Fe(CO)3-Promoted Isomerization of Olefin Esters
J . Org. Chem., Vol. 61, No. 22, 1996 7785
Fe(CO)3 as in A. Isolable Fe(CO)3 complexes of η4 R,â-
unsaturated aldehydes (RCHdCHC(dO)H), ketones
(RCHdCHC(dO)R′), and imines (RCHdCHC(dNR′)H)
are well known.16,17,18 The olefin esters, η4 methyl
acrylate and methyl trans-crotonate, also form η4 com-
plexes, Fe(CO)3(η4-H2CdCHCO2Me) and Fe(CO)3(η4-
CH3CHdCHCO2Me), that have been detected in ultra-
violet-irradiated, low-temperature matrices (10-12 K)
containing Fe(CO)5 and the R,â-unsaturated esters.19
Even at -5 °C, Fe(CO)3(η4-H2CdCHCO2Me) has been ob-
served in n-hexane where it is in equilibrium with
Fe(CO)3(η2-H2CdCHCO2Me)2, whose concentration de-
pends on the concentrations of the complex and added
methyl acrylate.20,21 These studies suggest that it may
be possible to stabilize the R,â-ester isomer of methyl
oleate by η4 coordination to Fe(CO)3 if the complex (of
type A) is produced at relatively low temperatures. This,
of course, means that Fe(CO)5 must catalyze the isomer-
ization at low temperature and that the inexpensive
Fe(CO)5 be used in stoichiometric amount so as to capture
the R,â-ester as its η4 complex.
In this paper, we show that Fe(CO)5 under UV pho-
tolysis at 0 °C promotes the reaction in eq 1 by isomer-
izing methyl oleate to the R,â-ester isomer that is trapped
as its η4 complex, Fe(CO)3(η4-R,â-ester). The R,â-ester
is removed from this complex by reaction with more
strongly coordinating ligands (CO or pyridine). This
method also converts methyl 3-butenoate and ethyl
4-methyl-4-pentenoate to their R,â-unsaturated ester
isomers in high yields.
representative reactions to determine product yields, which
are reproducible within (3%. For analyses of methyl oleate
and its isomers, the GC column temperature was held at 120
°C for 1 min, followed by a 50 °C/min ramp to 200 °C. After
a 1 min hold, a final ramp (2 °C/min) to 220 °C was used. For
analyses of ethyl 4-methyl-4-pentenoate and its isomers, the
column temperature was held at 50 °C for 5 min, followed by
a 20 °C/min ramp to 150 °C. The 1H NMR spectra were
recorded on a Nicolet NT-300 spectrometer using a deuteriated
solvent as the internal lock and internal reference (CDCl3: δ
7.25 ppm; C6D6: δ 7.16 ppm for 1H).
Ir r a d ia tion (UV) of F e(CO)5 a n d Meth yl tr a n s-Cr oto-
n a te. A solution of methyl trans-crotonate (28 µL, 0.27 mmol)
and Fe(CO)5 (35 µL, 0.27 mmol) in hexanes (20mL) was
allowed to come to temperature equilibrium at 0 °C for 30 min.
Irradiation (UV) of the slightly yellow solution with a continu-
ous nitrogen flow over the surface of the solution to remove
the evolved CO gave a red solution. Approximately every 30
min ca. 0.2 mL of the solution was removed by syringe for
immediate FTIR analysis.
Ir r a d ia tion of F e(CO)5 a n d Meth yl 3-Bu ten oa te. The
same procedures were used for this reaction as for methyl
trans-crotonate. After 6 h of irradiation, the solution was
treated with CO first at 0 °C for 1 h and then at 50 °C for 12
h as described in detail below in order to liberate the free
methyl crotonate; the resulting product mixture (with
a
capillary containing reference C6D6) was identified by its 1H
NMR spectrum as being 94% methyl trans-crotonate and 6%
of the starting methyl 3-butenoate. Their 1H NMR spectra
are as follows: methyl trans-crotonate (δ 7.47, d of q, J ) 15.6,
6.9 Hz, olefin H at C3; 6.35, d of q, J ) 15.6, 1.7 Hz, olefin H
at C2; 4.19, s, OCH3) and 6% of the starting methyl 3-butenoate
(δ 5.65, d of apparent q, J ) 10.2, 1.5, 1.5 Hz, olefin H at C4;
5.66, d of apparent q, J ) 17.1, 1.5, 1.5 Hz, olefin H at C4;
6.51, d of d of t, J ) 16.8, 10.2, 6.9 Hz, olefin H at C3; 3.57, d
of t, J ) 6.9, 1.5 Hz, H at C2; 4.17, s, OCH3). Although not
observed in this reaction, the 1H NMR spectrum of methyl cis-
crotonate in C6D6 is as follows: δ 7.47 (d of q, J ) 15.6, 6.9
Hz, olefin H at C3), 6.35 (d of q, J ) 15.6, 1.7 Hz, olefin H at
C2), 4.19 (s, OCH3).
Exp er im en ta l Section
All reactions were performed under a nitrogen atmosphere
in reagent-grade solvents using standard Schlenk techniques.
Hexanes and toluene were distilled under N2 from CaH2;
tetrahydrofuran (THF) was distilled from Na/benzophenone.
Silica gel (Davisil 62; 150 mesh, 58 Å) was purchased from
Davison Chemical. Fe(CO)5 was purchased from Aldrich and
used immediately after purification by bulb-to-bulb distilla-
tion. Methyl oleate (99%), methyl palmitate (99%), ethyl
4-methyl-4-pentenoate (98%), methyl trans-crotonate (98%),
methyl 3-butenoate (97%), pyridine, cis-cyclooctene (99%), and
Fe2(CO)9 (98%) were purchased from Aldrich and used without
further purification.
The photochemical reactions were performed in a quartz
tube equipped with a nitrogen bubbler using a 450-W Hanovia
high-pressure mercury UV lamp (Ace, cat. no. 7825-34). The
temperatures (-35 to +20 °C) of the photochemical reaction
solutions were maintained by a Lauda RK 20 constant tem-
perature circulator. Infrared spectra were obtained on a
Nicolet 710 FTIR spectrophotometer using a solution cell with
NaCl salt plates (0.2 mm). A Varian 3400 GC interfaced to a
Finnigan TSG 700 high-resolution magnetic sector mass
spectrometer with electron ionization (70 eV) was used for all
GC-MS measurements. Gas chromatographic analyses were
performed with a temperature-programmed Varian 3400 GC
using a 25 m HP-1 (cross-linked methylsilicone gum phase)
capillary column (OV-101) with a flame ionization detector.
Internal standards of reactants or products were used in
Ir r a d ia tion of F e(CO)5 a n d Meth yl Olea te. A solution
of methyl oleate (90 µL, 0.27 mmol) and Fe(CO)5 (35 µL, 0.27
mmol) in hexanes (20 mL) at 0 °C was irradiated under a
continuous nitrogen flow. The solution turned from pale
yellow to red during the irradiation. As the irradiation
was prolonged, the color of the solution turned to deep
red. Periodically, 0.5 mL of the solution was removed by
syringe for analysis. A portion of the solution (0.2 mL) was
syringed into the NaCl IR cell for immediate FTIR analy-
sis. The other portion of the solution (0.3 mL) was added to a
vial (1.5 mL) containing a 20-fold amount of pyridine; this
solution was stirred at room temperature for 10 min and then
analyzed by GC for the organic compounds. Similar proce-
dures were used in irradiation experiments with different
concentrations of Fe(CO)5 and methyl oleate in different
solvents (toluene and THF) and at a higher reaction temper-
ature (20 °C).
Ir r a d ia tion of F e(CO)5 a n d Eth yl 4-Meth yl-4-p en ten o-
a te. This reaction was performed as described for methyl
oleate.
Rea ction of CO w ith a n Ir r a d ia ted Solu tion of F e(CO)5
a n d Meth yl Olea te. A hexanes solution (20 mL) of Fe(CO)5
(0.27 mmol) and methyl oleate (0.27 mmol) was irradiated at
0 °C for 3 h under a continuous N2 flow to give Fe(CO)3(η4-
CH3(CH2)14CHdCHCO2Me) (6). Then CO was bubbled through
the solution at 0 °C for 1 h, followed by stirring under the CO
atmosphere at 50 °C for 12 h. The resulting Fe(CO)5 and
solvent were vacuum distilled (bulb-to-bulb) from the reaction
mixture. The residue was then chromatographed in hexanes
through a short (1 × 10 cm) column (silica gel) to remove the
iron impurities. The composition of the resulting solution was
analyzed by GC as described above. The compounds present
in the mixture were identified by the following 1H NMR
spectra in CDCl3, which are very similar to those reported in
(16) Adams, C. M.; Cerioni, G.; Hafner, A.; Kalchhauser, H.; von
Philipsborn, W.; Prewo, R.; Schwenk, A. Helv. Chim. Acta 1988, 71,
1116.
(17) Brodie, A. M.; J ohnson, B. F. G.; J osty, P. L.; Lewis, J . J . Chem.
Soc., Dalton Trans. 1972, 2031.
(18) Cardaci, G. J . Am. Chem. Soc. 1975, 97, 1412.
(19) Gerhartz, W.; Grevels, F.-W.; Klotzbu¨cher, W. E. Organome-
tallics 1987, 6, 1850.
(20) Grevels, F.-W.; Schulz, D.; Koerner von Gustorf, E. Angew.
Chem., Int. Ed. Engl. 1974, 13, 534.
(21) Koerner von Gustorf, E.; J aenicke, O.; Polansky, O. E. Z.
Naturforsch. 1972, 27b, 575.