Truxinates and truxillates: building blocks for architecturally complex polymers and advanced materials

Significant advancements in the syntheses of cyclobutane containing small molecules and polymers are described in the last 15 years. Small molecule cyclobutanes are under investigation for their diverse pharmacological activities, while polymers with cyclobutane backbones are emerging as novel mechanophores, stress-responsive materials, and sustainable plastics. Within these chemistries, [2 + 2] photocycloadditions to yield truxinates and truxillates are highly efficient offering a versatile strategy to access complex scaffolds. This article provides a comprehensive review on the synthetic methodologies, properties, and applications of polymer truxinates and truxillates, providing the background necessary to understand current developments and envision future applications. Additionally, we highlight the links between the development, discoveries, and synthetic methodologies of small molecules and cyclobutane polymers. We emphasize structure property relationships and discuss methods to control composition and structure for desired applications. We begin with a discussion of synthetic techniques for small molecule and polymer cyclobutanes followed by their greater applications, including pharmacological and material properties with examples including sustainable plastics and stimuli-responsive systems.

Solid-state [2+2] photodimerization of eniminium salts: stereoselective syntheses of 1,3-diacetylcyclobutanes

Solid-state [2+2] photodimerization of eniminium ions oriented in a head-to-tail manner controlled by cation-π interactions produced synHT dimers in high yields. As the resulting dimer is readily converted to 1,3-diacetylcyclobutane, the iminium serves as a removable orientation-controlling group for the conjugated ketones.

Characteristics and catalytic behavior of Ru-Sn bimetallic catalysts for TMCB hydrogenation to CBDO

A series of Ru-Sn/γ-Al2O3 catalysts were prepared by the immersion method for tetramethylcyclobutane-1,3-dione (TMCB) hydrogenation to prepare 2,2,4,4-tetramethyl-1,3-cyclobutanediol (CBDO). The effect of the preparation method and reaction technology on TMCB hydrogenation activity was discussed. The catalysts were analyzed by means of XRD, BET, H2-TPR, XPS, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and it was found that the synthesized Ru was distributed on the surface of the carrier in the form of nanoparticles, showing a good catalytic effect. The results showed that when Ru loading was fixed at 5%, Sn was used as an auxiliary agent, and Ru/Sn = 1 : 1 as the catalyst, the reaction conditions were 120 °C, 4 MPa, and 1 h, and the catalytic hydrogenation effect of TMCB on CBDO was the best. The selectivity was as high as 73.5%, and the cis-trans ratio was 1.11. It may be the strong interaction between Ru and Sn under this ratio condition, which leads to the largest number of nano-active centers of elemental Ru. Finally, the reaction mechanism of TMCB hydrogenation to CBDO is discussed.

A Biorenewable Cyclobutane-containing Building Block Synthesized from Sorbic Acid Using Photoenergy

A novel cyclobutane-containing diacid building block, CBDA-3, was synthesized from sorbic acid using clean, efficient [2 + 2] photocycloaddition. This photoreaction can be performed using commercially available germicidal lamps, which represent a form of ECO-UV. SC-XRD showed that the cyclobutane ring in CBDA-3 has a unique semi-rigid character, unlike more rigid aromatic rings or more flexible types of aliphatic rings. C=C bonds present in the structure of CBDA-3 provide opportunities for derivatization which could be used to alter the characteristics of polymers made from this monomer. Additionally, TGA and DSC analysis showed CBDA-3 to have excellent thermal stability. These characteristics make CBDA-3 a promising building block with the potential to be used as a sustainable alternative to traditional petroleum-derived diacids. Finally, a facile and reliable Fischer esterification of CBDA-3 was performed to tune its melting point and solubility for different applications and to demonstrate the applicability of this building block in polymer synthesis.

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A novel cyclobutane-containing diacid building block

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A potentially sustainable alternative to petroleum-derived diacids

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Photoreaction using ECO-UV (Energy-efficient, Cost-effective, and Operator-friendly)

A recyclable thermoset with built-in thermocleavable group developed from a cis-cyclobutane-1,2-dicarboxylic acid

A novel class of recyclable thermoset has been developed from cis-3,4-diphenylcyclobutane-1,2-dicarboxylic acid (CBDA-4) due to its thermocleavability at high temperature. This key CBDA-4 building block was synthesized from β-trans-cinnamic acid using a [2+2] photocycloaddition reaction. CBDA-4 was subsequently linked with glycerol via esterification to give a thermoset with T_g of 68 °C. The thermoset was heated to 300 °C to analyze its degradation. A key intermediate was successfully obtained after purification of the degraded polymer. NMR, FT-IR, HRMS, and single crystal X-ray diffraction confirmed the intermediate was glycerol cinnamate, which was the result of splitting cyclobutane in the polymer backbone at high temperature. Glycerol cinnamate was readily hydrolyzed reforming the starting materials glycerol and trans-cinnamic acid to complete the recycling loop.

Blended vinylogous urethane/urea vitrimers derived from aromatic alcohols

An in-depth investigation of the condensation, substitution and transamination reactions in blended vinylogous urethane/urea vitrimers derived from commercially relevant alcohols, proven by detailed model studies and 16 prepared vitrimer networks.

Terephthalic Acid Copolyesters Containing Tetramethylcyclobutanediol for High-Performance Plastics

There is a need for high-performance applications for terephthalic acid (TPA) polyesters with high heat resistance, impact toughness, and optical clarity. Bisphenol A (BPA) based polycarbonates and polyarylates have such properties, but BPA is an endocrine disruptor. Therefore, new TPA polyesters that are less hazardous to health and the environment are becoming popular. Tetramethylcyclobutanediol (TMCD) is a difunctional monomer that can be polymerized with TPA and other diols to yield copolyesters with superior properties to conventional TPA polyesters. It has a cyclobutyl ring that makes it more rigid than cyclohexanedimethanol (CHDM) and EG. Thus, TMCD containing TPA copolyesters can have high heat resistance and impact strength. TPA can be made from abundantly available upcycled polyethylene terephthalate (PET). Therefore, this review discusses the synthesis of monomers and copolyesters, the impact of diol composition on material properties, molecular weight, effects of photodegradation, health safety, and substitution of cyclobutane diols for future polyesters.

Crystal Engineering Construction of Caffeic Acid Derivatives with Potential Applications in Pharmaceuticals and Degradable Polymeric Materials

Natural products are precious feedstock in drug discovery and sustainable materials. This work using crystal engineering strategy, visible light, and solvent-free cycloaddition successfully constructed two caffeic acid derivatives, rel-(1R,2R,3S,4S)-2,4-bis(3,4-dihydroxyphenyl)cyclobutane-1,3-dicarboxylate and rel-(1R,2R,3S,4S)-2,4-bis(3,4-dihydroxyphenyl)cyclobutane-1,3-dicarboxylic acid. Because of the multiple stereocenters, it is challenging to prepare those compounds using traditional organic synthesis methods. The crystal engineering Hirshfeld surface analysis and 2D intermolecular interaction fingerprints were applied to synthetic route design. The light resources used in this work was visible LED or free, clean, and renewable sunlight. The evidence suggested that pure stereoisomer was obtained demonstrating the stereospecificity and efficiency of the topochemical cycloaddition reaction. The derivatives exhibited free radical scavenging and antioxidant biological activities, as well as the potential inhibitory activity of fatty acid binding proteins. One of the derivatives is the precursor of the natural product Shimobashiric acid C which paves the way for the total synthesis and further study of Shimobashiric acid C. In addition, the derivatives possess photodegradability at a specific wavelength, which is very attractive for "green" degradable polymeric materials.