The conversion of acyl dicarbonyl cyclopentadienyliron compounds Cp(CO)2Fe(COR) into aldehydes is not straightforward. It is shown here, mainly from electrochemical results, that the hydrogenolysis of the metal acyl bond can be efficiently achieved by trialkytin hydrides under very mild conditions, according to a chain reaction process initiated by an electron transfer to the acyl complex. The expected aldehyde is formed together with the heterobinuclear iron-tin complex

Amatore, Christian

Bayachou, Mekki

Buriez, Olivier

Verpeaux, Jean Noel

English

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Cleveland State University
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Chemistry Faculty Publications Chemistry Department
9-15-1998
Electron Transfer Catalysis of The Hydrogenolysis
of Acyl Dicarbonyl Cyclopentadienyliron
Complexes by Tributyltin Hydride
Christian Amatore
Processus d’Activation Moléculaire
Mekki Bayachou
Cleveland State University, M.BAYACHOU@csuohio.edu
Olivier Buriez
Processus d’Activation Moléculaire
Jean Noel Verpeaux
Processus d’Activation Moléculaire
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Recommended Citation
Amatore, Christian; Bayachou, Mekki; Buriez, Olivier; and Verpeaux, Jean Noel, "Electron Transfer Catalysis of The Hydrogenolysis of
Acyl Dicarbonyl Cyclopentadienyliron Complexes by Tributyltin Hydride" (1998). Chemistry Faculty Publications. 311.
https://engagedscholarship.csuohio.edu/scichem_facpub/311
Electron transfer catalysis of the hydrogenolysis of acyl dicarbonyl
cyclopentadienyliron complexes by tributyltin hydride
Christian Amatore , Mekki Bayachou, Olivier Buriez, Jean-Noe¨l Verpeaux
Introduction
The reductive cleavage of the metal carbon bond of
transition metal acyl compounds is an important reac-
tion, which occurs as the final step of many catalytic
processes involving CO activation and fixation to or-
ganic moieties. In some cases, a simple protonation can
release the expected aldehyde as happens with tetracar-
bonyl acylferrates deriving from acylation of Collman’s
reagent [1]. In other cases, however, the intermediate
hydroxycarbene complex [2] is stable enough to be the
product of the protonation.
Cp(CO)2Fe(COR)H [Cp(CO)2FeCR(OH)]
Since the compounds bearing the dicarbonyl cy-
clopentadienyliron unit are among the most extensively
used in preparative chemistry [3], various hydrides have
been tested as possible hydrogenolysis reagents. LiAlH4
and related derivatives reacted with one of the terminal
CO ligands [4]; more covalent hydrogen transfer
reagents, such as hydrogen, silicon or tin hydrides have
then been considered to achieve the reductive cleavage
of the metal acyl bond. Upon thermal or photochemi-
cal activation, trimethylsilane or stannane did cleave
the metal carbon bond, but the silyl or stannyl deriva-
tive of the alcohol was formed simultaneously, together
with bi or tri nuclear mixed organometallics [5]:
Cp(CO)2Fe(COR)HMMe3
RCH2OMMe3Cp(CO)2Fe(H)(MMe3)2
or Cp(CO)2FeMMe3 MSi or Sn
The best conditions for a selective formation of the
aldehyde was trimethyltin hydride under photochemical
inducement [6].
Because of the practical importance of this reaction
in catalysis, and also because of the analogy between
photochemical and electron transfer activation in terms
of labilization of a coordination shell [7], we decided to
explore the possibility of an electrochemical inducement
of the reductive cleavage of the acyl metal bond by tin
hydride, and the mechanism thereof.
025
Electrochemical investigation
Voltammetric methods
At room temperature, the cyclic voltammogram of
Cp(CO)2Fe(COR) in THF consists in a fully chemically
reversible reduction wave, at a potential scan rate of 1
V s1 or above. This wave was shown to be monoelec-
tronic, with a standard potential E0 being 1.83 V
versus SCE when RMe, and 1.80 V when R
CH2CH2Ph. At lower scan rates, the ratio iox:i red grad-
ually becomes smaller than unity. This is clearly
indicative that the so-formed anion
[Cp(CO)2Fe(COR)] has a half lifetime of a few tenths
of a second at a millimolar concentration. Some
CpFe(CO)2 anion, the oxidation of which is visible at
0.90 V, is formed upon decomposition of the initial
paramagnetic anion, see Fig. 1a.
When the same cyclic voltammetry experiment was
run in the presence of one molar equivalent of trib-
utyltin hydride, the voltammogram was drastically
modified. At 0.5 V s1, the reduction wave correspond-
ing to the formation of the anionic complex
[Cp(CO)2Fe(COR)] dropped to approximately 30% of
its previous size and also became chemically irre-
Fig. 2. Steady-state voltammogram of Cp(CO)2Fe(COCH2CH2Ph), 2
mM solution in THF:0.3 M nBu4BF4 at a gold electrode (25 mm
diameter); potential scan rate 10 mV s1. , complex alone; , in
the presence of 5 molar equivalents of Bu3SnH.
versible, see Fig. 1b. The oxidation wave at 0.90 V
also disappeared showing that CpFe(CO)2 was no
longer formed in the diffusion layer. Simultaneously, a
new wave appeared at a slightly more negative poten-
tial, the size of which was close to the size of the
reduction wave of the acyl complex in the absence of
tin hydride. This new wave is chemically reversible,
located at a standard potential E0 2.02 V, and
could be assigned to the reduction of the heterobinu-
clear complex Cp(CO)2FeSnBu3 by comparison with
cyclic voltammograms of an authentic sample of this
latter compound [8].
Steady state voltammograms obtained at a small
microelectrode (¥ 25 mm) and very slow potential scan
rate (10 mV s1) are perhaps better suited to illustrate
the phenomenon: an important decrease of the current
corresponding to the reduction of the acyl complex in
the presence of tin hydride, together with the apparition
of a new wave and the conservation of the overall
reduction current; compare the two waves in the pres-
ence and absence of Bu3SnH in Fig. 2.
Increasing the amount of tributyltin hydride at the
same potential scan rate just made the observations
depicted above still more visible, i.e. led to a more
pronounced decrease of the size of the first reduction
wave. This indicates that the overall process is con-
trolled by a bimolecular step involving
[Cp(CO)2Fe(COR)] and Bu3SnH. Conversely, increas-
ing the potential scan rate for a given excess of tin
hydride progressively restored the initial voltammetric
pattern. Fig. 3 illustrates that it is necessary to reach
200 V s1 at minimum to ‘freeze’ the kinetics of the
chemical process when a ten molar equivalent excess of
Fig. 1. Cyclic voltammetry of Cp(CO)2Fe(COCH3), 2 mM solution in
THF:0.3 M nBu4BF4 at a gold electrode (0.5 mm diameter); potential
scan rate 0.5 V s1. Curve (a) complex alone; curve (b) in the
presence of 1 molar equivalent of Bu3SnH.
026
Fig. 3. Cyclic voltammograms of Cp(CO)2Fe(COCH2CH2Ph), 3 mM solution in THF:0.3 M
nBu4BF4 in the presence of ten molar equivalents
of Bu3SnH, at a gold electrode (0.5 mm diameter) and various potential scan rates.
tributyltin hydride is used. It is also noteworthy that
the first reduction wave remains fully irreversible even
at a potential scan rate (50 V s1) where most of its
initial size has been regenerated.
All these observations establish that a very fast reac-
tion between the reduced form of the initial acyliron
and tributyltin hydride takes place in the diffusion
layer. The strong decrease of the size of the reduction
wave R1 points to a process where the electron initially
used to reduce the acyl complex is given back to the
system at the end of the reaction scheme, in other
words, that the electron acts as a catalyst [9].
Identification of the products formed; preparati6e
aspects
The organometallic product of the reaction could be
identified from its reversible reduction wave as the
heterobinuclear complex Cp(CO)2FeSnBu3. Further
confirmation of this assignment was obtained by in-
frared spectroscopy. Indeed, a solution of
Cp(CO)2Fe(COCH2CH2Ph) in THF was electrolyzed at
1.8 V in the presence of 5 molar equivalents of
tributyltin hydride, and the course of the reaction was
monitored by recording IR spectra of aliquots. As the
electrolysis proceeded, the three characteristic absorp-
tion bands of the starting acyliron complex, two bands
at 2016 and 1956 cm1 for the two coordinated CO
and one at 1648 cm1 for the carbonyl of the acyl
group [10] progressively decreased, and three new
waves appeared concomitantly; two of them, in the CO
region at 1980 and 1929 cm1, corresponded to
Cp(CO)2FeSnBu3 [8], and the third and weaker one, at
1724 cm1 could easily be assigned to dihydrocin-
namaldehyde. At the end of the electrolysis, a yield of
aldehyde in the range of 60–70% could be estimated
from the intensity of the absorption at 1724 cm1 but
this determination lacked of accuracy because of the
small value of the extinction coefficient of the aldehyde
(about six to seven times smaller than the o of the
027
carbonyl ligands). This yield was, however, confirmed
by GC analysis.
The complete conversion of the acyl iron complex
and subsequent formation of the two products in good
to high yield were achieved when only 0.25 Faradays
per mole of starting complex were passed through the
solution, in agreement with an electron transfer
catalysed process.
The chain reaction process
In voltammetric experiments, the drop in the size of
the reduction wave of Cp(CO)2Fe(COR) when tin hy-
dride is added indicates that a chemical process ini-
tiated on the foot of this reduction wave, i.e. when a
small amount of reduced anionic complex is generated
in the diffusion layer, consumes most of the starting
acyliron during the time needed to scan over this wave.
This behaviour which corresponds to an electron trans-
fer catalysed process has been recognised in many
ligand exchange reactions and reviewed [11]. A homo-
geneous intermolecular electron transfer from the re-
duced form of one of the products (here the
heterobinuclear complex) to the starting material (Eq.
(1) below) is responsible for the propagation of the
chain reaction in the diffusion layer, see Scheme 1.
When the potential of the electrode now reaches the
reduction potential of this heterobinuclear compound,
the latter is stoichiometrically reduced, as shown by the
one electron reversible reduction wave R2.
At the first reduction wave R1 (zero electron at a
sufficiently long time scale):
[Cp(CO)2Fe(COR)] Bu3SnH
 [Cp(CO)2FeSnBu3] RCHO (1)
[Cp(CO)2FeSnBu3] Cp(CO)2Fe(COR)
 [Cp(CO)2FeSnBu3] [Cp(CO)2Fe(COR)] (2)
At the second reduction wave R2 (one electron):
[Cp(CO)2FeSnBu3]e [Cp(CO)2FeSnBu3]
During the preparative electrolysis, a controlled flow
of electrogenerated acyl iron anions diffuse from the
electrode to the bulk of the solution where the homoge-
neous process represented in Scheme 1 takes place.
Unidentified termination reactions lead to a decrease of
the catalytic efficiency by deactivation of the chain.
These are more efficient in preparative scale electrolyses
(where the turn-over is 4) than under the conditions
of voltammetry experiments [12]. Because of these ter-
mination reactions, it was necessary to maintain a small
current flowing during the whole electrolyses; an inter-
ruption of the current leads to an interruption of the
reaction.
The second step (Eq. (2)) of the propagation is
controlled by the difference DE0 between the standard
potentials of the two redox couples; since the standard
potential of the binuclear product is more negative than
that of the starting compound, it is exergonic and
therefore very fast [13]. The first step (Eq. (1)) is
expected to be rate determining.
A quantitative study of the kinetics is the object of
current investigations. However the time scale in which
this reaction takes place, as illustrated by the series of
voltammograms in Fig. 3, shows that the reaction of
[Cp(CO)2Fe(COR)] with 5 molar equivalents of trib-
utyl tin hydride (in a 10 mmol range concentration) is
at least 100 times faster that the spontaneous decompo-
sition of this anion. This fact rules out any possibility
of SN1-type reaction involving a partial decoordination
or cleavage of this anion prior to its reaction with the
tin hydride.
Conclusion
Cyclopentadienyl dicarbonyl acyliron compounds are
rather inert toward trialkyltin hydride in standard con-
ditions. However, a very fast hydrogenolysis leading to
the corresponding aldehyde takes place when a small
fraction of the acyliron complex is reduced to its radical
anion by electron transfer. The overall kinetics of this
chain process are controlled by a bimolecular reaction
between this electrogenerated anion and the tin hy-
dride. Since these acyl iron complexes are very easily
prepared by acylation of Cp(CO)2Fe this new reac-
tion can be seen as an indirect but chimioselective
reduction of acyl chlorides RCOCl into aldehydes.
Acknowledgements
This work was supported by the CNRS (URA 1679)
and the Ecole Normale Supe´rieure. The authors wish to
express their gratitude to Dr L. Pospisil and Dr J.
Fiedler (J. Heyrovsky Institute of Physical Chemistry,
Prague, Czech Republic) for stimulating discussions;Scheme 1.
028
indeed, this preliminary work is further developed with
them as part of a European COST project D5:008:94.
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Electron Transfer Catalysis of The Hydrogenolysis of Acyl Dicarbonyl Cyclopentadienyliron Complexes by Tributyltin Hydride

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Electron Transfer Catalysis of The Hydrogenolysis of Acyl Dicarbonyl Cyclopentadienyliron Complexes by Tributyltin Hydride

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