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Other Nonnitrogenous Organocatalysts
Published in Andrew M. Harned, Nonnitrogenous Organocatalysis, 2017
Several details of the mechanistic work performed by Curci and Edwards would prove to be critical in developing a catalytic asymmetric variant. Foremost among these was the finding that Oxone® (caroate) can be used as the source of HSO5–. Oxone® is a so-called triple salt with the formula 2KHSO5•KHSO4•K2SO4. Control of pH is also important. If the pH is too low, then decomposition of the ketone by a Baeyer–Villiger reaction becomes a problem. If the pH is too high, then decomposition of the dioxirane by SO52– becomes a problem. Consequently, running the reaction with water that is buffered to pH 7.5 was found to be optimal. Also important was the addition of small amounts of Na2EDTA as a scavenger of trace metals that may also decompose peroxide intermediates. By following these precautions, Curci and Edwards were able to show that caroate and acetone could function as a useful combination for alkene epoxidation reactions (Figure 8.4), presumably through in situ formation of dimethyldioxirane (DMDO).10 Also important was Curci’s finding that the use of fluorinated ketones such as trifluoroacetone had a pronounced impact on the reactivity and stability of the corresponding dioxirane.11,12 Yang would show that this reactivity could be harnessed for practical alkene epoxidations by adapting Curci’s procedure for in situ formation of DMDO to work with trifluoroacetone (Figure 8.5).13 Especially important was the finding that NaHCO3 could effectively regulate the pH of the reaction without requiring constant monitoring.
[3 + 2] Cycloaddition reactions of nitrile oxides generated in situ from aldoximes with alkenes and alkynes under ball-milling conditions
Published in Green Chemistry Letters and Reviews, 2022
Run-Kai Fang, Zheng-Chun Yin, Jun-Shen Chen, Guan-Wu Wang
Oxone (2KHSO5·KHSO4·K2SO4) is a stable, environmentally friendly, non-toxic oxidant, which has been used in mechanochemical reactions (43–47). In our previous work, we reported that the dimerization of nitrile oxides generated in situ from aldoximes in the presence of Oxone, NaCl and base under ball-milling conditions could afford furoxans in high yields (Scheme 1a) (47). Inspired by this result, we envisioned that the addition of alkenes and alkynes to the in situ generated nitrile oxides may compete with the dimerization process and lead to isoxazoles and isoxazolines. Herein, we report the details for the mechanosynthesis of these [3 + 2] cycloaddition products under ball-milling conditions (Scheme 1b). We have found that various (hetero)aromatic and aliphatic aldoximes, diverse alkenes such as acrylate esters, acrylonitrile, chalcone, styrene, N-methylmaleimide and [60]fullerene, different alkynes including simple phenylacetylene and those containing functional group CH2OH, SiMe3, COCH3 or CO2CH3 can be utilized to generate the [3 + 2] cycloadducts in good yields.
Iodination of vanillin and subsequent Suzuki-Miyaura coupling: two-step synthetic sequence teaching green chemistry principles
Published in Green Chemistry Letters and Reviews, 2019
James J. Palesch, Beau C. Gilles, Jared Chycota, Moriana K. Haj, Grant W. Fahnhorst, Jane E. Wissinger
Our aim was to design a guided-inquiry experiment exemplifying similar learning outcomes to the nitration experiment through a greener, safer transformation. The ideal substrate would have multiple possible substitution positions so that EAS selectivity could be studied and would afford a crystalline product with instructive 1H NMR spectral features. Recently, we developed an oxidation of borneol to camphor using Oxone® and catalytic sodium chloride (3). This experiment has been a highly successful green addition to our organic chemistry laboratory curriculum. Oxone® is a stable triple salt consisting of 2KHSO5•KHSO4•K2SO4 which has found wide spread application in synthetic chemistry (4). One such application is the halogenation of aromatic rings using a combination of Oxone® and a halide salt in various solvents (5–7). This reaction works most efficiently with aromatic substrates containing one or more electron-donating substituents (8). The active oxidizing agent in Oxone® is potassium peroxymonosulfate (KHSO5) which is thought to react with halide salts (M + X-) to produce a source of the electrophilic X+ in the form of a hypohalous acid, HOX (4). After workup, the by-products of the reaction are environmentally-benign potassium sulfate salts.