Influence of HMPA on reducing power and reactivity of SmBr2

Article abstract of DOI:10.1021/ol0160957

(matrix presented) Addition of HMPA to SmBr₂ in THF produces a reactant with an estimated redox potential of -2.63 ± 0.01 V (vs Ag/AgNO₃). This powerful reductant is capable of reducing ketimines and alkyl chlorides at room temperature. Although the structure of the reductant has not been established, it is nonetheless a powerful addition to the arsenal of samarium-based reductants currently utilized.

Full text of DOI:10.1021/ol0160957

ORGANIC
LETTERS
2
001
Vol. 3, No. 15
321-2324
Influence of HMPA on Reducing Power
and Reactivity of SmBr2
2
Brian W. Knettle and Robert A. Flowers, II*
Department of Chemistry, UniVersity of Toledo, Toledo, Ohio 43606
rflower@uoft02.utoledo.edu
Received May 10, 2001
ABSTRACT
2 3
Addition of HMPA to SmBr in THF produces a reactant with an estimated redox potential of −2.63 ± 0.01 V (vs Ag/AgNO ). This powerful
reductant is capable of reducing ketimines and alkyl chlorides at room temperature. Although the structure of the reductant has not been
established, it is nonetheless a powerful addition to the arsenal of samarium-based reductants currently utilized.
Reagents based on samarium are widely employed in organic
2
selectivity than SmI in the addition of alkyl iodides to
1
,2
6
2
synthesis. Samarium diiodide (SmI ) is the most widely
ketones. Dicyclopentadienylsamarium produces stable or-
ganosamarium intermediates in reactions with allylic and
used Sm(II) reductant because it is soluble in electron-donor
organic solvents such as THF at moderate concentrations. It
is also stable for long periods of time if stored in an inert
7
benzyllic halides. Kagan and co-workers have shown that
SmBr
superior to both SmI
laboratory has shown that the combination of LiBr and SmI
produces soluble SmBr and that this reagent is a more
powerful reductant than SmI
2
is an excellent reagent for pinacol coupling and is
8
atmosphere. The general utility of SmI
2
lies in the fact that
2 2
and SmCl . Recent work in our
it can initiate tandem bond-forming reactions through radical
2
3
or anionic processes. Many traditional organic reactions such
2
9
as the Grignard or Reformatsky reactions can utilize SmI
in place of magnesium or zinc with improved yields.
Although SmI is a versatile reductant, it does have a number
of limitations. Many reactions of SmI require the addition
of HMPA (or other additives) to be promoted. While SmI
or SmI -additive combinations easily reduce alkyl and aryl
2
2
. In our present study, we
examine the influence of HMPA on SmBr
the reactivity of the resulting complex.
2
and investigate
2
2
Redox potentials are one of the most important parameters
that need to be measured for Sm(II) reducing agents because
they provide valuable insight into the reducing power of the
Sm(II) reductant. Recent studies with samarium diiodide have
shown that the redox potential can be changed by addition
of a coordinating cosolvent such as hexamethylphosphor-
2
2
iodides, bromides are reduced at a slower rate and alkyl
4
chlorides require photochemical conditions or high temper-
atures.5
10
Other Sm(II) reductants have been used for organic
transformations, including samarium(II) triflate, dicyclopen-
amide (HMPA) or other donor solvents and ligands. Recent
(
6) Fukuzawa, S.-i; Mutoh, K.; Tsuchimoto, T.; Hiyama, T. J. Org. Chem.
996, 61, 5400.
7) (a) Namy, J. L.; Collins, J.; Zhang, J.; Kagan, H. B. J. Organomet.
tadienylsamarium, and samarium dibromide (SmBr
2
). Sa-
1
marium(II) triflate was found to promote higher diastereo-
(
Chem. 1987, 328, 81. (b) Collin J.; Kagan, H. B. Tetrahedron Lett. 1988,
29, 6097.
(8) (a) Lebrun, A.; Namy, J. L.; Kagan, H. B. Tetrahedron Lett. 1993,
34, 2311. (b) Kagan, H. B.; Collin, J.; Namy, J. L.; Bied, C.; Dallemer, F.;
Lebrun, A. J. Alloys Compd. 1993, 192, 191.
(9) (a) Fuchs, J. R.; Mitchell, M.; Shabangi, M.; Flowers, R. A., II.
Tetrahedron Lett. 1997, 38, 8157. (b) Miller, R. S.; Sealy, J. M.; Shabangi,
M.; Kuhlman, M. L.; Fuchs, J. R.; Flowers, R. A., II. J. Am. Chem. Soc.
2000, 122, 7718.
*
Correspondence after July 1, 2001 should be sent to: Department of
Chemistry and Biochemistry, Texas Tech University, Box 41061, Lubbock,
TX 79409-1061.
(
1) Molander, G. A. Chem. ReV. 1992, 92, 29.
(2) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J. Synlett
1
992, 943.
3) Molander, G. A.; Harris, C. R. Chem. ReV. 1996, 96, 307.
J. Am. Chem. Soc. 1997, 119, 2745.
5) Inanaga, J.; Ishikawa, M.; Yamaguchi, M. Chem. Lett. 1987, 1485.
(10) Shabangi, M.; Sealy, J. M.; Fuchs, J. R.; Flowers, R. A., II.
Tetrahedron Lett. 1998, 39, 4429.
(
1
0.1021/ol0160957 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/27/2001

studies in this lab have shown that change of the coordinating
halide from iodide to bromide also changes the reducing
power of samarium complexes. The large number of
coordinating solvents and ligands makes it increasingly
important to obtain redox values for each, so that future
synthetic work can be performed in a more logical manner.
This will further increase the ability to fine-tune a reducing
agent for a particular reaction.
2
To examine SmBr complexes, cyclic voltammograms of
the reductant with increasing amounts of HMPA were
measured in THF. Figure 1 shows voltammograms obtained
Figure 2. Cyclic voltammograms of (1) SmI
SmBr -HMPA.
2
-HMPA and (2)
2
was initially identified by us as [SmI
of Daasbjerg and co-workers has provided evidence that the
solution structure is [Sm(THF) (HMPA) ]I , and a careful
VPO analysis in our lab concurs with this finding.
Furthermore, recent work of Daasbjerg provides strong
evidence that SmI containing greater than 10 equiv of
HMPA produces the octahedral complex [Sm(HMPA)
2 4
(HMPA) ]. Recent work
2
4
2
12
2
12
6
2
]I .
In their CV experiments, increasing the amount of HMPA
from 4 to 10 equiv shows a shift toward increasing
irreversibility of the Sm complex. The SmBr
not exhibit this irreversibility with increasing amounts of
HMPA. Comparison of the voltammograms of SmBr with
0 equiv of HMPA and SmI containing 50 equiv of HMPA
shows significant differences in both redox potential and
reversibility, indicating that the SmBr -HMPA complex is
2
complex does
2 2
Figure 1. Cyclic voltammograms of (1) SmBr and (2) SmBr -
HMPA.
2
5
2
for SmBr
2
and SmBr
2
containing 50 equiv of HMPA. It is
is
2
2
+
not [Sm(HMPA)
Solutions of SmBr
analyzed using UV-vis spectroscopy. Figure 3 shows the
2
spectra obtained for SmI , SmI -HMPA (50 equiv), SmBr ,
6
]
(Figure 2).
immediately apparent that the reducing ability of SmBr
2
and SmBr
2
-HMPA were further
greatly enhanced by addition of large amounts of HMPA
from the shift in the oxidation peak (520 mV) as well as the
shift in the reduction peak (510 mV) to more negative values.
Addition of more than 50 equiv of HMPA showed no further
change in the redox potential. Redox potentials were
estimated from the oxidation and reduction peaks of the
quasi-reversible voltammograms and found to be -2.07 (
2
2
2
0
(
.03 V for SmBr
vs Ag/AgNO reference).
It is interesting to compare the CVs of SmBr
2
and -2.63 ( 0.01 V for SmBr
2
-HMPA
3
2
-HMPA
and SmI -HMPA (Figure 2) and note the similarities and
2
differences between the two complexes. Both species interact
with HMPA in a manner that increases their reducing ability.
However, SmI
2
is known to show a change in reducing power
1
1
that shows an intermediate with a maximum at 4 equiv
and then another at greater than 10 equiv of HMPA,
1
2
whereas SmBr
its reducing power. The SmI
2
requires 50 equiv of HMPA to maximize
species at 4 equiv of HMPA
2
(
11) Shabangi, M.; Kuhlman, M. L.; Flowers, R. A., II. Org. Lett. 1999,
, 2133.
12) (a) Enemaerke, R. J.; Hertz, T.; Skrydstrup, T.; Daasbjerg, K. Chem.
Eur. J. 2000, 6, 3747. (b) Flowers, R. A., II. Unpublished results.
Figure 3. UV-vis spectra of (1) SmI
SmBr , and (4) SmBr -HMPA.
2 2
, (2) SmI -HMPA, (3)
1
(
2
2
2322
Org. Lett., Vol. 3, No. 15, 2001

and SmBr
three primary absorbance regions for SmBr
and 604 nm. For SmBr -HMPA, these peaks have been
2
-HMPA (50 equiv) in THF solvent. There are
2
, at 410, 548,
Table 1. SmBr -HMPA-Mediated Reactions
2
2
shifted to the shorter wavelengths 394, 526, and 582 nm.
The bands between 500 and 650 nm for Sm(II) complexes
have been ascribed to f-d transitions, although this assign-
ment is not a certainty.13 The two shortest wavelengths are
consistent in terms of relative intensities. However, the peak
2
at 604 nm for SmBr loses a significant amount of its relative
intensity upon the addition of HMPA. These shifts to shorter
wavelengths show that the Sm(II) complex is absorbing a
higher amount of energy. A reasonable conclusion is that
HMPA, being a more tightly bound ligand than THF, is
increasing the ligand field strength of the system. In transition
metals, increased ligand field strength increases the splitting
of the d-orbitals responsible for absorption.14 A similar
conclusion could be drawn here, that f-orbital splitting is
occurring to a larger extent, increasing the amount of energy
necessary for an excited state to be achieved. While 4f
orbitals are highly shielded in lanthanide elements, it has
been reported that perturbations of up to 0.1 eV (9.6 kJ
-
1
15
mol ) can occur from ligand effects. The maximum shift
of 22 nm by the SmBr absorptions upon the addition of
2
-
1
HMPA is equivalent to 4 kJ mol , well within this
established literature value. Our studies show there is clearly
a
Yields based on GC-MS analysis. b 2-Octanol found in 20% yield.
c
Isolated yields in parentheses. d General conditions: 0.25-1.0 mmol
2
a relationship between the addition of HMPA to SmBr and
the change in the energies of the ground and excited-state
orbitals, whatever the transitions may be.
substrate, 2-2.5 equiv SmBr2-HMPA (50 equiv HMPA), under nitrogen
atmosphere (See Supporting Information).
It is interesting to compare the UV-vis spectra of SmI
and SmBr and their HMPA complexes. Both SmI and
SmBr absorptions shift to shorter wavelengths upon addition
of HMPA. While SmI and SmBr complexes show absorp-
tions at 618 nm (SmI ) and 604 nm (SmBr ), these peaks
broaden significantly upon the addition of HMPA. Although
the exact species in solution for the bromide complexes
cannot be discerned from these data, careful examination of
2
2
2
reductant of choice for alkyl chloride reductions. We also
2
found that SmBr -HMPA is not useful in ester reduction.
2
2
2
All attempts to reductively couple alkyl chlorides with esters
resulted in reduction of the chloroalkane and recovery of
the ester.
2
2
Another reaction that we explored was the reduction of
ketimines. Although the reductive dimerization of aldimines
the UV-vis spectra in Figure 3 shows that both SmBr
2
and
1
6
2
has been carried out with SmI , there are few examples of
SmBr -HMPA are different than their SmI analogues. This
2
2
1
7
the reduction of ketimines. Treatment of the N-benzyl
imines of acetophenone and pinacolone with SmBr -HMPA
supports electrochemical evidence presented earlier in this
paper and clearly shows that addition of excess HMPA to
2
produced the corresponding amines in less than 15 min at
room temperature. The reduction product from the N-benzyl
imine of pinacolone was isolated in a 90% yield. We are
currently exploring the reduction of less hindered ketimines
SmBr
HMPA)
To examine the synthetic utility of SmBr
2
does not produce the octahedral complex [Sm-
2
+
(
6
] .
2
2
and SmBr -
HMPA, several reactions with functionalities that are typi-
cally difficult to reduce were performed. Table 1 lists some
with SmBr -HMPA to examine the possibility of carrying
2
out reductive coupling reactions that lead to vicinal diamines.
In conclusion, we have studied the influence of HMPA
substrates that were exposed to SmBr -HMPA, the obtained
2
products, and their respective yields. The pinacol coupling
of 2-octanone provides a 70% yield of product with the
balance of the reaction mixture being 2-octanol. More
interesting is the reduction of alkyl chlorides. Both 1-chlo-
rododecane and chlorocyclohexane are reduced quantitatively
to the corresponding alkanes in less than 2 h at room
temperature. Although chloroalkanes can be reduced by
on SmBr
a more powerful reductant in a manner analogous to SmI
the reductant is distinct from those formed between SmI
and HMPA. The combination of SmBr
2 2
. While coordination of HMPA to SmBr produces
2
,
2
2
and HMPA provides
a powerful reducing reagent capable of reducing carbon-
chlorine bonds and imines. We are currently carrying out a
SmI
2
-HMPA, the reaction requires elevated temperatures
-HMPA is the
(
16) (a) Enholm, E. J.; Forbes, D. C.; Holub, D. P. Synth. Commun. 1990,
0, 981. (b) Imamoto, T.; Nishimura, S. Chem. Lett. 1990, 1141.
17) Banik and co-workers utilized Sm metal and a catalytic amount of
and extended periods of time. Thus, SmBr
2
2
(
(
(
13) Okaue, Y.; Isobe, T. Inorg. Chem. Acta 1988, 144, 143.
14) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy; Dover
iodine to reduce and reductively couple aldimines and ketimines. The
identity of the reactive intermediate has not been established. Banik, B.
K.; Zegrocka, O.; Banik, I.; Hackfeld, L.; Becker, F. F. Tetrahedron Lett.
1999, 40, 6731.
Publications Inc.: New York, 1978; pp 397-398.
(15) Ortiz, J. V.; Hoffman, R. Inorg. Chem. 1985, 24, 2095.
Org. Lett., Vol. 3, No. 15, 2001
2323

careful spectroscopic and thermochemical study of Sm(II)
reductants in order to learn more about the relationship
between the structure and activity of these important reducing
reagents. The results of these experiments will be reported
in due course.
Chemical Society, for support of this work. We thank
Masangu Shabangi for initial investigation of the SmBr
HMPA system.
2
-
Supporting Information Available: Experimental pro-
cedures, UV-vis spectra, and cyclic voltammograms. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. R.A.F. is grateful to the National
Science Foundation (CHE-0094091) and the Petroleum
Research Fund (35762-AC1), administered by the American
OL0160957
2324
Org. Lett., Vol. 3, No. 15, 2001