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. 

For some excellent recent reviews of the utility of samarium diiodide in organic synthesis, see (a) Molander, G. A. Chem. Rev. 1992, 92, 29 (b) Soder-quist, J. A. Aldrichimica Acta 1991, 24, 15. 

SAE reaction see Sharpless asymmetric epoxidation Saegusa oxidation 390 samarium diiodide 496, 633, 638 saponification 49, 207 sativene 382 f. 

The blue samarium diiodide solutions are stable indefinitely in the absence of water and oxygen, and Sml and Ybl2 solutions have been employed as reducing agents in organic synthesis. 

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. 

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... 

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... 

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. 

Samarium diiodide is another powerful one-electron reducing agent that can effect carbon-carbon bond formation under appropriate conditions.257 Aromatic aldehydes and aliphatic aldehydes and ketones undergo pinacol-type coupling with Sml2 or SmBr2. 

Amino alcohols like (iS )-prolinol react with nitroalkenes very rapidly with very high facia] selectivity.31 Rapid and stereoselective reduction of the nitro function is essential for the conversion of the products to 1,2-diamine derivatives with the retention of the configuration. Samarium diiodide is recommended in the stereoselective reduction of thermally unstable 2-aminonitroalkanes to give a range of useful 1,2-diamines (Eq. 4.26).32... 

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. 

Treatment of iV-(a-aminobenzyl)benzotriazoles 648 with samarium diiodide generates radicals 649 that undergo coupling to form vicinal diamines 650 (Scheme 101) <1992TL4763>. Formation of intermediate radicals 649 at low temperature is confirmed by EPR <19990L1755>. Short-living radicals 649 are readily converted to more stable radicals 651 by treatment with 2-methyl-2-nitrosopropane. 

Crombie and Rainbow reported synthesis of terminal dienes of high. E-selectivity via samarium diiodide mediated scission of 2-vinyl-3-chlorotetrahydrofuran (equation 175) or the corresponding pyran derivatives296. Interestingly, both cis- and frans-2-substituted 3-chlorotetrahydrofurans give the same diene, indicating the involvement of identical intermediates formed by electron transfer from samarium diiodide. 

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]. 

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. 

This method was later applied to samarium diiodide initiated reactions [26] and to tita-nocene-catalyzed pinacol couplings [27]. The first examples of enantioselective reactions using Me3SiCl as a mediator for catalysis have very recently been reported by Cozzi et al., as shown in Scheme 12.14 [28]. 

Novel synthetic procedures for indolizidine alkaloids were developed via a samarium diiodide-promoted carbon-nitrogen bond cleavage as a key step. Application of the procedure led to the total synthesis of (+)-(8R, 8aR)-perhydro-8-indolizidinol <2006H193>. 

Scheme 2.58 Palladium-catalyzed reduction of propargylic acetates with samarium diiodide.

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. 

Derivatives incorporating the 230/269 motif as a substructure have often been invoked as reactive intermediates. An example is the reduction of the bisacetate 272 with samarium diiodide in the presence of a palladium catalyst (see above) (Scheme 5.41) [114],... 

Two convenient methods have been developed for the preparation of trifluoro-methyl-substituted alkoxyallenes. Reductive elimination of allylic acetates 30 with samarium diiodide leads to 31 (Scheme 8.11) [38], whereas reaction of Wittig cumu-lene 32 with phenyl trifluoromethyl ketone (33) and thermolysis of the intermediate 34 provides 35 (Scheme 8.12) [39]. 

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