Samarium (low valent)

Low valent metal compounds are common reducing agents. Samarium, which is most stable in the third oxidation state (also like all other lanthanides), is used in the second oxidation state as an one-electron reducing agent. Samarium(II) compounds can be produced in situ by a reaction of samarium(III) salts with metallic samarium. Also the derivatization from the commercially available SmI₂ to more reactive compounds is possible.

The fact that samarium compounds are very mild reducing agents were demonstrated impressively by the mentioned examples. Therefore, the Pinacol rearrangement can be accomplished in water.

Besides, in some reactions the affinity from samarium to oxygen is evident. This can be used for Barbier or Grignard-type reactions to cause the reaction of alkyl halides with ketones and aldehydes.

Recent Literature

The reduction of ketones and aldehydes with lanthanide metals (La, Ce, Sm, Yb) and a catalytic amount of iodine (5 mol %) in iPrOH proceeded smoothly to produce the corresponding alcohols as the major products in good yield, while in THF, methanol, and ethanol the pinacols were mainly produced. The yields of alcohols were improved most effectively by the use of Sm metal.
S.-I. Fukuzawa, N. Nakano, T. Saitoh, Eur. J. Org. Chem., 2004, 2863-2867.

Sm in aqueous HCl is an environmentally benign reducing agent. Whereas aromatic carbonyls underwent pinacol coupling reactions in an efficient manner in Sm/2 M HCl/THF, unimolecular reductions of aliphatic aldehydes provided alcohols.
S. Talukdar, J.-M. Fang, J. Org. Chem., 2001, 66, 330-333.

An intermolecular pinacol coupling of aromatic or aliphatic carbonyl compounds catalyzed by a complex of samarium diiodide (SmI₂) with tetraglyme in the presence of Me₂SiCl₂ and Mg is described. High diastereoselectivity has been achieved in reactions with aliphatic and aromatic aldehydes. De values of up to 99% have been achieved in intramolecular pinacol coupling reactions.
H. C. Aspinall, N. Greeves, C. Valla, Org. Lett., 2005, 7, 1919-1922.

Unexpected disproportionation of SmCl₃-Sm in water was observed via UV-vis spectroscopic analysis indicating that low-valent samarium(II) species can exist in water. SmCl₃-Sm and SmCl₃-Mg systems were found to act as good one-electron reducing agents in water.
S. Matsukawa, Y. Hinakubo, Org. Lett., 2003, 5, 1221-1223.

A highly efficient, mild and practical approach for the synthesis of optically pure β-amino alcohols by the SmI₂-induced reductive cross-coupling of various chiral N-tert-butanesulfinyl imines with aldehydes was developed.
Y.-W. Zhong, Y.-Z. Dong, K. Fang, K. Izumi, M.-H. Xu, G.-Q. Lin, J. Am. Chem. Soc., 2005, 127, 11956-11957.

A Michael addition type reaction between aroyl chlorides and chalcones was realized in the presence of samarium metal in N,N-dimethylformamide as solvent. Various 1,4-diketones were synthesized in moderate to good yields. A possible mechanism is also discussed.
Y. Liu, Y. Li, Y. Qi, J. Wan, Synthesis, 2010, 4188-4192.

Reduction of β-hydroxyketones by SmI₂/H₂O/Et₃N provided 1,3-diols in quantitative yields with no byproduct formation.
T. A. Davis, P. R. Chopade, G. Hilmersson, R. A. Flowers, Org. Lett., 2005, 7, 119-122.

The use of D₂O and SmI₂ as a mild single-electron donor enables a general single-electron transfer reductive deuteration of readily commercially available aromatic esters for the synthesis of α,α-dideuterio benzyl alcohols. This operationally convenient method features high deuterium incorporations and very good functional group tolerance.
S. Luo, C. Weng, Y. Ding, C. Ling, M. Szostak, X. Ma, J. An, Synlett, 2021, 32, 51-56.

The use of pentafluorophenyl esters as ketyl radical precursors, SmI₂ as a mild reducing agent, and D₂O as the deuterium source enables a highly chemoselective synthesis of α,α-dideuterio alcohols with exquisite incorporation of deuterium. This system tolerates a variety of functional groups.
H. Li, Y. Hou, C. Liu, Z. Lai, L. Ning, R. Szostak, M. Szostak, J. An, Org. Lett., 2020, 22, 1249-1253.

A highly chemoselective direct reduction of primary, secondary, and tertiary amides to alcohols in high yields in presence of SmI₂/amine/H₂O proceeds via C-N bond cleavage in a carbinolamine intermediate and shows excellent functional group tolerance. The expected C-O cleavage products are not formed under the reaction conditions. Notably, the method provides direct access to acyl-type radicals from unactivated amides under mild conditions.
M. Szostak, M. Spain, A. J. Eberhard, D. J. Procter, J. Am. Chem. Soc., 2014, 136, 2268-2271.

Activation of SmI₂ (Kagan’s reagent) with Lewis bases enables a mild general reduction of nitriles to primary amines under single electron transfer conditions. Activated samarium diiodide features excellent functional group tolerance and is therefore an attractive alternative to pyrophoric alkali metal hydrides. Notably, an electron transfer from Sm(II) to bench stable nitrile precursors generates imidoyl-type radicals.
M. Szostak, B. Sautier, M. Spain, D. J. Procter, Org. Lett., 2014, 16, 1092-1095.

A new, easy and versatile methodology for the deoxygenation of alcohols via the corresponding toluates offers a broad scope using simple and commercially available reagents such as toluolyl chloride and samarium(II) iodide. In addition, this methodology is also useful for radical cyclizations directly from toluate precursors.
K. Lam, I. E. Markó, Org. Lett., 2008, 10, 2919-2922.

Several ureates formed by treatment of the corresponding ureas with n-BuLi activate SmI₂ to a substantial extent toward dehalogenations of alkyl and aryl halides including substrates of low reactivity such as aryl fluorides.
C. E. McDonald, J. D. Ramsey, C. C. McAtee, J. R. Mauck, E. M. Hale, J. A. Cumens, J. Org. Chem., 2016, 81, 5903-5914.

The combination of HMPA and SmBr₂ in THF is a powerful reductant that is capable of reducing ketimines and alkyl chlorides at room temperature. The structure of this reductant has not been established.
B. W. Knettle, R. A. Flowers, II, Org. Lett., 2001, 3, 2321-2324.

B. W. Knettle, R. A. Flowers, Org. Lett., 2001, 3, 2321-2324.

A mild and efficient electron-transfer method for the chemoselective reduction of aromatic nitro groups using samarium(0) metal in the presence of a catalytic amount of 1,1'-dioctyl-4,4'-bipyridinium dibromide gives aromatic amines in good yield with selectivity over a number of other functional and protecting groups.
C. Yu, B. Liu, L. Hu, J. Org. Chem., 2001, 66, 919-924.

Selective cleavage of unsubstituted allyl ethers is provided by SmI₂/H₂O/i-PrNH₂ in very good yields. This method is useful in the deprotection of alcohols and carbohydrates.
A. Dahlen, A. Sundgren, M. Lahmann, S. Oscarson, G. Hilmersson, Org. Lett., 2003, 5, 4085-4088.

The use of SmI₂ in the reductive elimination of 1,2-acetoxy sulfones (Julia-Lythgoe olefination) and the reductive cleavage of vinyl sulfones is reported.
G. E. Keck, K. A. Savin, M. Weglarz, J. Org. Chem., 1995, 60, 3194-3204.

Samarium diiodide promotes a photoinduced metalation of nonactivated C-Cl bonds of O-acetyl chlorohydrins and a β-elimination which affords alkenes with total or high stereoselectivity.
J. M. Concellon, H. Rodriguez-Solla, C. Simal, M. Huerta, Org. Lett., 2005, 7, 5833-5835.

J. M. Concellon, H. Rodriguez-Solla, C. Simal, M. Huerta, Org. Lett., 2005, 7, 5833-5835.

A samarium-promoted synthesis of (E)-nitroalkenes from 1-bromo-1-nitroalkan-2-ols in high yields together with an efficient preparation of the 1-bromo-1-nitroalkan-2-ols constitutes a simple and advantageous alternative toward nitroalkenes with total E-stereoselectivity. A mechanism is proposed to explain the E-stereoselectivity of the β-elimination reaction.
J. M. Concellón, P. L. Bernad, H. Rodríguez-Solla, C. Concellón, J. Org. Chem., 2007, 72, 5451-5423.

A modification of the classical Julia-Lythgoe olefination using sulfoxides instead of sulfones affords 1,2-di-, tri-, and tetrasubstituted olefins in moderate to excellent yields and E/Z selectivity. The mild conditions involve in situ benzoylation and SmI₂/HMPA-mediated reductive elimination as key steps.
J. Pospisil, T. Pospisil, I. E. Marko, Org. Lett., 2005, 7, 2373-2376.

An easy, direct, general, and efficient samarium diiodide-mediated preparation of 3-hydroxyacids in high yield by reaction of different aldehydes or ketones with commercially available iodoacetic acid is described.
J. M. Concellón, C. Concellón, J. Org. Chem., 2006, 71, 4428-4432.

Addition of in situ generated samarium acetamide and chloroacetamide enolates to aldimines afforded 3-aminoamides and 3-amino-2-chloroamides in high yields. A mechanism is proposed to explain the synthesis and reactivity of samarium enolates of primary amides.
J. M. Concellón, H. Rodríguez-Solla, C. Concellón, C. Simal, N. Alvaredo, Synlett, 2010, 2119-2121.

A highly stereoselective synthesis of aromatic α,β-unsaturated amides was achieved by treatment of aromatic α,β-epoxyamides with samarium diiodide. α,β-epoxyamides are easily prepared by the reaction of enolates derived from α-chloroamides with carbonyl compounds at -78°C.
J. M. Concellón, E. Bardales, J. Org. Chem., 2003, 5, 9492-9295.

The stereoselective reaction of different aldehydes and ethyl dibromoacetate promoted by SmI₂ or CrCl₂ gives (E)-α,β-unsaturated esters by an an aldol-type reaction and a subsequent β-elimination reaction.
J. M. Concellon, C. Concellon, C. Mejica, J. Org. Chem., 2005, 70, 6111-6113.

The stereoselective reaction of different aldehydes and dibromoacetic acid promoted by SmI₂ gives (E)-α,β-unsaturated carboxylic acids. The mechanism is discussed.
J. M. Concellon, C. Concellon, J. Org. Chem., 2006, 71, 1728-1731.

The samarium diiodide promoted addition of α-halo-α,β-unsaturated esters to carbonyl compounds led to (Z)-2-(1-hydroxyalkyl)-2,3-alkenoates in good yields and very high stereoselectivity. A mechanism is proposed to explain this transformation.
J. M. Concellon, M. Huerta, J. Org. Chem., 2005, 70, 4714-4719.

Racemic 1-nitroalkan-2-ols are obtained by reaction of bromonitromethane with a variety of aldehydes promoted by SmI₂. Chiral N,N-dibenzyl amino aldehydes afford the corresponding enantiopure 3-amino-1-nitroalkan-2-ols with good diastereoselectivity.
J. M. Concellón, H. Rodríguez-Solla, C. Concellón, J. Org. Chem., 2006, 71, 7919-7922.

A samarium-promoted cyclopropanation can be carried out on unmasked (E)- or (Z)-α,β-unsaturated carboxylic acids. In all cases the process is completely stereospecific and stereoselective. A mechanism has been proposed.
J. M. Concellón, H. Rodríguez-Solla, C. Simal, Org. Lett., 2007, 9, 2685-2688.

A new convergent synthetic approach to the 2-hydroxypyran motif common to many naturally occurring structures includes the esterification of two fragments and a subsequent intramolecular reductive cyclization.
L. V. Heumann, G. E. Keck, Org. Lett., 2007, 9, 1951-1954.

A mild deprotection for notoriously difficult to unmask primary N-(p-toluenesulfonyl) amides occurs at low temperature by initial activation of the nitrogen with a trifluoroacetyl group, followed by reductive cleavage of the p-toluenesulfonyl group with samarium diiodide.
Z. Moussa, D. Romo, Synlett, 2006, 3294-3298.

A reductive cleavage of the N-O bond in oxime ether promoted by SmI₂ generates N-centered radicals, that undergo intramolecular cyclization to afford five-membered cyclic imines either via N-centered radical addition or N-centered anion nucleophilic substitution.
F. Huang, S. Zhang, Org. Lett., 2019, 21, 7430-7434.

SmI₂ promotes a formation of unstable 2,3,4-trien-1-ols from 4,5-epoxyalk-2-ynyl esters via reduction and elimination of an acetate or benzoate leaving group. A subsequent Pd(II)-catalyzed cycloisomerization provides furans in the presence of a proton source. The whole one-pot sequence takes place under mild reaction conditions and tolerates some useful functional groups.
J. M. Aurrecoechea, E. Pérez, M. Solay, J. Org. Chem., 2001, 66, 564-569.