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Organic and Inorganic Supramolecular Catalysts
Published in Jubaraj Bikash Baruah, Principles and Advances in Supramolecular Catalysis, 2019
When more than one reactive site undergoes chemical reactions, the use of a catalyst leads to different isomers as products from the same reagent. A catalyst binds differently to the substrate, changing the course of the reaction to provide chemoselectivity, or the catalyst causes specific site-selective reaction. For example, epoxidation of terpenoid 2.25a takes place at the double bonds connecting C11-C10, C7-C6, C3-C2, as shown in Figure 2.23, upon reaction with hydroperoxides. The catalyst 2.25b causes selective epoxidation with high enantiomeric excess at the C3-C2 position, while a catalyst 2.25c is used, changes the reactivity and the epoxidation reaction takes place more selectively at the C7-C6 position. These observations are attributed to the hydrogen-bonded intermediate involved in this reaction. The interaction of the terpenoid 2.25a with the catalyst 2.25c provides an adduct, which uses the ethereal oxygen and a peptide N─H bond to hold the O-H group. Hence, the catalyst adopt a locked geometry through intramolecular hydrogen bonds between a carbonyl of a proline unit and an NH of an amide unit. This geometry 2.25d prevents the attack of the peroxy compounds at the C3-C2 position, and the reaction takes place at the second favourable position at C6-C7 (Figure 2.25).
Atom Economy
Published in Aidé Sáenz-Galindo, Adali Oliva Castañeda-Facio, Raúl Rodríguez-Herrera, Green Chemistry and Applications, 2020
Kunnambeth M. Thulasi, Sindhu Thalappan Manikkoth, Manjacheri Kuppadakkath Ranjusha, Padinjare Veetil Salija, Nisha Vattakkoval, Shajesh Palantavida, Baiju Kizhakkekilikoodayil Vijayan
According to IUPAC, Chemoselectivity is defined as “the preferential reaction of a chemical reagent with one of two or more different functional groups” (Shenvi et al., 2009). In the absence of chemoselective reagents the reaction is indiscriminate and all possible products are formed, decreasing the atom economy of the process. Chemoselectivity becomes important when complex organic molecules with more than one reactive site or functional groups are involved in the reaction. In this case, the reagent has to be directed to the preferred site for the desired outcome. The discrimination ability of a reagent, the ability of the reagent to select sites to react with it, is called chemoselectivity (Trost, 1983).
The Use of Small Particle Catalysts in Pursuit of Green and Sustainable Chemistry
Published in Ahindra Nag, Greener Synthesis of Organic Compounds, Drugs and Natural Products, 2022
One of the controllable features of chemistry is chemoselectivity where a selective reactivity of a functional group in the presence of others can be elicited. An example of this amazing phenomenon can be found in the synthesis of anilines.94 Chemoselective catalytic reduction of nitro aromatic compounds is the prominent means for conversion to synthesize functionalized anilines.95 These amines are integral feedstock for the synthesis of dyes, pigments, pharmaceuticals, and agrochemicals. The hydrogenation conversion of 4-nitrostyrene portrayed in Scheme 9.2 depicts the products that can be formed by nanocatalyst selection.
Diaryl sulfides synthesis: copper catalysts in C–S bond formation
Published in Journal of Sulfur Chemistry, 2019
Lian Chen, Ali Noory Fajer, Zhanibek Yessimbekov, Mosstafa Kazemi, Masoud Mohammadi
In one of the most efficient and valuable protocols for synthesis of diaryl sulfides, copper(I) oxide (Cu2O) was introduced as an inexpensive and commercially available catalyst for the performance of cross coupling of aryl halides with aromatic thiols [51]. In this protocol, ethyl 2-oxocyclohexanecarboxylate was used as an effective ligand to synthesize diaryl sulfides in good to excellent yields (Scheme 8). Several valuable indices such as the use of an inexpensive and efficient catalyst, very high yields, high chemoselectivity and good functional group tolerance demonstrated the excellent efficiency of this methodology to synthesize diaryl sulfides. In the year of 2011, Boshun and Haolong synthesized a series of carbonyl-phosphine oxide ligands (1a–1c) from 2-bromophenylaldehyde to carry out C–S coupling reactions [52]. They studied and compared the effects of all prepared ligands to select the best ligand for C–S cross coupling reaction of aryl iodides with thiols in the presence of Cu-catalyst. The best results were obtained when 1b used as a ligand. In this protocol, copper(I) iodide in the presence of cesium carbonate and acetonitrile was acted as a suitable catalyst to synthesize diaryl sulfides (Scheme 9). A diverse range of diaryl sulfides was generated with good functional group tolerance.
An ionic liquid supported on magnetite nanoparticles as an efficient heterogeneous catalyst for the synthesis of alkyl thiocyanates in water
Published in Journal of Sulfur Chemistry, 2021
Mehdi Fallah-Mehrjardi, Soheil Sayyahi
It is well known that sulfur-containing architectures exhibit significant functions in general organic synthesis and thus are utilized in the pharmaceutical industry as well as materials science [1]. Organic thiocyanates are the members of a chemical class of organic chalcogen cyanates where O, Se, S, or Te, heteroatoms are bound via a single bond to the organic substituent such as alkyl or aryl, and simultaneously to a CN group. As a result of the frequent use of the XCN functional group especially as a leaving group, alkyl thiocyanates are frequently regarded as organic pseudo-halides. They have been considered to be beneficial functional motifs found in diverse natural products as well as bio-active compounds and have widespread utilization as the synthetic intermediate for accessing different worthwhile sulfur-containing compounds. Hence, a considerable number of attempts have focused on innovative methodologies in order to incorporate a thiocyano-group into organic molecules. Most frequently these compounds are prepared from alcohols [2], thiols [3] and isothiocyanates [4], but nucleophilic substitution reaction of thiocyanate anion with alkyl halides is still the simplest and most common procedure for their synthesis [5–12]. It is obvious that various electrophilic as well as nucleophilic thiocyanating and cyanating agents have been provided and may be utilized in such transformations. Nonetheless, a main caveat of a majority of them is to apply excessively strong oxidants which lowers chemoselectivity and regioselectivity, and results in a narrow substrate scope. Hence, it is very crucial to devise a novel efficacious thiocyanation reaction to synthesize SCN containing organic molecules.
Magnetically recoverable ferromagnetic 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres as an efficient and ligand-free catalyst for C–S bond formation in poly (ethylene glycol)
Published in Journal of Sulfur Chemistry, 2020
Ahad Vatandoust Namanloo, Batool Akhlaghinia, Arezou Mohammadinezhad
Next, to explore the scope of the C–S coupling reaction via this modified procedure (Table 1, entry 20), the reaction of various aryl halides with thiourea (as a commercial available and economic affordable reagent for the synthesis of aryl sulfides) was studied. (Table 2) As summarized in Table 2, the electronic nature of substitution played a major role in governing the reactivity of the aryl halides. Generally, synthesis of aryl thioethers with electron-withdrawing groups requires shorter reaction time than electron-donating ones. (Table 2, entries 2–4 vs. entries 5–6, entry 8 vs. entry 9 and entries 13–14 vs. entries 15–16) Our results reveal that aryl chlorides were not as reactive as aryl bromides and aryl iodides. (Table 2, entries 1–2 and 7–8 vs. entries 12–13) In these cases, the observed yields are less and there was no improvement in the yields even after prolonged reaction time. As can be seen, the reaction rate is sensitive to steric effects of the aryl halides. A sterically hindered ortho-substituted aryl halide proved to be problematic in the presence of 3D hierarchical core-shell Fe3O4@NiO/Co3O4 microspheres, and no cross-coupled product was detected even after 24 h (Table 2, entries 16–17). In the case of naphthyl and heterocyclic halides, the reaction time is high to obtain a reasonable yield of the corresponding symmetrical thioethers (Table 2, entries 10–11). To investigate the chemoselectivity of the present method, the reaction of 1-chloro-4-iodobenzene (as dihalogenated aryl halide) with thiourea was also tested which the iodide showed more reactivity (Table 2, entry 4). This fact was confirmed by the mass spectrometry (Figure 9, Supporting Information file).