The availability of dysprosium diiodide as a powerful reducing agent in organic synthesis: Reactivity studies and structural analysis of DyI2((DME)3 and its naphthalene reduction product [13]

Full text of DOI:10.1021/ja0034949

J. Am. Chem. Soc. 2000, 122, 11749-11750
The Availability of Dysprosium Diiodide as a
11749
Powerful Reducing Agent in Organic Synthesis:
Reactivity Studies and Structural Analysis of
DyI2((DME)3 and Its Naphthalene Reduction
Product1
William J. Evans,* Nathan T. Allen, and Joseph W. Ziller
Department of Chemistry, UniVersity of California
IrVine, California 92697-2025
ReceiVed September 26, 2000
Figure 1. Structure of DyI2(DME)3 with thermal ellipsoids drawn at
the 50% probability level. Selected bond lengths (Å) and angles (deg):
Dy1-O1, 2.587(2); Dy1-O2, 2.603(2); Dy1-O3, 2.622(2) Dy1-I1,
3.2628(3); Dy2-O4, 2.585(2); Dy2-O5, 2.630(2); Dy2-O6, 2.678(21);
Dy2-I2, 3.2371(3); I1-Dy1-I1′, 157.322(12); I2-Dy2-I2,′ 180.000(9).
Metal-based reduction chemistry in organic synthesis may be
divided into two general regimes based on available reagents and
their reduction potentials.2 One class of reductants consists of
the extremely powerful reagents that allow Birch reductions,3 for
example, the alkali metals in liquid ammonia, NaK and Na(Hg)
alloys, and molten alkali metals. These are rather intolerant of
functional group diversity. Another class of reducing systems
involves SmI2(THF)x, which accommodates a wide range of
functional groups, but is less reducing.4 A variety of methods
have been used to increase the reduction potential of SmI2-
(THF)x,5,6 the most common being the addition of hexamethyl-
phosphoramide (HMPA). In addition, a new divalent lanthanide
reagent, TmI2(THF)x,7 has recently been reported which partially
narrows the gap between the Birch reductions and SmI2, since
Tm(II) is 0.8 V more reducing than Sm(II).8 Tm(II) reagents were
not considered earlier since it was only in 1997 that the first
molecular complex of this highly reactive ion was definitively
identified.9 The isolation of that complex, TmI2(DME)3, led to
the development of Tm(II) chemistry which now includes facile
and Dy(III) ions coincidentally have the same magnetic moment
and definitive structural data were elusive. Since several incorrect
claims of unusual oxidation states of f element complexes have
been made in the past based only on spectroscopy, we wanted
crystallographic confirmation of the existence this ion before its
use in organic synthesis was examined.
We report here the first crystallographic data on a molecular
complex of Dy(II) as well as a more convenient synthesis of solid
DyI2 that allows it to be stored as a solid and conveniently
solubilized for use in organic syntheses. We also describe some
preliminary reactions that demonstrate the utility of this reagent
in organic synthesis including the reduction of naphthalene.
Crystals of DyI2(DME)3,12 prepared by the method of Boch-
karev, were grown from DME at -20 °C in a nitrogen-containing
glovebox over a period of 36 h. These crystals were found to be
isomorphous with SmI2(DME)3.13 Since we were suspicious that
we could actually isolate such a reactive species, the same crystal
examined by X-ray diffraction was also examined by energy
dispersive absorption X-ray spectroscopy (EDAX) which con-
firmed that it was truly a dysprosium complex. DyI2(DME)3, like
its samarium analogue, has an unusual structure in which there
are two independent molecules in the unit cell, one of which has
a linear I-Dy-I component and the other which has a bent
I-Dy-I moiety, Figure 1. Each molecule has three chelating
DME ligands that generate a hexagonal bipyramidal geometry in
the linear I-Dy-I case and a distorted dodecahedral geometry
in the bent I-Dy-I system. This differs from the structure of
TmI2(DME)3 which contains one η1-DME and is 7-coordinate.8
This difference is consistent with the smaller radial size of Tm.14
Once the existence of DyI2(DME)3 was crystallographically
established, we sought a more convenient preparation that would
allow it to be used as a routine reagent. Solid DyI2 can be
conveniently prepared in multigram quantities by reacting dys-
prosium filings and iodine in an alumina or quartz crucible
contained within a quartz tube connected to a Schlenk line.15 The
solid mass of DyI2 that forms can be easily separated from residual
metal. The DyI2 can be crushed with a mortar and pestle under
an inert atmosphere and stored for long periods under nitrogen
until needed.
7
in situ synthesis of TmI2(THF)X analogous to the commonly
used SmI2(THF)2.4
To further narrow the gap between the Birch-type reductants
and the Sm(II)/Tm(II) reagents using lanthanide metals would
require access to Ln(II) ions significantly more difficult to isolate
than Tm(II). Although divalent ions of most of the lanthanides
have been reported to form under extreme conditions in solid-
state lattices,10 isolation of easily accessible, soluble forms that
would be used in organic synthesis seemed unlikely. However,
Bochkarev and co-workers have recently reported that a soluble
Dy(II) complex can be obtained from Dy/I2 reactions which occur
at temperatures up to 1500 °C.11 Although the analytical and
spectroscopic data were consistent with Dy(II), the existence of
this ion could not be established by magnetic data since the Dy(II)
(1) Presented in part at Contemporary Inorganic Chemistry II, College
Station, Texas, March 2000.
(2) Encyclopedia of Reagents for Organic Synthesis; L. A. Paquette, Ed.;
John Wiley: New York 1995.
(3) Rabideau, P. W.; Marcinow, Z. Org. React. 1992, 42, 1.
(4) For reviews, see: (a) Kagan, H. B.; Namy, J. L. Tetrahedron 1986,
42, 6573-6614. (b) Soderquist, J. A. Aldrichimica Acta 1991, 24, 15-23.
(c) Molander, G. A. Chem. ReV. 1992, 92, 29-68. (d) Krief, A.; Laval, A.
M. Chem. ReV. 1999, 99, 745-777. (e) Molander, G. A.; Harris, C. R. In
Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A., Ed.;
Wiley: New York, 1995;Vol. 6, pp 4428-4432.
(5) Curran, D. P.; Hasegawa, E. J. Org. Chem. 1993, 58, 5008-5010. (b)
Inanaga, J.; Ishikawa, M.; Yamaguchi, M. Chem. Lett. 1987, 1485. (c) Kagan,
H. B.; Namy, J.-L. In Lanthanides: Chemistry and use in Organic Synthesis;
Kobayashi, S., Ed.; Springer: Berlin, 1999, 156-198.
(6) (a) Otsubo, K.; Inanaga, J.; Yamaguchi, M. Tetrahedron Lett. 1986,
57, 4437. (b) Flowers, R. A.; Shabangi, M. Tetrahedron Lett. 1997, 38, 1137-
1140.
(7) Evans, W. J.; Allen, N. T. J. Am. Chem. Soc. 2000, 122, 2118-2119.
(8) Aqueous LnIII/LnII reduction potentials (vs NHE): Dy (-2.5 V), Tm
(-2.3 V), and Sm (-1.55 V). See: Morss, L. R. Chem. ReV. 1976, 76, 827-
841.
(12) Experimental data for DyI2(DME)3: IR (Nujol): 1471m, 1444s, 1374s,
1301w, 1282w, 1239w, 1193w, 1116m, 1058s, 1027m, 860s, 722s cm-1; UV/
Vis (DME): λ ) 412, 476, 577, 716 nm; Calcd for DyIC12H30O6: C, 20.99;
H, 4.40; I, 36.96; Dy, 23.66. Found: C, 20.81; H, 4.29; I, 37.11; Dy, 24.4.
DyI2(DME)3 crystallizes in the space group C2/c with a ) 24.9195(13) Å, b
) 13.0439(7) Å, c ) 14.4466(8) Å, R ) 90°, â ) 115.8070(10)° γ ) 90°, V
) 4227.5(4) Å3 and Fcalcd ) 2.158 g/cm3 for Z ) 8 at 163 K. At convergence,
wR2 ) 0.0592 and GOF ) 1.033 for 246 variables refined against 5091 data
(As a comparison for refinement on F, R1 ) 0.0232 for those 4501 data with
I > 2.0σ(I)).
(9) Bochkarev, M. N.; Fedushkin, I. L.; Fagin, A. A.; Petrovskaya, T. V.;
Ziller, J. W.; Broomhall-Dillard, R. N. R.; Evans, W. J. Angew. Chem., Int.
Ed. Engl. 1997, 36, 133-135.
(10) Meyer, G. Chem. ReV. 1988, 88, 93-107 and references therein.
(11) Bochkarev, M. N.; Fagin, A. A. Chem. Eur. J. 1999, 5, 2990-2992.
(13) Evans, W. J.; Broomhall-Dillard, R. N. R.; Ziller, J. W. Polyhedron
1998, 17, 3361-3370.
(14) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
10.1021/ja0034949 CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/11/2000

11750 J. Am. Chem. Soc., Vol. 122, No. 47, 2000
Communications to the Editor
Since addition of D2O generated 1,4-dideuteronaphthalene, eq 2,
this reaction is equivalent to a Birch reduction.
(2)
The isolation of a naphthalenide dianion complex is not unique
in f element chemistry and several examples are known.19
However, in each case, these have been formed by reduction of
naphthalene with an alkali metal and reaction of the alkali metal
naphthalene dianion with an f element halide or by reaction of
the lanthanide metal with naphthalene in liquid ammonia. This
is the first case in which a low-oxidation state lanthanide metal
complex has reduced naphthalene directly in ethereal solvents.
DyI2(DME)3 alone does not appear to reduce benzene or
anisoles, but it will reduce alkynes. Reduction of PhCtCPh
followed by hydrolysis forms exclusively cis-stilbene, eq 3.
Figure 2. Structure of (C10H8)DyI(DME)2 with thermal ellipsoids drawn
at the 50% probability level. Selected bond lengths (Å) and angles
(deg): Dy-O1, 2.467(3); Dy-O2, 2.562(4); Dy-O3, 2.411(3); Dy-
O4, 2.430(3); Dy-C1, 2.510(5); Dy-C2, 2.599(5); Dy-C3, 2.605(5);
Dy-C4, 2.486(5); Dy-C5, 2.992(5); Dy-C10, 3.002(4); Dy-I, 3.1382(4).
Since solutions of DyI2 are so reactive, they should be prepared
immediately before use. DyI2 may be used conveniently by
making a saturated solution in THF or DME at temperatures below
-20 °C. The solubility of DyI2 in THF is 0.025 M at -45 °C.
The utility of DyI2(THF)x in ketone coupling reactions with alkyl
halides appears to be similar to that of TmI2(THF)x 7 except that
Dy(II) is much more reactive. Hence, reductions of alkyl chlorides
that take 60 min at 0 °C with TmI2(THF)x and would not occur
at all with SmI2(THF)x/HMPA,16 proceed immediately at -45
°C with DyI2(THF)x. For example, addition of 2-chloroethylben-
zene to DyI2(THF)x at -45 °C immediately dissipates the dark
green color. After reaction with cyclohexanone an 88% isolated
yield of the coupled product, phenethylcyclohexanol, is obtained
(eq 1).17
(3)
In summary, the existence of a molecular complex of Dy(II),
namely DyI2(DME)3, has been established and a convenient
protocol has been developed so that DyI2 can be used like SmI2-
(THF)x in THF or DME. Preliminary reactivity studies show that
this reagent provides an option for metal-based reductions in
organic synthesis that is intermediate between Birch reducing
agents and Sm(II)/Tm(II) reagents. It is expected that, as in the
case of SmI2(THF)x,4 the unique utility of DyI2 in reductive
organic synthesis will become more fully defined as this reagent
is more thoroughly examined.
Acknowledgment. We thank the National Science Foundation for
support of this research and Michael Zach and Professor Reginald Penner
for help with the EDAX measurements.
(1)
Supporting Information Available: Coupling procedure, tables of
crystal data, positional parameters, bond distances and angles, and thermal
parameters (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
DyI2 in DME is a strong enough reductant to immediately react
with naphthalene at -45 °C to form a dark purple solution from
which (C10H8)DyI(DME)2,18 Figure 2, crystallizes at -20 °C.
(15) DyI2. In air, Dy filings (3.0 g, 18.4 mmol) were spread evenly in a
layer on the bottom of a 10-mL alumina or quartz crucible. Iodine (4.75 g,
18.7 mmol) was then layered on top of the metal. The charged crucible was
placed in a 20 × 4.5 cm quartz test tube fitted with a 55/50 joint which is
connected to a trap and a Schlenk line in a well-shielded hood. The tube was
evacuated and refilled with N2 three times to remove oxygen. During the
synthesis, the apparatus was under a nitrogen atmosphere connected to a
mineral oil bubbler. The apparatus was tipped at a 60° angle to avoid directly
heating the joint and the bottom of the tube containing the crucible was heated
with a Meeker burner at maximum heat. The tube initially fills with I2 vapor
and a bright light flash indicates ignition of the metal. Heating is continued
at the bottom of the tube for about 2 min, during which time I2 is consumed.
Vacuum is then applied to transfer unreacted I2 to the trap. A nitrogen
atmosphere is reestablished in the vessel and heating is continued for 15 min
during which time part of the mixture melts. The system is allowed to cool to
room temperature and the apparatus is transferred to a nitrogen-filled glovebox.
The solid mass of DyI2 is removed from the crucible with a spatula and crushed
with a mortar and pestle. The unreacted metal (1.25 g), which is left as a
nugget, is easily identified and removed while the DyI2 is being ground. DyI2
(3.8 g, 85% based on metal consumed) is recovered as a violet powder suitable
for synthetic reactions. Since solutions in DME or THF are light and
temperature sensitive, they should be prepared immediately before use.
(16) (a) Ogawa, A.; Nanke, T.; Takami, N.; Sumino, Y.; Ryu, I.; Sonoda,
N. Chem. Lett. 1994, 379-380. (b) Ogawa, A.; Sumino, Y.; Nanke, T.; Ohya,
S.; Sonoda, N.; Hirao, T. J. Am. Chem. Soc. 1997, 119, 2745-2746. (c) Ogawa,
A.; Ohya, S.; Hirao, T. Chem. Lett. 1997, 3, 275-276.
JA0034949
(18) (C10H8)DyI(DME)2. In a glovebox, addition of naphthalene (0.017 g,
0.13 mmol) to a dark khaki green solution of DyI2 (0.14 g, 0.33 mmol) in
DME (15 mL) at -20 °C caused an immediate color change to violet. After
0.5 h, light gray insoluble materials (90 mg, 90% if DyI3(DME)x) were
removed. The solvent was removed in vacuo leaving a purple solid (75 mg,
95%). Dissolution of the solid in DME followed by cooling to -20 °C yielded
crystals suitable for X-ray diffraction. IR (Nujol): 1459s, 1378s, 1339w,
1270m, 1239w, 1185w, 1112m, 1027s, 976w, 849s, 780m, 760w, 737m, 695w
cm-1; UV/Vis (DME): λ ) 508, 904, 939 nm. Calcd for C18H28O4DyI: Dy,
27.18. Found: Dy 28.4. (C10H8)DyI(DME)2 crystallizes in the space group
Pca21 with a ) 15.0111(7) Å, b ) 9.8564(5) Å, c ) 13.8731(7) Å, R ) 90°,
â ) 90°, γ ) 90°. V ) 2052.60(18) Å3 and Fcalcd ) 1.934 g/cm3 for Z ) 4
at 163 K. The hydrogen atoms associated with carbon atoms C(1) and C(4)
could not be located or placed accurately and were not included in the
refinement. At convergence, wR2 ) 0.0538 and GOF ) 1.068 for 218
variables refined against 4717 independent data (As a comparison for
refinement on F, R1 ) 0.0257 for those 4368 data with I > 2.0σ(I)).
(19) (a) Bochkarev, M. N.; Fedushkin, I. L.; Fagin, A. A.; Schumann, H.;
Demtschuk, J. Chem. Soc., Chem. Commun. 1997, 1783 (b) Bochkarev, M.
N.; Fedushkin, I. L.; Schumann, H.; Esser, L.; Kociok-Kohn, G. J. Organomet.
Chem. 1995, 489, 145. (c) Bochkarev, M. N.; Trifonov, A. A.; Fedorova, E.
A.; Emelyanova, N. S.; Basalgina, T. A.; Kalinina, G. S.; Razuvaev, G. A. J.
Organomet. Chem. 1989, 372, 217. (d) Protchenko, A. V.; Zakharov, L. N.;
Bochkarev, M. N.; Struchkov, Y. T. J. Organomet. Chem 1993, 447, 209 (e)
Fryzuk, M. D.; Jafarpour, L.; Kerton, F. M.; Love, J. B.; Rettig, S. J. Angew.
Chem., Int. Ed. 2000, 39, 767-770.
(17) The procedure follows that in ref 7 and is described in the Supporting
Information.