Mischmetall: An efficient and low cost coreductant for catalytic reactions of samarium diiodide

Full text of DOI:10.1021/jo9820667

2944
J . Org. Chem. 1999, 64, 2944-2946
Ta ble 1. Ca ta lytic Ba r bier -Typ e Rea ction s
Misch m eta ll: An Efficien t a n d Low Cost
Cor ed u cta n t for Ca ta lytic Rea ction s of
Sa m a r iu m Diiod id e
Florence He´lion and J ean-Louis Namy*
Laboratoire des Re´actions Organiques Se´lectives,
associe´ au CNRS, Institut de Chimie Mole´culaire d’Orsay,
Universite´ Paris-Sud, 91405 Orsay, France
reaction
isolated yields
of products (%)
run
RX
conditionsa
1
2
3
4
5
6
allyl iodide
1.5 h then 0.5 h
3.5 h then 0.5 h
3.0 h then 0.5 h
3.5 h then 1 h
6.5 h then 1 h
2.5 h then 1 h
90
91
91
52
67
67b
Received October 14, 1998
allyl bromide
benzyl bromide
ethyl iodide
ethyl iodide
ethyl iodide
Introduced for the first time to synthetic organic
chemistry in 1977,1 samarium diiodide has emerged as
one of the most useful reagents. Except in reactions
where it is the precursor of a Sm(III) catalyst,2 it is used
in stoichiometric amounts. Its preparation requires sa-
marium powder or ingots,1,3 neither of which are very
expensive.4 However, the cost of samarium diiodide could
be considered as a major drawback, potentially limiting
the development of its chemistry.
Very recently, interesting attempts have been reported
to run reactions with catalytic quantities of SmI2 and an
in situ regeneration system for the Sm(II) species.
Magnesium has been employed with SmI2 for the pina-
a
b
See ref 10. With 0.001 equiv of NiI2.
dymium, or praseodymium fit all of the above require-
ments, except for the first one.
Fortunately, the alloy of the light lanthanides (La 33%,
Ce 50%, Nd 12%, Pr 4%, Sm and others lanthanides 1%)
known as mischmetall (or cerium mixed metal) is avail-
able at a low price.9
We report here some results on the use of mischmetall
in samarium diiodide-catalyzed Barbier-type reactions,
reduction of organic halides, pinacolic coupling of aceto-
phenone, and the coupling of an acid chloride.
The results concerning Barbier-type reactions are
gathered in Table 1.10
Samarium diiodide is used in catalytic quantities (0.2
equiv, 10 mol %). In the absence of SmI2, no reaction
occurs, except in the case of allyl iodide where 20% of
the tertiary alcohol and 10% of the pinacols from 2-oc-
tanone are obtained. The slow addition of a THF/ketone/
organic halide mixture to the THF/SmI2/mischmetall
suspension leads to the persistence of the deep blue SmI2
color. A quick addition leads to the disappearance of the
color though it is, however, restored within 1-3 h
depending on the halide; in these cases, small amounts
(∼5%) of byproducts (2-octanol and pinacol coupling
products of 2-octanone) are detected. Yields of tertiary
alcohols are close to those obtained under stoichiometric
conditions.1,11 Addition of HMPA (4 equiv with respect
5
colic coupling of aromatic aldehydes and ketones and
for the coupling of imines.6 Corey has described the use
of a zinc amalgam with SmI2 for the deoxygenation of
styrene oxide, the cyclization of 1-iodo-6-phenyl-5-hexyne,
and the annulation of ketones and acrylate esters to give
γ-lactones. However, in the latter example, the cleavage
of the Sm(III)-O alkoxide bond must be performed with
trimethylsilyl triflate, an expensive reagent.7 Moreover,
magnesium and zinc are reactive toward many organic
substrates and, therefore, not suitable for use in many
reactions mediated by SmI2.
Ideally, a system for in situ regeneration of SmI2
should be (1) cheaper than samarium metal, (2) able to
quickly reduce samarium(III) species into samarium(II)
ones, which implies the ability to cleave Sm(III)-O
bonds, (3) unreactive toward organic substrates, and (4)
simple and usable in a variety of the organic reactions
mediated by SmI2.
Due to their thermochemical properties (redox proper-
ties and strength of the Ln-O bonds) and the low
reactivity of lanthanide metals (if they are not activated
with iodine or mercury salts),8 cerium, lanthanum, neo-
(9) Mischmetall as small ingots (about 5 g) is purchased from Fluka
(about $6 per 500 g). It is an industrial material used, for example, as
an additive for iron-smelting. It is also the main component of flints.
Mischmetall ingots are easily scraped in air with a rasp; the powder
is kept under argon (average MW of mischmetall is 140).
* To whom correspondence should be addressed: Tel: (+33) 1 69
15 47 43. Fax: (+33) 1 69 15 46 80. E-mail: jelonamy@icmo.u-psud.fr.
(1) Namy, J . L.; Girard, P.; Kagan, H. B. Nouv. J . Chim. 1977, 1,
5-7.
(10) P r oced u r e for Sa m a r iu m Diiod id e-Ca ta lyzed Rea ction s:
Barbier-Type Reactions. In a Schlenk tube under argon, 5 mmol (0.7
g, 1.4 equiv.) of mischmetall powder were suspended in THF (7 mL),
with 0.7 mmol (0.2 equiv) of SmI2. A solution of 2-octanone (3.5 mmol)
and organic halide (4.2 mmol, 1.2 equiv) in THF (7 mL) was slowly
added to the THF/SmI2/mischmetall suspension at such a rate that
the deep blue color of SmI2 was maintained (see Table 1). The mixture
was then stirred for an additional period of 0.5 or 1.0 h (see Table 1)
and then quenched with HCl (1 M) and stirred for 15 min to obtain a
clear solution that was extracted with ether. The combined extracts
were washed with sodium thiosulfate and brine. The organic layer was
dried over MgSO4, and the solvents were removed under reduced
pressure. The crude material was purified by flash chromatography
on silica gel. Similar procedures were used for other SmI2-catalytic
reactions: reduction of organic halides, pinacol coupling of acetophe-
none, and coupling of 1-methyl-1-cyclohexanecarboxylic acid chloride.
The dl/meso ratio of pinacol coupling products was determined from
previously published NMR data.14
(2) (a) Prandi, J .; Namy, J . L.; Menoret, G.; Kagan, H. B. J .
Organomet. Chem. 1985, 285, 449-460. (b) Hydrio, J .; Van de Weghe,
P.; Collin, J . Synthesis 1997, 68-72.
(3) In contrast, the preparation of SmBr2 can be performed either
from samarium metal or from samarium oxide: Lebrun, A.; Rantze,
E.; Namy, J . L.; Kagan, H. B. New J . Chem. 1995, 19, 699-705.
(4) Samarium chunks (99.9%) can be purchased from Acros (about
$6 per 50 g).
(5) Nomura, R.; Matsuno, T.; Endo, T. J . Am. Chem. Soc. 1996, 118,
11666-11667.
(6) Annunziata, R.; Benaglia, M.; Cinquini, M.; Cozzi, F.; Raimondi,
L. Tetrahedron Lett. 1998, 39, 3333-3336.
(7) Corey, E. J .; Zheng, G. Z. Tetrahedron Lett. 1997, 38, 2045-
2048.
(8) Imamoto, T.; Kusumoto, T.; Tawayarama, Y.; Sugiura, Y.; Mita,
T.; Hatanaka, Y.; Yokoyama, M. J . Org. Chem. 1984, 49, 3904-3912.
(11) Girard, P.; Namy, J . L.; Kagan, H. B. J . Am. Chem. Soc. 1980,
102, 2693-2698.
10.1021/jo9820667 CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/26/1999

Notes
J . Org. Chem., Vol. 64, No. 8, 1999 2945
Sch em e 1
Sch em e 2
to SmI2) presumably impedes the reduction of Sm(III)
species into Sm(II) ones; no turnover of SmI2 can be
effected and only about 10% of the tertiary alcohol is
detected. In contrast, the beneficial effect on Barbier-type
reactions of very small quantities of NiI2 (0.001 equiv) is
reinforced again (run 6).12 We have also verified that
cerium or lanthanum metal can be used instead of
mischmetall; under the conditions of run 1 or 2, the
tertiary alcohol is obtained in similar yields.
A possible reaction pathway involves a Barbier-type
reaction mediated by SmI2, through an organosamarium
compound that leads to a samarium(III) alkoxide and
SmI3. These species are then reduced by Ln (La, Ce, Pr,
or Nd) metals to give Ln(III) alkoxides and regeneration
of samarium diiodide (Scheme 1).
Sch em e 3
This assumption is supported by the following observa-
tions. Treatment of a yellow suspension of (t-Bu)OSmI2
with mischmetall in THF leads to the fast formation (10
min) of the deep blue SmI2 color. Samarium diiodide is
regenerated from SmI3 in a similar fashion.
Sch em e 4
Alternatively, a transmetalation could take place after
formation of an organosamarium compound, leading to
another organometallic compound such as R3Ln (Ln other
than Sm) and regeneration of samarium diiodide. Addi-
tion of R3Ln to the ketone could furnish Ln(III) alkoxides
(Scheme 2).
The samarium diiodide-catalyzed reaction of 1-iodo-
dodecane has also been studied (Scheme 3).
Slow addition (over 3 h) of the halide to the THF/SmI2/
mischmetall suspension allows the persistence of the
deep blue color of SmI2. A mixture of dodecane and
1-dodecene is quantitatively obtained in a 2/1 ratio.
Hydrolysis with D2O does not lead to any deuterium
incorporation, and without SmI2 the halide is quantita-
tively recovered. The presence of 1-dodecene suggests the
formation of either an organometallic species that un-
dergoes a â-hydrogen elimination or a radical dispropor-
tionation reaction.
Samarium diiodide-catalyzed reactions of 4-(t-Bu)-
benzyl bromide have also been performed (Scheme 4).
Quenching of the reaction mixture with D2O gives
4-tert-butyltoluene with incorporation of deuterium and
small amounts of 4,4′-di-tert-butyldibenzyle. Trapping
with butanone furnishes the tertiary alcohol in good yield
(10% of 4,4′-di-tert-butyldibenzyle and 18% of 4-tert-
butyltoluene are also detected). These experiments
strongly support the formation of stable organometallic
species. As it is known that 4-(tert-butyl)benzylsamarium
diiodide obtained from the reaction of 4-(tert-butyl)benzyl
bromide with SmI2 is not stable in THF,13 the formation
of an organometallic compound such as R3Ln (Ln other
(12) Machrouhi, F.; Hamann, B.; Namy, J . L.; Kagan, H. B. Synlett
1996, 633-634.
(13) Bied, C.; Collin, J .; Kagan, H. B. Tetrahedron 1992, 48, 3877-
3890.

2946 J . Org. Chem., Vol. 64, No. 8, 1999
Notes
Sch em e 5
Sch em e 6
ing to a samarium(III) pinacolate, the reduction of which
by mischmetall regenerates SmI2. Without samarium
diiodide, no reaction occurs.
The coupling of acid chlorides to give R-ketols, medi-
ated by Sm(II) species has been extensively studied.15 We
found that it can be also performed with the (SmI2-
catalytic)/mischmetall system under the conditions in-
dicated in Scheme 6. The use of a very small amount of
NiI2 avoids the formation of byproducts.12 Mischmetall
alone is unreactive (Scheme 6).
To conclude, we have found an efficient, simple, and
low-cost system to perform catalytically the reactions of
samarium diiodide. Many transformations mediated by
this useful reagent could be conducted under these
conditions. We are currently exploring the scope of
potential applications for the (SmI2-catalytic)/mischmet-
all system, especially in the case of polyfunctionalized
substrates.
than Sm; R ) 4-(tert-butyl)benzyl) is likely. This indicates
that the reaction pathway proposed in Scheme 2 for the
SmI2-catalyzed Barbier reactions cannot be ruled out.
Without SmI2 only 4,4′-di-tert-butyldibenzyle is obtained.
Surprisingly, an equimolar mixture of 2-octanone and
4-(tert-butyl)benzyl bromide is unreactive with misch-
metall alone.
The pinacolic coupling of acetophenone can also be
readily performed using SmI2 as a catalyst and misch-
metall as a coreductant (Scheme 5). In contrast to the
previously reported procedure that uses magnesium as
a coreductant, there is no need to add trimethylsilyl
chloride to cleave the Sm(III)-O bonds.5 The pinacolic
coupling products are obtained in good isolated yields,
and small amounts (less than 5%) of 1-phenylethanol are
also detected. The dl/meso ratio (80/20)10,14 is the same
as that measured when the reaction is performed under
stoichiometric conditions (79/21). This result is in good
agreement with a pinacolization mediated by SmI2 lead-
Ack n ow led gm en t. We thank the University of
Paris-Sud and the CNRS for their financial support and
Professor H. B. Kagan for fruitful discussions.
(14) Seebach, D.; Oei, H.-A.; Daum, H. Chem. Ber. 1977, 110, 2316-
2333.
(15) Collin, J .; Namy, J . L.; Dallemer, F.; Kagan, H. B. J . Org. Chem.
1991, 56, 3118-3122.
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