employed as the acylating agent in the strictly acidic [emim]I–
(AlCl3)x derived ionic liquid afford diacetylated ferrocene,
bis(acetylcyclopentadienyl)iron() 2a, in a reasonable yield
(Table 2).
with 1-ethyl-3-methylimidazolium iodide (5.0 g, 21 mmol).
While the powdery white solid was agitated with a magnetic
stirbar, AlCl3 (2.8 g to 8.4 g, 21 mmol to 63 mmol) was added
very slowly to afford the ionic liquids of variable Lewis acidity
to be used for the acidity study. For [emim]I–(AlCl3)2:
δH(CDCl3): 0.84 (t, 3H, J = 7.3), 3.2 (s, 3H), 3.5 (q, 2H, J = 7.3
Hz), 6.6 (s, 1H), 6.7 (s, 1H), 7.6 (s, 1H).
The cyclic anhydrides, succinic anhydride and maleic
anhydride, afforded no Friedel–Crafts products (Table 2). In
these reactions unreacted starting material, ferrocene, was isol-
ated after work-up and chromatographic separation.
An increasingly higher number of chiral cyclopentadienyl
complexes of transition metals are being prepared and used as
catalysts or ligands in asymmetric synthesis.12 Frequently, the
precursors to these chiral catalysts, such as bis(diethylcarb-
amoylcyclopentadienyl)iron() 3, can be difficult and expensive
to prepare using conventional methods. Hence, we endeavored
to prepare 3 in a more facile manner using [emim]I–(AlCl3)x
derived ionic liquids as solvents. Unfortunately, our attempts to
prepare 3 by the reaction of ferrocene with N,N-diethylcarb-
amoylchloride in [emim]I–(AlCl3)x derived ionic liquids failed
(Table 2).
Studies directed at the synthesis of important synthetic pre-
cursors to chiral transition metal catalysts, such as 3, continue
in our laboratory. Investigations into other Lewis acid catalyzed
reactions, including Friedel–Crafts reactions with other metallo-
cenes and metal arenes, that can be performed in these reac-
tion media as well as development of methods in which the
ionic liquid reaction media can be recycled are currently
underway in our laboratories.
Typical reaction of ferrocene with acetic anhydride in ionic
liquids
Ferrocene (0.25 g, 1.35 mmol) was added to the ionic liquid
(as prepared above to afford
a
ratio of [emim]I–
(AlCl3)2 :AlCl3 = 1.0:2.0) in a glove box to afford a greenish-
brown solution that was allowed to stir for 10 minutes. The
reaction vessel was sealed with a septum, removed from the
glove box, and placed in an ice bath at 0 ЊC. Acetic anhydride
(0.254 mL, 2.70 mmol, 2 equiv. with respect to ferrocene) was
added via syringe to afford a purple solution that was allowed
to stir for 2 hours at 0 ЊC. The reaction was then quenched
using 10 mL of 2 M hydrochloric acid and extracted with 3 × 25
mL dichloromethane. The combined organic phases were dried
(MgSO4), filtered, and concentrated in vacuo. The resulting
crude oil was purified using column chromatography with silica
gel and hexanes–ethyl acetate (10:1). The polarity of the solv-
ent system was increased gradually to hexanes–ethyl acetate
(1:1) to elute any diacylated product. Spectra recorded for both
1a and 2a were in agreement with those reported in the liter-
ature.12 Monoacylated ferrocene, 1a (0.050 g, 16%): δH(CDCl3):
2.39 (s, 3H), 4.20 (s, 5H), 4.50 (dd, 2H, J = 1.83), 4.77 (dd, 2H,
J = 1.83 Hz). δC(CDCl3): 27.5, 69.6, 69.9, 72.4, 79.3, 202.2.
Mass spectrum (EI, 70 eV): m/z (%): 228 (Mϩ, 90), 213 (35), 185
(100), 152 (3), 129 (35), 121 (14). Diacylated ferrocene, 2a
(0.145 g, 40%): δH(CDCl3): 2.35 (s, 6H), 4.51 (m, 4H), 4.77 (m,
4H). δC(CDCl3): 27.8, 71.2, 73.8, 80.9, 201.3. Mass spectrum
(EI, 70 eV): m/z (%): 270 (Mϩ, 100), 255 (36), 227 (6), 199 (28),
163 (2), 121 (6).
Experimental
General methods
All glassware and syringes were dried in an oven overnight at
120 ЊC and glassware was flame dried and flushed with argon or
nitrogen immediately prior to use. Transfer of solid reagents
and formation of ionic liquids was performed inside a glove box
(VAC HE-63-P) under argon atmosphere or with syringes
equipped with stainless steel needles. Methylimidazole, iodo-
ethane and AlCl3 (99.99%) were used as purchased from
Aldrich Chemical Company, Inc. Ferrocene (Caledon Labora-
tories Ltd.) was purified by sublimation just prior to use. Acetic
anhydride (Caledon Laboratories Ltd.), acetyl chloride (Fisher
Scientific), and benzoyl chloride (Kodak) were distilled shortly
before use. Isobutyryl chloride and valeryl chloride were used
as purchased from Aldrich Chemical Company, Inc. Maleic
anhydride (Kodak) and succinic anhydride (Aldrich Chemical
Company, Inc.) were dissolved in chloroform, filtered to remove
any insoluble impurities, and the solvent removed in vacuo prior
to use. Purification of all the products was done using column
chromatography with Silica Gel 60 (Caledon Laboratories
LTD., 63–200 mesh). 1H and 13C NMR spectra were recorded in
ppm relative to tetramethylsilane at 250.13 MHz or 400.13
MHz for 1H and 62.9 MHz or 100.6 MHz for 13C.
Typical reaction of ferrocene with acid chlorides in ionic liquid
An oven dried round bottomed flask in a glove box was charged
with 1-ethyl-3-methylimidazolium iodide (5.0 g, 21 mmol).
While the powdery white solid was agitated with a magnetic
stirbar, AlCl3 (5.6 g, 42 mmol) was added very slowly to afford
the ionic liquid. Ferrocene (0.25 g, 1.35 mmol) was added to the
ionic liquid in the glove box to afford a greenish-brown solution
that was allowed to stir for 10 minutes. The reaction vessel was
sealed with a septum, removed from the glove box, and placed
in an ice bath at 0 ЊC. The acylating agent (1, 2 or 10 equiv.)
(Table 2) was added via syringe to afford a purple solution that
was allowed to stir for 2 hours at 0 ЊC. The reaction was then
quenched using 10 mL of 2 M hydrochloric acid and extracted
with 3 × 25 mL dichloromethane. The combined organic
phases were dried (MgSO4), filtered, and concentrated in vacuo.
The resulting crude oil was purified using column chrom-
atography with silica gel and hexanes–ethyl acetate (10:1) as
eluent. The polarity of the eluent was increased gradually to
hexanes–ethyl acetate (1:1) to elute any diacylated product.
Spectra recorded for all compounds were in agreement with
those reported in the literature.12
Preparation of 1-ethyl-3-methylimidazolium iodide, [emim]I
To an oven dried round bottomed flask containing 200 mL of
freshly dried and distilled THF was added methylimidazole
(31.9 mL, 0.400 mol) and iodoethane (38.4 mL, 0.480 mol) via
syringe. The mixture was allowed to stir while refluxing for 4
hours. Removal of THF in vacuo, resulted in a white solid that
was protected from the light by wrapping the round bottomed
flask in an aluminium foil. The white solid was then heated in
vacuo at 100 ЊC for 6 hours to remove residual solvent and
any unreacted starting materials (yield: 95.0 g, 100%; mp 79–
80 ЊC).13
Benzoylcyclopentadienyl(cyclopentadienyl)iron(II) 1b
δH(CDCl3): 4.2 (s, 5H), 4.6 (s, 4H), 4.9 (s, 4H), 7.5–7.9 (m, 5H).
δC(CDCl3): 70.5, 71.8, 72.8, 78.3, 128.3, 128.5, 131.7, 104.1,
199.4. Mass spectrum (EI, 70 eV): m/z (%): 290 (Mϩ, 100), 261
(4), 213 (3), 105 (4), 77 (1).
Preparation of 1-ethyl-3-methylimidazolium halogenoaluminate
Bis(benzoylcyclopentadienyl)iron(II) 2b
ionic liquids, [emim]I–(AlCl3)x
δH(CDCl3): 4.6 (m, 4H), 4.9 (m, 4H), 7.3–7.8 (m, 10H).
δC(CDCl3): 73.1, 74.6, 79.6, 128.1, 128.4, 132.0, 139.2, 198.0.
An oven dried round bottomed flask in a glove box was charged
J. Chem. Soc., Dalton Trans., 1999, 63–66
65