Base‐Selective Five‐ versus Six‐Membered Ring Annulation in Palladium‐Catalyzed C–C Coupling Cascade Reactions: New Access to Electron‐Poor Polycyclic Aromatic Dicarboximides

Palladium‐catalyzed base‐selective annulation of dibromonaphthalimide to different aryl boronate esters by combined Suzuki–Miyaura cross‐coupling and direct C−H arylation afforded a series of new five‐ and six‐membered ring annulated electron‐poor polycyclic aromatic hydrocarbons. Cesium carbonate (Cs₂CO₃) as auxiliary base in these C−C coupling cascade reactions led exclusively to six‐membered ring annulation, while the use of organic base diazabicycloundecene (DBU) afforded the corresponding five‐membered ring annulated products. This base‐dependent selective mode of annulation is attributed to different mechanistic pathways directed by the applied base. The selective annulation was revealed by single crystal X‐ray analysis of the respective five‐ and six‐membered ring annulated products. The optical and redox properties of the new polycyclic aromatic dicarboximides were characterized by UV/Vis absorption and fluorescence spectroscopy and cyclic voltammetry.     

Catalytic Double Carbon–Boron Bond Formation for the Synthesis of Cyclic Diarylborinic Acids as Versatile Building Blocks for π‐Extended Heteroarenes

The first catalytic synthesis of cyclic diarylborinic acids is developed using a dihydroaminoborane reagent as the boron source. Unlike previously reported methods that use organolithium reagents, this method allows the easy synthesis of cyclic diarylborinic acids bearing a range of functionalities including CN, CO₂Et, CONEt₂ and NMeCO₂^t Bu. Furthermore, these cyclic diarylborinic acids provide rapid access to skeletal diversity, in particular they enable the synthesis of six‐ to nine‐membered π‐extended heteroarenes through simple cross‐coupling reactions, which are important synthetic targets in both advanced materials and pharmaceuticals.     

Synthesis of Di‐, Tri‐, and Tetrasubstituted Oxetanes by Rhodium‐Catalyzed OH Insertion and CC Bond‐Forming Cyclization

Oxetanes offer exciting potential as structural motifs and intermediates in drug discovery and materials science. Here an efficient strategy for the synthesis of oxetane rings incorporating pendant functional groups is described. A wide variety of oxetane 2,2‐dicarboxylates were accessed in high yields, including functionalized 3‐/4‐aryl‐ and alkyl‐substituted oxetanes and fused oxetane bicycles. Enantioenriched alcohols provided enantioenriched oxetanes with complete retention of configuration. The oxetane products were further derivatized, while the ring was maintained intact, thus highlighting their potential as building blocks for medicinal chemistry.     

Synthesis of Di‐, Tri‐, and Tetrasubstituted Oxetanes by Rhodium‐Catalyzed O?H Insertion and C?C Bond‐Forming Cyclization

Oxetanes offer exciting potential as structural motifs and intermediates in drug discovery and materials science. Here an efficient strategy for the synthesis of oxetane rings incorporating pendant functional groups is described. A wide variety of oxetane 2,2‐dicarboxylates were accessed in high yields, including functionalized 3‐/4‐aryl‐ and alkyl‐substituted oxetanes and fused oxetane bicycles. Enantioenriched alcohols provided enantioenriched oxetanes with complete retention of configuration. The oxetane products were further derivatized, while the ring was maintained intact, thus highlighting their potential as building blocks for medicinal chemistry.     

Expected and unexpected reactions in ring‐opening (co)polymerization

This review covers most of the authors' work on ring‐opening polymerization and copolymerization of heterocyclic monomers during the time of their cooperation since 1985. The mechanistic aspects of anionic ring opening polymerization of cyclic carbonates with a variety of functional groups are described first. By sequential polymerization of first styrene, methyl methacrylate or suitable heterocyclic monomers and then secondly a cyclic carbonate, the site transformation is highlighted. The influence of the chemical nature of macroinitiators with identical active sites on the course of polymerization of cyclic carbonates was studied for poly(ethylene oxide), poly(tetrahydrofuran), and poly(dimethylsiloxane) macroinitiators. For the copolymerization of cyclic carbonates with lactones and lactide the dependence of the polymer microstructure on the polymerization conditions is discussed on the basis of the copolymerization mechanism. The copolymerization of cyclic carbonates with ε‐caprolactam and with tetramethylene urea results in an alternating copolymer, i. e. a poly(ester urethane) and an [m, n]‐polyurethane, respectively, the key step being the insertion of the lactam or the cyclic urea into the carbonate chain. The cationic ring opening polymerization of cyclic six and seven membered carbamates leading to [4]‐ and [5]‐polyurethane with uniform microstructure is reported with respect to kinetic, mechanistic, and thermodynamic aspects. This new access to [n]‐polyurethanes by a chain growth reaction allows the synthesis of well defined polymer architectures with polyurethane sequences. Sequential polymerization of tetrahydrofuran and the cyclic carbamate with mono‐ and bifunctional initiators leads to the respective A–B and B–A–B block copolymers. Site transformation from the oxonium to the immonium active species is the key step in the polymerization mechanism. Finally, mechanistic aspects of the ring‐opening polymerization of cyclic ester‐amides are presented.     

Polyaddition of bis(seven‐membered cyclic carbonate) with diamines: A novel and efficient synthetic method for polyhydroxyurethanes

This article deals with the polyaddition of a novel bis(seven‐membered cyclic carbonate), 1,2‐bis[3‐(1,3‐dioxepan‐2‐one‐5‐yl)‐propylthio]ethane, with the diamines 4,9‐dioxa‐1,12‐dodecanediamine and p‐xylylenediamine. The polyaddition was carried out at 30–70 °C for 6–24 h in dimethyl sulfoxide to obtain the corresponding polyhydroxyurethanes with number‐average molecular weights of 10,900–35,700 in good yields. The reaction of a monofunctional seven‐membered cyclic carbonate, 5‐allyl‐1,3‐dioxepan‐2‐one (7CC), with monoamines was also carried out to examine the reactivity in comparison with that of six‐ and five‐membered cyclic carbonates. The reaction rate constants of 7CC with n‐hexylamine and benzylamine were estimated to be 48.5 and 11.0 L/mol · h, respectively, in dimethyl sulfoxide‐d₆ (initial reagent concentration = 1 M) at 30 °C. The seven‐membered cyclic carbonate ring was 2.98 and 5.82 kcal/mol more strained than those of the six‐ and five‐membered cyclic carbonates, respectively, according to a semiempirical molecular orbital calculation with the PM3 Hamiltonian. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4091–4100, 2001     

Siloxy Alkynes in Annulation Reactions

Siloxy alkynes are a family of versatile species in organic synthesis. This account reviews the annulation reactions of siloxy alkynes for the synthesis of a variety of carbo‐ and heterocyclic products. With various dipolarophiles or dipolarophile‐like reaction partners, siloxy alkynes are capable of forming small (three‐ to six‐membered) rings. Recently, we have expanded the scope to the synthesis of medium‐ and large‐ring lactones, enabled by the design of new amphoteric molecules as well as a new ring‐expansion strategy. These annulation reactions provide not only practically useful syntheses of cyclic molecules, but also important understanding of the fundamental reactivity of siloxy alkynes.     

Tunable poly(hydroxyl urethane) from CO2‐Based intermediates using thiol‐ene chemistry

CO₂‐based, crosslinked poly(hydroxyl urethane)s (PHUs) are accessed via a set of efficient reactions based on the addition chemistry of thiol‐ene and amines‐cyclic carbonates. This strategy to utilize 5‐membered cyclic carbonates produced from CO₂ is robust, facile, modular, and atomically efficient in nature. The thiol‐ene reaction was utilized to access bis(cyclic carbonate), tris(cyclic carbonate), and tetrakis(cyclic carbonate) in quantitative yield from 4‐vinyl‐1,3‐dioxolan‐2‐one and thiols. Multi‐functional cyclic carbonates were simply mixed with diethylenetriamine and/or 1,6‐diaminohexane to generate crosslinked PHUs from 25 to 80 °C. These materials are easy to scale‐up and are potential candidates in many applications such as coatings, binders, and resins. The resulting polymers have glass transition temperatures between ?1 and 16 °C and thermal decomposition temperatures from 190 to 230 °C. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Efficient DMF‐Catalyzed Coupling of Epoxides with CO2 under Solvent‐Free Conditions to Afford Cyclic Carbonates

To develop a simple, low‐molecule, and cost‐effective organocatalyst for the coupling of epoxides with CO₂, we have screened this coupling reaction in different organic solvents and found that DMF is an efficient organic catalyst for the coupling of epoxides with CO₂ to give cyclic carbonates in high yield. In some cases, the catalytic activity of DMF can be significantly increased by the addition of catalytic amount of H₂O.     

Bifunctional Boron Phosphate as an Efficient Catalyst for Epoxide Activation to Synthesize Cyclic Carbonates with CO2

Development of inexpensive, easily prepared, non‐toxic, and efficient catalysts for the cycloaddition of CO₂ with epoxides to synthesize five‐membered cyclic carbonates is a very attractive topic in the field of CO₂ transformation. In this work, we conducted the first work on the cycloaddition of CO₂ with epoxides to produce cyclic carbonates catalyzed by a binary catalyst system consisting of KI and boron phosphate (BPO₄), which are both inexpensive and non‐toxic, and various corresponding cyclic carbonates could be produced with high yields (93–99 %) at 110 °C with a CO₂ pressure of 4 MPa under solvent‐free conditions. In the BPO₄/KI catalyst system, BPO₄, a Brønsted and Lewis acid hybrid, played the role of activating the epoxy ring through the formation of hydrogen bonds with Brønsted acidic sites and the interaction with Lewis acidic sites simultaneously, and thus enhanced the activity of KI for the cycloaddition of CO₂ with epoxides significantly. Additionally, the activity of the BPO₄/KI catalyst system showed no noticeable decrease after being reused five times, indicating that the BPO₄ was stable under the reaction conditions.     

Unusual Skeletal Reorganization of Oxetanes for the Synthesis of 1,2‐Dihydroquinolines

Skeletal reorganization is a type of fascinating transformations owing to their intriguing mechanisms and utility in complex molecule synthesis. However, only a limited amount of examples are known for most functional groups. Herein, we describe such an unusual process of oxetanes. In the presence of In(OTf)₃ as catalyst, oxetane‐tethered anilines reacted unexpectedly to form 1,2‐dihydroquinolines. This process not only provides expedient access to dihydroquinolines, but also represents a new reaction of oxetane. Mechanistically, it is believed that the reaction proceeds through initial nitrogen attack rather than arene attack followed by a series of bond cleavage and formation events. Control experiments provided important insights into the mechanism.     

One‐pot carboxylative cyclization of propargylic alcohols and CO2 catalysed by N‐heterocyclic carbene/Ag systems

A series of N ‐heterocyclic carbene (NHC)/Ag systems were developed for the carboxylative assembly of propargylic alcohols and carbon dioxide (CO₂). With the catalysis of these catalytic systems, a variety of target α‐alkylidene cyclic carbonates could be obtained smoothly under atmospheric CO₂ pressure in straightforward one‐pot processes. Particularly, these reactions could be performed without any stoichiometric addition of bases or additives. Further mechanistic investigation reveals that the excellent activities are attributed to the effective activations of CO₂ accomplished by the NHCs via the formation of the NHC‐CO₂ adducts.     

Direct Synthetic Processes for Cyclic Carbonates from Olefins and CO<Subscript>2</Subscript>

This short review presents the recent developments in the direct synthesis of cyclic carbonates from olefins and CO₂. The straightforward synthesis of cyclic carbonates from olefins instead of epoxides, also called one-pot “oxidative carboxylation” of olefins, can be viewed as the coupling of two sequential reactions of epoxidation of olefins and CO₂ cycloaddition to epoxides formed. The facile synthetic approach would make carbonate synthesis simpler and even cheaper with industrial potential from environmental and economic points of view. Some progresses have been made on this direct synthetic reaction for cyclic carbonates with homogeneous and heterogeneous catalysts, however, this reaction system is still at a preliminary stage. Among the catalysts reported, only a few can be considered as effective for the direct oxidative carboxylation of olefins to cyclic carbonates. Thus active and selective catalysts should be explored to put the direct synthesis of cyclic carbonates into practical applications.     

Carbon Dioxide as a Protecting Group: Highly Efficient and Selective Catalytic Access to Cyclic cis‐Diol Scaffolds

The efficient and highly selective formation of a wide range of (hetero)cyclic cis‐diol scaffolds using aminotriphenolate‐based metal catalysts is reported. The key intermediates are cyclic carbonates, which are obtained in high yield and with high levels of diastereo‐ and chemoselectivity from the parent oxirane precursors and carbon dioxide. Deprotection of the carbonate structures affords synthetically useful cis‐diol scaffolds with different ring sizes that incorporate various functional groups. This atom‐efficient method allows the simple construction of diol synthons using inexpensive and accessible precursors and green metal catalysts and showcases the use of CO₂ as a temporary protecting group.     

Fragmentation pathways of protonated coumarin by ESI‐QE‐Orbitrap‐MS/MS coupled with DFT calculations

Coumarin is one of the basic structures of naturally oxygen heterocyclic compound, which was investigated in this paper for its gas‐phase fragmentation behaviors using electrospray quadrupole extractive orbitrap mass spectrometry in the positive mode. The possible fragmentation pathways were proposed based on electrospray ionization (ESI)‐ mass spectrometry (MS)/MS data and theory calculation. The elimination of two CO and CO₂ was observed for protonated coumarin, which was followed by the formation of a stabilized seven‐, six‐, and five‐membered ring carbocation by loss of C2H2. The possible protonation sites occurred at Oxygen 11 atom of coumarin were the main fragmentation pathways. The relative abundance of characteristic fragment ions and the energy‐resolved breakdown curves were used to confirm the cleavage mechanism of protonated coumarin. The methodology and results of present work would contribute to the chemical structure identification of other coumarins.     

Synthesis and cationic ring‐opening polymerization of oxetane monomer containing five‐membered cyclic carbonate moiety via highly chemoselective addition of CO2

The bicyclic amidinium iodide effectively catalyzed the reaction of carbon dioxide and the epoxy‐containing oxetane under ordinary pressure and mild conditions with high chemoselectivity to give the corresponding oxetane monomer containing five‐membered cyclic carbonate quantitatively. The cationic ring‐opening polymerization of the obtained monomer by boron trifluoride diethyl ether proceeded to give linear polyoxetane bearing five‐membered cyclic carbonate pendant group in high yield. The molecular weight of the polyoxetane was higher than that of polyepoxide obtained by the cationic ring‐opening polymerization of epoxide monomer containing five‐membered cyclic carbonate. The cyclic carbonate functional crosslinked polyoxetanes were also synthesized by the cationic ring‐opening copolymerization of cyclic carbonate having oxetane and commercially available bisoxetane monomers. Analyses of the resulting polyoxetanes were performed by proton nuclear magnetic resonance, size exclusion chromatography, thermogravimetric analysis, and differential scanning calorimetry. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2606–2615     

Formation of nitrogen‐containing heterocycles using di(imidazole‐1‐yl)methanimine

A mild and efficient synthesis of five‐ and six‐membered nitrogen containing heterocyclic compounds, in which di(imidazole‐1‐yl)methanimine serves as a one‐carbon source, is reported.     

Total Synthesis of 1‐Hydroxytaxinine

1‐Hydroxytaxinine ( 1 ) is a cytotoxic taxane diterpenoid. Its central eight‐membered B‐ring possesses four oxygen‐functionalized centers (C1, C2, C9, and C10) and two quaternary carbon centers (C8 and C15), and is fused with six‐membered A‐ and C‐rings. The densely functionalized and intricately fused structure of 1 makes it a highly challenging synthetic target. Reported here is an efficient radical‐based strategy for assembling 1 from A‐ and C‐ring fragments. The A‐ring bearing an α‐alkoxyacyl telluride moiety underwent intermolecular coupling with the C‐ring fragment by a Et₃B/O₂‐promoted decarbonylative radical formation. After construction of the C8‐quaternary stereocenter, a pinacol coupling reaction using a low‐valent titanium reagent formed the B‐ring with stereoselective installation of the C1,C2‐diol. Subsequent manipulations at the A‐ and C‐rings furnished 1 in 26 total steps.     

Reactivity comparison of five‐ and six‐membered cyclic carbonates with amines: Basic evaluation for synthesis of poly(hydroxyurethane)

The reaction of six‐ and five‐membered cyclic carbonates, 5‐(2‐propenyl)‐1,3‐dioxan‐2‐one ( 1 ) and 4‐(3‐butenyl)‐1,3‐dioxolan‐2‐one ( 2 ) with hexylamine and benzylamine was carried out in N,N‐dimethylacetamide at 30, 50, and 70 °C. The six‐membered cyclic carbonate 1 proceeded quantitatively with hexylamine at 30 °C for 24 h, while the five‐membered cyclic carbonate 2 converted in 34%. The reaction rate constants at 50 °C are evaluated as follows; 1.42 L/mol · h ( 1 with hexylamine) > 0.29 L/mol · h ( 1 with benzylamine) > 0.04 L/mol · h ( 2 with hexylamine) > 0.01 L/mol · h ( 2 with benzylamine). The activation energies in the reactions of 1 and 2 with hexylamine were estimated to be 10.1 and 24.6 kJ/mol, respectively. The ring‐strain energy was calculated by the semi‐empirical method using the PM3 Hamiltonian. The ring‐strain energy of the six‐membered cyclic carbonate was 2.86 kcal/mol larger than that of five‐membered one. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 162–168, 2001     

Beryllepin,C6H6Be,and “Beryllium‐Inserted Benzenes,” C6H6Ben,n = 2–6: A Density Functional Computational Investigation

The seven‐membered beryllium‐containing heterocycle beryllepin, C₆H₆Be, has been examined computationally at the B3LYP/6‐311++G** density functional level of theory. Beryllepin is best described as a planar singlet heterocyclic conjugated triene with marginal aromatic character containing a C–Be–C moiety forced to be nonlinear (∠C‐Be‐C = 146.25°) by the cyclic constraints of the seven‐membered ring. The molecule can be considered to be derived from a benzene‐like system in which a neutral beryllium atom has been inserted between two adjacent carbon atoms. The 11 other possible “beryllium‐inserted benzenes,” C₆H₆Be_n, n = 2–6, have also been investigated. Only two of these heterocyclic systems, the eight‐membered 1,4‐diberyllocin and the nine‐membered 1,4,7‐triberyllonin, were found to be stable, singlet‐ground‐state systems, albeit with little aromatic character. Of the remaining nine beryllium‐inserted benzenes, with the exception of the 11‐membered ring containing five beryllium atoms and the 12‐membered ring containing six beryllium atoms, which were calculated to exist as a ground state pentet and septet, respectively, all were calculated to be ground state triplet systems.