LETTER
Synthesis and Use of LnCl3·7H2O in Luche-Type Reduction
Workup Conditions (pH 7)
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allyl alkyl ethers under acidic workup conditions. Their
formation is only ascribable to the acidity of the media.
However, in this field, the formation of allylic ethers as
byproducts upon acidic catalysis is frequently observed
with aromatic enones, though rather poorly documented.7
The reaction mixture was treated by a decimolar solution of HCl
until pH 7, then extracted three times with Et2O. Combined organic
layers were washed with brine, then dried over MgSO4. Solvents
were evaporated, then the crude product was purified by flash
chromatography over silica gel.
Workup Conditions (pH 1)
OH
OR3
Ph
HCl (pH 1), R3OH, 20 min
The reaction mixture was treated by a 1 M solution of HCl until pH
1, then extracted three times with Et2O. Combined organic layers
were washed with brine, then dried over MgSO4. Solvents were
evaporated, then crude product was purified by flash chromatogra-
phy over silica gel.
Ph
Ph
Ph
R3 = Me
82% yield
85% yield
R
R
3 = Et
3 = i-Pr
76% yield (+ 12% starting material)
Scheme 5 Synthesis of various allyl alkyl ethers upon acidic
hydrolysis
Selected Spectral Data
(E)-4-(2,6,6-Trimethylcyclohex-1-enyl)-but-3-en-2-ol Obtained
Under Workup Conditions A
1H NMR (250 MHz, CDCl3): d = 0.91 [6 H, s, C(6¢)(CH3)2], 1.11–
1.71 [4 H, m, C(4¢)H2, C(5¢)H2], 1.22 [3 H, d, C(1)H3, J1–2 = 6.4
Hz], 1.59 [3 H, s, C(2¢)CH3], 1.90 [2 H, t, C(3¢)H2, J3¢–4¢ = 6.4 Hz],
2.16 [1 H, d, C(2)OH, JOH-2 = 7.3 Hz], 4.28 [1 H, m, C(2)H], 5.40
[1 H, dd, C(3)H, J3–4 = 16.1 Hz, J3–2 = 6.4 Hz], 5.95 [1 H, d, C(4)H,
J4–3 = 16.1 Hz] ppm. 13C NMR (62.9 MHz, CDCl3): d = 19.1, 21.3,
23.5, 28.6, 32.6, 33.8, 39.3, 69.3, 127.2, 128.5, 136.6, 137.7 ppm.
FTIR (CaF2): nmax = 3339, 2965, 2928, 2865, 2828, 1456, 1371,
1360, 1336, 1287, 1204, 1145, 1058 cm–1. GC-MS (IE): m/z (%
base peak) = 194 (13), 179 (20), 176 (11), 161 (44), 136 (29), 123
(31), 121 (95), 119 (34), 109 (30), 107 (27), 105 (38), 95 (42), 93
(53), 91 (43), 81 (36), 79 (35), 77 (28), 69 (34), 67 (22), 55 (40), 53
(19), 43 (100), 41 (54), 39 (16).
In 1999 Barluenga et al.8 described a similar phenomenon
in the Luche-type reduction of divinylketones. Acidic
workup of the reaction mixture gave an E,E-dienyl ethyl
ether formed by the recombination of the intermediate
carbocation. Considering our work (Scheme 5) in the light
of Barluenga’s study, led us to suggest a mechanistic
scheme involving a carbocation, where the etherification
reaction occurs selectively in cases where the carbocation
is stabilized, namely when the C=C double bond is conju-
gated to another unsaturation (aromatic ring, C=C double
bond).
Mischmetall trichloride hydrates were synthesized in a
one-step procedure from the inexpensive alloy Misch-
metall in order to be tested in a classical organic reaction.
These complexes exhibited the same reactivity as cerium
trichloride in the sodium borohydride reduction of highly
conjugated ketones (Luche-type reduction). In addition,
we described conditions for easy access to allyl methyl
ethers, using acidic hydrolysis conditions.
2-[(E)-3-Methoxybut-1-enyl]-1,3,3-trimethylcyclohexene
Obtained Under Workup Conditions B
1H NMR (250 MHz, CDCl3): d = 0.98 [6 H, s, C(3)(CH3)2], 1.25 [3
H, d, C(4¢)H3, J4¢–3¢ = 6.4 Hz], 1.64–1.36 [4 H, m, C(4)H2, C(5)H2],
1.66 [3 H, s, C(1)CH3], 1.96 [2 H, t, C(6)H2, J6–5 = 6.1 Hz], 3.28 [3
H, s, C(3¢)OCH3], 3.73 [1 H, dq, C(3¢)H, J3¢–2¢ = 7.8 Hz, J3¢–4¢ = 6.4
Hz], 5.25 [1 H, dd, C(2¢)H, J2¢–1¢ = 15.6 Hz, J2¢–3¢ = 7.8 Hz], 5.99 [1
H, d, C(1¢)H, J1¢–2¢ = 15.6 Hz] ppm. 13C NMR (62.9 MHz, CDCl3):
d = 19.2, 21.4, 21.9, 28.7, 32.6, 33.8, 39.3, 55.9, 78.8, 128.7, 129.9,
135.4, 136.8 ppm. FTIR (CaF2): nmax = 2968, 2928, 2860, 2825,
2109, 1733, 1458, 1371, 1260. GC-MS (IE): m/z (% base
peak) = 208 (64), 193 (39), 176 (7), 161 (80), 151 (10), 137 (24),
133 (26), 121 (26), 107 (31), 106 (23), 105 (60), 95 (15), 93 (29), 91
(46), 85 (28), 79 (23), 77 (23), 69 (22), 59 (100), 55 (29), 43 (28),
41 (33).
This example is the first application of Mischmetall
trichloride hydrates in organic synthesis. Obviously, the
use of this material could be extended to a wide variety of
reactions involving lanthanide salts.
Typical Experimental Procedure for Synthesis of LnCl3
In a two-necked flask fitted with a reflux condenser and a dropping
funnel ingots of Mischmetall (4 g, 28.6 mmol), were placed in H2O
(20 mL). Then, 6 M HCl (9.6 mL, 57.3 mmol) was added dropwise
under stirring. A rise temperature was observed together with a gas
evolution. After 30 min, pH must be neutral. The solution was then
filtered off in order to eliminate the excess of Mischmetall, then the
filtrate was distilled to eliminate H2O and evaporation was complet-
ed with the rotatory evaporator. Mischmetall trichloride hydrates
were dried for 12 h under primary vacuum to give LnCl3·7H2O
(m = 6.75 g, 95% yield).
Acknowledgment
We thank the University of Paris-Sud and CNRS for their financial
support.
References
(1) (a) Hélion, F.; Namy, J.-L. J. Org. Chem. 1999, 64, 2944.
(b) di Scala, A.; Garbacia, S.; Hélion, F.; Lannou, M.-I.;
Namy, J.-L. Eur. J. Org. Chem. 2002, 2989. (c) Lannou,
M.-I.; Helion, F.; Namy, J.-L. Tetrahedron Lett. 2002, 43,
8007.
Typical Experimental Procedure for Luche-Type Reduction
Mischmetall trichloride (373 mg, 1 mmol) was placed in MeOH
(2.5 mL) in a 10 mL flask. Ketone (1 mmol) was added under
stirring at 0 °C. Sodium borohydride (57 mg, 1.5 mmol) was added
portionwise, then the reaction mixture was stirred for another 15
min at r.t.
(2) Lannou, M.-I.; Hélion, F.; Namy, J.-L. Tetrahedron 2003,
53, 10551.
Synlett 2007, No. 17, 2707–2710 © Thieme Stuttgart · New York