Formation of acylruthenium promoted by coordination of AlMe3 to (η4-cyclopentadienone)Ru(CO)3

Article abstract of DOI:10.1039/b802019c

The reaction of AlMe₃ with (η⁴- tetraphenylcyclopentadienone)Ru(CO)₃ leads to rapid and quantitative formation of an adduct arising from coordination of the enone oxygen to aluminium, which undergoes alkylation at the Ru(CO)₃ moiety to give (η⁵-C₄Ph₄C(OAlMe₂))Ru(CO) ₂(COMe) concomitant with a change of hapticity of the dienone ligand. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802019c

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Formation of acylruthenium promoted by coordination of AlMe3 to
(g4-cyclopentadienone)Ru(CO)3†
Sensuke Ogoshi,* Tetsuya Kato, Masato Ohashi and Hideo Kurosawa
Received 4th February 2008, Accepted 11th February 2008
First published as an Advance Article on the web 25th February 2008
DOI: 10.1039/b802019c
4
The reaction of AlMe3 with (g -tetraphenylcyclopentadie-
none)Ru(CO)3 leads to rapid and quantitative formation of
an adduct arising from coordination of the enone oxygen
to aluminium, which undergoes alkylation at the Ru(CO)3
5
moiety to give (g -C4Ph4C(OAlMe2))Ru(CO)2(COMe) con-
comitant with a change of hapticity of the dienone ligand.
Scheme 1 Reaction of 1 with AlX3.
Although reaction of transition metal carbonyl complexes with
alkyllithium or alkylmagnesium gives acylate complexes by
alkylation of carbon monoxide on the transition metal,1 less
nucleophilic alkylmetal reagents do not react with transition metal
carbonyl complexes under similar reaction conditions. Thus, a new
method for enhanced reactivity between transition metal carbonyl
complexes and less nucleophilic alkylmetal reagents is required.
One solution to this issue is suggested by the observation that a
trialkylaluminium can undergo alkylation of the enone to give a
conjugate addition product in the presence of a transition metal
catalyst. As one possible reaction mechanism, we proposed that
the coordination of alkyl metals to the carbonyl group in the
enone triggered oxidative bond formation between the transition
metal center and the terminal carbon of a,b-unsaturated carbonyl
compounds, facilitating alkyl group transfer.2 From this point
5
Fig. 1 Zwitterionic g -structure.
comparison of 13C NMR and IR spectra with those of [(g -
hydroxytetraphenylcyclopentadienyl)Ru(CO)3][OTf] (3) (Fig. 2),
which was prepared by the reaction of 1 with CF3SO3H and
5
5
identified as having the g -structure by X-ray crystallography. As
a result of coordination of the carbonyl oxygen of 1 to aluminium
compounds, the CO stretching absorption of the coordinated
carbon monoxide in the infrared spectrum moved to a higher
wavenumber, and the resonance of the carbonyl carbon in the
TPCPD group moved to higher magnetic field in the 13C NMR
spectrum (Table 1). Moreover, these parameters progressively
approached those of 3 with increasing acidity of the aluminium
compounds. The above observation indicates that coordination
of the aluminium compounds increases electron donation from
ruthenium to TPCPD ligands. The positive charge on ruthenium
inhibits back donation to carbon monoxide. A similar influence on
a palladium-bound enone ligand by coordination of a Lewis acid
to enone oxygen was rationalized based on the MO calculation.2
4
of view, we assumed that an g -cyclopentadienone ruthenium
carbonyl complex would confirm if alkylation of a carbonyl ligand
with alkylaluminium is possible. In fact, it has been proposed that
4
5
inter-exchange of hapticity between g and g by coordination of
a Brønsted acid to oxygen plays an important role in catalytic
hydrogenation of unsaturated compounds.3 Therefore, formation
5
of a Lewis acid–cyclopentadienone complex would generate an g -
cyclopentadienyl group accompanied by enhanced electrophilicity
of the M–CO moiety. Here, we wish to report the reaction of
4
(g -tetraphenylcyclopentadienone)Ru(CO)3 with alkylaluminium
compounds, leading to the formation of acylruthenium complexes
triggered by the coordination of alkylmetal compounds to the
carbonyl oxygen of cyclopentadienone.
4
The reaction of (g -TPCPD)Ru(CO)3 (1) (TPCPD
=
tetraphenylcyclopentadienone) with aluminium compounds
quantitatively gave the corresponding coordination products
(Scheme 1, 2a: AlMe2Cl, 2b: AlMeCl2, 2c: AlCl3). The ex-
Fig. 2
5
tent to which the zwitterionic g -structure (Fig. 1) contributes
to complexes 2a, 2b and 2c could be estimated from the
4
The reaction of (g -TPCPD)Ru(CO)3 1 with 1 equiv of AlMe3
was also carried out. In the early stage of the reaction, quantitative
formation of the intermediate complex (2d) (Scheme 2) could be
deduced by observation of a doublet peak at d 7.50 in C6D6 in 1H
NMR. This doublet corresponds to the ortho protons of 2- and 5-
phenyl groups of the TPCPD ligand. The appearance of the peak
in this region seems to be indicative of coordination of AlX3 to
the carbonyl group in TPCPD; the ortho proton was observed at
Department of Applied Chemistry, Faculty of Engineering, Osaka University,
Suita, Osaka, 565-0871, Japan. E-mail: ogoshi@chem.eng.osaka-u.ac.jp;
Fax: (+81) 6-6879-7394; Tel: (+81) 6-6879-7392
† Electronic supplementary information (ESI) available: General proce-
dure and spectral data for all new compounds; crystallographic data in
CIF format (CCDC reference numbers 668259 (3) and 668260 (5)). See
DOI: 10.1039/b802019c
2232 | Dalton Trans., 2008, 2232–2234
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Table 1 13C NMR and IR data
explained by the contribution of alumoxy-substituted carbene
structure 4ꢀ to 4. The range of chemical shifts of usual carbene
carbons in transition metal carbene complexes occurs at a lower
magnetic field than that of acyl carbons in acyl complexes.5
As mentioned above, the coordination complex 2d is generated
quantitatively during the early stage of the reaction shown in
Scheme 2. Thus, the decrease in the coordination complex 2d
was followed using 1H NMR. The plot of −ln([2d]/[2d0]) vs. time
gave a straight line over four half-lives. Almost identical kobs were
obtained for three different concentrations of 1 in the presence
of 1 equiv of AlMe3 (= 2d) ([2d0] = 1.46 × 10−2 M, kobs = 2.2 ×
10−3 s−1;[2d0] = 2.93 × 10−2 M, kobs = 2.3 × 10−3 s−1; [2d0] = 5.85 ×
10−2 M, kobs = 2.4 × 10−3 s−1). These observations indicate that the
reaction proceeds in an intramolecular manner. For comparison,
Complex
Additive
dC= (ppm)a
mC≡O/cm−1b
O
1
none
173.0
158.1
155.1
150.8
148.6
2080, 2029, 2004
2104, 2056, 2035
2109, 2059, 2041
2111, 2061, 2042
2120, 2070,
2a
2b
2c
3
AlMe2Cl
AlMeCl2
AlCl3
TfOH
2050c
a CDCl3. b C6H6. c KBr.
4
6
the reaction of (g -cyclohexadiene)Ru(CO)3 with AlMe3 did not
proceed at all, while AlMe3 reacted with [CpRu(CO)3][OTf]7 very
slowly to give the corresponding acylruthenium complex, as well
as a small amount of methylruthenium complex8 (Scheme 4).
Scheme 2 Formation of acylruthenium complex.
d 7.48 in 1, d 7.56 in 2a, d 7.55 in 2b and d 7.50 in 2c. Complex 2d
easily underwent alkylation of carbon monoxide to quantitatively
give the expected acylruthenium complex (4). The spectrum of 4
in C6D6 showed the following characteristic signals: resonance at
289.8 ppm for acyl carbon in 13C NMR; the singlet at 2.36 ppm
corresponds to CH3 in acyl groups in 1H NMR; and the signal at
−0.55 ppm was assigned to OAlMe2 in 1H NMR.
Protonolysis of 4 gave the corresponding acylruthenium com-
plex (5),‡ the structure of which was determined by X-ray
crystallography (Scheme 3, Fig. 3). The short distance between two
Scheme 4 Control experiment.
4
˚
oxygen atoms (2.54 A) indicates the presence of hydrogen bonds.
The reaction of 5 with AlMe3 regenerated 4 quantitatively, which is
also consistent with the structure of 4. The resonances of carbonyl
carbon in the acyl group (d 289.8) and alumoxy-substituted carbon
in the cyclopentadienyl ring (d 141.3) in 4 appear at lower magnetic
fields than those (d 252.1, 130.5) in 5. This observation can be
The alkylation reaction might proceed as follows (Scheme 5).
The coordination of AlMe3 to carbonyl oxygen occurs very rapidly
to generate coordination complex 2d. Then, the electron density
on the ruthenium center is reduced by AlMe3 coordination to
TPCPD, resulting in the formation of zwitterionic intermediate
A. Then, direct nucleophilic attack of the carbon monoxide might
occur, since the nucleophilicity of the methyl group on aluminium
and the electrophilicity of the Ru–CO ligand are enhanced due
to the zwitterionic form. Alternatively, transmetallation might
proceed more readily, concomitant with slippage of the cyclopen-
tadienyl ring. Then, insertion of carbon monoxide would generate
a 16-electron acyl complex followed by ring slippage to give the
acyl complex 4. In general, alkyllithium and Grignard reagents
have been used for nucleophilic alkylation of metal carbonyls.8
Therefore, the types of functional group that can co-exist in
nucleophilic alkylation of metal carbonyl are limited. In contrast,
AlMe3 is more tolerant of a wide variety of functional groups.
Thus, the development of this reaction system into a general
Scheme 3 Hydrolysis of 4.
Fig. 3 Molecular structure of 5 with thermal ellipsoids at the 30%
probability level. Phenyl groups and hydrogen atoms except for H1 are
omitted for clarity. The location of the H1 hydrogen at O1 unequivocally
established the existence of an intramolecular O1–H1 · · · O2 hydrogen
bond.
Scheme 5 Plausible mechanism.
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method that is applicable to other alkylmetals would expand the
utility of the carbonylation reaction in organic synthesis.
In conclusion, we demonstrated the formation of an acyl
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complex by the reaction of (g -TPCPD)Ru(CO)3 with AlMe3.
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In this reaction, the most important step appears to be the
coordination of alkylmetal compounds to the carbonyl oxygen
of the TPCPD ligand. The Lewis acidity of alkylaluminium
compounds induces a positive charge on the ruthenium center,
which generates the zwitterionic intermediate. As a result, the
carbonylation reaction proceeds smoothly.
This research was supported by a Grant-in-Aid for Scientific
Research. Grant-in-Aid for Scientific Research on Priority Areas,
“Chemistry of Concerto Catalysis,” from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan is also
gratefully acknowledged.
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Notes and references
4
=
‡ To a solution of (g -C4Ph4C O)Ru(CO)3 (200 mg, 0.351 mmol) in 10 mL
of toluene was added 370 lL (1.0 M) of a solution of AlMe3 (0.370 mmol)
in n-hexane at room temperature. After 1 h, aqueous HCl (5 mL, 1.0 N)
was added to the solution and the mixture was stirred for 30 min at room
temperature. The solution was dried (MgSO4), filtered, and concentrated
in vacuo to give a yellow solid quantitatively. The solid was washed with
n-hexane and dried in vacuo to give a yellow solid 5 (194 mg, 94%). An
analytical sample was prepared by recrystallization from C6H6–n-hexane
1
solution. H NMR (C6D6, 400 MHz): d 2.45 (s, 3H) 6.80–6.81 (m, 6H),
6.88–6.95 (m, 6H), 7.11 (dd, J = 7.6, 8.0 Hz, 4H), 7.45 (d, J = 6.4 Hz,
4H), 10.5 (s, 1H). 13C NMR (C6D6, 100 MHz): d 50.8 (COCH3), 96.8 (C
3,4 of Cp), 107.0 (C 2,5 of Cp), 130.2–133.8 (aromatic), 202.2 (CO), 250.4
(COCH3). 1H NMR (CDCl3, 400 MHz): d 2.76 (s, 3H), 7.00–7.02 (m, 4H),
7.09 (t, J = 7.6 Hz, 4H), 7.15 (d, J = 7.6 Hz, 2H), 7.18–7.21 (m, 10H), 9.72
(s, 1H). 13C NMR (CDCl3, 100 MHz): d 51.1 (COCH3), 96.5 (C 3,4 of Cp),
106.4 (C 2,5 of Cp), 127.7–132.2 (aromatic), 130.5 (C1 of Cp), 201.3 (CO),
252.1 (COCH3). Anal. calcd for C33H24O4Ru: C, 67.68; H, 4.13. Found: C,
67.41; H, 4.23%.
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2234 | Dalton Trans., 2008, 2232–2234
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The Royal Society of Chemistry 2008
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