Samarium reagents reduction

Figure 4.11. Examples of redox-initiated radical reactions. Samarium diiodide reduction of the bromide gives a radical that cyclizes faster than the second reduction reaction. Manganese triacetate oxidation of the P-keto ester gives an enol radical that is not further oxidized by the manganese reagent. The IBX oxidizes anilides to the corresponding radicals. Hexamethylphosphoramide = HMPA and Tetrahydrofuran = THE.

Ring opening of epoxides using selenolates that are generated by other methods, such as reduction with samarium reagents [19], electrochemical reduction [22], reduction with phosphine [25] or reduction with NaBH4 under microwave irradiation [34], have also been reported. 

KAGAN-MOLANDER Samarium reagent Lanthanide reagents, specifically samarium, for generation of tree radicals useful m cydizations, reductions... 

The two reactions discussed here are new to you—they are reductive processes where an alkali or alkaline earth metal supplies electrons, to convert carbonyl compounds into radical anions, which then couple together on the metal surface. The first is the pinacol coupling reaction of ketones, which, classically, uses magnesium metal as an electron source. The reaction mechanism is shown in Figure 20.8. In recent years, many other metals have been used similarly titanium and samarium reagents have been particularly popular, and some applications are shown in Figure 20.9. 

Another useful reagent for reduction of a-acetoxyketones and similar compounds is samarium diiodide.233 Sml2 is a strong one-electron reducing agent, and it is believed that the reductive elimination occurs after a net two-electron reduction of the carbonyl group. 

Another well-known transformation of carbonyl derivatives is their conversion to pinacols (1,2-diols) via an initial one-electron reduction with highly active metals (such as sodium, magnesium, aluminum, samarium iodide, cerium(III)/ I2, yttrium, low-valent titanium reagents (McMurry coupling), etc.), amines, and electron-rich olefins and aromatics as one-electron donors (D).43 Ketyl formation is rapidly followed by dimerization44 (equation 22). 

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

More recently, Doris et al. have described the reductive ring-opening of a-keto epoxides [16]. In this manner, p-hydroxy ketones can be obtained in high yields. The synthesis of enantiomerically pure compounds can easily be realized. The titanocene] 111) reagents are distinctly superior to samarium diiodide, which is also known to induce this transformation. 

The samarium diiodide-mediated reductive cyclization of a sugar-derived oxime ether provides a trace amount of an unexpected cyclized product <1997JOC7397>. It is thought that its formation probably arises from an intermediate (methylthio)methyl ether formed with the excess of Swern reagent used to generate the aldehyde in situ after cyclization (Equation 59). 

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