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.
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.
The combination of SmI2/H2O/MeOH promotes a selective
reduction of quinolin-2(1H)-ones to afford 3,4-dihydroquinoline-2(1H)-ones
under mild conditions with good to excellent yields.
D. Xie, S. Zhang, J. Org. Chem., 2022, 87,
8757-8763.