Unusual anion effects in the iron-catalyzed formation of 3-hydroxyacrylates from aromatic aldehydes and ethyl diazoacetate

Article abstract of DOI:10.1002/ejic.201000240

Due to the lability of one of the CO ligands in irans-[Fe(PNP)(CO) ₂Cl]⁺ this compound is an efficient catalyst for the coupling of a series of aromatic aldehydes with ethyl diazoacetate (EDA), which give, in most cases, selectively 3-hydroxyacrylates rather than β-oxo esters. This reaction is strongly dependent on the nature of the counterion, Whereas with BF₄^- the reaction proceeds with conversions up to 90%, in the case of the counterions NO₃^-, CF ₃COO^-, CF₃SO₃^-, SbF ₆^-, and BAr'₄^- [Ar' = 3,5-(CF ₃)₂C₆H₃] no reaction took place. In the case of PF₆^- only up to 20% conversion was achieved. A conceivable mechanism for the coupling of aromatic aldehydes with EDA was established by means of DFT/B3LYP calculations, which allowed the rationalization of both the chemoselectivity and the role of the counterions.

Full text of DOI:10.1002/ejic.201000240

FULL PAPER
DOI: 10.1002/ejic.201000240
Unusual Anion Effects in the Iron-Catalyzed Formation of 3-Hydroxyacrylates
from Aromatic Aldehydes and Ethyl Diazoacetate
Luis Gonçalo Alves,[a] Georg Dazinger,[a] Luis F. Veiros,[b] and Karl Kirchner*[a]
Dedicated to Professor Kurt Mereiter on the occasion of his 65th birthday
Keywords: Iron / Pincer complexes / Aminophosphanes / C-C coupling / Anions / Density functional calculations
–
Due to the lability of one of the CO ligands in trans-[Fe(PNP)-
(CO)2Cl]+ this compound is an efficient catalyst for the cou-
pling of a series of aromatic aldehydes with ethyl diazoacet-
the case of the counterions NO3–, CF3COO–, CF3SO3
,
SbF6–, and BArЈ4 [ArЈ = 3,5-(CF3)2C6H3] no reaction took
place. In the case of PF6 only up to 20% conversion was
–
–
ate (EDA), which give, in most cases, selectively 3-hydroxy- achieved. A conceivable mechanism for the coupling of aro-
acrylates rather than β-oxo esters. This reaction is strongly
matic aldehydes with EDA was established by means of
DFT/B3LYP calculations, which allowed the rationalization
of both the chemoselectivity and the role of the counterions.
dependent on the nature of the counterion. Whereas with
–
BF4 the reaction proceeds with conversions up to 90%, in
Introduction
Likewise, Kanemasa et al. also reported the synthesis of
I and II using ZnCl2 in the presence of chlorotrimethylsil-
ane as catalyst.[6] Very recently, Pérez and co-workers have
reported the use of N-heterocyclic carbene based gold com-
plexes, which are capable of mediating this reaction with
low catalyst loadings and high reaction rates.[7] In all these
examples, however, a major problem is the selectivity since
typically mixtures of I and II are obtained. In this context
we have shown[8] that cationic iron pincer complexes of the
types [Fe(PNP)(CH3CN)3](BF4)2 and [Fe(PNP)(CO)-
(CH3CN)2](BF4)2 (PNP are various tridentate pincer-type
ligands based on 2,6-diaminopyridine), which contain labile
CH3CN ligands, are active catalysts for the coupling of aro-
matic aldehydes with EDA to selectively form I.[9] The for-
mation of II is typically below 3%.
Aromatic aldehydes are known to react with ethyl diazo-
acetate (EDA) in the presence of Lewis acids such as
BF3, ZnCl2, AlCl3, GeCl2, and SnCl4 to give mainly ethyl
3-aryl-3-oxopropanoates (β-oxo esters) I in high yields
(Scheme 1).[1–3] Hossain and co-workers have found that the
dicarbonyl(cyclopentadienyl) complex [FeCp(CO)2(thf)]-
BF4 is an excellent catalyst for the coupling of aromatic
aldehydes with EDA to afford ethyl 2-aryl-3-hydroxy-
acrylates (3-hydroxyacrylates) II as the main product
(Scheme 1).[4] They applied this methodology to the synthe-
sis of the Naproxen precursor 2-(6-methoxy-2-naphthyl)-
propenoic acid and other related compounds.[4b] Later on
the same authors also showed[5] that even the simple
Brønsted acid HBF4·Et2O is an active catalyst for the prep-
aration of I and II.
As part of our ongoing research on the synthesis and
reactivity of iron(II) pincer complexes[10] we have recently
prepared cationic complexes of the types trans-
[Fe(PNP)(CO)2Cl]+.[11] These complexes are unusual as
they feature two CO ligands in a mutual trans position
making one of them comparatively labile, which may thus
be replaced by other potential ligands. Accordingly, trans-
[Fe(PNP)(CO)2Cl]+ complexes may be promising candi-
dates as precatalysts for Lewis acid catalyzed reactions.
Here we report on our investigations on the catalytic ac-
tivity of the iron pincer complex trans-[Fe(PNP-iPr)(CO)2-
Scheme 1.
[a] Institute of Applied Synthetic Chemistry, Vienna University of
Technology,
–
–
–
–
Cl]X with X = NO3 , CF3COO–, CF3SO3 , BF4 , PF6 ,
Getreidemarkt 9, 1060 Vienna, Austria
Fax: +43-1-58801-16399
–
–
SbF6 , BArЈ4 [ArЈ = 3,5-(CF3)2C6H3] in the coupling of
aromatic aldehydes with EDA. Mechanistic aspects of this
reaction will be supported by DFT/B3LYP calculations.
E-mail: kkirch@mail.zserv.tuwien.ac.at
[b] Centro de Química Estrutural, Instituto Superior Técnico,
1049-001 Lisboa, Portugal
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Anion Effects in the Iron-Catalyzed Formation of 3-Hydroxyacrylates
the these compounds we first focused on 1·BF4 as precata-
lyst for the coupling of various aromatic aldehydes and
EDA. The results of this study are provided in Table 1. Op-
erationally, EDA and aldehyde were mixed in a 1:1 ratio
together with 10 mol-% of 1·BF4 in CH3NO2 as the solvent,
and the solution was stirred at room temperature for 16 h.
Although the catalytic reactions were routinely performed
under argon, admission of air did not affect the yields. In
most cases 3-hydroxyacrylates II, as judged from the 1H
NMR spectra of the crude reaction mixture, were formed
selectively. The formation of β-oxo esters I is typically
Results and Discussion
In the present study complexes of the type trans-
–
[Fe(PNP-iPr)(CO)2Cl]X (1·X) with X–= NO3 , CF3COO–,
–
–
–
–
–
CF3SO3 , BF4 , PF6 , SbF6 , and BArЈ4 were used as pre-
catalysts. They were prepared by treating [Fe(PNP-iPr)Cl2]
with 1 equiv. of the respective silver or sodium salts in the
presence of CO. It has to be noted that the synthesis of the
corresponding cis isomer, cis-[Fe(PNP-Ph)(CO)2Br]BPh4,
has been described recently and was found to be catalyti-
cally inactive for the coupling of aromatic aldehydes and
EDA.[9] In order to evaluate the scope and limitations of
Table 1. Yields of 3-hydroxyacrylates from the reactions of aromatic aldehydes with EDA catalyzed by trans-[Fe(PNP-iPr)(CO)2Cl]-
BF4 (1·BF4).[a,b]
[a] Reaction conditions: 1 equiv. aldehyde, 1 equiv. EDA, 10 mol-% catalyst, CH3NO2 as solvent, room temp., reaction time 16 h; yields
represent isolated yields (average of at least three experiments). [b] The yield of β-oxo ester is Ͻ3% unless indicated.
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L. G. Alves, G. Dazinger, L. F. Veiros, K. Kirchner
FULL PAPER
Ͻ3%, whereas the formation of epoxides was not observed Scheme 2), and methyl diazoacetate (MDA) were chosen. A
in any of these reactions. The yields of 3-hydroxyacrylates mechanistic proposal for the coupling of aromatic alde-
are in the range of 34–90%. The catalytic effect of the iron hydes with MDA catalyzed by A with no counterion inter-
–
complexes was confirmed by running the reaction of p-anis- actions considered – as would be the case for BArЈ4 and
–
aldehyde and EDA without catalyst. No product was possibly also SbF6 – is depicted in Scheme 3 and Figure 2.
formed, and only starting materials were isolated from the
reaction mixture.
In Lewis acid catalyzed transformations the nature of the
counterion sometimes has a strong effect on reaction rates,
yields, and even product distribution.[12–15] Therefore, we
also investigated the influence of the counterion on the cou-
pling of aromatic aldehydes with EDA by using trans-
[Fe(PNP-iPr)(CO)2Cl]X (1·X) complexes as catalyst precur-
–
sors (containing the counterions X– = NO3 , CF3COO–,
Scheme 2.
–
–
–
–
CF3SO3 , PF6 , SbF6 , BArЈ4 ) and p-anisaldehyde as
model substrate. Complexes 1·X bearing the coordinating
Since CO exhibits a strong trans effect (and trans influ-
anions NO3 , CF3COO– or CF3SO3– turned out to be com- ence) one of the two CO ligands is substitutionally labile.[11]
pletely inefficient and no conversion was observed (Fig- Accordingly, facile dissociation of one CO ligand and sub-
ure 1). This may be explained by competition of the anion sequent addition of an aldehyde molecule (which is present
and the substrate for the Lewis acid site.[11] If this is the in large excess under catalytic conditions, benzaldehyde in
case for all counterions, one would expect the activity of the calculations) affords [Fe(PNP-Me)(CO)(Cl){κ1(O)-
–
–
the precatalyst to increase in the order NO3 Ͻ CF3COO– benzaldehyde}]+ (A).
–
–
–
–
–
Ͻ CF3SO3 Ͻ BF4 Ͻ PF6 Ͻ SbF6 Ͻ BArЈ4 . Surpris-
Nucleophilic attack of MDA to the coordinated aldehyde
ingly, this is not the case. In fact, complexes 1·X bearing the yields, via transition state TSAB, intermediate B. Upon N2
–
non-coordinating anion BArЈ4 or the poorly coordinating loss the carbenium ion is stabilized by a comparatively
–
anion SbF6 turned out to be completely inefficient, strong C···Cl (1.87 Å) interaction. Once TSAB is reached the
whereas in the case of 1·X with PF6– as weakly coordinating changes associated with this step are already visible. The
counterion the conversion was merely 20%. In order to ex- new C–C bond formed is 1.57 Å, while at the same time N2
plain this phenomenon we propose that the weakly coordi- is moving away from the original MDA molecule with a C–
nating anions interact simultaneously with either catalyst, N separation of 1.77 Å. Despite the fact that a new C–C
coordinated aldehyde and/or EDA by hydrogen-bonding in- bond is formed and N2 is liberated, the activation barrier
teractions, which must be highly sensitive to the nature of for this process is as high as 41.4 kcal/mol. Consequently,
the anion. This observation is in strong contrast to the reac- despite the exergonicity (–3.2 kcal/mol), the kinetic barrier
tion of aromatic aldehydes and EDA catalyzed by both for the formation of B is prohibitively high and thus an
[Fe(PNP)(CH3CN)3](BF4)2 and [Fe(PNP)(CO)(CH3CN)2]- unlikely scenario. Nevertheless, preferential migration of
(BF4)2, where no anion dependence was observed.[9]
the aryl substituent (energy of activation is 13.9 kcal/mol)
over hydride migration (energy of activation is 27.4 kcal/
mol) leads eventually to C featuring a κ1(O)-coordinated
aldehyde–ester ligand, going over TSBC, while hydrogen mi-
gration affords D featuring a κ1(O)-coordinated β-oxo ester
ligand, passing through TSBD. The structures of all transi-
tion states are intermediate between the minima involved in
each path. Thus, in TSBD the transferring hydride is bridg-
ing the two C atoms with distances of 1.16 and 1.73 Å,
while in TSBC the migrating phenyl group exhibits C–CPh
distances of 1.64 and 1.96 Å. After liberation of the ester–
aldehyde in C by coordination of incoming aldehyde sub-
strate, that molecule rapidly tautomerizes to yield the corre-
sponding thermodynamically more stable 3-hydoxyacryl-
ates. This mechanism may explain why the reaction is not
proceeding in the case of non- and very weakly coordinat-
Figure 1. Plot of yield vs. counterion (X–) of the coupling of EDA
with p-anisaldehyde catalyzed by trans-[Fe(PNP-iPr)(CO)2Cl]X.
–
ing counteranions, i.e., BArЈ4– and SbF6 , since it was calcu-
lated in the absence of the counterion, and the energy bar-
In order to gain insight into to mechanism of this cata- rier involved is too high for a feasible reaction given the
lytic process and the subtle and unusual anion effect, experimental conditions (room temperature, 16 h).
–
DFT calculations were performed. As model systems
The possible participation of the counterion (BF4 ) in
[Fe(PNP-Me)(CO)(Cl){κ1(O)-benzaldehyde}]+ (A), [Fe- the reaction was also investigated computationally, and the
(PNP-Me,iPr)(CO)(Cl){κ1(O)-benzaldehyde}]+ (AЈ) (see results are presented in Figure 3. The pathway obtained for
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Anion Effects in the Iron-Catalyzed Formation of 3-Hydroxyacrylates
Scheme 3.
Figure 2. Energy profile for the coupling of [Fe(PNP-Me)(CO)(Cl){κ1(O)-benzaldehyde}]+ (A) and methyl diazoacetate (MDA) in the
–
absence of the counterion BF4 (energies in kcal/mol, numbers in parentheses are solvent-corrected energies; solvent: CH3NO2).
–
the formation of κ1(O)-coordinated β-oxo ester BH is repre- in a cyclic arrangement: complex(NH)···BF4 ···MDA···
sented on the left-hand side of Figure 2. In the reagent AH coordinated benzaldehyde. From AH the reaction follows a
there is a network of hydrogen bonds connecting the single-step mechanism to the formation of the final product
–
counterion BF4 with the complex and the MDA molecule BH showing that the overall process is slightly facilitated
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L. G. Alves, G. Dazinger, L. F. Veiros, K. Kirchner
FULL PAPER
Figure 3. Energy profile for the coupling of [Fe(PNP-Me,iPr)(CO)(Cl){κ1(O)-benzaldehyde}]+ (AЈ) and methyl diazoacetate (MDA) in-
–
volving of the counterion BF4 through hydrogen bonding (AH and AAr). Left-hand side: hydrogen migration; right-hand side: phenyl
migration (energies in kcal/mol, numbers in parentheses are solvent-corrected energies; solvent: CH3NO2).
with respect to the mechanism calculated without anion of the β-oxo ester complex BH (38.9 kcal/mol) indicating
that requires two steps to accomplish the reaction (Fig- that the aldehyde should be the product formed in the reac-
ure 2). In the transition state TSH the new C–C bond is tion, in agreement with the experimental observations.
AB
–
already formed (1.51 Å), while loss of N2 is underway
It should be added that the effect of PF6 as counterion
(dC–N = 1.93 Å). At the same time, the shifting hydrogen was also investigated. However, we were unable to locate
atom is starting to move from the carbonyl carbon atom any transition states, which may have led to experimentally
(dC–H = 1.13 Å) towards the adjacent carbon atom (dC–H observed products. In addition, solvation effects were evalu-
= 1.93 Å). The activation energy calculated for this path ated with the polarized continuum model (PCM), leading
(38.9 kcal/mol) is lower than the one obtained for the to similar results with only 2–4 kcal/mol differences (see
“anion-free” reaction (41.4 kcal/mol).
Figures 2 and 3, numbers in parentheses refer to solvent-
The energy profile calculated for the formation of the corrected energies, solvent = CH3NO2).
κ1(O)-coordinated aldehyde BAr, resulting from aryl mi-
–
gration with participation of the BF4 anion is represented
on the right-hand side of Figure 2. It should be noted that
Conclusion
this pathway was calculated with the model complex trans-
[Fe(PNP-Me,iPr)(CO)2Cl]+ (AЈ, see Scheme 2) where one of
We have shown that the trans-dicarbonyl complex trans-
–
the four P-methyl substituents is replaced by a bulkier iso- [Fe(PNP-iPr)(CO)2Cl]X with X = BF4 is an efficient pre-
propyl group. This is a more realistic model, closer to the catalyst for the coupling of aromatic aldehydes with ethyl
real system, and its use was necessary in order to prevent diazoacetate. A general and efficient protocol for this pro-
–
BF4 to form a strong hydrogen bond to the acidic N–H cess has been developed and various aromatic aldehydes
proton of the PNP ligand in the transition state, thereby have been used as substrates. In most cases, 3-hydroxy-
promoting aryl migration rather than hydrogen migration. acrylates rather than β-oxo esters are selectively obtained.
Computational limitations prevent us from using the real The catalyst is part of a new generation of air-stable, well-
complex (with four isopropyl groups) due to the size of the defined iron PNP pincer-type systems. However, this reac-
system. The calculated path starts from the aggregate of the tion turned out to be strongly dependent on the nature of
–
–
three species: the Fe complex, the BF4 anion, and the the counterion. Whereas with BF4 the reaction proceeds
–
MDA molecule held together by means of hydrogen interac- with conversions up to 90%, in the case of NO3 ,
–
–
–
tions (AAr). From here, the reaction is accomplished in one CF3COO–, CF3SO3 , SbF6 , and BArЈ4 no reaction took
–
single step through transition state TSArAB. In TSAr the place. With PF6 about 20% conversion was observed. A
AB
new C–C bond is already formed (1.52 Å), and loss of one conceivable mechanism for the coupling of aromatic alde-
N2 molecule is still proceeding (dC–N = 1.83 Å), while at the hydes with EDA was established by means of DFT/B3LYP
same time the phenyl group starts to migrate between the calculations, rationalizing both the chemoselectivity and the
two adjacent carbon atoms [dC–C(Ph) = 1.58 and 2.29 Å]. role of the counterions. The calculated mechanism indicates
The activation energy associated with this path is 32.1 kcal/ that the selective formation of 3-hydroxyacrylates II over β-
mol and thus lower than the one obtained for the formation oxo esters I, as experimentally observed, results from a
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Anion Effects in the Iron-Catalyzed Formation of 3-Hydroxyacrylates
afforded a red solid, which was dried under vacuum. Yield: 164 mg
lower energy barrier and, thus, a more favorable reaction
pathway that is stabilized by means of hydrogen-bond for-
mation with the BF4 anion.
(45%). IR (ATR): ν = 2012 (νC=O) cm–1.
˜
–
General Procedure for the Iron(II)-Catalyzed Synthesis of 3-Hy-
droxyacrylates: In a typical experiment, aldehyde (ca. 3–5 mmol,
1 equiv.) and ethyl diazoacetate (ca. 3–5 mmol, 1 equiv.) were
added to a solution of the catalyst (ca. 0.3–0.5 mmol, 10 mol-%) in
CH3NO2 (5 mL), and the mixture was stirred at room temperature
for 16 h. The mixture was then filtered through a plug of silica to
remove the catalyst, and the product was purified by flash
chromatography (silica, CH2Cl2). In the case of p-(dimethylamino)-
benzaldehyde, CH3CN was used as eluent.
Experimental Section
General: All manipulations were performed under argon by using
Schlenk techniques. Solvents were purified according to standard
procedures.[16] NaBArЈ4 was prepared according to the literature.[17]
Deuterated solvents were purchased from Aldrich and dried with
molecular sieves (4 Å). 1H, 13C{1H}, and 31P{1H} NMR spectra
were recorded with a Bruker Avance-250 spectrometer and were
referenced to SiMe4 (1H, 13C) and H3PO4 (85%) (31P).
Computational Details: Calculations were performed by using the
Gaussian 03 software package,[18] and the B3LYP functional[19]
without symmetry constraints. The optimized geometries were ob-
tained with the Stuttgart/Dresden ECP (SDD) basis set[20] to de-
scribe the electrons of the iron atom. For all other atoms the 6-
31g** basis set was employed.[21] Frequency calculations were per-
formed to confirm the nature of the stationary points, yielding one
imaginary frequency for the transition states and none for the min-
ima. Each transition state was further confirmed by following its
vibrational mode downhill on both sides, and obtaining the minima
presented on the energy profiles. Solvent effects (CH3NO2) were
considered through single-point energy calculations with the opti-
mized geometries by using the Polarizable Continuum Model
(PCM) initially devised by Tomasi and co-workers[22] as im-
plemented in Gaussian 03.[23,24] The molecular cavity was based on
the united atom topological model applied on UAHF radii, opti-
mized for the HF/6-31G(d) level.
trans-[Fe(PNP-iPr)(CO)2Cl]BF4 (1·BF4): Carbon monoxide was
bubbled through a solution of Fe(PNP-iPr)Cl2 (1.18 g, 2.52 mmol)
in CH3NO2 (60 mL) at room temperature, whereupon the brownish
solution turned into a blue suspension. After 2 min, AgBF4
(0.494 g, 2.54 mmol) was added. The bubbling was continued for
additional 10 min allowing the bluish suspension to turn into a red
suspension. After filtration through a syringe filter, the solvent was
removed under reduced pressure to afford a red solid, which was
washed with diethyl ether and dried under vacuum. Yield: 1.12 g
(77%). C19H33ClF4FeN3O2P2 (564.73): calcd. C 39.65, H 5.78, N
1
7.30; found C 39.60, H 5.81, N 7.27. H NMR (CD3NO2, 20 °C):
δ = 7.29 (t, J = 7.8 Hz, 1 H, py4), 6.63 (s, 2 H, NH), 6.21 (d, J =
7.8 Hz, 2 H, py3,5), 3.25 [m, J = 6.7 Hz, 4 H, CH(CH3)2], 1.56 [s,
12 H, CH(CH3)2], 1.53 [s, 12 H, CH(CH3)2] ppm. 13C{1H} NMR
(CD3NO2, 20 °C): δ = 211.6 (t, J = 24.4 Hz, CO), 161.4 (t, J =
6.2 Hz, py2,6), 141.2 (py4), 100.0 (t, J = 3.0 Hz, py3,5), 31.7 [vt, J =
12.2 Hz, CH(CH3)2], 16.9 [d, J = 3.7 Hz, CH(CH3)2] ppm. 31P{1H}
NMR (CD3NO2, 20 °C): δ = 119.2 ppm. IR [ATR (attenuated total
Acknowledgments
reflection)]: ν = 2020 (νC=O) cm–1.
˜
The support by COST Action D24 (“Sustainable Chemical Pro-
cesses: Stereoselective Transition Metal Catalyzed Reactions”) is
kindly acknowledged.
trans-[Fe(PNP-iPr)(CO)2Cl]CF3SO3 (1·CF3SO3): This complex
was prepared analogously to 1·BF4 with Fe(PNP-iPr)Cl2 (354 mg,
0.76 mmol) and AgCF3SO3 (198 mg, 0.77 mmol) as the starting
materials. Yield: 288 mg (60%). IR (ATR): ν = 2015 (νC=O) cm–1.
˜
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trans-[Fe(PNP-iPr)(CO)2Cl]CF3COO (1·CF3COO): This complex
was prepared analogously to 1·BF4 with Fe(PNP-iPr)Cl2 (318 mg,
0.68 mmol) and AgCF3COO (153 mg, 0.69 mmol) as starting mate-
rials. Yield: 137 mg (34%). C21H33ClF3FeN3O4P2 (601.75): calcd.
C 41.92, H 5.53, N 6.98; found C 41.80, H 5.41, N 6.87. IR (ATR):
ν = 2012 (νC=O) cm–1.
˜
trans-[Fe(PNP-iPr)(CO)2Cl]NO3 (1·NO3): This complex was pre-
pared analogously to 1·BF4 with Fe(PNP-iPr)Cl2 (212 mg,
0.45 mmol) and AgNO3 (80 mg, 0.47 mmol) as the starting materi-
als. Yield: 152 mg (61%). IR (ATR): ν = 2005 (νC=O) cm–1.
˜
trans-[Fe(PNP-iPr)(CO)2Cl]PF6 (1·PF6): This complex was pre-
pared analogously to 1·BF4 with Fe(PNP-iPr)Cl2 (219 mg,
0.47 mmol) and AgPF6 (123 mg, 0.49 mmol) as the starting materi-
als. Yield: 219 mg (74%). IR (ATR): ν = 2020 (νC=O) cm–1.
˜
trans-[Fe(PNP-iPr)(CO)2Cl]SbF6 (1·SbF6): This complex was pre-
pared analogously to 1·BF4 with Fe(PNP-iPr)Cl2 (300 mg,
0.64 mmol) and AgSbF6 (230 mg, 0.67 mmol) as the starting mate-
rials. Yield: 324 mg (70%). IR (ATR): ν = 2016 (νC=O) cm–1.
˜
[9] D. Benito-Garagorri, J. Wiedermann, M. Pollak, K. Mereiter,
K. Kirchner, Organometallics 2007, 26, 217.
trans-[Fe(PNP-iPr)(CO)2Cl]BArЈ4 (1·BArЈ4): NaBArЈ4 was added
to a suspension of 1·BF4 (156 mg, 0.27 mmol) in toluene (40 mL)
at room temperature and was stirred for 2 h, whereupon the solu-
tion turned red, and a precipitate was formed. Then, the mixture
was filtered through a syringe filter, and the solvent was evaporated
under reduced pressure to afford a red oil. Washing with n-pentane
[10] a) D. Benito-Garagorri, L. G. Alves, M. Puchberger, L. F.
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Received: February 28, 2010
Published Online: May 21, 2010
3166
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