Asymmetric synthesis and catalysis have been actively pursued in the chemical and material sciences for some time. An increasing number of drugs and pesticides contain chiral structural units in their structures because drug actions require conformational matching to increase their potency and selectivity for receptors and other active targets within and on the surface of the cells.
The structural structure of pharmaceuticals plays an important role in reducing or avoiding severe side effects during their action processes. Meanwhile, more advanced materials, especially nano and photoelectronic materials have micro- and macro-chiral units that require different levels of asymmetric factors. Therefore, control of chirality will continue to play an important role in the aforementioned fields.
There are four methods to control chirality including the use of chiral auxiliaries, reagents, solvents, and catalysts documented in the literature and textbooks. Among them, asymmetric catalysts are usually divided into homogeneous and heterogeneous catalysis.
The Texas Tech University/Nanjing University Li Guigen team reports a new approach to controlling chirality—asymmetric synthesis (AIAS) and asymmetric catalysis (AIAC) using chiral aggregates. This method differs from the five reported asymmetric synthesis methods: chiral auxiliaries, chiral reagents, chiral solvents and chiral catalysts. These five methods are based on the reaction behavior of individual molecules in the reaction solution.
The aggregation-induced asymmetric catalysis (AIAC) method is different from the traditional homogeneous catalysis (single molecular behavior) and heterogeneous catalysis. This is a new type of asymmetric catalysis in between. AIAS is through the accumulation of chiral auxiliary raw materials, AIAC is through the accumulation of chiral catalysts to achieve the purpose of chirality control.
Both of the new methods can be used to react with low or non-toxic solvents, and achieve the purpose of improving stereoselectivity or controlling the opposite chiral configuration.
This work not only presents a new concept, but also confirms it in asymmetric reaction processes through direct observation and evidence of molecular aggregation.
The Sharpless asymmetric catalytic dihydroxylation reaction (AD) was chosen to represent this new concept and method because of its simplicity with predictable asymmetric control through well-known and commercially available chiral ligands. Commercial AD-mixes with ligands (DHQ)2PHAL and (DHQD)2PHAL, called AD-mix-α and AD-mix-b, respectively, were directly used for the reaction at room temperature.
For the synthesis, the co-solvents of THF-H2Or commonly adopted AIE (aggregation-induced emission) and AIP (aggregation-induced polarization) are used for the current study instead of the traditional AD solvents of acetone, acetonitrile, alcohol with water. Among the many AD substrates, styrene and its derivatives are considered because they are best mechanistically investigated in AD due to their simplicity.
The correlation of chiral aggregates to aggregation-induced polarization (AIP) was further investigated to make the optical rotation enhancement or adjustment as more polar solvents were added to the co-solvents. AIP measurements of chiral dimeric quinine ligands were performed using a Rudolph polarimeter (Rudoph Research Analytica APIV/2W) with a sodium lamp emitting light at 589 nm as the light source.
The samples were prepared at a constant concentration (c = 2 mg/mL) of cosolvents commonly used AIE, and also for the present asymmetric dihydroxylation was used. The average specific values of the rotation taken from the three measurements are considered the most real results and are further plotted on the relationship curves with the specific rotation on the Y axis.
The fraction of water is placed in the component of 5% (v/v) on the X horizontal axis. (DHQD)2PHAL shows a trend in specific optical rotation values with slowly increasing water fractions, ranging from – 186.5or to -132.5or during the change of water fraction from 0% to 65%. In contrast, (DHQ)2PHAL shows an opposite trend, showing a steady downward trend, where the optical rotation value falls from 292.5.or to 250 or.
This strategy has shown good results in other reactions. For example, in the same co-solvent system containing THF and H2Or, organocatalytic asymmetric Diels-Alder reaction between 1,3-diphenylisobenzofuran and (E)-but-2-enal described opposite enantioselectivity with respect to the endo-isomer by increasing the fw value. This synthesis strategy will serve as a greener and more environmentally friendly tool to control the chirality of products compared to the classical methods previously mentioned.
In the latter, to create products of the opposite chirality, the chirality of the reactants, as well as the catalysts or chiral solvents, must be reversed by making the synthesis of these materials, causing manpower and energy and producing more a lot of garbage.
The research was published in the journal Research.
Yao Tang et al, Aggregation-Induced Catalysis: Asymmetric Catalysis with Chiral Aggregates, Research (2023). DOI: 10.34133/research.0163
Citation: Aggregation-induced catalysis: Asymmetric catalysis with chiral aggregates (2023, July 18) retrieved on July 18, 2023 from https://phys.org/news/2023-07-aggregation-induced-catalysis-asymmetric-chiral -aggregates.html
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