Selective reduction of carbon-carbon double and triple bonds in conjugated olefins mediated by SmI2/H2O/amine in THF

Article abstract of DOI:10.1016/S0040-4039(03)00369-1

Conjugated double and triple bonds are reduced into alkenes using non-hazardous SmI₂/H₂O/amine mixtures as reducing agents in THF. Isolated alkenes are not reduced during these reductions. All the reactions studied are quantitative and are completed in less than five minutes.

Full text of DOI:10.1016/S0040-4039(03)00369-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2661–2664
Selective reduction of carbonꢀcarbon double and triple bonds in
conjugated olefins mediated by SmI2/H2O/amine in THF
Anders Dahle´n and Go¨ran Hilmersson*
Organic Chemistry, Department of Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden
Received 18 December 2002; revised 28 January 2003; accepted 7 February 2003
Abstract—Conjugated double and triple bonds are reduced into alkenes using non-hazardous SmI2/H2O/amine mixtures as
reducing agents in THF. Isolated alkenes are not reduced during these reductions. All the reactions studied are quantitative and
are completed in less than five minutes. © 2003 Elsevier Science Ltd. All rights reserved.
Recently there has been a rapid development of SmI2-
mediated transformations.1 SmI2 has become a versatile
and selective reagent that mediates a number of differ-
ent reduction and coupling reactions.2 Recently we
reported that a mixture of H2O/amine as co-solvent
strongly accelerates the rate of reduction of ketones,
a,b-unsaturated esters and imines promoted by SmI2.3
Encouraged by the large acceleration of these reactions,
we explored the use of SmI2 in other reduction reac-
tions. SmI2/H2O/amine3 or SmI2/HMPA4 mixtures
readily reduce a,b-unsaturated esters, but to the best of
our knowledge, SmI2 has so far proved to be too weak
a reductant for conjugated olefins.
Styrene is almost immediately reduced to ethyl benzene
in quantitative yield in the presence of SmI2/H2O/amine
(Table 1), leaving a white precipitate of samarium- and
amine-salts. There is no reduction of the phenyl group.
The terminal double bonds of styrene are conjugated
with the aromatic ring and hence more reactive, there-
fore it was not surprising that both cis- and trans-stil-
bene were reduced to the corresponding saturated
compound, dibenzyl.
Table 1. Reduction of styrene derivatives by SmI2/H2O/
PMDTA
In our exploration of SmI2/H2O/amine mixtures in the
reduction of various functional groups we have discov-
ered that conjugated olefins can be selectively reduced
in the presence of isolated double bonds. Herein we
present the use of SmI2/H2O/amine mixtures in THF,
using either of the amines Et3N, N,N,N%,N%-tetra-
methylethylene diamine (TMEDA) or N,N,N%,N%%,N%%-
pentamethylethylene triamine (PMDTA), for the
reduction of conjugated olefins.
As we demonstrated in our earlier studies, the facile
SmI2/H2O/amine mediated reduction of ketones can be
understood from the equation described below. We
believe that this equation also applies for conjugated
olefins.
The transfer of two deuteriums from D2O to the double
bonds of styrene and stilbene was also confirmed by
GC/MS. Measurement of the initial rates of the reduc-
tion of styrene with SmI2/H2O/amine and SmI2/D2O/
amine, respectively, showed no difference in rates, i.e.
there was no primary kinetic isotope effect (kH/kD=1).
This indicates that the proton transfer is not involved in
the rate-determining step; therefore the formation of a
conjugated radical or anion is most likely the rate-
determining step.
* Corresponding author. Tel.: +46-31-772 2904; fax: +46-31-772 3840;
e-mail: hilmers@organic.gu.se
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00369-1

2662
A. Dahle´n, G. Hilmersson / Tetrahedron Letters 44 (2003) 2661–2664
Addition of phenylacetylene to the SmI2/H2O/amine
mixture resulted in rapid reduction of the triple bond to
styrene in ten seconds. Styrene was then further
reduced into ethyl benzene within 5 min, provided that
an excess of SmI2/H2O/amine was used (Table 2). Like-
wise was diphenylacetylene readily reduced into diben-
zyl via stilbene. It should be noted that approximately
2.5 equiv. of SmI2 is necessary to ensure complete
reduction of each conjugated bond.
A different triene is myrcene, which has two of its
double bonds conjugated while the third is isolated
(Table 4, entry 1). Reduction of myrcene with SmI2/
H2O/amine gives a mixture of four compounds accord-
ing to GC/MS analysis. The masses (m/z) of these
compounds are identical (M+=138) corresponding to
the reduction of one conjugated double bond in
myrcene. The formation of four products indicates that
the reaction is not concerted, and the reaction must
proceed via several competing intermediates. A tenta-
tive assignment of the four signals in the chromatogram
was made based on fragmentation patterns and fitted to
a database (NIST)5 after EI ionization. This assignment
was also supported by GC analysis on achiral and
chiral columns.6 The E and Z isomers have approxi-
mately the same retention times. One of the alkenes
exists as a racemate, which was resolved using chiral
GC (Table 4, entry 1a). Analysis of the product compo-
sition indicated that the reduction of the terminal dou-
ble bond is slightly favored (56%). Interestingly, both
the E and the Z stereoisomers were formed in equal
quantities. The product composition shows that the
reductions may occur via both 1,2- and 1,4-addition to
the diene and, yet again, the isolated double bond was
not affected.
Table 2. Reduction of phenylacetylene derivatives by
SmI2/H2O/PMDTA
These unsaturated systems are all conjugated with an
aromatic ring, but SmI2/H2O/amine mixtures are also
capable of reducing conjugated dienes and trienes
(Table 3). The conjugated diene 1,3-cyclohexadiene was
rapidly reduced into cyclohexene with SmI2/H2O/
amine, but not further reduced into cyclohexane. It
appears that the double bonds must be conjugated,
otherwise there is no reduction, i.e. 1,4-cyclohexadiene
does not undergo reduction with SmI2/H2O/amine.
Another conjugated cyclic alkene, 1,3,5-cyclohepta-
triene, with three conjugated double bonds was also
reduced to the corresponding cycloalkene. After 10 s in
a SmI2/H2O/amine mixture at room temperature all the
cycloheptatriene was reduced into 1,3-cycloheptadiene
and smaller amounts of cycloheptene. The only
detected product was cycloheptene after approximately
five minutes. Hence it is possible to reduce this triene
into the corresponding diene, if desired, or all the way
to the corresponding alkene.
Table 4. Reduction of alkene derivatives by SmI2/H2O/
Et3Na
Table 3. Reduction of cyclic alkene derivatives by SmI2/
H2O/PMDTA
To confirm the importance of conjugation a mixture of
cis- and trans-1,3-hexadiene was also reduced to a
mixture of 1-hexene, cis- and trans-2-hexene and cis-

A. Dahle´n, G. Hilmersson / Tetrahedron Letters 44 (2003) 2661–2664
2663
and trans-3-hexene (Table 4, entry 2). We could not
resolve completely these isomers on GC, however it is
clear that the formation of 2- and 3-hexene were
favored over 1-hexene. None of these alkenes was fur-
ther reduced to hexane with SmI2/H2O/amine. Further-
more, the unconjugated 1,4- and 1,5-hexadienes were
not affected by the SmI2/H2O/amine mixtures (Table 4,
entries 3 and 4).
In conclusion, the present development of SmI2/H2O/
amine mixtures for the selective reduction of conjugated
carbonꢀcarbon double bonds is very promising. But a
disadvantage is the formation of mixtures of products,
which will be obtained from unsymmetrical substrates,
such as myrcene. However, this method of reducing
double bonds does not require any pyrophoric cata-
lysts,7 H2-gas8 or alkali metal (Li or Na),9,10 and the
reagent mixture SmI2/H2O/amine is considered non-
hazardous. In addition the reductions are fast and
performed under 1 atm at room temperature. Further-
more, the use of H2O as the hydrogen source allows a
simple and inexpensive way for incorporating deu-
terium from D2O. High selectivity is a major advantage
in SmI2 reactions, however, the SmI2/H2O/amine mix-
ture may prove too powerful for selective reactions
involving multifunctional substrates, and this has yet to
be determined.
Treatment of cis-stilbene with SmI2 only for 24 h did
not result in any isomerization to the more stable
trans-isomer. Hence the reactions do not proceed via a
preequilibrium between the alkene and an organo-
samarium intermediate, since such an intermediate
would be expected to isomerize to some extent. The
addition of SmI2 to the conjugated double bond
requires both water and an amine.
A mechanism for the reduction of conjugated double
and triple bonds is given in Scheme 1. The mixture
SmI2/H2O/amine is suggested to generate an electron
and a proton which, based on these observations, read-
ily add to the conjugated alkene. The radical that is
formed is stabilized by delocalization of the unpaired
electron into the conjugated p-system. The reduction
using SmI2, being a single electron transfer reagent,
must therefore proceed in two steps. The first involves
the formation of p- or alkene-stabilized radical interme-
diates. We suggest that this first step in the reduction
reaction proceeds via either a 1,2- or 1,4-addition mech-
anism. The observation of both the 1,2- and 1,4-reduc-
tion products is also explained by equilibrium between
two pairs of resonance-stabilized radicals. The driving
force in these reduction reactions appears to be the
precipitation of Sm(OH)3 and R3N·HI.3 The radical
intermediate reacts with a second electron and proton
from SmI2 and water to generate the alkene. In order to
operate by this mechanism only conjugated olefins or
olefins with aromatic groups can be reduced.
Experimental
The SmI2 solutions were purchased from Aldrich (0.1
M SmI2 in THF). All substances, including starting
materials and products, were purchased from commer-
cial resources. The products of reduced myrcene were
not available; therefore a tentative assignment of these
products was carried out based on GC/MS, the NIST
Mass Spectral Database, and chiral GC analysis. Verifi-
cation of the retention times of the products with
authentic samples along with GC/MS analysis iden-
tified all other products. The non-volatile compounds
1
were also identified using H NMR.
General procedure for the reduction of a conjugated
double and/or triple bond: SmI2 in THF (0.1 M,
Aldrich, 2.5 equiv.) was added to a dry Schlenk tube,
fitted with a septum and containing a magnetic stirrer
bar, inside a glove box with a nitrogen atmosphere. The
amine, i.e. Et3N (5 equiv.), TMEDA (2.5 equiv.) or
PMDTA (1.7 equiv.), and the alkene (1 equiv.) were
added at 20°C. The proton donor, i.e. H2O (6.25
equiv.), was added dropwise using a gastight syringe.
Small portions of the mixture (100 mL) were removed
via a syringe and quenched with I2 in n-hexane (0.1 M,
0.1 mL). To the quenched solution diethyl ether (1 mL)
and HCl (0.12 M, 0.1 mL) were added to dissolve the
inorganic salts and finally Na2S2O3 to remove excess
iodine. The organic layer was transferred to a vial and
the yield of the reaction was analyzed with GC. All
products were analyzed by GC/MS and also compared
with authentic samples by GC.
All the reduction reactions reported proceed with any
of the tested amines, i.e. triethylamine, TMEDA or
PMDTA. However, there is a slightly higher rate of
reduction using PMDTA. In the absence of either water
or amine none of the reported SmI2-mediated reduc-
tions of any of the alkenes could be observed.
The rate measurements of the primary kinetic isotope
effect of styrene (1 equiv.) were performed using SmI2
in THF (0.1 M, 7 equiv.), Et3N (14 equiv.) and H2O or
D2O (35 equiv.). Samples from the reaction mixture
were quenched every ten seconds, work-up was then
carried out as described above, and the yield was
determined by GC.
All reported yields are based on GC analysis; however,
performing reductions on a larger scale showed that the
isolated yields are also nearly quantitative. Reactions
Scheme 1. Suggested mechanism for the reduction of 1,3-
hexadiene.

2664
A. Dahle´n, G. Hilmersson / Tetrahedron Letters 44 (2003) 2661–2664
on larger scale may be purified by dissolving the inor-
ganic samarium-salts and precipitated amines in HCl
(0.1 M), extracting the water layer with diethyl ether
and finally washing the ether layer with Na2S2O3 and
brine. The ether layer is then dried over Na2SO4 and,
after evaporation, the product was sufficiently pure for
further use.
2. (a) Molander, G. A.; Etter, J. B. Synth. Commun. 1987,
17, 901–912; (b) Huang, Z.; Jin, H.; Duan, D. Synth.
Commun. 2002, 32, 565–570; (c) Namy, J. L.; Souppe, J.;
Kagan, H. B. Tetrahedron Lett. 1983, 24, 765–766; (d)
Miller, R. S.; Sealy, M. S.; Shabangi, M.; Kuhlman, M.
L.; Fuchs, J. R.; Flowers, R. A., II. J. Am. Chem. Soc.
2000, 122, 7718–7722; (e) Taniguchi, N.; Uemura, M. J.
Am. Chem. Soc. 2000, 122, 8301–8302; (f) Kito, M.;
Sakai, T.; Haruta, N.; Shirahama, H.; Matsuda, F. Syn-
lett 1996, 11, 1057–1060; (g) Molander, G. A.; Harris, C.
R. J. Am. Chem. Soc. 1996, 118, 4059–4071.
A faster and more convenient work-up procedure for
isolation of the crude product is centrifugation of the
precipitated salts followed by evaporation of THF.
3. (a) Dahle´n, A.; Hilmersson, G. Tetrahedron Lett. 2002,
43, 7197–7200; (b) Dahle´n, A.; Hilmersson, G. Chem.
Eur. J. 2003, in press.
Acknowledgements
4. (a) Inanaga, J.; Sakai, S.; Handa, Y.; Yamaguchi, M.;
Yokoyama, Y. Chem. Lett. 1991, 12, 2117–2118; (b) Lin,
T.-Y.; Fuh, M.-R.; Chan, I. S. J. Chin. Chem. Soc. 2001,
48, 843–847; (c) Cabrera, A.; Alper, H. Tetrahedron Lett.
1992, 33, 5007–5008.
The authors would like to thank Dr. Anna Bo¨rje for a
sample of myrcene. Financial support from the Carl
Trygger foundation and the Swedish Natural Research
Council (VR) is also gratefully acknowledged.
5. NIST Mass Spectral Search Program, Microsoft Corpora-
tion®.
6. GC–MS column: CP-Sil 8 CB Low Bleed column (¥=
0.25 mm, length=30 m), Achiral GC column: CPWAX
52CB column (¥=0.25 mm, length=25 m), Chiral GC
column: CP Chirasil-Dex CB column (¥=0.25 mm,
length=25 m).
7. Pyrophoric catalyst e.g. LiAlH4: Magoon, E. F.; Slaugh,
L. H. Tetrahedron 1967, 23, 4509–4515, or Lindlar’s
catalyst: Lindlar, H.; Dubuis, R. Org. Synth. 1973, V,
880.
8. H2, Pt: Siegel, S.; Smith, G. V. J. Am. Chem. Soc. 1960,
82, 6082–6087.
9. Alkali metal (Li or Na), e.g.: Caine, D.; Chao, S. T.;
Smith, H. A. Org. Synth. 1977, 56, 52–58.
References
1. (a) Molander, G. A. In Organic Reactions; Paquette, L.
A., et al., Eds.; John Wiley & Sons 1994; Vol. 46 (Reduc-
tions with Samarium(II) Iodide), pp. 211–367; (b) Krief,
A.; Laval, A.-M. Chem. Rev. 1999, 99, 745–777; (c)
Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 96,
307–338; (d) Molander, G. A. Chem. Rev. 1992, 92,
29–68; (e) Nomura, R.; Endo, T. Chem. Eur. J. 1998, 4,
1605–1610; (f) Kim, S. M.; Byun, S., II; Kim, Y. H.
Angew Chem. 2000, 39, 728–731; (g) Nakamura, Y.;
Takeuchi, S.; Ohira, A.; Ohgo, Y. Tetrahedron Lett. 1996,
37, 2805–2808; (h) Takeuchi, S.; Nakamura, Y.; Ohgo,
Y.; Curran, D. P. Tetrahedron Lett. 1998, 39, 8691–8694;
(i) Pedersen, L. H.; Christensen, T. B.; Enemaerke, R. J.;
Daasbjerg, K.; Skrydstrup, T. Eur. J. Org. Chem. 1999, 3,
565–572; (j) Steel, P. S. J. Chem. Soc., Perkin Trans. 1
2001, 21, 2727–2751.
10. General references for reduction of various functional
groups: Catalytic Hydrogenation in Organic Synthesis:
Procedures and Commentary; John Wiley & Sons, New
York, 1978; Nylander, P. N. Hydrogenation Methods,
Academic Press: Orlando, FL, 1985.