Ketyls samarium diiodide

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

The acetate function of 98 was then cleaved by treatment with samarium diiodide in methanol in high yield (81 %) [44], A potential mechanism for this transformation is shown in Scheme 3.18. Reduction of the ketone function forms a samarium ketyl radical (103). Transfer of a second electron forms a carbanion (104) which undergoes p-elimination of acetate to generate the samarium enolate 105. Protonation and tautomerization then affords the observed product 107. 

Samarium diiodide has been used by Enholm and co-workers for the generation and cyclization of ketyl radicals from aldehydic substrates [Eq. (9) and (10) 33], As noted, the... 

Alkenyl halides and aromatic halides are unreactive with ketones in the presence of Sml2 in THF. Pinacolic coupling products can be detected in 10-20% yield under these conditions. In THF/HMPA, io-dobenzene reacts in the presence of a ketone to generate a phenyl radical, which abstracts a hydrogen from THF. Samarium diiodide induced coupling of the THF radical to the ketone (or ketyl) provides the major observed product (equation 21). ... 

Samarium diiodide-induced ketyl couplings are of significant synthetic interest, and Reissig el al. have reported an elegant application of the reaction in the case of the precursor 18. Reaction of 18 with 2 equivalents of samarium diodide and HMPA gave the 2-benzazepine 19 from an S-endo-dig cyclization reaction of the ketyl radical from the... 

Recently, enantioselective additions of samarium diiodide-generated ketyl radicals to olefins have been demonstrated [15]. As illustrated in Eq. (10), the reductive coupling of acetophenone with methyl acrylate (31a) in the presence of chiral phosphine ligand 29 (i -BINAPO) gives somewhat low yields (mostly under 50%) but moderate to good levels of enantioselectivity (60-70% ee) for the y-butyrolactone products 34. 

Since samarium diiodide is only a one electron donor, two equivalents of the metal are required in order for the reaction to proceed. The first electron donated from the samarium produces a chiral ketyl radical 30 which undergoes enantioselective addition to the acrylate according to the chelated transition state shown in 32. The second electron donation then provides a chiral samarium enolate intermediate 33 that can potentially undergo stereoselective proton transfer in the formation of a second chiral center. 

Samarium diiodide has also proven to be a useful reagent for this type of transformation with cyclopropyl ketones [11], In this case a ketyl radical is generated adjacent to the cyclopropane ring. Ring opening takes place as readily as with simple cyclopropyl carbinyl radicals. 

Our first specific Wessely oxidation approach is outlined in Scheme 2. Following an aldol-type reaction between an appropriately protected resorcinol fragment and an aldehyde, we expected the Wessely oxidation to selectively dearomatize at the ortho position of both phenols. The intramolecular Diels-Alder cycloaddition reaction was then expected to form the tricyclic core, which could then be converted to the critical tetracyclic cage via a samarium diiodide(II)-type 6-exo-trig ketyl radical cyclization reaction. 

Molander and co-workers have studied the stereoselective intramolecular addition of ketyl radicals to olefins [95JOC872]. The ketyl radicals are generated from ketone by treatment with samarium(II) diiodide. A similar reaction sequence using 61 gave only elimination products. 

Samarium(H) diiodide mediated ketyl radical addition to alkyne results in the formation of an alkene after radical cyclization [95SL277]. Compound 65, a product from one such reaction, has been converted to the carbohydrate portion of miharamycin A, a nucleoside antibiotic. 

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