CO insertion in lithiated diene-tricarbonyliron complexes

Article abstract of DOI:10.1039/c0cc00461h

Lithiated 1,3-diene-iron complexes undergo a CO insertion in the presence of trimethyl phosphite via a ferrate intermediate to give 3-formyl or 3-acetyl-1,3-diene-Fe(CO)₂P(OMe)₃ complexes. Formylation proceeds when the reaction is quenched by a strong Bronsted acid, whereas the ketone is obtained after the treatment with iodomethane.

Full text of DOI:10.1039/c0cc00461h

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CO insertion in lithiated diene–tricarbonyliron complexesw
Tatsuo Okauchi,* Takao Teshima, Mitsuru Sadoshima, Hidekazu Kawakubo,
Kouta Kagimoto, Yoko Sugahara and Mitsuru Kitamura
Received 18th March 2010, Accepted 10th May 2010
First published as an Advance Article on the web 4th June 2010
DOI: 10.1039/c0cc00461h
Lithiated 1,3-diene–iron complexes undergo a CO insertion in
the presence of trimethyl phosphite via a ferrate intermediate
to give 3-formyl or 3-acetyl-1,3-diene-Fe(CO)2P(OMe)3
complexes. Formylation proceeds when the reaction is quenched
by a strong Brønsted acid, whereas the ketone is obtained after
the treatment with iodomethane.
Z4-1,3-Diene–tricarbonyliron complexes are quite common in
Scheme 2 syn/anti Isomerization of 1a.
organometallic chemistry oriented toward organic synthesis
because of the wide range of substituted dienes, the thermal
stability of the complex, and the high availability of the
starting iron species. Over recent years the reactivity of the
diene complexes has been extensively investigated and these
complexes have become useful reagents in organic synthesis.1,2
We previously reported that regioselective proton abstraction
occurred at C-3 in 1,3-diene–Fe(CO)3 complexes bearing a
phosphonic amide group at C-2 by using LDA and the
resulting anionic intermediate was trapped intermolecularly.3
To explore the proposed CO insertion, we carried out a
model reaction of Z4-1,3-diene–iron complex 1b. The complex
1b was treated with LDA in the presence of CO or a
phosphorus ligand at ꢀ78 1C, followed by the addition of
H2O (Table 1, entries 1–4).z When P(OMe)3 and PMe3 were
used as a ligand, CO insertion products 2b and 2b0 were
formed, respectively, albeit in low yields. The IR spectrum
of 2b showed strong carbonyl absorptions at 1988, 1928, and
1678 cmꢀ1. The first two peaks corresponding to CO ligands
appeared at lower wavenumbers relative to the peaks of 1b
(2049 and 1963 cmꢀ1) due to the electron-donating character
of P(OMe)3. The CO insertion was confirmed by the presence
of a carbonyl absorption at 1678 cmꢀ1. Addition of excess
P(OMe)3 had little effect on the yield (Table 1, entry 5). This is
the first example of CO insertion of deprotonated 1,3-diene–iron
complexes.5,6 In all reactions of entries 1–5 in Table 1, the
reaction was quenched with H2O as a Brønsted acid and a fair
amount of 3b and metallic iron were obtained. These results
suggest that the nucleophilic attack of H2O on the iron atom of
the deprotonated intermediate would give 3b. In order to avoid
decomposing the iron complex, various Brønsted acids were
examined. We discovered that the reaction progress was highly
dependent on the Brønsted acid employed.7 As shown in Table 1,
entries 5–9, the yield of 2b was improved and the formation of 3b
was suppressed when a strong acid was used due to the lower
nucleophilicity of its counter anion. The generation of ferrate B
was slow and led to the formation of 2b in low yield when the
reaction time was shortened to 30 min (entry 10).
In
a subsequent screening of trapping of the anionic
intermediate, we found that when syn- or anti-1a was treated
with LDA at ꢀ78 1C and was quenched with D2O, about a
3 : 7 mixture of syn and anti deuterated 1a was obtained
regardless of the stereochemistry of the starting material
(Scheme 1).4
The observed syn/anti isomerizations indicated that the
ferrate intermediate B may be produced in this reaction
(Scheme 2). Deuteration of syn- and anti-A, or B would
provide the mixture of syn- and anti-1a-d. To the best of our
knowledge, the conversion of diene–iron complexes into
ferrates is unknown. This finding prompted us to examine
the possibility of CO insertion into a Fe–C bond of the ferrate
B in Scheme 2. We now report the CO insertion into the anion
intermediate generated from diene–iron complexes.
Various diene–iron complexes having an amidophosphate
group were subjected to treatment with LDA in the presence of
P(OMe)3, followed by protonation using CF3SO3H. The results
are summarized in Table 2. The CO insertion was compatible
with a variety of cyclic and acyclic iron–diene complexes. The
syn/anti ratios of the product 2a (Table 2, entries 1 and 2) were
similar to those of deuterated 1a (Scheme 1). These results
suggests that 2a would arise from similar intermediates shown
in Scheme 2. When the complexes bearing a substituent at C-4 of
the diene moiety were used, a slight decrease in the yields was
observed. This is presumably because of the steric interaction
between the substituent and iron in the formation of ferrate B.
Scheme 1 Deuteration of syn-1a and anti-1a.
Department of Applied Chemistry, Kyushu Institute of Technology,
1-1, Sensui-cho, Tobata, Kitakyushu 804-8550, Japan.
E-mail: okauchi@che.kyutech.ac.jp; Fax: (+81)-(0)93-884-3314
w Electronic supplementary information (ESI) available: Details of
experimental procedures and characterization data. See DOI: 10.1039/
c0cc00461h
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Table 1 CO Insertion in the presence of a liganda
Table 2 CO insertion of iron complex 1
Entry Iron complex Product
Yield (%)a
Yield (%)c
2b or 2b0
3b
Entry
Ligand
H+b
1
2
3
4
5
syn-1a
anti-1a
2a
2a
70 (25 : 75)b
66 (23 : 77)b
1
2
3
4
5
6
7
8
9
10
COd
PPh3
H2Oe
—
—
8
17
16
19
33
82
43
16
29
64
62
52
54
—
—
42
H2Oe
PMe3
H2Oe
P(OMe)3f
P(OMe)3
P(OMe)3
P(OMe)3
P(OMe)3
P(OMe)3
P(OMe)3
H2Oe
H2Oe
CH3CO2H
CF3CO2H
CH3SO3H
CF3SO3Hg
CF3SO3Hg
quant.
29
1c
1d
1e
1f
2c
2d
2e
2f
99
54
94
57
a
All reactions were performed using 2.2 equiv. of LDA, 1.2 equiv.
b
of phosphorus ligand under N2, unless otherwise noted. 5 equiv. of
c
d
H+ was added as a THF solution. Isolated yield. After the
treatment of 1b with LDA for 1h under N2, CO gas was introduced
e
and the mixture was stirred for 1 h. 10 equiv. of H2O was added.
f
g
4.0 equiv. of P(OMe)3 was added. Added as an Et2O solution.
h
1b was treated with LDA and P(OMe)3 for 30 min.
Replacement of CO by a phosphorus ligand in a diene–iron
complex generally requires oxidative,9 photochemical,10 or
thermal conditions.11 On the other hand, the reactions observed
in this study occur under mild conditions (ꢀ78 1C, LDA). This
indicates that a coordinatively unsaturated iron species would be
generated from the deprotonated diene–iron complexes. Thus, the
plausible mechanism for this reaction is depicted in Scheme 3.
Deprotonation of the iron complex 1 leads to vinyllithium I, and
equilibration occurs to generate ferrate II. In the presence of
P(OMe)3, the CO insertion5 takes place in the Fe–C bond of the
ferrate II to give acyliron complex III and this insertion is
reversible.12 The addition of a Brønsted acid provides a diene–iron
complex bearing a formyl group. Now, NMR studies directed
toward the detection of these intermediates (I–III) are in progress.
Next, trapping the acyliron complex III (Scheme 3) by an alkyl
halide was tried.13 We previously reported that alkylation of the
anion I using MeI gave a methylated diene–tricarbonyliron
complex.3 Thus, the similar reaction was carried out in the
presence of P(OMe)3. The complex 1b was treated with LDA
and P(OMe)3 under the same conditions as shown in Table 2,
and then MeI was added into the reaction mixture instead of
trifluoromethanesulfonic acid (eqn (1)). The methylation
required a higher temperature (ꢀ78 1C for 0.5 h, then 0 1C
for 2 h) than protonation. The corresponding methylketone 4
was obtained in good yield.y
6
a
b
Isolated yield. The syn : anti ratio was determined by 1H NMR.8
Scheme 3 Proposed mechanism.
In conclusion, lithiated 1,3-diene–iron complexes undergo a CO
insertion in the presence of a phosphorus ligand via a ferrate
intermediate. Formylation proceeds when the reaction is
quenched by a strong Brønsted acid, whereas the ketone is
obtained after the treatment with iodomethane. This is the first
example of CO insertion of deprotonated diene–iron complexes.
This procedure provides
a new approach to prepare a
1,3-diene–iron complex bearing a carbonyl group at C-3.
We gratefully acknowledge financial support for this
research by Grant-in-Aid for Scientific Research from
MEXT. We also thank the Center for Instrumental Analysis
KIT for the measurement of analytical data. We also
thank Central Glass Co., Ltd. for a generous gift of
trifluoromethanesulfonic acid.
ð1Þ
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Notes and references
z Selected data. 1b: yellow oil, IR (neat) 2049, 1963, 1467, 1313, 1240,
1170 cmꢀ1 1H NMR (500 MHz; CDCl3; Me4Si) d 0.96 (3H, s), 0.99
;
4 The ratio and the stereochemistry were determined by 1H NMR
spectroscopy. The chemical shift of the CH3 group attached to an
sp3 carbon in syn-1a (d 1.03) is slightly higher than that of the
CH3 group in anti-1a (d 0.98) in agreement with the generally
observed deshielding effect of Fe(CO)3 on syn-substituents, see:
B. M. R. Bandara, A. J. Birch and W. D. Raverty, J. Chem. Soc.,
Perkin Trans. 1, 1982, 1745–1753; A. J. Birch, K. B. Chamberl,
M. A. Haas and D. J. Thompson, J. Chem. Soc., Perkin Trans. 1,
1973, 1882–1891.
(3H, s), 1.62 (1H, dd, J = 3.1, 14.7 Hz), 1.73 (1H, dd, J = 3.1, 15.0 Hz),
2.47 (1H, d, J = 6.7 Hz), 2.69 (3H, s), 2.71 (3H, s), 2.73 (3H, s), 2.75
(3H, s), 3.45 (1H, dt, J = 2.4, 2.8 Hz), 5.57 (1H, dd, J = 1.8, 6.4 Hz);
13C NMR (126 MHz; CDCl3; Me4Si) d 30.7, 34.1, 35.1, 36.5
(d, JP–C = 4.1 Hz), 36.6 (d, JP–C = 4.1 Hz), 43.2, 57.3 (d, JP–C
=
3.1 Hz), 64.6, 75.5 (d, JP–C = 4.1 Hz), 130.7 (d, JP–C = 2.1 Hz), 211.2.
Anal. Calcd for C15H23FeN2O5P: C, 45.25; H, 5.82; N, 7.04. Found:
C, 45.34; H, 5.89; N, 6.99. 2b: yellow solid, IR (ATR) 1988, 1928,
1678, 1377, 1309, 1171 cmꢀ1; H NMR (500 MHz; CDCl3; Me4Si) t
1
0.93 (3H, s), 1.06 (3H, s), 1.78–1.84 (2H, m), 2.57 (1H, d, J = 5.5 Hz),
2.70 (6H, d, J = 10.5 Hz), 2.77 (6H, d, J = 10.5 Hz), 3.59 (9H, d, J =
11.5 Hz), 3.70 (1H, s), 10.27 (1H, s); 13C NMR (126 MHz; CDCl3;
Me4Si) t 31.0, 34.0 (d, J PꢀC = 2.1 Hz), 34.2, 36.4 (d, J PꢀC = 4.2 Hz),
5 CO insertion of alkyl iron complexes, see: M. F. Semmelhack,
J. W. Herndon and J. K. LIu, Organometallics, 1983, 2, 1885–1888;
M. F. Semmelhack and J. W. Herndon, Organometallics, 1983, 2,
363–372.
36.7 (d, J
= 4.2 Hz), 43.6, 52.0 (d, J
= 4.0 Hz), 60.6
PꢀC
PꢀC
6 CO insertion in the presence of
a phosphorus ligand, see:
(d, J PꢀC = 8.3 Hz), 60.7, 81.1 (d, J PꢀC = 3.2 Hz), 132.5, 191.8; MS
(FAB/NPOE) m/z 523 (M+H+); HRMS (FAB/NPOE) calcd for
C18H33O8N2P2Fe (M+H+) 523.1062, found 523.1089.
J. P. Collman, J. N. Cawse and J. I. Brauman, J. Am. Chem.
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H. Ogino, Chem. Lett., 1999, 119–120.
y Data for 4: yellow oil, IR (ATR) 1994, 1920, 1673, 1462, 1360, 1311,
1225, 1155 cmꢀ1; 1H NMR (400 MHz; CDCl3; Me4Si) d 0.94 (3H, s),
1.04 (3H, s), 1.25 (1H, brs), 1.70 (1H, d, J = 15.1 Hz), 1.76 (1H, d,
J = 14.8 Hz), 2.46 (3H, s), 2.71 (6H, d, J = 10.3 Hz), 2.76 (6H, d,
J = 9.9 Hz), 3.58 (9H, d, J = 11.5 Hz), 3.71 (1H, brs); 13C NMR
(101 MHz; CDCl3; Me4Si) t 29.7, 31.0, 34.1, 34.2 (d, J PꢀC = 2.9 Hz),
7 Acceleration of the CO insertion with Brønsted acids, see:
S. B. Butts, T. G. Richmond and D. F. Shriver, Inorg. Chem.,
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8 The chemical shift of the CH3 group attached to an sp3 carbon in
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36.5 (d, J
= 5.0 Hz), 36.7 (d, J
= 5.5 Hz), 58.3 (t, J
= 2.6 Hz), 202.4; MS (FAB/NPOE) m/z 537 (M+H+);
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PꢀC
= 10.2 Hz), 62.3, 81.3, 130.1
PꢀC
PꢀC
(d, J
(d, J
PꢀC
PꢀC
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