Iron(II) complexes bearing tridentate PNP pincer-type ligands as catalysts for the selective formation of 3-hydroxyacrylates from aromatic aldehydes and ethyldiazoacetate

Article abstract of DOI:10.1021/om060802o

Several new iron(II) complexes of the types [Fe(PNP)(CO)(CH ₃CN)₂]²⁺, [Fe(PNP)(CO)₂Br] ⁺, and Fe(PNP)Cl₂ containing tridentate PNP pincer-type ligands have been prepared and characterized.

Full text of DOI:10.1021/om060802o

Organometallics 2007, 26, 217-222
217
Iron(II) Complexes Bearing Tridentate PNP Pincer-Type Ligands as
Catalysts for the Selective Formation of 3-Hydroxyacrylates from
Aromatic Aldehydes and Ethyldiazoacetate
David Benito-Garagorri,† Julia Wiedermann,† Martin Pollak,† Kurt Mereiter,‡ and
Karl Kirchner*,†
Institute of Applied Synthetic Chemistry and Institute of Chemical Technologies and Analytics, Vienna
UniVersity of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
ReceiVed September 4, 2006
Several new iron(II) complexes of the types [Fe(PNP)(CO)(CH3CN)2]2+, [Fe(PNP)(CO)2Br]+, and Fe-
(PNP)Cl2 containing tridentate PNP pincer-type ligands have been prepared and characterized. Some of
them turned out to be efficient catalysts for the coupling of aromatic aldehydes with ethyldiazoacetate
(EDA) to give 3-hydroxyacrylates. These reactions are highly chemoselective, proceed under mild
conditions, exhibit good scope, and circumvent the need of slow addition of EDA at low temperature.
Scheme 1
Introduction
Aromatic aldehydes are known to react with ethyldiazoacetate
(EDA) in the presence of Lewis acids such as BF3, ZnCl2, AlCl3,
GeCl2, and SnCl4 to give mainly 3-oxo-3-arylpropanoic acid
ethyl esters (â-ketoesters) (A) in high yields (Scheme 1).1-3
Recently, Hossain and co-workers have found that the cyclo-
pentadienyl dicarbonyl Lewis acid [FeCp(CO)2(THF)]BF4 is an
active catalyst for the coupling of aromatic aldehydes with EDA
to afford 3-hydroxy-2-arylacrylic acid ethyl esters (3-hydroxy-
acrylates) (B) as the main product (Scheme 1).4 Similar results
were obtained by the same authors with the Bro¨nsted acid HBF4‚
Et2O as catalyst.5 This methodology was applied to the synthesis
of a naproxen precursor, a highly valued anti-inflammatory
drug.4b Moreover, the coupling of salicylaldehydes with EDA,
also reported by Hossain and co-workers, gives access to the
preparation of benzofurans, which can be used as building blocks
in the synthesis of several biologically active compounds.6
However, in these cases slow addition of EDA over a period
of 6-7 h at low temperature is required, and in addition
substantial amounts of â-keto esters are typically formed. Similar
results were also described recently by Kanemasa et al. by
utilizing ZnCl2 in the presence of chlorotrimethylsilane as
catalyst.7 Herein we describe a new class of iron catalysts8 that
are capable of mediating the direct coupling of aromatic
aldehydes with EDA to give chemoselectively 3-hydroxyacry-
lates in high isolated yields. The iron compounds tested as
catalysts bear PNP pincer ligands based on 2,6-diaminopyridine
and 2,6-diamino-4-phenyl-1,3,5-triazine as shown in Chart 1.
The ability to coordinate to metal centers very strongly makes
pincer ligands excellent candidates for the design of well-defined
catalysts since electronic, steric, and even stereochemical
properties can be varied easily in modular fashion.9
Results and Discussion
Synthesis of Fe(PNP) Complexes. Treatment of [Fe(PNP-
Ph)(CH3CN)3]2+ (1a) and [Fe(PNP-iPr)(CH3CN)3]2+ (1b) with
CO affords the monocarbonyl complexes cis-[Fe(PNP-Ph)-
(CO)(CH3CN)2]2+ (2a) and cis-[Fe(PNP-iPr)(CO)(CH3CN)2]2+
(2b) in 88 and 93% isolated yields, respectively (Scheme 2).
There was no evidence for the formation of dicarbonyl
complexes [Fe(PNP)(CO)2(CH3CN)]2+. On the other hand, with
[Fe(PNP-BIPOL)(CH3CN)3]2+ (1c), where the PNP ligand
contains the stronger π-accepting BIPOL units, no reaction took
place.
Characterization of complexes 2a and 2b was accomplished
by elemental analysis and 1H, 13C{1H}, and 31P{1H} NMR and
IR spectroscopy. In addition, the solid-state structure of cis-
[Fe(PNP-iPr)(CO)(CH3CN)2](BF4)2 (2b) was determined by
single-crystal X-ray diffraction. These complexes exhibit a
* Corresponding author. E-mail: kkirch@mail.zserv.utwien.ac.at.
† Institute of Applied Synthetic Chemistry.
‡ Institute of Chemical Technologies and Analytics.
(1) (a) Holmquist, C. R.; Roskamp, E. J. J. Org. Chem. 1989, 54, 3258.
(b) Holmquist, C. R.; Roskamp, E. J. Tetrahedron Lett. 1992, 33, 1131.
(2) (a) Padwa, A.; Hornbuckle, S. F.; Zhang, Z.; Zhi, L. J. Org. Chem.
1990, 55, 5297. (b) Sudrik, S. G.; Balaji, B. S.; Singh, A. P.; Mitra, R. B.;
Sonawane, H. R. Synlett 1996, 369.
(3) Nomura, K.; Iida, T.; Hori, K.; Yoshii, E. J. Org. Chem. 1994, 59,
488.
(4) (a) Mahmood, S. J.; Hossain, M. M. J. Org. Chem. 1998, 63, 3333.
(b) Mahmood, S. J.; Brennan, C.; Hossain, M. M. Synthesis 2002, 1807.
(5) Dudley, M. E.; Morshed, Md. Monzur; Brennan, C. L.; Islam, M.
S.; Ahmad, M. S.; Atuu, M.-R.; Branstetter, B.; Hossain, M. M. J. Org.
Chem. 2004, 69, 7599.
(6) Dudley, M. E.; Morshed, M. M.; Hossain, M. M. Synthesis 2006,
10, 1711.
(7) Kanemasa, S.; Kanai, T.; Araki, T.; Wada, E. Tetrahedron Lett. 1999,
40, 5055.
(8) Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Lackner, W.;
Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Organometallics 2006,
25, 1900.
(9) (a) Albrecht, M.; van Koten, G. Angew. Chem. 2001, 113, 3866;
Angew. Chem. Int. Ed. 2001, 40, 3750. (b) van der Boom, M. E.; Milstein,
D. Chem. ReV. 2003, 103, 1759. (c) Rybtchinski, B.; Milstein, D. Angew.
Chem. 1999, 111, 918; Angew. Chem. Int. Ed. 1999, 38, 870. (d) Singleton,
J. T. Tetrahedron 2003, 59, 1837. (e) Vigalok, A.; Milstein, D. Acc. Chem.
Res. 2001, 34, 798. (f) Milstein, D. Pure Appl. Chem. 2003, 75, 2003. (g)
Jensen, C. M. Chem. Commun. 1999, 2443.
10.1021/om060802o CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/24/2006

218 Organometallics, Vol. 26, No. 1, 2007
Benito-Garagorri et al.
Chart 1
Scheme 2
Figure 1. Structural view of cis-[Fe(PNP-iPr)(CO)(CH3CN)2]-
(BF4)2·(CH3)2CO (2b·(CH3)2CO) showing 50% thermal ellipsoids
(BF4-, (CH3)2CO, and C-bound H atoms omitted for clarity).
Selected bond lengths (Å) and angles (deg): Fe-C(18) 1.7635-
(16), Fe-N(1) 1.9785(13), Fe-N(4) 1.9286(14), Fe-N(5) 1.9786-
(14), Fe-P(1) 2.2670(4), Fe-P(2) 2.2674(5), P(1)-N(2) 1.6902-
(14), P(1)-C(6) 1.8458(16), P(1)-C(9) 1.8428(16), P(2)-N(3)
1.6936(14), P(2)-C(12) 1.8411(17), P(2)-C(15) 1.8404(16), O(1)-
C(18) 1.141(2); P(1)-Fe-P(2) 167.59(2).
singlet resonance in the 31P{1H} NMR spectrum at 95.9 and
114.5 ppm, respectively. In the 13C{1H} NMR spectrum the
CO ligand exhibits a low-intensity triplet resonance at 212.4
(J ) 27.0 Hz) and 215.4 ppm (J ) 25.9 Hz). The methyl groups
of the two inequivalent CH3CN ligands in 2a give rise to singlet
resonances at 5.1 and 3.4 ppm. In the case of 2b the resonance
of the two MeCN ligands exhibits signals at 3.8 ppm. In the 1H
NMR spectrum the methyl protons of the CH3CN ligands show
signals at 2.58 and 1.58 ppm (2a) and 2.53 and 2.40 ppm (2b).
All other resonances are unremarkable and are not discussed
here. In the IR spectrum the CO stretching frequencies for 2a
and 2b are observed at 2021 and 1995 cm-1, respectively,
indicating a weaker π-interaction in the case of 2a than in 2b
(cf. 2143 cm-1 in free CO).
Scheme 3
Scheme 4
An ORTEP diagram of 2b is depicted in Figure 1 with
selected bond distances and angles reported in the caption.
Complex 2b adopts a distorted octahedral geometry around the
metal center with the CO ligand being in trans position to an
acetonitrile ligand. The PNP ligand is coordinated to the iron
center in a typical tridentate meridional mode, with a P-Fe-P
angle of 167.6(1)°. The bond lengths around Fe are Fe-P(1)
2.267(1) Å, Fe-P(2) 2.267(1) Å, Fe-Npy 1.979(2) Å, Fe-C(18)
1.764(2), Fe-Ntrans 1.929(2), and Fe-Ncis 1.979(2) Å. The
Fe-N bond of the nitrile trans to the pyridine nitrogen of the
PNP ligand is slightly shorter than the one cis to the PNP moiety
due to the strong trans influence of the CO ligand.
iPr and PNP-iBu in THF at room temperature, the pentaco-
ordinated dichloro complexes [Fe(PNP-iPr)(Cl)2] (4a) and
[Fe(PNP-tBu)(Cl2)] (4b), respectively, were obtained in 90 and
84% isolated yields (Scheme 4). These yellow complexes are
paramagnetic and display contact-shifted 1H NMR spectra with
relatively narrow line widths at room temperature. In acetone-
A dicarbonyl complex, viz., cis-[Fe(PNP-Ph)(CO)2Br]+ (3),
could be obtained by reacting Fe(CO)4Br2 with 1 equiv of PNP-
Ph in the presence of NaBPh4 in CH2Cl2 (Scheme 3). If the
reaction was performed with PNP-iPr and PNP-tBu under the
same reaction conditions, no clean reaction took place, yielding
a mixture of isomers giving rise to several signals in the 31P-
{1H} NMR spectrum. Since all attempts to isolate any of these
compounds in pure form failed, this reaction was not further
investigated. Complex 3 exhibits a singlet resonance in the 31P-
{1H} NMR spectrum at 100.5 ppm. In the 13C{1H} NMR
spectrum the two CO ligands exhibit triplet resonances at 209.8
and 206.5 ppm with coupling constants of 20.2 and 23.1 Hz,
respectively, clearly indicating that the two CO ligands are cis
to one another.
1
d6, the H NMR spectrum of 4a displays all expected ligand
resonances, which can be readily assigned on the basis of
integration. Diastereotopic isopropyl methyl groups appear at
7.00 (12H) and 13.13 ppm (12H) and the CH protons give rise
to a signal at 140.35 ppm (4H), whereas the pyridine hydrogen
atoms are centered at 47.49 (2H) and -21.26 (1H). The NH
1
protons exhibit a signal at 75.30 ppm (2H). A concurrent H
NMR spectrum is obtained for 4b.
A structural view of 4b is depicted in Figure 2 with selected
bond distances and angles given in the caption. The coordination
geometry of the iron center is distorted square pyramidal with
the iron atom lying 0.615(1) Å out of the basal plane (defined
by N(1)-P(1)-P(2)-Cl(1), which show a rms aplanarity of
Upon treatment of anhydrous FeCl2 with 1 equiv of PNP-

Iron(II) Complexes Bearing Tridentate PNP Ligands
Organometallics, Vol. 26, No. 1, 2007 219
Changing the counterion in 2b from the weakly coordinating
BF4- to the noncoordinating anion tetrakis(3,5-ditrifluorometh-
ylphenyl)borate (BArF) (Table 1, entry 7) led to similar yields
and reaction rates. Thus, the nature of counterion is apparently
not affecting the coupling reaction, as has been frequently
observed for Lewis-acid-catalyzed transformations.14
Since this reaction is known to be catalyzed also by simple
Lewis acids such as HBF4‚Et2O,4 the acidic NH hydrogen atoms
of complexes 1-4 may release H+, being thus an additional or
even the sole source of the catalytically active species. In order
to rule out this possibility, complex 1d ([Fe(PNP-PhHex)(CH3-
CN)3](BF4)2), which lacks acidic NH hydrogen atoms, was also
utilized as a catalyst showing even higher activity than 1a, which
features acidic NH protons at the PNP ligand but bears also
phenyl substituents at the phosphorus atoms.
Figure 2. Structural view of [Fe(PNP-iPr)(Cl)2]·(C2H5)2O (4a·
(C2H5)2O) showing 30% thermal ellipsoids ((C2H5)2O and C-bound
H atoms omitted for clarity). Selected bond lengths (Å) and angles
(deg): Fe-N(1) 2.250(2), Fe-Cl(1) 2.3708(7), Fe-Cl(2) 2.3040-
(6), Fe-P(1) 2.4631(7), Fe-P(2) 2.4844(7), P(1)-N(2) 1.686(2),
P(1)-C(6) 1.877(3), P(1)-C(9) 1.819(3), P(2)-N(3) 1.691(2),
P(2)-C(12) 1.832(3), P(2)-C(15) 1.845(3); P(1)-Fe-P(2) 146.56-
(3), N(1)-Fe-Cl(1) 146.13(6), Cl(2)-Fe-N(1) 106.24(5), Cl(2)-
Fe-P(1) 105.19(3), Cl(2)-Fe-P(2) 100.48(3), Cl(2)-Fe-Cl(1)
107.60(3), N(1)-Fe-P(1) 75.95(5), N(1)-Fe-P(2) 76.55(5), Cl-
(1)-Fe-P(1) 93.75(3), Cl(1)-Fe-P(2) 98.60(3).
The effect of catalyst loading and reaction rate was then
determined using the same reaction conditions (Figure 3). The
reaction proceeds well with 10 mol % of catalyst 2b, showing
after 24 h a conversion of 95 and 83% for p-fluorobenzaldehyde
and p-anisaldehyde, respectively, with TONs in the range from
2.2 to 8.9. However, the conversion decreases to 60 and 50%
after the catalyst loading is decreased to 1 mol % (TONs: 23
to 60), which may be attributed to catalyst decomposition.
To investigate the scope and limitations of the iron PNP
system, complex 2b was used as catalyst. A survey of the
reactivity of EDA with various aromatic aldehydes under
catalytic conditions is provided in Table 2. Operationally, both
EDA and aldehyde were added together in a 1:1 ratio in CH3-
NO2 as the solvent and the solution was stirred for 16 h at room
temperature. Although the catalytic reactions were routinely
performed under an inert Ar atmosphere, admission of air did
not affect the yields. In all cases 3-hydroxyacrylates, as judged
0.056 Å) in the direction of the apical atom Cl(2). This feature
is similar to that reported for the five-coordinate complexes [Fe-
(2,6-bis(diisopropylphosphinomethyl)pyridine)(Cl)2],10 [Fe(2,6-
bis(dimethylaminomethyl)pyridine)(Cl)2],11 [Fe(2,6-bis(2,6-
diisopropylphenylaminomethyl)pyridine)(Cl)2],12 and [Fe(2,6-
bis(1-(2,6-diisopropylphenylimino)ethyl)pyridine)(Cl)2].13 The
bond distance of Fe-P(1) (2.4631(7) Å) is slightly shorter than
that of Fe-P(2) (2.4844(7) Å). The Fe-Cl(1) (basal) bond,
2.371(1) Å, is significantly longer than the distance to the apical
chlorine (Fe-Cl(2) 2.304(1) Å).
Catalytic Studies. Some of the complexes of the type [Fe-
(PNP)(CH3CN)3]2+ reported previously9 as well as the new
complexes described here were tested as catalyst precursors for
the coupling of aromatic aldehydes with EDA. A survey of
catalyst activity with p-anisaldehyde as model substrate proved
that both electronic and steric properties of the ligands are
important (Table 1). The most effective catalyst was found to
be cis-[Fe(PNP-iPr)(CO)(CH3CN)2]BF4 (2b), bearing both
strongly electron-donating and bulky iPr substituents in CH3-
NO2 as solvent. No reaction took place in CH3CN, suggesting
that initial dissociation of CH3CN to form the highly reactive
16e complex [Fe(PNP-iPr)(CO)(CH3CN)]+ is important. The
use of 2b leads to very high isolated yields of 3-hydroxyacrylate
(84%) with only a trace amount of â-keto ester being formed
(<3%). The trisacetonitrile complexes 1a-e are all less efficient
catalysts. Likewise, also complexes cis-[Fe(PNP-Ph)(CO)2Br]-
BPh4 (3) and [Fe(PNP-iPr)Cl2] (4b) in both the absence and
presence of AgSbF6 as halide scavenger turned out to be
inefficient as catalysts presumably due to decomposition of the
complexes. For comparison, with [FeCp(CO)2(THF)]BF4 as
catalyst a mixture of 3-hydroxyacrylate (60%) and â-keto ester
(20%) was obtained (Table 1, entry 12).
1
from the H NMR spectra of the crude reaction mixture, were
formed selectively. The formation of â-keto esters is <3%, while
the formation of epoxides was not observed in any of these
reactions. This is in contrast to the catalytic reactions of [FeCp-
(CO)2(THF)]BF4 with aromatic aldehydes and EDA, where
mixtures of 3-hydroxyacrylates and â-keto esters were observed.
For instance, the reaction of benzaldehyde and EDA at 25 °C
with [FeCp(CO)2(THF)]BF4 as catalysts (10 mol %) affords a
mixture of 3-hydroxy-2-phenylacrylic acid ethyl ester and 3-oxo-
3-phenylpropionic acid ethyl ester in 58 and 25% yields.4a The
same reaction performed with 2b yields almost exclusively
3-hydroxy-2-phenylacrylic acid ethyl ester in 91% yield (Table
2, entry 7). With the exception of p-dimethylaminobenzaldehyde
(entry 5), the yields of 3-hydroxyacrylates are in the range 74
to 91%. The catalytic effect of the iron complexes was
confirmed by running the standard reaction, i.e., p-anisaldehyde
and EDA, without catalyst. No product was formed and only
starting materials were isolated from the reaction mixture.
A tentative mechanistic proposal for the coupling of aromatic
aldehydes with EDA catalyzed by cis-[Fe(PNP-iPr)(CO)(CH3-
CN)2]2+ is presented in Scheme 5, which is similar to the one
suggested by Hossain et al.4a Since CO exhibits a much stronger
trans effect (and trans influence) than pyridine, the CH3CN
ligand trans to CO is more labile than the one trans to the
pyridine moiety of the PNP ligand. This fact may also account
for the higher reactivity of 2b as compared to 1b. Accordingly,
substitution of this ligand by an aldehyde molecule (which is
present in large excess under catalytic conditions) may afford
[Fe(PNP-iPr)(CO)(CH3CN)(κ1(O)-aldehyde)]2+ (A). Nucleo-
philic attack of EDA to the coordinated aldehyde via a SN2-
(10) Zhang, J.; Gandelman, M.; Herrman, D.; Leitus, G.; Shimon, L. J.
W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2006, 359, 1955.
(11) O’Reilly, R. K.; Gibson, V. C.; White, A. J. P.; Williams, D. J.
Polyhedron 2004, 23, 2921.
(12) Britovsek, G. J. P.; Gibson, V. C.; Mastroianni, S.; Oakes, D. C.
H.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J.
Inorg. Chem. 2001, 431.
(13) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.
1998, 120, 4049.
(14) Ku¨ndig, E. P.; Saudan, C. M.; Bernardinelli, G. Angew. Chem., Int.
Ed. 1999, 38, 1219.

220 Organometallics, Vol. 26, No. 1, 2007
Benito-Garagorri et al.
Table 1. Catalyst Screeninga
entry
catalyst
yield (%) A
yield (%) B
1
2
3
4
5
6
7
8
9
[Fe(PNP-Ph)(CH3CN)3](BF4)2 (1a)
<3
<3
<3
<3
<3
<3
<3
0
<3
<3
<3
20
21
66
53
49
44
84
80
0
13
26
37
60
[Fe(PNP-iPr)(CH3CN)3](BF4)2 (1b)
[Fe(PNP-BIPOL)(CH3CN)3](BF4)2 (1c)
[Fe(PNP-PhHex)(CH3CN)3](BF4)2 (1d)
[Fe(PNPT-Ph)(CH3CN)3](BF4)2 (1e)
cis-[Fe(PNP-iPr)(CO)(CH3CN)2](BF4)2 (2b)
cis-[Fe(PNP-iPr)(CO)(CH3CN)2](BArF)2 (2b′)b
cis-[Fe(PNP-Ph)(CO)2(Br)]BPh4 (3)
cis-[Fe(PNP-Ph)(CO)2(Br)]BPh4 (3) + AgSbF6 (1 equiv)
[Fe(PNP-iPr)(Cl)2] (4) + AgSbF6 (1 equiv)
[Fe(PNP-iPr)(Cl)2] (4) + AgSbF6 (2 equiv)
10
11
12
c
[FeCp(CO)2(THF)]BF4
a Reaction conditions: 1 equiv of aldehyde, 1 equiv of EDA, rt, CH3NO2 as the solvent, reaction time 16 h; yields represent isolated yields (average of
b
c
at least three experiments). BArF ) tetrakis(3,5-ditrifluoromethylphenyl)borate. 1.2 equiv of aldehyde, 1 equiv of EDA, 0 °C, CH2Cl2 as the solvent,
overall reaction time 16 h; isolated yields (ref 4a).
less likely but cannot be completely ruled out at this stage.
Likewise, the intermediacy of carbenes may have been taken
into account. In this context it has to be mentioned that 2b does
not react with EDA in the absence of aldehydes.
In summary, a general and efficient protocol for the coupling
of aromatic aldehydes with EDA has been developed. The
catalyst is part of a new generation of air-stable, well-defined
iron PNP pincer-type systems. Investigations into the use of
these compounds as catalysts in the coupling of EDA with
ketones, lactones, and lactams are ongoing. A detailed mecha-
nistic investigation based on DFT/B3LYP calculations is
currently also being pursued and will be reported in due course.
Experimental Section
Figure 3. Plot of yield versus time for the formation of 3-hy-
droxyacrylates from p-fluorobezaldehyde and p-anisaldehyde with
EDA utilizing 10 and 1 mol %, respectively, of cis-[Fe(PNP-iPr)-
(CO)(CH3CN)2](BF4)2 (2b) as catalyst.
General Procedures. All manipulations were performed under
an inert atmosphere of argon by using Schlenk techniques. The
solvents were purified according to standard procedures.16 N,N′-
Bis(diphenylphosphino)-2,6-diaminopyridine (PNP-Ph), N,N′-bis-
(diisopropylphosphino)-2,6-diaminopyridine (PNP-iPr), N,N′-bis-
(di-tert-butylphosphino)-2,6-diaminopyridine (PNP-tBu), [Fe(PNP-
Ph)(CH3CN)3](BF4)2 (1a), [Fe(PNP-Pri)(CH3CN)3](BF4)2 (1b),
[Fe(PNP-BIPOL)(CH3CN)3](BF4)2 (1c), [Fe(PNP-PhHex)(CH3-
CN)3](BF4)2 (1d), and [Fe(PNPT-Ph)(CH3CN)3](BF4)2 (1e) were
prepared according to the literature.9 The deuterated solvents were
type mechanism yields, via transition state B, intermediate C.
In the course of this step N2 is liberated. Preferential migration
of the aryl substituent over hydride migration leads eventually
to intermediate D, featuring an κ1(O)-coordinated aldehyde ester
ligand. After liberation of the ester aldehyde by incoming
aldehyde substrates, this molecule rapidly tautomerizes to yield
the thermodynamically more stable respective 3-hydoxyacry-
lates.
In principle, an alternative pathway could proceed via
formation of the epoxide ethyl-3-arylglycidate from complex
C and subsequent rearrangement to the corresponding enol ester,
since it is kown that transition metal complexes are able to
convert epoxides to ketones and aldehydes.15 This mechanism
was dismissed however since formation of 3-hydroxy-2-phe-
nylacrylic acid ethyl ester from ethyl-3-phenylglycidate in the
presence 2b under the same reaction conditions was not
observed.
1
purchased from Aldrich and dried over 4 Å molecular sieves. H,
13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker
AVANCE-250 spectrometer and were referenced to SiMe4 and H3-
PO4 (85%), respectively. 1H and 13C{1H} NMR signal assignments
1
were confirmed by H-COSY, 135-DEPT, and HMQC(1H-13C)
experiments.
cis-[Fe(PNP-Ph)(CO)(CH3CN)2](BF4)2 (2a). Carbon monoxide
was bubbled into a solution of 1a (0.5 g, 0.6 mmol) in acetone (30
mL) for 15 min, whereupon the color of the solution turned from
orange to yellow, and the mixture was stirred overnight at room
temperature under CO atmosphere. After removal of the solvent
under reduced pressure, the remaining yellow solid was washed
twice with Et2O (20 mL) and dried under vacuum. Yield: 0.4 g
(88%). Anal. Calcd for C34H31B2F8FeN5OP2: C, 49.98; H, 3.82;
N, 8.57. Found: C, 49.93; H, 3.71; N, 8.40. 1H NMR (δ, acetone-
d6, 20 °C): 9.21 (s, 2H, NH), 7.99-7.84 (m, 7H, Ph and py4),
A mechanism where in the initial step both aldehyde and EDA
are coordinated, as proposed by Kanemasa et al., seems to be
(15) (a) Maruoka, K.; Nagahara, S.; Ooi, T.; Yamamoto, H. Tetrahedron
Lett. 1989, 30, 5607. (b) Miyashita, A.; Shimada, T.; Sugawara, A.; Nohira,
H. Chem. Lett. 1986, 1323.
(16) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon: New York, 1988.

Iron(II) Complexes Bearing Tridentate PNP Ligands
Organometallics, Vol. 26, No. 1, 2007 221
7.56-7.40 (m, 14H, Ph), 6.70 (d, J ) 7.4 Hz, 2H, py3,5), 2.58 (s,
3H, CH3CN), 1.58 (s, 3H, CH3CN). 13C{1H} NMR (δ, acetone-d6,
20 °C): 212.4 (t, J ) 27.0 Hz, CO), 162.6 (t, J ) 8.3 Hz, py2,6),
143.6 (py4), 140.2 (Ph1), 133.4 (CH3CN), 133.1 (CH3CN), 132.1-
131.7 (m, Ph), 130.7-130.4 (m, Ph), 103.2 (t, J ) 4.1 Hz, py3,5),
5.1 (CH3CN), 3.4 (CH3CN). 31P{1H} NMR (δ, acetone-d6, 20 °C):
95.9. IR (KBr, cm-1): 2021 (νCdO).
Table 2. Yields of 3-Hydroxyacrylates from the Reactions of
Aromatic Aldehydes with EDA Catalyzed by
cis-[Fe(PNP-iPr)(CO)(CH3CN)2](BF4)2 (2)a,b
cis-[Fe(PNP-iPr)(CO)(CH3CN)2](BF4)2 (2b). This complex has
been prepared analogously to 2a with 1b (1.5 g, 2.2 mmol) and
CO as the starting materials. Yield: 1.4 g (93%). Anal. Calcd for
C22H39B2F8FeN5OP2: C, 38.80; H, 5.77; N, 10.28. Found: C, 38.90;
1
H, 5.71; N, 10.21. H NMR (δ, acetone-d6, 20 °C): 7.39 (s, 1H,
py4), 6.90 (s, 2H, NH), 6.34 (s, 2H, py3,5), 3.17 (s, 4H, CH(CH3)2),
2.53 (s, 3H, CH3CN), 2.40 (s, 3H, CH3CN), 1.56-1.50 (m, 24H,
CH(CH3)2). 13C{1H} NMR (δ, acetone-d6, 20 °C): 215.4 (t, J )
25.9 Hz, CO), 162.1 (t, J ) 6.6 Hz, py2,6), 141.8 (t, J ) 38.8 Hz,
py4), 139.8 (CH3CN), 100.59 (t, J ) 3.5 Hz, py3,5), 27.87 (t, J )
13.0 Hz, CH(CH3)2), 17.7 (d, J ) 29.7 Hz, CH(CH3)2), 17.0 (d, J
) 29.1 Hz, CH(CH3)2), 3.8 (d, J ) 10.1 Hz, CH3CN). 31P{1H}
NMR (δ, acetone-d6, 20 °C): 114.7. IR (KBr, cm-1): 1995 (νCd
O).
cis-[Fe(PNP-iPr)(CO)(CH3CN)2](BArF)2 (2b′). To a stirred
suspension of 2b (100 mg, 0.14 mmol) in CH2Cl2 (10 mL) was
added sodium tetrakis(3,5-ditrifluoromethylphenyl)borate(260 mg,
0.29 mmol), and the mixture was stirred for 2 h. During that time
the solution turned yellow and a white solid (NaBF4) was formed.
Solid materials were removed by filtration. The solvent was then
removed under reduced pressure, affording a yellow solid, which
was dried under vacuum. Yield: 275 mg (88%). Anal. Calcd for
C86H63B2F48FeN5OP2: C, 46.24; H, 2.84; N, 3.14. Found: C, 46.19;
1
H, 2.90; N, 3.25. H NMR (δ, CD2Cl2, 20 °C): 7.72 (bs, 16 H,
BArF), 7.57 (bs, 8H, BArF), 7.33 (t, J ) 8.4 Hz, 1H, py4), 6.30 (d,
J ) 8.4 Hz, 2H, py3,5), 5.68 (s, 2H, NH), 3.08 (bs, 2H, CH(CH3)2),
2.90 (bs, 2H, CH(CH3)2), 2.46 (s, 3H, CH3CN), 2.30 (s, 3H, CH3-
CN), 1.55-1.24 (m, 24H, CH(CH3)2. 13C{1H} NMR (δ, CD2Cl2,
20 °C): 219.9 (CO), 161.7 (q, J ) 49.7 Hz, BArF), 161.1 (py2,6),
143.1 (py4), 134.7 (BArF), 133.6 (CH3CN), 128.8 (q, J ) 32.2
Hz, BArF), 124.5 (q, J ) 270.7 Hz, BArF), 117.5 (BArF), 102.1
(py3,5), 29.6 (t, J ) 10.4 Hz, CH(CH3)2), 28.9 (t, J ) 12.3 Hz,
CH(CH3)2), 17.6 (t, J ) 5.7 Hz, CH(CH3)2), 4.65 (CH3CN), 4.47
(CH3CN). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 114.5.
cis-[Fe(PNP-Ph)(CO)2(Br)]BPh4 (3). To a solution of Fe-
(CO)4Br2 (250 mg, 0.76 mmol) and NaBPh4 (261 mg, 0.76 mmol)
in CH2Cl2 was added PNP-Ph (364 mg, 0.76 mmol), resulting in
significant gas evolution. The solution was stirred until no further
gas evolution was observed. Insoluble materials (NaBr) were then
removed by filtration. After removal of the solvent under reduced
pressure, the remaining orange solid was washed twice with Et2O
(20 mL) and dried under vacuum. Yield: 417 mg (82%). Anal.
Calcd for C55H45BBrFeN3O2P2: C, 66.83; H, 4.59; N, 4.25.
1
Found: C, 66.94; H, 4.66; N, 4.35. H NMR (δ, CD2Cl2, 20 °C):
7.90 (s, 2H, NH), 7.79-7.77 (m, 3H, Ph), 7.66-7.64 (m, 4H, Ph),
7.48-7.45 (m, 20H, Ph and py4), 7.02-7.00 (m, 10H, Ph), 6.85-
6.82 (m, 4H, Ph), 6.59-6.56 (d, J ) 7.9 Hz, py3,5). 13C{1H} NMR
(δ, CD2Cl2, 20 °C): 209.8 (t, J ) 20.2 Hz, CO), 206.5 (t, J ) 23.1
Hz, CO), 164.0 (q, J ) 49.32 Hz, BPh4), 159.3 (t, J ) 8.67 Hz,
py2,6), 142.4 (py4), 136.0 (BPh4), 132.2 (PPh2,6), 132.0 (PPh1), 130.0
(PPh4), 129.9 (PPh3,5), 125.8 (BPh4), 122.0 (BPh4), 102.7 (py3,5).
31P{1H} NMR (δ, CD2Cl2, 20 °C): 100.5.
[Fe(PNP-iPr)(Cl)2] (4a). FeCl2 (180 mg, 1.46 mmol) was added
to a solution of PNP-iPr (500 mg, 1.46 mmol) in THF, and the
reaction mixture was stirred overnight at room temperature. The
yellow solution was then filtered, and the solvent was evaporated
under reduced pressure. The remaining yellow solid was washed
twice with Et2O (10 mL) and dried under vacuum. Yield: 615 mg
(90%). Anal. Calcd for C17H33Cl2FeN3P2: C, 43.61; H, 7.10; N,
8.98. Found: C, 43.70; H, 7.16; N, 8.87. 1H NMR (δ, acetone-d6,
a Reaction conditions: 1 equiv of aldehyde, 1 equiv of EDA, 10 mol %
catalyst, CH3NO2 as the solvent, rt, reaction time 16 h. Yields represent
isolated yields (average of at least three experiments). The yield of â-keto
ester is <3%.
b

222 Organometallics, Vol. 26, No. 1, 2007
Benito-Garagorri et al.
Scheme 5
F2 was carried out with the program SHELXL97.17 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
inserted in idealized positions and were refined riding with the atoms
to which they were bonded.
20 °C, all peaks appear as broad singlets): 140.35 (CH(CH3)2),
75.30 (NH), 47.49 (py3,5), 13.13 (CH(CH3)2), 7.00 (CH(CH3)2),
-21.26 (py4). The identity of the NH protons was determined by
adding 0.2 mL of D2O to the NMR tube, whereupon the signal at
75.30 ppm disappeared.
Crystal data: 2b‚(CH3)2CO: C25H45B2F8FeN5O2P2, Mr ) 739.07,
monoclinic, space group P21/c (no. 14), T ) 100 K, a ) 10.8683-
(9) Å, b ) 10.4976(8) Å, c ) 29.963(2) Å, â ) 99.796(2)°, V )
3368.7(4) Å3, Z ) 4, µ ) 0.619 mm-1. Of 44 797 reflections
collected, 9753 were independent; final R indices: R1 ) 0.047 (all
data), wR1 ) 0.098 (all data). Crystal data of 4a‚(C2H5)2O: C21H43-
Cl2FeN3OP2, Mr ) 542.27, orthorhombic, space group P212121 (no.
19), T ) 100 K, a ) 9.6583(5) Å, b ) 14.5653(7) Å, c ) 20.2207-
(10) Å, V ) 2844.6(2) Å3, Z ) 4, µ ) 0. 0.847 mm-1. Of 32 002
reflections collected, 8261 were independent; final R indices: R1
) 0.043 (all data), wR1 ) 0.113 (all data). Further details (atomic
coordinates, displacement parameters, etc.) and geometric data are
given in the Supporting Information in CIF format.
[Fe(PNP-tBu)(Cl)2] (4b). This complex has been prepared
analogously to 4a with FeCl2 (100 mg, 0.78 mmol) and PNP-tBu
(314 mg, 0.78 mmol) as the starting materials. Yield: 343 mg
(84%). Anal. Calcd for C21H41Cl2FeN3P2: C, 48.11; H, 7.88; N,
8.01. Found: C, 48.21; H, 7.79; N, 8.11. 1H NMR (δ, acetone-d6,
20 °C, all peaks appear as broad singlets): 64.68 (NH), 51.57 (py3,5),
17.46 (CH3), -20.18 (py4).
General Procedure for the Iron(II)-Catalyzed Synthesis of
3-Hydroxyacrylates. Aldehyde (1 equiv) and ethyldiazoacetate (1
equiv) were added to a solution of the catalyst (10 mol %) in CH3-
NO2 (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).
Acknowledgment. Financial support by the “Fonds zur
Fo¨rderung der Wissenschaftlichen Forschung” is gratefully
acknowledged (Project No. P16600-N11). D.B.-G. thanks the
Basque Government (Eusko Jaurlaritza/Gobierno Vasco) for a
doctoral fellowship.
X-ray Structure Determination. X-ray data for cis-[Fe(PNP-
iPr)(CO)(CH3CN)2](BF4)2‚(CH3)2CO (2‚(CH3)2CO) and [Fe(PNP-
iPr)(Cl)2]‚(C2H5)2O (4a‚(C2H5)2O) were collected on a Bruker Smart
APEX CCD area detector diffractometer at T ) 100 K using
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) and
0.3° ω-scan frames covering complete spheres of the reciprocal
space with θmax ) 30°. After data integration with the program
SAINT corrections for absorption, λ/2 effects, and crystal decay
were applied with SADABS.17 The structures were solved by direct
methods using the program SHELXS97.18 Structure refinement on
Supporting Information Available: Spectroscopic data of all
organic compounds. Complete crystallographic data and technical
details in CIF format for 2b‚(CH3)2CO and 4b‚(C2H5)2O. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
OM060802O
(17) Bruker programs: SMART, version 5.629; SAINT, version 6.45;
SADABS, version 2.10; SHELXTL, version 6.14; Bruker AXS Inc.: Madison,
WI, 2003.
(18) Sheldrick, G. M. SHELX97, Program System for Crystal Structure
Determination; University of Go¨ttingen: Go¨ttingen, Germany, 1997.