Affordable and Clean Energy (SDG 7), Responsible Consumption and Production (SDG 12)
Bimolecular Nucleophilic Substitution Reactions of Organophosphorus Compounds: Computational Chemistry Complementing Synthesis N. Savoo* 1 , L. Rhyman 1,2* and P. Ramasami 1,2* 1 Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of Mauritius, Mauritius 2 Centre for Natural Product Research, Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa In an article titled “IUPAC Top Ten Emerging Technologies in Chemistry 2022”, Fernando Gomollón-Bel reported that computational chemistry helps to expand the possibilities of synthesis and the study of more complex reactions [1]. In this research work, the findings of an experimental research involving S N 2 reaction were investigated. An S N 2 reaction occurs when a nucleophile attacks a reactive atom centre in a molecule and causes the departure of part of the molecule (leaving group). The attack and departure happen in a simultaneous manner. Two types of S N 2 attacks are possible which result in the inversion and retention pathways. In the inversion pathway, the nucleophile (Nu in Figure 1) attacks the reactive centre from the side opposite to the leaving group (LG in Figure 1). In the retention pathway, the nucleophile attacks the reactive centre from the same side as the leaving group. These two types of attacks cause an inversion of configuration in the product of the inversion pathway and a retention of configuration in the product of the retention pathway. Figure 1 depicts the inversion and retention pathways.
Figure 1: Inversion (black) and retention (purple) pathways of the S N 2 reaction. The stereochemical outcome of S N 2 reactions is determined by factors such as the reactive centre, the nucleophile and the solvent. In a study by Ye et al., aliphatic and aromatic Grignard reagents dictated the stereochemical outcome of the SN2 reaction at the phosphorus atom (SN2@P) of organophosphorus compounds [2]. These compounds are extensively studied as they are employed in asymmetric synthesis as ligands of metal catalysts and as organocatalysts [3]. Ye et al. proposed that an inversion pathway occurred with the aliphatic Grignard reagent, whereas the aromatic Grignard reagent attacks through the retention pathway. In this research work [4], the SN2 reactions, which were proposed by Ye et al., were studied using the B3LYP/6-31++G(d,p) method. The results showed that the aliphatic Grignard reagent reacts through the backside SN2@P pathway. However, the aromatic Grignard reagent leads to a novel SN2@Cl mechanism, which is followed by a frontside SN2@C mechanism. The findings of our theoretical research work help in determining which reagents should be used in the synthesis of organophosphorus compounds so that a stereoselective reaction occurs. Hence, this work contributes to green chemistry by minimising trial and error, and wastage of resources. In this way, this work also helps in achieving SDG 12 (responsible consumption and production). References: 1. F. Gomollón-Bel, Chemistry International, 44, 2022, 4-13. 2. J.-J. Ye, S.-Z. Nie, J.-P. Wang, J.-H. Wen, Y. Zhang, M.-R. Qiu and C.-Q. Zhao, Organic Letters, 19, 2017, 5384-5387. 3. S. Kotani, M. Nakajima, Tetrahedron Letters, 2020, 61, 151421. 4. N. Savoo, L. Rhyman and P. Ramasami, RSC Advances, 12, 2022, 9130-9138.
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