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Free Radical Polymerization of Expandable Oxaspiro Monomers
Published in Rajender K. Sadhir, Russell M. Luck, Expanding Monomers, 2020
A possible alternative to the use of acrylate-based cyclic monomers to achieve improved reactivity involves the utilization of new 2,3-bis(methylene)tetraoxaspiro compounds. Several examples of these monomers (25 and 26; Figure 25), which fit a substituted 1,3-butadiene model, have been prepared34 utilizing procedures similar to those utilized in the synthesis of monomers such as 13 with the single exocyclic methylene substituent. In addition to the more reactive conjugated system, this approach provides a fixed s-cis diene conformation which may further enhance polymerizability since radical addition would allow for the release of steric strain. There are literature reports37-38 which indicate the ready polymerizability of 4,5-bis(methylene)-l,3-dioxolan-2-one (27; Figure 26), which apparently occurs primarily through 1,4-addition with no ring opening. A somewhat related monomer (28, Figure 27), which combines a 1,3-butadiene group with a ketene acetal ring system, has been reported to undergo efficient free radical single ring-opening in solution polymerizations.39
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Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
Unlike polyacetylene, polysilylpropynes are air stable.1 Air stability may also be attributed to the lack of π bond conjugation. Electronically, polysilylpropyne’s main-chain, double bonds are similar to isolated double bonds. But chemically, polysilylpropyne’s double bonds are inert to many reactions that are characteristic for isolated double bonds, such as bromination, epoxidation, carbene addition, hydroboration, hydrogenation, hydrosilylation, and so on. One possibility is that the crowdedness of pendant substituents shields the double bond from attacking reagents. Another possibility is that these type of reactions change the main chain carbon orbital hybridization from sp2 to sp3. This transformation will increase the proximity between the neighboring pendant substituents and increase steric strain (Structure 4).
Cipargamin: Biocatalysis in the Discovery and Development of an Antimalarial Drug
Published in Peter Grunwald, Pharmaceutical Biocatalysis, 2019
Thomas Ruch, Elina Siirola, Radka Snajdrova
When designing the new route, aspects of how to increase yield, reduce cost, improve process safety and green metrics, had to be considered. It is clear that the low yield of a late stage Pictet-Spengler of amine 4 and 5-chloroisatine, had to be addressed. If starting from racemic tryptamine 4, all four diastereoisomers of the final product were formed, as the stereochemistry of the tryptamine set the stereochemistry of the spiroindolone system. In another words, if chiral amine S-4 could be used, only 2 out of 4 possible diastereoisomers would be formed—1R, 3S as the major diastereomer (KAE609) and the 1S, 3S as minor diastereomer (Scheme 15.5). This diastereoselectiviy can be rationalized by the avoidance of steric strain in the transition states leading to the diastereomers (Zhou, 2012). Stereochemistry of the Pictet-Spengler reaction.
Energetic competition in the complexation affinity of paracetamol with water and oxalic acid
Published in Molecular Physics, 2023
Amanda Studinger, Loredana Valenzano-Slough
An evaluation of the configuration energies of the complexes can provide useful insights into the presence of energy barriers related to changes in molecular geometry as small molecular complexes build up into the actual crystalline structure. Changes in bond lengths and angles can alter torsional and steric strain and can act as a possible hindrance between molecules. The effect of implicit solvation has a clear impact on the configuration energies of PCA-OXA. After optimisation of the complexes in vacuum, averaged configuration energies ranged from −67.1 to −53.4 kJ/mol as shown in Figure 8. This difference shows that the energy of the relaxed structure is lower than that of the complexes in the crystal, indicating an energy barrier. Changes to bond lengths, angles, and molecular positions contribute to this change in energy. With complexation energies of −58.3 and −67.1 kJ/mol, results indicate that positions 1 and 4, respectively, are under the most geometric strain in the crystal configuration. These results agree with the geometric analysis reported earlier. Results for position 4 are in line with earlier geometry observations because relaxed structures in the aqueous solvent do not form a secondary hydrogen bond. From the results obtained, the PCA-OXA complex is more stable in an aqueous environment, which means that there is a lower configuration energy barrier present for nucleation.
Solar cells sensitised by push–pull azo dyes: dependence of photovoltaic performance on electronic structure, geometry and conformation of the sensitizer
Published in International Journal of Ambient Energy, 2018
Bismi Basheer, Temina Mary Robert, K. P. Vijayalakshmi, Dona Mathew
The I-V characteristics given in Figure 5 shows the superior performance of DSSC with sensitizer DR1 and DR2. The poor performance of 2,4-dinitro-substituted sensitizers, DR3 and DR4 is attributed to the distortion of the conjugation sphere due to the loss of planarity as predicted by the computational modelling studies. The planarity is lost in DR3 and DR4 due to the repulsion between the lone pairs of oxygen in nitro group and the lone pairs of nitrogen in azo group, leading to out-of-plane orientation of second nitro group in the dinitro-substituted phenyl ring, as shown in Figure 6 (Ottersbatch, Schnakenburg, and Gutschow 2012). This effect is similar to atropisomerism which results from hindered rotation about single bonds where the steric strain acts as a barrier to rotation. Figure 6 shows the partial obstruction in conjugation in the chromophore DR4 hindered the expected red shift in optical absorption due to extended conjugation in it.