A new preparation of samarium dibromide and its use in stoichiometric and catalytic pinacol coupling reactions

Article abstract of DOI:10.1016/S0040-4039(03)01229-2

A new convenient preparation of samarium dibromide in THF is reported. Pinacol coupling reactions using SmBr₂ in catalytic amounts together with mischmetall as a coreductant have been performed with a variety of carbonyl compounds.

Full text of DOI:10.1016/S0040-4039(03)01229-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5507–5510
A new preparation of samarium dibromide and its use in
stoichiometric and catalytic pinacol coupling reactions
Florence H e´ lion, Marie-Isabelle Lannou and Jean-Louis Namy*
Laboratoire de Catalyse Mol e´ culaire, associ e´ au CNRS, ICMO, Bat 420, Universit e´ Paris-Sud, 91405 Orsay, France
Received 9 May 2003; revised 16 May 2003; accepted 19 May 2003
Abstract—A new convenient preparation of samarium dibromide in THF is reported. Pinacol coupling reactions using SmBr in
2
catalytic amounts together with mischmetall as a coreductant have been performed with a variety of carbonyl compounds. © 2003
Elsevier Science Ltd. All rights reserved.
1
0
1
. Introduction
additive and magnesium as a coreductant. We have
also described a catalytic pinacolization with samarium
diiodide and mischmetall but this system is efficient
Intermolecular and intramolecular pinacol coupling
reactions give an efficient way of forming car-
bonꢀcarbon bonds. Lanthanide metals have been used,
sometimes in the presence of additives such as TMSCl
or HMPA, to achieve in most cases coupling of aro-
matic ketones, while divalent samarium compounds
allow pinacolic coupling of aromatic as well as aliphatic
ketones and aldehydes. Although samarium diiodide
has been widely employed in pinacolization, samarium8
dibromide seems to be a more powerful reagent.
Besides, we have recently reported that mischmetall (a
cheap alloy of the light lanthanides used in large
amounts for industrial applications) can act as a core-
ductant for catalytic reactions of some organic sub-
11
only for aromatic ketones.
1
2
2. Results and discussion
3–7
Before studying catalytic pinacolic coupling, we first
tried to find an easy and reliable method of preparation
of samarium dibromide. This compound can be pre-
pared in several steps starting from samarium oxide or
directly from samarium metal and 1,2-dibromoethane
in THF. However, the latter reaction usually needs a
long induction time and small amounts of diiodoethane
must be added to ensure reproducible results. To over-
come this drawback, we attempted to use 1,1,2,2-tetra-
bromoethane instead of 1,2-dibromoethane. Mixing of
samarium with one equivalent of C H Br in THF
12
9
strates with samarium diiodide. This led us to examine
the possibility of performing catalytic pinacol coupling
reactions with samarium dibromide and mischmetall as
a coreductant. A catalytic pinacolization has been pre-
viously reported with samarium diiodide, TMSCl as an
2
2
4
resulted in a fast exothermic reaction, evolution of a
Scheme 1.
Keywords: pinacolic coupling; catalysis; 1,1,2,2-tetrabromoethane; mischmetall; samarium dibromide.
*
Corresponding author. Fax: +33-1-69-15-4680; e-mail: jelonamy@icmo.u-psud.fr
0
040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01229-2

5
508
F. H e´ lion et al. / Tetrahedron Letters 44 (2003) 5507–5510
gas, and disappearance of samarium metal. A cream-
coloured solid was formed which failed for the coupling
of 5-nonanone into diol. It was identified as being
Samarium dibromide prepared from Sm and 0.5 equiv.
tetrabromoethane in THF was next tested for the pina-
colization of some representative carbonyl compounds,
results are gathered in Table 1.
13
samarium tribromide. With half an equivalent of
,1,2,2-tetrabromoethane a gas was also produced and
1
a dark blue solid, which was slightly soluble in THF,
was obtained. Its reaction with 5-nonanone furnished
the expected diol in 87% isolated yield. These properties
As expected, all reactions afford diols in good to excel-
lent isolated yields, thus demonstrating the value of the
preparation.
16
are quite in agreement with those of SmBr . The gas
was identified as being acetylene by trapping with
2
For economic reasons, it is highly desirable to develop
methods that use the mixture of lanthanide metals. We
considered then the possibility of performing catalytic
aqueous Cu(NH ) Cl (brick red cuprous acetylide was
3
2
formed). In addition, we observed that Sm reacts slowly
within 3 days) at 20°C in THF with a mixture of
(
1
pinacolizations using SmBr
2
in catalytic amounts and
,2-dibromoethene (Z/E=20/80) in THF affording
mischmetall as a coreductant.
14
samarium dibromide with evolution of acetylene.
Scheme 1 sums up the results.
In SmI catalytic Barbier-type reaction, we had previ-
2
ously observed that a simple experimental procedure,
which starts from a mixture of samarium metal and
mischmetall treated by 1,2-diiodoethane gave excellent
The reaction of Sm with 1,1,2,2-tetrabromoethane
clearly differs from that of Zn, which furnishes ZnBr
2
9
15
results. Here we have used this methodology, changing
and a mixture of (Z)- and (E)-1,2-dibromoethene. It
can be assumed that Sm reacts with tetrabromoethane
to give acetylene directly. Tentatively, the pathway
depicted in Scheme 2 can be proposed.
the diiodide for the tetrabromide.
The results are collected in Table 2.
This procedure involving a mixture of Sm and misch-
metall has allowed us to isolate diols in moderate to
good yields. In some cases a slow addition of the
carbonyl compound was necessary in order to allow
regeneration of Sm(II) species thus preventing forma-
tion of by-products. With cyclohexanone and cyclopen-
tanone a fast addition gave a complex mixture of
products, which were formed in aldol-type reactions
while with octanal a mixture of trioxanes arising from
trimerization of the aldehyde was obtained (identified
by GC–MS). This feature was not unexpected since it is
known that these reactions are efficiently catalysed by
Reaction of mischmetall with an excess of 1,1,2,2-tetra-
bromoethane in THF was also investigated. As with
samarium an exothermic reaction occurred, but acetyl-
ene was given off slowly and 1,2-dibromoethenes were
detected. A yellow solid, identified as a mixture of
lanthanide tribromides, was obtained after disappear-
ance of the metal.
1
3
17
lanthanide(III) species. The quantities of each reagent
mischmetall, samarium, tetrabromoethane) have been
optimized according to the procedure (fast or slow
(
18,19
addition of the carbonyl compounds).
We observed
in some cases that an excess of tetrabromoethane, with
respect to samarium, has a beneficial effect on the
Scheme 2.
Table 1. Stoichiometric pinacolization with SmBr2
Entry
Carbonyl compound
Yields (%)a
1
2
3
4
5
5-Nonanone
87
74
94
62
78
Cyclopentanone
Acetophenone
Octanal
b
c
1,3-Dibenzoyl-propaned
e
a
Isolated yields.
dl/meso=80/20.
dl/meso=50/50.
b
c
d
e
SmBr /carbonyl compound=0.5.
2
cis-1,2-Diphenyl-cyclopentan-1,2-diol.

F. H e´ lion et al. / Tetrahedron Letters 44 (2003) 5507–5510
5509
yields in pinacol (entries 10–14), probably in making
easier regeneration of samarium dibromide (more bro-
mide ions are available).
Scheme 3.
In the absence of samarium, the diols were not
obtained, either the carbonyl compound was recovered
or a complex mixture of products was formed. Without
tetrabromoethane, no reaction took place.
Table 2. Catalytic pinacolizations with SmBr and mis-
chmetall
2
The procedure (mixing of the metals prior to addition
of tetrabromoethane) was a success because either
samarium metal is much more reactive with tetra-
bromoethane than mischmetall or it is able to reduce
LnBr into Ln giving SmBr (Scheme 3).
3
2
Lack of solubility of SmBr and LnBr in THF does
Conditionsa Yields (%)b
2
3
Entry Carbonyl compounds x/y/z
not permit a check of this latter assumption. However,
it was possible to observe that mixing of LnI (Ln=
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Cyclobutanone
Cyclopentanone
Cyclohexanone
Cyclooctanone
Benzaldehyde
Phenylacetone
4-Phenylbutane-2-one 5/1.4/0.7
Octanal
Pivalaldehyde
Acetophenone
1,3-Dibenzoylpropane 5/0.7/1.4
5/1.4/0.7
5/1.4/0.7
5/1.4/0.7
5/1.4/0.7
5/1.4/0.7
5/1.4/0.7
Slow
Slow
Slow
Slow
Slow
Slow
Fast
Slow
Slow
Fast
Fast
Slow
Fast
Fast
66
42
55
69
62
32
60
61
63
72
70
72
54
68
3
mischmetall) with samarium metal in THF results in
the formation of SmI , the deep blue-green color of
2
which is easy to detect. This fact gives support to the
above hypothesis.
c
d
d
d
e
The following catalytic scheme can be proposed. In
5/1.4/0.7
5/1.4/0.7
5/0.7/1.4
contrast to the previously reported SmI -catalytic pina-
2
10
colization, there is no need to use TMSCl together
f
0
1
2
3
4
with a coreductant since cerium, lanthanum and
neodymium (the main components of mischmetall) are
able to cleave the SmꢀO bond and to reduce Sm(III)
species into Sm(II) ones (Scheme 4).
g
h
d
d
2-Octanone
3-Octanone
5-Nonanone
5/0.7/1.4
5/0.7/1.4
5/0.7/1.4
a
Slow: carbonyl compound was slowly added within 14 h with a
syringe pump, the mixture was then stirred for an additional period
of 2 h. Fast: carbonyl compound was quickly added, the mixture
was then stirred for 16 h.
Isolated yields.
dl/meso=57/43.
dl/meso=50/50.
dl/meso=100/0.
dl/meso=80/20.
Carbonyl compound: 2 mmol.
In conclusion, we have found a rapid and convenient
method of preparation of SmBr . A new reaction of
2
samarium metal with 1,1,2,2-tetrabromoethane has
been explored. In contrast to zinc, samarium metal
gives acetylene directly. The efficiency of SmBr2 in
pinacolic coupling of a variety of carbonyl compounds
has been demonstrated. It can be employed as well in
stoichiometric as in catalytic amounts. In the latter
case, mischmetall is a suitable coreductant which avoids
the use of TMSCl.
b
c
d
e
f
g
h
cis-1,2-Diphenylcyclopentan-1,2-diol.
Scheme 4.

5
510
F. H e´ lion et al. / Tetrahedron Letters 44 (2003) 5507–5510
material was purified by flash chromatography on silica
Acknowledgements
gel.
17. Namy, J. L.; Souppe, J.; Collin, J.; Kagan, H. B. J. Org.
We are indebted to Dr. J. Collin for fruitful discussions.
We thank the University of Paris-Sud and the CNRS
for their financial support.
Chem. 1984, 49, 2045–2049.
18. Fast addition: In a Schlenk tube under argon, 5 mmol (0.7
g) of mischmetall powder were suspended in THF (2
mL), with 0.7 mmol of Sm (0.105 g) and 1.4 mmol of
Br CHCHBr at room temperature within 2 h. Ketone or
2
2
References
aldehyde (4 mmol) in THF (12 mL) was then added to
the THF/SmBr /mischmetall suspension. The mixture
was stirred for a period of 16 h. Workup was performed
as described above.
2
1
. Robertson, G. M. In Comprehensive Organic Synthesis;
Trost, B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 3,
pp. 563–610.
. Ogawa, A.; Takeuchi, H.; Hirao, T. Tetrahedron Lett.
1
Slow addition: In a Schlenk tube under argon, 5 mmol
2
3
4
5
6
(
0.7 g) of mischmetall powder were suspended in THF (2
999, 40, 7113–7114.
. Namy, J. L.; Souppe, J.; Kagan, H. B. Tetrahedron Lett.
983, 24, 765–766.
. Molander, G. A.; Kenny, C. J. Am. Chem. Soc. 1989,
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mL), with 1.4 mmol of Sm (0.21 g) and 0.7 mmol of
Br CHCHBr at room temperature within 2 h. THF (5
2
2
1
mL) was then added. Ketone or aldehyde (4 mmol) in
THF (7 mL) was slowly added within 14 h to the
1
THF/SmBr /mischmetall suspension. The mixture was
2
. Hanamoto, T.; Sugimoto, Y.; Sugino, A.; Inanaga, J.
Synlett 1994, 377–378.
. Pedersen, H. L.; Christensen, T. B.; Enemærk, R. J.;
Daasbjerg, K.; Skrydstrup, T. Eur. J. Org. Chem. 1999,
then stirred for an additional period of 2 h. Workup was
performed as described above.
1
9. Selected spectral data: cis-1,2-Diphenylcyclopentane-1,2-
diol. Purification: column chromatography on silica gel
5
65–572.
(
7
EtOAc–pentane, 20:80) afforded pure alcohol (0.383 g;
7
8
. Jong, S.-J.; Fang, J.-M. Org. Lett. 2000, 2, 1947–1949.
. Lebrun, A.; Namy, J. L.; Kagan, H. B. Tetrahedron Lett.
1
0% yield): R =0.4; dl/meso: 0/100 H NMR (250 MHz,
f
CDCl ): l 7.05 (m, 10H, 2 Ph), 3.00 (s, 2H, 2 OH), 2.58
3
1
993, 34, 2311–2314.
13
(
m, 2H), 2.45–2.00 (m, 4H); C NMR (63 MHz, CDCl3):
9. Di Scala, A.; Garbacia, S.; H e´ lion, F.; Lannou, M. I.;
l 142.4, 127.2, 126.9, 126.2, 85.7 (C-1, C-2), 36.9, 20.0;
GC/MS: m/z (%)=55 (19), 77 (27), 104 (50), 105 (100),
Namy, J. L. Eur. J. Org. Chem. 2002, 2989–2995.
1
1
1
1
0. Nomura, P.; Matsuno, T.; Endo, T. J. Am. Chem. Soc.
996, 118, 11666–11667.
1. H e´ lion, F.; Namy, J. L. J. Org. Chem. 1999, 64, 2944–
946.
1
20 (19), 133 (13), 218 (6), 236 (2), 254 (4); HRMS (m/z):
1
+
calcd for C H O (M ), 254.1307; found, 254.1310;
17
18
2
FTIR (CaF /CCl ): wmax=3614, 3567, 3093, 3064, 3041,
2
4
2
3029, 2953, 2879, 1497, 1369, 1195, 1124, 1065, 1035.
2. Lebrun, A.; Rantze, E.; Namy, J. L.; Kagan, H. B. New
Bicyclobutyl-1,1%-diol. Purification: column chromatogra-
J. Chem. 1995, 19, 699–705.
phy on silica gel (EtOAc–pentane, 40:60) afforded pure
3. Titrations were performed as previously described:
Namy, J. L.; Girard, P.; Kagan, H. B.; Caro, P. E. New
J. Chem. 1981, 5, 479–484.
4. A reductive debromination of alkenes with Sm in
methanol has been previously reported: Yanada, R.;
Negoro, F. Tetrahedron Lett. 1996, 37, 9313–9316.
5. Noyes, R. M.; Noyes, W. A.; Steinmetz, H. J. Am. Chem.
Soc. 1950, 72, 33–34.
1
alcohol (0.188 g; 66% yield): R =0.27; H NMR (250
f
MHz, CDCl ): l 2.14 (s, 2H, 2 OH), 2.00 (m, 6H), 1.63
3
13
(
m, 2H); C NMR (63 MHz, CDCl ): l 78.4, 30.5, 12.8;
3
1
+
HRMS (m/z): calcd for C H O (M −H O), 124.0888;
8
12
2
found, 124.0885. Anal. calcd for C H O : C, 67.57; H,
8
14
2
9.92; O, 22.50. found: C, 67.31; H, 9.91; FTIR (CaF2/
1
1
CCl ): wmax=3625, 3573, 2989, 2948, 2870, 2841, 1458,
4
1417, 1355, 1301, 1251, 1151, 1125, 1041. 2,2,5,5-Tetra-
6. In a Schlenk tube under argon, 4.4 mmol of Sm (0.66 g)
powder were suspended in THF (10 mL), with 2.2 mmol
of Br CHCHBr at room temperature within 4 h. Ketone
methyl-hexane-3,4-diol. Purification: column chromato-
graphy on silica gel (EtOAc–pentane, 10:90) afforded
2
2
pure alcohol (0.219 g; 63% yield): R =0.34; dl/meso:
or aldehyde (2 mmol) in THF (30 mL) was then added to
f
1
1
2
1
00/0 H NMR (250 MHz, CDCl ): l 3.33 (d, J=6.7 Hz,
the THF/SmBr suspension. The mixture was stirred for
3
2
H, 3-H , 4-H ), 2.34 (d, J=6.7 Hz, 2H, 2 OH), 0.90 (s,
a period of 24 h, then diluted with ether, quenched with
HCl (1 M) and stirred for 15 min to obtain a clear
solution, which was extracted with ether. The combined
extracts were washed with brine, sodium thiosulfate and
2
2
1
3
8H, 9 CH3); C NMR (63 MHz, CDCl ): l 74.8 (C-3,
3
C-4), 35.2 (C-2, C-5), 25.8 (CH ); HRMS (m/z): calcd for
3
+
C H O (M ), 174.1620; found, 174.1620; FTIR (CaF /
10 22
2
2
brine. The organic layer was dried over MgSO , and the
CCl ): wmax=3642, 3543, 2961, 2909, 2870, 1477, 1465,
4
4
solvents were removed under reduced pressure. The crude
1394, 1365, 1240, 1181, 1088, 1060, 1009.