Reduction samarium iodide

Appropriate functionalization of C=N bonds can greatly assist their asymmetic reduction. In particular, the reduction of N-acyl hydrazones with a rhodium complex of the ligand DuPHOS (P13) represents an outstanding example. In this process (Scheme 62) a product of up to 97% e.e. is obtained in high yield. After the reduction, samarium-iodide cleavage of the N—N bond gives the product amine273,274. 

It has been shown recently that the selective reductive homo-coupling polymerization of aromatic diisocyanates via one electron transfer promoted by samarium iodide in the presence of hexamethylphosphoramide [PO(NMe2)3] (HMPA) can produce poly(oxamide)s in nearly quantitative yield (Scheme 9). 

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). 

Zard and coworkers have developed a synthesis of substituted dienes by reductive elimination of allylic nitroacetates (equation 33)66. Allylic nitroacetates can be prepared by condensation of nitromethane with the carbonyl compound followed by addition of formaldehyde and acetylation67. Reductive elimination can be carried out by employing either chromous acetate or samarium iodide. 

When 1,2-dihydroquinazolines are actually desired as the reaction product, they can often be contaminated with their conjugated analogs due to spontaneous dehydrogenation <2000JME4479>, but this can be avoided under reductive conditions, as demonstrated by the preparation of 2-substituted-l,2-dihydroquinazolines 815 by the reductive condensation of 2-nitrobenzamides 814 with aldehydes in the presence of samarium iodide <2002JHC1271>. 

The reductive elimination of 0-hydroxyimidazoyl sulfones by samarium iodide was claimed to be an improved variation of the Julia olefin synthesis [421]. 

From epoxides by reduction Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281... 

Reduction of A,(V-di bcnzy lam i noalkyl chloromethyl ketones 133, followed by spontaneous intramolecular ring closure of the resulting 7-chloroamines 134, afforded azetidinium salts 135, which were subsequently deprotected toward chiral 3-hydroxyazetidines 136 and 137 (Scheme 29) <1997JOC1815>. In a similar approach, ct-amino aldehydes 138 were converted into 3-hydroxyazetidinium salts 139 upon treatment with diiodomethane and samarium iodide, followed by stabilization using AgBF4. Ar-Dealkylation with Pd afforded enantiopure azetidines 140 (Scheme 30) <2000TL1231>. 

The amides derived from /3-hydroxy-a-amino acids, obtained from the reaction of the latter with resin-bound hydroxylamine, have been cyclized under the Mitsunobu conditions to afford 3-aminoazetidin-2-ones. The free azetidin-2-ones were cleaved from the resin by reduction with samarium iodide <20010L337>. 

Samarium iodide catalyzes the reduction of halides in damp THF120-122 and yields may be significantly improved if HMPA is added to the reaction mixture121,123. a-Halocarbonyl compounds are readily hydrodehalogenated by the use of this and similar methods124-126. Bis-cyclopentadiene complexes of samarium catalyze the reduction of benzylic and ally-lie halides via an organosamarium complex intermediate127-129. This reaction may be controlled so that monodeuteration occurs if the reaction is carried out under dry aprotic conditions followed by addition of D20 (equation 12). 

In the stereoselective synthesis of epothilone A (11), Carreira used a syn -reduction methodology in the synthesis of the key intermediate (14)9 (Scheme 4.If). Reduction of the isoxazoline 12 with samarium iodide at 0 C in THF gave the ketone 13. Narasaka reduction of the P-hydroxy ketone 13 using triethylb-orane/sodium borohydride afforded the. syn-diol 14 in high yield and with high diastereoselectivity. 

Fluoro- and chloro-substituted tricarbonyl chromium arene complexes can be reductively coupled with ketones, in the presence of samarium iodide, to give tertiary alcohols (Scheme 109). A related reaction of a tethered alkoxyimine furnished a tricyclic compound (Scheme 110). Samarium also promotes the formation of a benzyhc ketyl... 

Divalent samarium is known to reduce alkyl halides. However, reductions of iodides and bromides in tetrahydrofuran (THF) require a long reaction time and chlorides are not reduced even at refluxing temperature. 

Samarium iodide is a reagent capable of highly selective reduction of epoxy ketones and esters to the corresponding alcohols (Table 12). Diene monoepoxides are converted with high regio- and stereo-selectivity to the ( )-allylic alcohols, and the reaction can be carried out at -90 C under neutral conditions. As a result functional groups such as alkoxycarbonyl, carbonyl and cyano survive. 

Finally, it can be stated that many reactivity patterns of free radical ions are equally found in oxidative and reductive transformations involving initial inner-sphere ET, such as in reactions with samarium iodide [389], low valent titanium [390] and titanocene complexes [391], manganese(III) [392], and CAN [393]. 

Samarium iodide has been used to bring about reductive photo-dehalogenation of 1,1-dichlorocyclopropane. The results and the efficiency of the reaction are shown in Scheme 5. The process, a Barbier reaction, is brought about using visible light. The yields of product are enhanced by the addition PhSH as a hydrogen donor. 

Reduction of cyano groups under radical conditions is carried out employing tin hydrides such as /-IJu3SnH.286 Unfortunately, the reaction is so far restricted to malonitriles. Samarium iodide is a valid alternative to tin hydrides since in the presence of HMPA it promotes the reductive decyanation of malononitriles and a-cyanoesters in high yields (Eq. 165).287 -289... 

The samarium iodide promoted reduction of substrate 242 also led to ring opening to yield hydroxycyclohexenone 243 in De Clercq s synthesis of a precursor to the A-ring of 1 a-hydroxyvitamin D3, Eq. 148 [196]. 

Apart from sodium borohydride, which is frequently used in water or water-alcohol mixtures to reduce ketones or aldehydes selectively, water is rarely used as the solvent in reductions, because of incompatibility with most reducing agents. However, samarium iodide reduction of ketones, as well as alkyl and aryl iodides is accelerated in water [99]. Likewise, the a-deoxygenation of unprotected aldonolac-tones is efficient when the SmI2-tetrahydrofuran-water system is used [100],... 

Samarium iodide has been utilized in the synthesis of the antineoplastic macrolidc antibiotic cytovaricin 55. The high equatorioselectivity and chemoselecti vity make samarium iodide36 the reagent of choice for this reduction. 

Other reducing agents, such as sodium/ammonia or diphenylstannane are unsatisfactory55. Since stereoselective reductions of this type have been a problem in the synthesis of milbemycins and avermectins57. the utility of the radical anion reduction via samarium diiodide/proton donor remains to be appreciated generally. For routine applications on a multigram scale, the price of samarium iodide is currently too high (1 mol of samarium costs ca. 25 times more than 1 mol of lithium). 

Meerwein-Ponndorf-Verley reduction. The samarium iodide is a catalyst tor asymmetric reduction (36-96% yield, up to 97% ee) of carbonyl compounds by / PrOH. 

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