Synthesis and Structure Revision of Goupiolone A: A Benzotropolone Natural Product

Goupiolone A, a benzotropolone natural product, has been synthesized by assembling a benzocyclobutene derivative and a silyl‐substituted cyclopropane unit, followed by thermal ring enlargement. The synthetic sample did not correspond to the reported data. On the basis of biogenetic considerations, an alternative structure with a catechol moiety was proposed, and the synthesis established it as the correct structure.     

Covalent stabilization of nanostructures: Robust block copolymer templates from novel thermoreactive systems

Structurally robust block copolymer templates with feature sizes of approximately 10 nm were prepared from functionalized poly(methyl methacrylate)‐b‐polystyrene block copolymers. By the inclusion of benzocyclobutene crosslinking groups in the polystyrene block, the covalent stabilization of thin films to both thermal treatment and solvent exposure became possible. In addition, the crosslinking of the poly(styrene‐benzocyclobutene) domains at 220 °C, followed by the removal of poly(methyl methacrylate), provided a robust, crosslinked nanostructure with greater processing and fabrication potential. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1028–1037, 2005     

Synthesis of poly(silmethylene)s via ring‐opening polymerization of benzocyclobutene functionalized disilacyclobutene and their low‐dielectric and thermal properties

In this work, benzocyclobutene‐introduced poly(silmethylene)s were synthesized by ring‐opening copolymerization of benzocyclobutene (BCB) functionalized disilacyclobutene with phenyldisilacyclobutene. The incorporation ratio of BCB was tailored by changing feeding ratio of 1,3‐dimethyl‐1,3‐dibenzocyclobutene‐1,3‐disilacyclobutane and 1,3‐dimethyl‐1,3‐diphenyl‐1,3‐disilacyclobutane, thus enabling the control of ultimate properties of cured resins. With increasing the incorporation ratios of BCB from 9.5 to 28.9 mol%, T5% of cured resins is improved from 404°C to 462°C, while the dielectric constant is decreased from 2.59 to 2.41 at 10 MHz. The excellent thermal stability and low‐dielectric performance make such thermosets potential inter‐layered or inter‐lined low‐dielectric media. Copyright © 2017 John Wiley & Sons, Ltd.     

Benzocyclobutenes: A New Class of High Performance Polymers

Benzocyclobutenes are a family of thermally polymerizable monomers which can be classified into two groups: 1) monomers which contain only benzocyclobutene moieties and 2) monomers which contain sites of unsaturation in addition to benzocyclobutene moieties. The monomers can be partially polymerized (B-staged) by heating to form oligomers having processing advantages for various composite fabrication techniques. The polymerization proceeds through the thermally initiated cyclobutene ring opening to yield an o-quinodimethane intermediate (calculated to be a ground state singlet). Preliminary characterization of the network structures indicates that monomers which contained multiple benzocyclobutene moieties, optionally with sites of unsaturation, were transformed into multifunctional network junctions when the thermosets were fully cured. The 3-maleimidobenzocyclobutenes thermally polymerize to yield substantially linear, high glass transition temperature (T_g) polymers. Thus benzocyclobutene polymers encompass materials which have properties ranging from high T_g, thermosets to those of substantially linear thermoplastics. Some polymers exhibit an excellent retention of their room temperature mechanical properties to at least 200-250°C, making them useful as high performance polymers for applications in the aerospace industry. Other polymers have outstanding electrical properties including very low dielectric constant and water pickup, making them useful in electronic applications.     

An Mo theoretical study on the reaction of benzocyclobutene with ethylene. Concerted vs stepwise

In an attempt to determine the reaction mechanism of the Diels-Alder type cycloaddition reaction of benzocyclobutene with dienophiles, the stabilities for the assumed intermediate structures were examined by using MINDO/3, STO-3G, and 4-31G methods. The potential energies of the ring-opening reaction of the benzocyclobutene and cycloaddition reaction of quinodimethane with a dienophile were obtained by MINDO/3 and discussed in relation to the controversial reaction mechanism of the cycloaddition, concerted vs stepwise mechanisms. The results lead to a conclusion that the reaction involves a biradical intermediate followed by a stepwise cycloaddition.     

Easy and regioselective synthesis of highly functionalized o-quinodimethide precursors from Fischer carbene complexes and isocyanides

Highly functionalized cis- and trans-1,2-bis(trialkylsilyloxy)benzocyclobutene derivatives have been synthesized in a completely regioselective fashion from easily available Fischer alkenylcyclobutenyl(alkoxy)carbene complexes and isocyanides. This methodology can be viewed as an alternative to existing methods of synthesis of this kind of compounds and overcomes the main problem of introducing functionality in the aromatic ring. Both cis and trans derivatives underwent the expected fragmentation to generate o-quinodimethide intermediates upon thermal treatment, affording complex polycyclic structures in a stereoselective manner when heating was carried out in the presence of a range of electron deficient dienophiles.     

Synthesis of polymethacrylate‐bearing benzocyclobutene structure and extension to networked polymer based on thermal isomerization

1‐Benzocyclobutenyl methacrylate‐bearing methacrylate (BCBMA) backbone has been synthesized, and radical polymerization of the monomer was performed by utilizing 2, 2′‐azobisisobutyronitrile (AIBN) as initiator to result poly‐BCBMA. Differential scanning calorimetry (DSC) measurement of the derived poly‐BCBMA revealed the lowering of thermal isomerization temperature from that of nonsubstituted benzocyclobutene. The thermal decomposition temperature of BCBMA before and after thermal treatment was confirmed by thermogravimetric analysis (TGA). The results of the TGA observation did not show significant difference in both 5% and 10% weight loss temperature (T_d5 and T_d10). This result suggests that the thermal conversion of the poly‐BCBMA to the networked polymer take place without thermal decomposition of the main chain based on the methacrylate framework. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2175–2180     

New polymers derived from 4‐vinylsilylbenzocyclobutene monomer with good thermal stability,excellent film‐forming property,and low‐dielectric constant

A series of benzocyclobutene (BCB) polymers derived from a new readily available monomer, 4‐(1′,1′‐dimethyl‐1′‐vinyl) silylbenzocyclobutene (4‐DMVSBCB), were conveniently prepared by radical and anionic polymerization. The homo‐ and co‐polymerization results show that the reactivity of 4‐DMVSBCB in anionic polymerization is relatively higher compared with radical polymerization. The molecular weight of 4‐DMVSBCB polymers and content of 4‐DMVSBCB can be controlled by anionic copolymerization. The introduction of rigid and crosslinkable BCB building blocks in side chains and carbosilanes in molecule gives rise to insulating materials with good film‐forming property, smooth and flat film surface, and low‐dielectric constants of 2.41–2.45, as preserving good thermal stability. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthetic approach to grayanotoxins and asebotoxins: a new construction of the a-homograyanotoxane ring system

The tetracyclic compounds ～3 and ～4 corresponding to A-homograyanotoxane system were synthesised by a thermolysis of the benzocyclobutene ～8, followed by a Wagner-Meerwein rearrangement of the kaurane type of compounds ～15 and ～16.     

Synthesis and Study of the Thermal and Chiro-Optical Properties of Polyacetylenes with Bulky Side Groups: Poly(1-ethynyl-4-biphenyl), Poly(1-ethynyl-4- phenoxybenzene) and Poly(1-ethynyl-4-pentylbenzene)

The monosubstituted acetylenes 1-ethynyl-4-biphenyl, 1-ethynyl-4-phenoxybenzene and 1-ethynyl-4-pentylbenzene were polymerized to the corresponding polyacetylenes with bulky side substituents: PEBIP, PEPHENO and PEPB, respectively, using a Rh(I) complex as polymerization catalyst. The FT-IR and UV-VIS spectra of PEBIP, PEPHENO and PEPB were studied using the corresponding spectra of polyphenylacetylene (PPA), polytolylacetylene (PETOL) and poly(1-ethynyl-naphthalene) PEN as reference. Based on the spectral analysis, high cis-trasoid structures were assigned to PEBIP, PEPHENO and PEPB. The specific optical rotation of PEBIP, PEPHENO and PEPB was measured in the chiral solvent α (-)pinene and, only for PEPB, also in α (+)pinene. It was found that even at low concentration PEBIP, PEPHENO and PEPB cause changes in the specific optical rotation of the chiral medium where they have been dissolved. This phenomenon has not been observed when toluene or atactic polystyrene are dissolved in the same chiral solvent in similar concentration. This suggests that the polyacetylenes with bulky side groups are able to rearrange in a helical configuration (prevalently right-handed or vice-versa, depending on the type of chiral medium used) once hosted in a chiral medium giving a contribution to the chirality of the solvent where they have been dissolved. The thermal stability of PEBIP, PEPHENO and PEPB was studied under N₂ flow by thermal analysis (TGA, DTG and DTA) in comparison to reference monosubstituted polyacetylenes PPA, PETOL and PEN. The results show that the thermal stability of monosubstituted polyacetylenes is greatly improved by the presence of bulky substitutents as theoretically expected, and similarly, also the char-forming tendency increases with the bulkiness of the side groups.     

Polymeric Carbon Nitride with Chirality Inherited from Supramolecular Assemblies

The facile synthesis of chiral materials is of paramount importance for various applications. Supramolecular preorganization of monomers for thermal polymerization has been proven as an effective tool to synthesize carbon and carbon nitride-based (CN) materials with ordered morphology and controlled properties. However, the transfer of an intrinsic chemical property, such as chirality from supramolecular assemblies to the final material after thermal condensation, was not shown. Here, we report the large-scale synthesis of chiral CN materials capable of enantioselective recognition. To achieve this, we designed supramolecular assemblies with a chiral center that remains intact at elevated temperatures. The optimized chiral CN demonstrates an enantiomeric preference of ca. 14 %; CN electrodes were also prepared and show stereoselective interactions with enantiomeric probes in electrochemical measurements. By adding chirality to the properties transferrable from monomers to the final product of a thermal polymerization, this study confirms the potential of using supramolecular precursors to produce carbon and CN materials and electrodes with designed chemical properties.     

The reductive metalation of 1,2-dihalobenzocyclobutenes by NaFe(CO)2Cp and HFe(CO)2Cp

The reactions between cis- and trans-1,2-dibromo- or 1,2-diidobenzocyclobutene and NaFe(CO)₂Cp (NaFp) were investigated. The mechanism of formation of 1,2-bis(cyclopentadienyldicarbonyliron)benzocyclobutene (I) and 1-(cyclopentadienyldicarbonyliron)benzocyclobutene (II) is thought to involve initial formation of benzocyclobutadiene, addition of a Fp radical to benzocyclobutadiene and subsequent addition of a second Fp radical to form I, or hydrogen abstraction from FpH to form II.     

Synthetic approach to grayanotoxins: a new method for the construction of the a-homograyanotoxane ring system

The A-homograyanotoxane ring system was constructed by a thermolysis of benzocyclobutene, followed by a Wagner-Meerwein rearrangement of the resulting kaurane type of compound.     

基于L-谷氨酸的手性硅胶球的制备及其应用

无机介孔硅球因其具有足够的机械强度、热稳定性,以及适应多种流动相的优点,成为高效液相色谱(HPLC)柱填料中使用最广泛和最重要的材料。但在此研究领域中,并未见球形的全无机手性硅胶用作HPLC手性固定相。该文以无机球形介孔硅胶作为研究对象,通过堆砌硅珠法,以硅溶胶为原料,L-谷氨酸(L-Glu)为手性源,在手性环境中制造出脲醛树脂与胶体二氧化硅混合的小球,在550 ℃高温下煅烧除去树脂部分,制备基于L-Glu的无机介孔硅胶球。通过元素分析、红外光谱、扫描电镜、透射电镜和氮气吸附等表征证明这是一种具有规则球形的手性硅胶球,其手性来源于硅胶球自身的骨架和孔结构。将L-Glu手性硅胶球作为固定相制备了HPLC色谱柱,以正己烷-异丙醇(9∶1, v/v)作为流动相,流速为0.1 mL/min,考察了该手性柱对一系列外消旋化合物的拆分性能。实验表明,该手性柱拆分了15种外消旋化合物,其中特罗格尔碱、吡喹酮、3-苄氧基-1,2-丙二醇、1,2-环氧己烷、3-羟基-2-丁酮、2-甲基四氢呋喃-3-酮、异丙基缩水甘油醚达到基线分离;还分离了10种苯系位置异构体,o,m,p-氨基苯酚、o,p-氯苯酚、o,m,p-碘苯胺、o,m,p-甲苯胺、o,m,p-二硝基苯、o,m,p-氯苯胺、o,m,p-硝基苯酚、o,m,p-溴苯胺达到基线分离。实验表明,L-Glu手性硅胶球在手性分离方面具有良好的可行性,与普通硅胶相比不需要进一步修饰就可以有较好的手性分离效果,是一种低成本、制备便捷的手性无机硅胶固定相。     

Chiral oxazoline route to enantiomerically pure biphenyls: magnesio and copper mediated asymmetric hetero- and homo-coupling reactions

A series of chiral biphenyls were prepared via asymmetric reactions involving chiral oxazolines. One series of chiral biphenyls was reached by the magnesium mediated coupling of aryl bromides (via their Grignard reagents) with o-methoxyaryl oxazolines. In this case only the o-methoxy group was replaced by the ArMgBr. The initially formed biphenyl adducts were obtained in de's as high as 92:8. These adducts could be manipulated to various other chiral biphenyls. Another series of chiral biphenyls were obtained via an asymmetric Ullmann reaction, which was shown to be thermodynamically controlled. The de's of this copper mediated process were also in the range of 90% and could be utilized to reach various derivatives. Racemization, thermal stability, and atropisomerization characteristics were also studied.     

Rationally Designed Axially Chiral Diarylethene Switches with High Helical Twisting Power

Three rationally designed axially chiral diarylethene switches were synthesized and their application as chiral dopants for phototunable cholesteric liquid crystal devices was investigated. Design of these molecules was based on the combination of photochromic dithienylcyclopentene core with bridged binaphthyl units as chiral precursors. Aromatic groups were introduced to the molecules at 6,6′‐positions of binaphthyls through a Suzuki–Miyaura coupling reaction. Their helical twisting powers (HTPs) are significantly higher than those of the known chiral diarylethenes reported as chiral dopants so far. Photocyclization of these molecules upon light irradiation brought out dramatic variation in HTPs between different states. The primary colors, red, green, and blue, were obtained in reflection on light irradiation and with thermal stability. Moreover, a multi‐switchable photodisplay was demonstrated using one of these chiral molecular switches.     

Thermal and light control of the chiral order of azopolymers

The control of the chiroptical properties of two azopolymers, which contain chiral terminal alkyl chains, by means of thermal and light irradiation processes has been studied. Both UV–vis and CD spectra of films and dichloromethane (DCM)/hexane solutions of the polymers have been registered and analyzed before and after different irradiation conditions: 488 nm circularly polarized light (CPL) and 365 nm unpolarized light. The chiroptical properties of the polymer containing chiral 1-methylheptyloxy terminal chains depended on the thermal history of the sample. As a result, the photocontrol of the chiral response in the bulk material by CPL irradiation has been evaluated on samples cooled from the isotropic state to room temperature at different rates. The chiroptical properties of these azopolymers show an intriguing combination of control from both the supramolecular and molecular chirality level as well as the thermal history of the sample and CPL irradiation.     

Feasibility of thermal crosslinking of polyarylate gas-separation membranes using benzocyclobutene-based monomers

Current literature results suggest that crosslinking may be a useful route to balance the productivity/selectivity tradeoff of polymeric gas-separation membranes. This study explores the use of benzocyclobutene chemistry to thermally crosslink polyarylate membranes based on 1,2-dihydrocyclobutabenzene-3,6-dicarbonyl dichloride (XTA-Cl), fluorene bisphenol (FBP) and 5-t-butyl isophthalic acid dichloride (tBIA). Very high temperatures were required to produce significant crosslinking which also caused significant degradation of the polymers. The result of this heat treatment was small increases in gas-permeability coefficients and small decreases in permselectivity; this is opposite to what is typically observed for simple crosslinking. It is concluded that the thermal treatment required for crosslinking is too severe to utilize this approach successfully for polyarylate gas-separation membranes.     

Synthesis, curing kinetics and thermal properties of novel difunctional chiral and achiral benzoxazines with double chiral centers

Novel difunctional chiral and achiral benzoxazine monomers were synthesized from the reaction of bisphenol A with paraformaldehyde and primary amines, including S-(+)-3-methyl-2-butylamine and rac-(±)-3-methyl-2-butylamine, by solventless method. The chemical structures of chiral and achiral benzoxazines were identified by fourier transform infrared, nuclear magnetic resonance (¹H NMR and ¹³C NMR). The curing behavior and non-isothermal curing kinetics of chiral and achiral benzoxazine monomers were investigated by differential scanning calorimeter (DSC). Isoconversional methods based on Friedman and Kissinger–Akahira–Sunose were applied to analyze the curing process of chiral and achiral benzoxazines. The thermal properties of cured polymers were characterized by DSC and thermogravimetry. The results suggested that the optical purity and stereo-configuration for chiral and achiral benzoxazines have definite influence on curing behavior and thermal properties despite the same chemical structure. Chiral benzoxazine displayed typical characteristics of difunctional benzoxazines. Achiral benzoxazine showed distinctly double peaks in DSC exotherms due to the presence of racemic and mesomeric isomers. The thermal properties of achiral polybenzoxazine were slightly higher than those of chiral polybenzoxazine, and were much higher than those of other bisphenol A-C₃–C₈ linear aliphatic amine-based polybenzoxazines because of tight packing, low free volume, and abundant intramolecular and intermolecular hydrogen bonds in network structure of polymers.     

Reappraisal of the diastereoselectivity of intramolecular Diels-Alder reactions of some o-quinodimethanes generated by benzocyclobutene thermolysis: some complementary results and an improvement

In a synthetic approach to 19-nor steroids and in order to compare the diastereoselectivities observed for the intramolecular Diels-Alder reactions of o-quinodimethanes generated either from 1 or 2, the thermolysis of benzocyclobutenes such as 2 was reexamined. The IMDA diastereoselectivity was highly dependent on the nature of the protective group of the hydroxyl substituent at the unique chiral stereocentre of the o-quinodimethane intermediate, in a position α to the double bond of the dienophile. Consistently with previous results reported for benzocyclobutenes 2, the trans-fused cycloadducts were the major products, the trans syn or trans anti predominant isomer being determined by the hydroxyl protective group. In contrast with these previous reports, the cis-fused cycloadducts were always formed competitively, although as minor products. In the present work, the diastereoselectivity was improved by achieving the thermolysis of a benzocyclobutene 2 having a lithium alkoxide which afforded the trans syn adduct in high yield.