Samarium diiodide alkenes

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... 

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]. 

To a stirred solution of samarium diiodide (1.5 mmol) in THF (12 ml) was rapidly added a solution of a (3-hydroxy sulfone (0.5 mmol) in THF (6 ml) under an argon atmosphere. After 15 min at room temperature the reaction mixture was still blue, due to an excess of Sml2. The reaction mixture was then poured into a 10% solution of Na2S203 (20ml) and extracted with ethyl acetate. The residue was chromatographed over silica gel (hexane/ethyl acetate 99 1) to give the alkene (55-82%) as a mixture of ( )/(Z) isomers. 

Reduction of multiple bonds with samarium diiodide has been reviewed. Chemo-and stereo-selective reduction of various compounds such as conjugated alkenes, c/,/3-unsaturated carboxylic acids, activated alkynes, carbonyl, azides, nitriles, and nitro compounds, under mild conditions, has been discussed. Recent developments in the use of samarium metal in this field have also been discussed.381... 

A further, more complicated example of the stereoselective reductive ketone-alkene coupling is shown in the samarium diiodide promoted cyclization of unsaturated ji-oxo estersl2, giving 1.2,3-trialkyl-2-hydroxycyclopentanecarboxylates. The configuration of the two newly formed stereogcnic centers is determined by chelation of the intermediate radical by samarium(III). 

The reductive elimination reaction of the /3-hydroxy imidazolyl sulfone derivatives with sodium amalgam or samarium diiodide provided mainly the desired -alkenes in good yields. 

The MBH adduct shown in Scheme 3.162, prepared from aromatic aldehydes and methyl acrylate, can directly undergo smooth dehydroxylation with concomitant olefin isomerization with low-valent titanium (LVT, prepared from TiCls-LAH-THF) reagent to afford the trisubstituted alkenes 364 with high ( )-selectivity. However, it is unsatisfactory in view of the low yield and the purity of products.More recently, Zhang et al. developed the samarium diiodide-promoted hydroxyl elimination of MBH adducts to form trisubstituted alkenes 364 with total ( )-stereoselectivity in good to excellent yields. This method also provided a new route to synthesizing a class of 1,5-hexadiene derivatives 365 by temperature tuning (Scheme 3.162). ... 

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... 

Modification of the Julia-Lythgoe olefination reaction between ketones and primary sulfones leads to trisubstituted alkenes in good overall yields reported by Marko and co-workers. Samarium diiodide/HMPA shown to play a crucial role in the reductive elimination step. Starting from sulfone 40, key intermediate 42 was produced in 93% yield, which was further converted to 43 in 73% yield with E/Z ratio of 2 1. More complicated intermediate 44 was similarly reduced by Smh to 45 in 68% yield. 

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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