Diels-Alder reactions in confined spaces: the influence of catalyst structure and the nature of active sites for the retro-Diels-Alder reaction

Potential use of the Diels-Alder reaction in biomedical and nanomedicine applications

The Diels-Alder reaction and its retro breakdown has garnered increasing research focus due to several of its advantageous properties including, atomic conservation, reversibility, and substituent retention. This is especially true in biomedical application and nanomedicine development which display a preference for rapid, efficient, and clean "click" chemistry reactions allowing for delivery of active ingredients and subsequent release upon temperature elevation. There are multiple variations on the Diels-Alder reaction based around substitution position and materials being coupled which can affect the temperature threshold for and rate of the retro reaction reversal. Hence, the Diels-Alder reaction offers a simple coupling reaction for active ingredients with tailorable release. In this review the incorporation of the Diels-Alder chemistries and linkers within the biomedical and nanomedicine field will be discussed, as well as its use in future potential technologies.

Carbazole-functionalized hyper-cross-linked polymers for CO2 uptake based on Friedel-Crafts polymerization on 9-phenylcarbazole

To systematically explore the effects of the synthesis conditions on the porosity of hyper-cross-linked polymers (HCPs), a series of 9-phenylcarbazole (9-PCz) HCPs (P1-P11) has been made by changing the molar ratio of cross-linker to monomer, the reaction temperature T 1, the used amount of catalyst and the concentration of reactants. Fourier transform infrared spectroscopy was utilized to characterize the structure of the obtained polymers. The TG analysis of the HCPs showed good thermal stability. More importantly, a comparative study on the porosity revealed that: the molar ratio of cross-linker to monomer was the main influence factor of the BET specific surface area. Increasing the reaction temperature T 1 or changing the used amount of catalyst could improve the total pore volume greatly but sacrificed a part of the BET specific surface area. Fortunately changing the concentration of reactants could remedy this situation. Slightly changing the concentration of reactants could simultaneously obtain a high surface area and a high total pore volume. The BET specific surface areas of P3 was up to 769 m2 g-1 with narrow pore size distribution and the CO2 adsorption capacity of P11 was up to 52.4 cm3 g-1 (273 K/1.00 bar).

Conceptual similarities between zeolites and artificial enzymes

Inspired by artificial enzymes, we have designed zeolites acting as scaffolds for a given Diels–Alder reaction using mimics of the reaction transition state.

By using a Diels–Alder (DA) reaction as a base case, we show that a pure silica zeolite acting as an entropy-trapping scaffold can be synthesized with an organic structure directing agent (OSDA) analogue of the transition state (TS) of the DA reaction. A cavity stabilization of the TS is observed with the corresponding decrease in the activation energy of the reaction. A lower enthalpy of activation and a larger decrease in entropy are obtained with the zeolite synthesized with the analogue of the DA TS when compared with other zeolitic structures. Those differences are maintained, while catalytic activity is increased, when active sites are introduced in the zeolite. The catalytic zeolitic system synthesized with the OSDA analogue of the TS shows conceptual similarities with “de novo design” of an artificial enzyme to perform DA reactions, in where a suitable scaffold of existing proteins is chosen, and computationally designed active sites able to catalyze the cycloaddition reaction are introduced.