 1,3‐dipoles such as azides, nitrones, and nitrile oxides, are of interest for the functionalization of polymers. In this study, we have explored the use of a 4‐dibenzocyclooctynol (DIBO)‐containing chain transfer agent in reversible addition–fragmentation chain transfer polymerizations. The controlled radical polymerization resulted in well‐defined DIBO‐terminating polymers that could be modified by 1,3‐dipolar cycloadditions using nitrones, nitrile oxides, and azides having a hydrophilic moiety. The self‐assembly properties of the resulting block copolymers have been examined. The versatility of the methodology was further demonstrated by the controlled preparation of gold nanoparticles coated with the DIBO‐containing polymers to produce materials that can be further modified by strain‐promoted cycloadditions.     

Bio‐Based Polyketones by Selective Ring‐Opening Radical Polymerization of α‐Pinene‐Derived Pinocarvone

The most abundant naturally occurring terpene, α‐pinene, which cannot be directly polymerized into high polymers by any polymerization method, was quantitatively converted under visible‐light irradiation into pinocarvone, which possesses a reactive exo methylene group. The bicyclic vinyl ketone was quantitatively polymerized in fluoroalcohols by selective (99 %) ring‐opening radical polymerization of the four‐membered ring, which results in unique polymers containing chiral six‐membered rings with conjugated ketone units in the main chain. These polymers display good thermal properties, optical activities, and contain reactive conjugated ketone units. Reversible addition fragmentation chain transfer (RAFT) polymerization was successfully accomplished by using appropriate trithiocarbonate RAFT agents, enabling the synthesis of thermoplastic elastomers based on controlled macromolecular architectures.     

Two isomeric 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]hexahydro­isoindole‐1,3‐dione compounds

The molecular structures of 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]‐1,3,4,5,6,7‐hexahydro­isoindole‐1,3‐dione, C₁₇H₁₃ClFNO₃, (I), and the isomeric compound 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]‐cis‐1,3,3a,4,7,7a‐hexahydro­isoindole‐1,3‐dione, (II), are, as anticipated, significantly different in their conformations and in the distances between the farthest two atoms. The six‐membered ring of the 1,3,4,5,6,7‐hexahydro­isoindole‐1,3‐dione moiety in (I) adopts a half‐chair conformation. The dihedral angle between the five‐membered dione ring of (I) and the benzene ring is 50.96 (7)°. The six‐membered ring of the cis‐1,3,3a,4,7,7a‐hexahydro­isoindole‐1,3‐dione moiety in (II) adopts a boat conformation. The dihedral angle in (II) between the five‐membered dione ring and the benzene ring is 61.03 (13)°. In the crystal structures, the molecules are linked by C—H⋯O hydrogen bonds and weak π–π interactions. Compound (I) is a much more potent herbicide than (II). The Cl⋯H distances between the farthest two atoms in (I) and (II) are 11.37 and 9.97 Å, respectively.     

Polymerization of an epoxy‐containing silacyclobutane and its application to networked polymer synthesis

Polymerization of a silacyclobutane having an epoxy moiety and its application to networked polymer synthesis were examined. Four‐membered ring‐opening polymerization of silacyclobutane having a 3,4‐epoxybutyl group on the silicon atom (OBMSB) was achieved by using a platinum vinyldisiloxane complex with keeping the epoxy ring unchanged. Copolymerization of 1,1‐diethylsilacyclobutane (DESB) with OBMSB by using the same catalyst effectively gave the corresponding copolymers [poly(DESB‐co‐OBMSB)]. Thermal properties of the polyOBMSB, polyDESB, and poly(DESB‐co‐OBMSB) were investigated by DSC and TGA. Cast films of the obtained polymers with 1‐naphthylmethylmethyl‐p‐hydroxyphenylsulfonium hexafluoroantimonate, a small amount of thermally latent acid generator were prepared. Heating the films at 80 °C for 2 h gave crosslinked networked polycarbosilanes through cationic ring‐opening of the epoxy moieties. Thermal and mechanical properties of the networked polymers were investigated by TGA, DSC, and tensile strength measurements. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3400–3405     

Azulene‐Based Macrocyclic Receptors for Recognition and Sensing of Phosphate Anions

Herein we report the synthesis and detailed studies of the anion‐binding properties of two 20‐membered macrocyclic tetramide receptors: one symmetrical, containing two identical azulene‐based bisamide units, the other a hybrid, containing a dipicolinic bisamide unit and an azulene‐based bisamide unit. Analysis of the crystal structures of the macrocyclic receptors revealed their preference for adopting similar well‐preorganized bent‐sheet conformations, both as free receptors and in their complexes with anions. Studies of the optical properties of both receptors revealed abilities to selectively sense phosphate anions (H₂PO₄^?, HP₂O₇^3?), allowing for naked‐eye detection of the presence of these guests in DMSO. Binding studies in solution confirmed that the receptors bind strongly to a series of anions even in highly demanding media, such as mixtures of DMSO with water or with methanol. Comparison of the anion affinity of linear analogues with that of the macrocyclic receptors evidenced the importance of macrocyclic topology. Quantitative analysis revealed that the macrocyclic receptors are selective for H₂PO₄^? over other anions. The affinity to H₂PO₄^? seen for the symmetrical receptor, containing two azulene‐based subunits, is much higher than for the hybrid macrocycle containing both the azulene‐based and pyridine‐derived subunits. This highlights that the azulene‐based building block serves efficiently as both a binding site and a structure‐preorganizing motif.