Reduction, by: samarium diiodide

The asymmetric synthesis of unsymmetrical vicinal diamines by samarium diiodide induced reductive coupling of nitrones derived from aUphatic aldehydes with optically pure N-tert-butanesulfinyl aromatic imines has been recently reported [41]. For example, the reaction between nitrone 55 and... 

J. L. Chiara, J. Marco-Contelles, N. Khira, P. Gallego, C. Destabel, and M. Bemabe, Intramolecular reductive coupling of carbonyl-tethered oxime ethers promoted by samarium diiodide A powerful method for the stereoselective synthesis of aminocyclopentitols, J. Org. Chem., 60 (1995) 6010-6011. 

As an alternative to the Bu3SnH-phenylseleno glycoside chemistry, radicals may also be generated by samarium diiodide reduction of anomeric arylsulfonyl groups [115], (R=Ph] (O Scheme 55). 

Chiara, J. L. New reductive carbocyclizations of carbohydrate derivatives promoted by samarium diiodide. Carbohydrate Mimics 1998, 123-156. 

As shown in Fig. 3.33, starting from the Diels-Alder product, reduction for removal of acetoxyl group by samarium diiodide was first tried. Compound 3.68 was obtained in 90 % yield. Using CCLj as the solvent and benzoyl peroxide as the... 

Sc(OTf)3 (4 mol%)-catalyzed addition of 1-trimethylsilyl nitropropanate to aryl and alkylimines was described [109]. The use of N-(4-methoxyphenyl)imines gave superior yields and/or diastereoselectivities of P-nitro amines in most cases (72-99% yield, antijsyn = 5/3-9/1). The products could be reduced to 1,2-diamines by samarium diiodide reduction, formation of the cyclic urea and cleavage of the 4-methoxyphenyl group with ceric ammonium nitrate (CAN). This methodology was later employed for the synthesis of pseudo-C2-symmetric triamines as candidates for human immunodeficiency virus (HIV) protease inhibition [110]. 

Samarium diiodide serves as an effective reductant for a variety of functional groups. Conjugated double bonds are readily reduced by Sml2 in the presence of proton donors such as methanol (Kagan and Namy, 1986). Inanaga (1990) has shown that alkynes are reduced efficiently to alkenes by samarium diiodide in the presence of catalytic amounts of cobalt (Co) complexes. Selective synthesis of (Z)-alkenes is possible due to the chemoselective nature of... 

Kende AS, Mendoza JS. An improved variant of the Julia olefin synthesis-reductive elimination of (3-hydroxy imida-zolyl sulfones by samarium diiodide. Tetrahedron Lett. 1990 31(49) 7105-7108. 

Molander GA, Kenny C. Intramolecular reductive coupling reactions promoted by samarium diiodide. J. Am. Chem. Soc. 1989 lll(21) 8236-8246. 

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. 

Moreover, a dramatic increase of the reaction rate was observed when the coupUng of aromatic imines mediated by samariiun diiodide was carried out in the presence of both water and a tertiary amine or tetramethylethylene-diamine (TMEDA) [29], causing the almost instantaneous formation of the 1,2-diamine, although with undetermined diastereoselectivity. Similarly, the samarium diiodide promoted reductive coupling of iminiiun ions formed in situ by reacting ahphatic aldehydes with secondary amines and benzotriazole occurred at temperatures as low as - 70 °C [30]. Even in this case a mixture of diastereomers with undetermined ratio was obtained nevertheless, the item of diastereoselectivity induced by a chiral amine (auxiliary) is worthy of investigation. 

A variety of solvents was investigated for this reaction, as shown in Table 15.1. As inferred from Table 15.1, the hydrogenolysis performance is best in more polar solvents snch as acetonitrile, acetone, ethyl acetate, and acetic acid. Only in o-dichlorobenzene is the rate of reaction ranch lower than predicted by the dielectric constant. The presence of nonpolar solvents snch as hexane and the thiol product resulted in large amonnts of the disnlfide intermediate. It has been shown that the disnlfide is the intermediate in stoichiometric rednctions such as samarium diiodide reduction of alkyl thiocyanates to thiols (11) so it is reasonable to expect it as the... 

The Julia olefination involves the addition of a sulfonyl-stabilized carbanion to a carbonyl compound, followed by elimination to form an alkene.277 In the initial versions of the reaction, the elimination was done under reductive conditions. More recently, a modified version that avoids this step was developed. The former version is sometimes referred to as the Julia-Lythgoe olefination, whereas the latter is called the Julia-Kocienski olefination. In the reductive variant, the adduct is usually acylated and then treated with a reducing agent, such as sodium amalgam or samarium diiodide.278... 

Opening of a cyclobutane ring fused to a quinolizine system under reductive conditions has been described. Thus, the previously mentioned compound 128 was obtained by treatment of 132 with samarium diiodide (Equation 8)... 

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. 

Burk et al. showed the enantioselective hydrogenation of a broad range of N-acylhydrazones 146 to occur readily with [Et-DuPhos Rh(COD)]OTf [14]. The reaction was found to be extremely chemoselective, with little or no reduction of alkenes, alkynes, ketones, aldehydes, esters, nitriles, imines, carbon-halogen, or nitro groups occurring. Excellent enantioselectivities were achieved (88-97% ee) at reasonable rates (TOF up to 500 h ) under very mild conditions (4 bar H2, 20°C). The products from these reactions could be easily converted into chiral amines or a-amino acids by cleavage of the N-N bond with samarium diiodide. 

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]. 

Moreover, propargyl oxiranes 202 were found to react with samarium diiodide and ketones to form a,a -dihydroxyallenes 203 with moderate to high anti-diastereo-selectivities (Scheme 2.62). Aurrecoechea and co-workers [99] reported this reductive coupling to proceed smoothly in the absence of a palladium catalyst, i.e. a direct electron transfer from the samarium(II) to the substrate has to take place in order to generate an allenyl/propargyl samarium intermediate of type 184/185, which is then regioselectively trapped by the electrophile. 

Reduction of diaryltellurium dichlorides with samarium diiodide (typical procedure). Diaryl tellurium dichloride (1 mmol) was added to the deep blue solution of Sml2 (2.2 mmol) in THF (22 mL) at room temperature under nitrogen with stirring. The deep blue colour of the solution disappeared immediately and became yellow. The resulting solution was stirred at room temperature under nitrogen for 30 min. To the solution was added dilute hydrochloric acid, and the mixture was extracted with ether. The ethereal solution was washed with brine and dried over MgS04. The solvent was evaporated in vacuo, and the residue was purified by preparative TLC on silica gel (petroleum ether-methylene dichloride as eluent). 

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