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![Three examples of the many potential product outcomes for a Schiff base macrocycle reaction; pictured here are the [1+1], [2+2] and [4 + 4] macrocyclic products](publication/359642964/figure/fig1/AS:1155451064463378@1652492409454/Three-examples-of-the-many-potential-product-outcomes-for-a-Schiff-base-macrocycle.png)
Three examples of the many potential product outcomes for a Schiff base macrocycle reaction; pictured here are the [1+1], [2+2] and [4 + 4] macrocyclic products
Source publication
Schiff base imine condensations are a useful tool for macrocycle synthesis and applications within supramolecular chemistry. Here we address the mixtures of products that can arise from template free synthesis using dicarbonylheterocycles and diamines, and look to develop metal-free template methods for selective macrocycle formation. A range of al...
Citations
... 17 Strategies for selective formation of a specific imine macrocycle include templating, or tuning reaction conditions to isolate a specific macrocycle via crystallisation. [18][19][20][21] For these systems, there have also been limited reports of more sustainable methods of synthesis: López-Periago et al. used supercritical carbon dioxide (scCO2) as a green solvent to synthesise an unsubstituted trianglimine 22 and microwave-assisted synthesis has been reported for Schiff base macrocycles. [23][24][25] However, a general method for imine macrocycle synthesis has not been developed, and the success of such strategies can be challenging to predict and control. ...
... In all cases, the cleanest NMR spectra were obtained at 5 minutes (SI, [18][19][20][21][22][23][24][25][26][27][28][29] and the majority product in each case was the desired macrocycle. Macrocycle 3 underwent full conversion and would not require any further purification; no aldehyde peaks were observed at any of the time points. ...
Macrocycles are candidates for wide-ranging applications, yet their synthesis can be low-yielding, poorly reproducible, and resource-intensive, limiting their use. Here, we explore the use of Non-Thermal Plasma (NTP) as an...
... In this situation, it is necessary to remove water as a byproduct to force the reaction to produce the imine only. To eliminate water from the reaction mixture during the reaction, some authors [36] use the distillation method, and this is not without ambiguity when the water formed presents an azeotrope with one of the components of the mixture. The Dean-Stark apparatus is also used to remove water from the reaction mixture. ...
A comparative study of a Schiff base reaction involving benzaldehyde and n-butylamine was carried out to improve the yield of the resulting imine. This reaction was carried out at different temperatures without and with the elimination of the water produced during the process by the pervaporation (PV) technique using a typical cylindrical cell. To reach this goal, different dense membranes made of crosslinked poly(vinyl alcohol) with different oxalic acid (crosslinker) contents were prepared by the solvent casting method. Different parameters influencing the performance of the membrane in the separation process including swellability, diffusivity, crosslinking density, and thermal properties were investigated. The total and partial cumulative transmembranar fluxes as well as the separation factor were studied and the separation process was monitored by HPLC analysis. The n-butyl-1-phenylmethanimine produced was characterized by FTIR and 1HNMR analyses. The results obtained were a clear improvement in the yield of the reaction. For example, the yield obtained from the Schiff base reaction occurring without assistance by PV varied from 58 to 84 wt% when the temperature changed from 5 to 45 °C. On the other hand, when the PV process was used to eliminate water from this reaction mixture, the yield went from 90.4 to 98.6% by weight in this same temperature order. The cumulative total and partial fluxes significantly decreased with time. On the other hand, the separation factor reached a maximum at about one hour at 5, 15, and 45 °C. At 25 °C, the maximum total flux was reached at about 2 h of the PV process. The best selectivity of the PVA-0.5 membrane with regard to water was obtained at 15 °C. It was also revealed from the results obtained that the cumulative total and partial flux decreased rapidly with time and the separation factor reached a maximum at one hour into the PV process, in which 1.51 × 104 was reached at 15 °C, 6.25 × 103 and 3.50 × 103 at one hour of the separation process, and 10.23 × 103 at 25 °C at 2 h of the water removal by PV.
... Experimental and computational evidence both supported this thesis. Indeed, the lack of evidence for oligomeric or polymeric species was surprising because such products are commonly observed in dynamic covalent chemistry [15][16][17][18][19]. Scheme 1. ...
In the absence of preorganization, macrocyclization reactions are often plagued by oligomeric and polymeric side products. Here, a network of hydrogen bonds was identified as the basis for quantitative yields of macrocycles derived from the dimerization of monomers. Oligomers and polymers were not observed. Macrocyclization, the result of the formation of two hydrazones, was hypothesized to proceed in two steps. After condensation to yield the monohydrazone, a network of hydrogen bonds formed to preorganize the terminal acetal and hydrazine groups for cyclization. Experimental evidence for preorganization derived from macrocycles and acyclic models. Solution NMR spectroscopy and single-crystal X-ray diffraction revealed that the macrocycles isolated from the cyclization reaction were protonated twice. These protons contributed to an intramolecular network of hydrogen bonds that engaged distant carbonyl groups to realize a long-range order. DFT calculations showed that this network of hydrogen bonds contributed 8.7 kcal/mol to stability. Acyclic models recapitulated this network in solution. Condensation of an acetal and a triazinyl hydrazine, which adopted a number of conformational isomers, yielded a hydrazone that adopted a favored rotamer conformation in solution. The critical hydrogen-bonded proton was also evident. DFT calculations of acyclic models showed that the rotamers were isoenergetic when deprotonated. Upon protonation, however, energies diverged with one low-energy rotamer adopting the conformation observed in the macrocycle. This conformation anchored the network of hydrogen bonds of the intermediate. Computation revealed that the hydrogen-bonded network in the acyclic intermediate contributed up to 14 kcal/mol of stability and preorganized the acetal and hydrazine for cyclization.
